bacteria in yogurt experiment

• 2 tablespoons (30 ml) of yogurt with live bacteria
• 1 L (33.8 fl oz) of milk
• 1 thermometer going to 85 °C (185 °F)
• 1 bowl with lid
• 1 tablespoon measuring spoon (15 mL)

Short explanation

Long explanation.

• What kind of milk gives the best results?
• What kind of yogurt gives the best result?
• What will be the result if you use a yogurt with flavor?
• At what temperature does the yogurt thicken the fastest?

Yeast and a balloon

Rainbow milk

Screaming dry ice

Dry ice in a balloon

Special: Dry ice color change

Dry ice smoking soap bubble snake

Dry ice giant crystal ball bubble

Dry ice in water

Gummy bear osmosis

Floating ping pong ball

Rotating Earth

Special: Colored fire

Special: Fire bubbles

Water cycle in a jar

Egg drop challenge

Taking the pulse

Orange candle

Glass bottle xylophone

Warped spacetime

Homemade rainbow

Water implosion

Warm and cold plates

Plastic bag kite

Tamed lightning

Forever boiling bottle

Moon on a pen

Moon in a box

Inexhaustible bottle

Crystal egg geode

Magic ice cut

Leaf pigments chromatography

Heavy smoke

Popsicle stick bridge

Micrometeorites

Special: Fire tornado

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Dancing water marbles

Brownian motion

Flying static ring

Water thermometer

String telephone

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Disappearing styrofoam

Special: Burning money

Special: Burning towel

Salt water purifier

Fish dissection

Hovering soap bubble

Homemade sailboat

Water mass meeting

Plastic bag and pencils

Water sucking bottle

Water sucking glass

Mentos and coke

Aristotle's illusion

Spinning spiral snake

Imploding soda can

Carbon dioxide extuingisher

Plastic bag parachute

Dental impression

Impact craters

Rolling static soda can

Static paper ghost

Color changing flower

Upside down glass

Shrinking chip bag

Solar system model

Strawberry DNA

Electric motor

Flashy electric motor

Bouncing soap bubbles

Toilet paper roll maraca

Cloud in a bottle 1

Cloud in a bottle 2

Balloon rocket

Water whistle

Homemade yogurt

Special: Screaming gummy bear

Homemade compass

Trash airplane

Wind-up spinner toy

Tea bag rocket

Balancing soda can

Lung volume test

Fireproof balloon

Baking powder popper

Expanding space

Straw propeller

Wooden cutlery

Levitating match

Human reflexes

Electromagnet

Soil layers

Straw potato

Straw rocket launcher

Traveling flame

Water bowls

Straw duck call

Solar eclipse

Silo of salt

Balloon skewer

Newspaper tower

Microwave light bulb

Heavy paper

Rubber chicken bone

Homemade marble run

Drops on a coin

Cartesian diver

Content of website.

Incorporate STEM journalism in your classroom

• Exercise type: Activity
• Topic: Life

How is yogurt made?

• Download Yogurt Activity Guide
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Class time: 30 to 50 minutes during two class periods.

Mostly Microbes

A biologist mom shares stories of our microbial world.

The Great Yogurt Experiment – Test 1

It’s taken too long for me to actually do it, but now I HAVE!! I’ve made homemade yogurt! Correction – WE experimented with making homemade yogurt! Despite my love of all things microbial and my dislike of spending lots of time cooking, I’ve not really made much fermented foods other than pickles about 15 years ago. Those days are changing! Several friends along the way have encouraged me to make homemade yogurt, but reading the book The Good Gut pushed me over the edge. Certainly we go through a ton of yogurt and heck – what kind of microbe-lover am I to not make fermented foods?

Ever since getting pregnant with Emily, my mid-morning snack has been a cup of plain Greek style yogurt that I add fruit to. Yogurt, especially the thicker Greek-style, is the one thing all members in my family enjoy – and so do our microbes. It’s a fabulous substitute for sour cream and often adds depth to butternut squash soup. Funny – reading this – it sounds like I like to cook and am a foodie. Truth is, that’s my husband, but I guess he’s teaching me to appreciate food more nowadays.

So – how did we do this? Simple. All it takes is heating up some milk, plopping in some bacteria to do all the “cooking”, and I get to sleep (hopefully) while the bacteria do all the work. Like one friend said, “It’s like magic”. Oh so true!

• Get out your culturing containers. We used glass mason jars and a cooler. One friend does his culturing in an insulated thermos. I’ve heard of people using crockpots.

• Stir so milk doesn’t stick to the bottom.
• Let the milk cool to 115 °F.
• Pour into jars.
• Add ¼ c of starter culture, the contents of a probiotic capsule, or a commercial yogurt starter.Waiting for the milk to cool before putting into the jars. Note the starter cultures lurking in the background.
• Whisk, gently

• Cap the jar.
• Incubate overnight in a pot or cooler with a few inches of water around 105-115°F.

• Sleep (my favorite step!)
• In the morning move the jars to the fridge to set up a bit more.
• Have a taste test

Easy peasy – eh? Yeah, the trickiest thing is getting the temperature right and I was probably a little more attentive to that than one needs to be. Sometimes my kitchen is more like my lab bench.

So with our experiment, we decided to play a little with different starter cultures. This is just me. I can’t simply make yogurt for the first time – I have to do an experiment with several different cultures. Simple isn’t really what I do. Thankfully, my girls were really excited about this and with lots of starter cultures everyone got to do something. Here’s the list of starter cultures:

• Greek yogurt containing
• A single strain probiotic
• A multi-strain probiotic
• Regular yogurt
• Unboiled milk

In another post I’ll talk about the actual microbial members doing the work.

Moment of Truth – The Taste Test

Finally – the taste test. Taste tests are something my husband often suggests and are lots of fun. Previous house hold taste tests: What apple varieties do we all like? Does Jac REALLY like white bagels more than multigrain (answer is NO – she doesn’t! ha!!)

To make it fair, I randomly numbered the tops of the jars and scooped a sample into bowls. One thing was quickly apparent – where fermentation had occurred, the texture was grainy or lumpy. The boiled milk was smooth. It was also really sweet. That was the milk sugar or lactose that had been broken down by the boiling. When that lactose is fermented – or digested by the bacteria – it is turned into lactic acid. It’s the lactic acid that gives yogurt a sour or tart taste.

Tasting helped us hone in more as to which bowl had what yogurt starter. Two bowls tasted like unflavored yogurt; they were the ones that had had the yellowish liquid on top. We all liked the flavor of the greek yogurt the best. The kefir-starter was the tartest tasting yogurt, with the multi-strain probiotic running a close second. The single strain probiotic was nasty tasting. In looking up more about this species in general, it was initially isolated from coagulated evaporated milk and can form lactic-acid. Perhaps it does, but we don’t care for the other by-products!

Issues and “Modifications for Future Experiments”

The one draw-back was the texture wasn’t very good. Also in reading the food science book, I’m wondering if I should have perhaps cooked the milk longer than I did. Maybe that would help?

I did drain some of the yogurt in cheese cloth to thicken it up a bit more and whisked it a bit and that helped a little. Still not the same texture as store-bought yogurt, but it sounds like commercial yogurt has additional thickeners. Still – I enjoyed it and so did the family.

Next – I’m going to mix a few different yogurts and probiotics together and see how that changes the taste. Might even break down and buy bacterial cultures from Cultures For Health – we’ll see . I’m now routinely using Cultures for Health starters and loving them! Click the banner for a good deal. Also thinking of going to some of the fabulous local organic dairies around our house and using some of their farm fresh milk.

** Updated 8/17/20 Take a look at The Great Yogurt Experiment – Take 2 – a blog post about what I’m doing with the General Microbiology class I’m teaching at Towson University. **

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• For Foodies

Hey Yogurt-Maker, Where'd You Get Those Microbes?

Dan Charles

Historic yogurt-making cultures held by Mirjana Curic-Bawden. Dan Charles/NPR hide caption

Historic yogurt-making cultures held by Mirjana Curic-Bawden.

Yogurt is a truly living food. The bacteria that transform milk into this thick and sour food also provide a sense of mystique.

For Atanas Valev , they carry the taste and smell of his homeland, Bulgaria. "It's just the smell of the fermented milk. It's tart, tangy tart. That's what yogurt should taste like," he says.

The secret to that taste, he says, is the bacteria that Bulgarian yogurt-makers have used for thousands of years. So when he flew to the U.S. in 1991, he brought with him, in his luggage, two jars of those precious bacterial cultures.

"It was homemade yogurt in Bulgaria," he says. "Sheep milk yogurt. I got it from a shepherd." He kept that yogurt and used it as a "mother culture" to make more, for himself and his friends.

The process is simple. Add yogurt to warm milk, and the bacteria in it multiply, consuming lactose and turning it into lactic acid. Gradually, the milk becomes more acidic and eventually sets in a gel.

Valev is now trying to bring the taste of his boyhood to America with his company , called Trimona Bulgarian yogurt.

Siggi Hilmarsson, founder of a company that makes, of course, siggi's Icelandic-style yogurt, has also tried to duplicate the taste of his childhood. "To begin with, I just bought yogurt off the shelf and tried to incubate the cultures from those," he says.

The 'Immortal' Homemade Yogurt That Traveled 'Round The World

Many small yogurt companies tell stories of starting with bacterial cultures handed down from previous generations. "My uncle, a long time ago, got his own" yogurt-making cultures, says Hannibal Murray, operations manager of White Mountain Foods in Austin, Texas.

In fact, though, it's not feasible to carry out commercial production the old-fashioned way, using existing yogurt to inoculate each new batch. This process, called backslopping, is inefficient and can raise the risk of contamination.

If you're in the commercial yogurt business, you need a microbe manufacturer, and that means a company like the Danish firm Christian Hansen . Its North American headquarters is in Milwaukee.

Mirjana Curic-Bawden is the house expert on yogurt-making microbes at Christian Hansen. She, too, has childhood memories of homemade yogurt. "I grew up in Belgrade, in Serbia, and my grandmother lived by the Bulgarian border and she made yogurt by herself," she says. "My grandmother would be really proud of me. She never understood why I needed to go to school to make yogurt."

Siggi Hilmarsson, founder of siggi's Icelandic-style yogurt, calls Mirjana Curic-Bawden at Christian Hansen "the doyenne of cultures." Dan Charles/NPR hide caption

Siggi Hilmarsson, founder of siggi's Icelandic-style yogurt, calls Mirjana Curic-Bawden at Christian Hansen "the doyenne of cultures."

What Grandma didn't realize is how science can change the taste and texture of this food.

Despite the wild proliferation of yogurt labels these days — say hello, please, to Icelandic yogurt, Bulgarian yogurt and Australian yogurt — they all are made using a very similar recipe.

By law, anything called "yogurt" must be made from a few common ingredients: milk, of course, plus two species of bacteria called Lactobacillus bulgaricus and Streptococcus thermophilus . (Those are the essential ingredients; yogurt can also include other bacteria, as well as fruit and flavorings.)

So what makes Yogurt A different from Yogurt B?

Curic-Bawden explains that there's lots of variation within these two bacterial species, just as there's immense variation within our species, Homo sapiens . Some of these little creatures gobble up lactose faster than others; some release more of that sour, tangy flavor.

So her company, Christian Hansen, has assembled a kind of microscopic zoo: 60 different strains of yogurt-making bacteria. They were originally collected in the ancestral homelands of yogurt, including Greece, Turkey, Bulgaria, the Balkans and the Caucasus region. "We blend them in different ratios to achieve a certain texture and flavor," Curic-Bawden says.

Yogurt-makers with a particular vision for their yogurt make pilgrimages to Curic-Bawden's workplace, looking for the bacterial blend that's just right for them. "She's the doyenne of cultures," says Hilmarsson.

A typical yogurt-making culture contains four to six strains of bacteria. Each company's exact mix of microbes, however, is a closely guarded secret.

Deciding on that mix can be complicated. Douglas Stewart, co-founder of Smari Organics , which makes Icelandic-style yogurt, says his company had to adopt a different bacterial culture when the first version produced yogurt that the company's yogurt-straining equipment couldn't handle. "If we found something that worked better, we'd switch," Stewart wrote in an email.

Many yogurt-makers add additional species of bacteria to the mix, such as Lactobacillus acidophilus, Bifidus regulari s and Lactobacillus casei . These "probiotics" may improve intestinal health (although the evidence for this is mixed), but they don't affect the yogurt's flavor very much, says Murray.

It's possible to get your hands on yogurt-making cultures that, do, in fact, trace their lineage back to someone's kitchen. There's a community of culture-sharing yogurt enthusiasts, and a company called Cultures for Health sells various yogurt starters, some of them labeled as Greek, Bulgarian and Finnish. But Julie Feickert, the company's founder, says she acquired these bacterial cultures from "people I know": fellow yogurt-makers near Portland, Ore., where she started the company, and elsewhere in the U.S.

The labels on these starter cultures, she says, refer to their historical origins, but their actual source is a matter of "legends and stories."

Curic-Bawden, for her part, believes that true "heirloom" yogurt cultures are now almost impossible to find. She says that most home yogurt cultures these days actually trace their ancestry to yogurt that someone bought in a store, which in turn came from the bacterial collections of companies like Christian Hansen.

Christian Hansen grows those microbes on a grand scale. Bacteria from this one company ferment 40 percent of all the yogurt sold in America.

Max McGloughlan, one of the chemists in charge of production at the company, shows me 8,000-gallon tanks where the bacteria multiply, and machines that concentrate the microbes into a thick soup. "After it is concentrated, we bring it over to our freezing area for pelletizing, and we make small droplets of frozen bacteria," he explains.

There are 100 million individual microbes in each little pellet. Each pound of pellets will make 1,000 gallons of yogurt.

They leave the factory in big insulated boxes: a few hundred pounds of microbe pellets packed together with a few hundred pounds of dry ice, on their way to yogurt companies across the country.

They are the living heart of the yogurt business.

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12-2 Lactic acid bacteria experiment

( 102659 Reads)

Below is the protocol that we will follow for investigation of the Lactic Acid Bacteria .

Cultures of the following grown in APT Broth1 or Milk2:

Samples of juice (diluted 1/10) from 4 or 5-day-old and 10 to 12-day-old sauerkraut fermentations (Each pair uses either the younger or the older sample.)

2 tubes of APT Broth (approx. 8-10 ml)

2 tubes of pasteurized or sterilized milk (exactly 5 ml)

5 plates HIAG (HIA plus 5% glucose)

5 plates HIAS (HIA plus 5% sucrose, 0.5% glucose and 0.02% sodium azide)

4 dilution blanks (9 ml)

Pipettors and sterile tips

1 small, clean Erlenmeyer flask

Titration apparatus (with 0.1M NaOH and phenolphthalein)

• Gram reaction and morphology. Prepare heat-fixed smears from the cultures. After performing the inoculations below, gram-stain the smears and determine the gram reaction, shape and arrangement of the cells for each culture. Reocrd your observations. A stained background will be seen for the organisms which had been grown in milk.
• Differentiation between homo- and heterofermentative lactic acid bacteria. This simple test which detects CO 2 production by the heterofermentative lactics is helpful in the identifications of the genera. We will differentiate between the almost morphologically-identical genera Lactococcus and Leuconostoc.
• Inoculate one tube of APT Broth with Lactococcus lactis and another tube with Leuconostoc mesenteroides .
• Incubate the tubes in your desk drawer for 1-2 days. If the next lab period is more than 2 days away, place the tubes in the rack on the stage. (The tubes will be refrigerated until the day before the next lab period, then incubated at 30°C.)
• Fermentation of milk. Some (not all!) lactic acid bacteria ferment lactose, the milk sugar.
• The production of yogurt is accomplished by two organisms, Lactobacillus bulgaricus and Streptococcus thermophilus . The resulting low pH from lactose fermentation causes the coagula-tion of the milk protein (casein), forming a curd. The two organisms produce some flavoring compounds and also growth factors which assist their mutual growth. The following production of pseudo-yogurt will demonstrate the effect of these organisms on milk:
• To each tube of milk (A and B), add 0.1 ml of L. bulgaricus and 0.1 ml of S. thermophilus . Incubate tube B at 37°C until the next period.
• Add the contents of tube A to a clean Erlenmeyer flask. Obtain some distilled water in a clean glass from the special tap in the sink, and pipette 5 ml of the water into the flask. At the titration station, add 4 drops of phenolphthalein solution, mix well and titrate with 0.1M NaOH until the indicator shows a trace of pink color persisting for at least 15-20 seconds. Record the volume of NaOH used. Dispose the flask into the tray on the discard cart.
• Slime production from sucrose. This procedure will demonstrate how some lactic acid bacteria produce slime from sucrose but not from most other sugars. In the case of Leuconostoc mesenteroides and certain other lactics, the enzyme dextran sucrase splits sucrose into glucose and fructose and also polymerizes the glucose into dextran.
• Streak the culture of L. mesenteroides for isolated colonies on one plate each of glucose and sucrose-containing media, i.e., HIAG and HIAS. (See Materials. The sodium azide in the latter medium will be of no significance in this test.)
• Incubate at 30°C until the next period.
• Lactic acid bacteria in the sauerkraut fermentation. Sauerkraut is essentially shredded cabbage which has been altered by the growth and activities of bacteria indigenous to the cabbage (see the introduction). Important in the activities of these organisms and the reduction of spoilage organisms are anaerobic conditions and the initial addition of NaCl (2.5% of the total weight). Proceed as follows with one of the sauerkraut samples provided:
• Prepare dilutions and platings of the sample (already a 1/10 dilution) onto HIAG and HIAS such that one plate of each medium is made for each of these plated dilutions: 10 -3 , 10 -4 , 10 -5 and 10 -6 .
• Optionally, make a streak plate of the sauerkraut sample, plating onto HIAS and HIAG.
• Note that we are using Heart Infusion Agar plus glucose (HIAG) - an all-purpose medium - for the total count.
• The other medium (HIAS) contains sodium azide and will support the growth of only lactic acid bacteria when incubated aerobically.
• As HIAS contains sucrose, it will help to differentiate between the two major lactic acid genera in sauerkraut - the slime-forming Leuconostoc and the non-slime-forming Lactobacillus.

Figure 12.1. Titration results, day 1 . Titration results of an unincubated milk medium tubes. Even though the starter culture could not possible have produced any acid, the milk has a significant buffering capacity. The initial titration will measure the strength of this buffering.

Figure 12.2. Gram stains of various strains . The Gram stains of various lactic acid bacteria. Av, Aerococcus viridans ; Lb, Lactobacillus bulgaricus ; Ll, Lactococcus lactis ; Lm, Leuconostoc mesenteroides ; Lp, Lactobacillus plantarum ; St, Streptococcus thermophilus .

1 pipette (5 ml)

titration apparatus (with 0.1M NaOH and phenolphthalein)

• Differentiation between homo- and heterofermentative lactic acid bacteria. For each tube, plunge a red-hot inoculating loop fully into the culture. The evolution of gas indicates soluble CO 2 in the medium, a result of heterofermentative metabolism . This test will differentiate effectively between Leuconostoc and the other chain-forming genera of cocci in the lactic acid bacteria group. Observe Figure 12-2 to see a movie of the hot loop test. Record your results (Additional note: Pediococcus and Aerococcus are both homofermentative; Lactobacillus species are either homo- or heterofermentative.)
• Analysis of the milk fermentation. Note the odor and texture of the pseudo-yogurt in tube B incubated since last period. Titrate as you did for tube A; the 5 ml of distilled water can be added to the tube to help loosen the solid curd.
• Compare the results of both tubes. Has acid been produced? What effect has this had on the milk?
• From the amount of NaOH used, the percent lactic acid in the milk can be calculated according to the following formula: (Recall that the sample volume was 5 ml.)

Lactic Acid Formula:

• Lactic acid bacteria in the sauerkraut fermentation. If the colonies are difficult to see, reincubate the plates at room temperature and make your observations next period.
• From the colony counts on HIAG and HIAS, determine the CFUs per ml in the original, undiluted sample for the total count and the count of lactic acid bacteria.
• Optionally, observe your streak plates of the sauerkraut on the streak plates of HIAS or HIAG.
• Are any relatively slimy or mucoid colonies apparent on the medium containing sucrose (HIAS)? If so, determine the CFUs per ml of Leuconostoc in the sample.
• Compare your results with those who used the other sauerkraut sample. What may the relative differences in the various counts mean?

Figure 12.3. Titration results, day 2 . Titration results from the second day. Why does it take more NaOH to neutralize the milk broth after incubation?

Figure 12.4. L. mesenteroides growing on HIAS and HIAG . Note the large amount of slimed produced by L. mesenteroides on HIAS. In contrast no such slime is made on the HIAG plate. This is a convenient method of tentatively determining lactic acid bacterial types in various substances.

Figure 12.5. Colonies present in sauerkraut . The different types of colonies present in sauerkraut. Not the presence of slimy and non-slimy colonies on the HIAS. What genus do you think the slimy colonies belong to?

An example of a hot loop test

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Science Projects > Chemistry Projects > How To Grow Bacteria and More  

How To Grow Bacteria and More

If learning how to grow bacteria in a petri dish interests you, read on.

How Can Bacteria Help Us? How Can Bacteria Harm Us? What Are Antibacterial Agents? Experiment #1: Cheek Cell Swab Experiment #2: Testing Antibacterial Agents Experiment #3: Soap Survey Experiment #4: Bacteria in the Air Experiment #5: Homemade Yogurt More Experiment Ideas

Bacteria Overview

Bacteria are one-celled, or unicellular, microorganisms . They are different from plant and animal cells because they don’t have a distinct, membrane-enclosed nucleus containing genetic material. Instead, their DNA floats in a tangle inside the cell.

Individual bacteria can only be seen with a microscope, but they reproduce so rapidly that they often form colonies that we can see. Bacteria reproduce when one cell splits into two cells through a process called binary fission. Fission occurs rapidly in as little as 20 minutes. Under perfect conditions a single bacterium could grow into over one billion bacteria in only 10 hours! (It’s a good thing natural conditions are rarely perfect, or the earth would be buried in bacteria!)

Growing and testing bacteria is a fun any-time project or a great science fair project. Bacteria are everywhere, and since they reproduce rapidly they are easy to study with just a few simple materials. All you need are some petri dishes , agar, and sterile swabs or an inoculating needle . Agar is a gelatinous medium that provides nutrients and a stable, controlled environment for bacteria growth . Most bacteria will grow well using nutrient agar , but some more fastidious bacteria (those with more complex nutrient requirements like Bacillus stearothermophilus , Branhamella catarrhalis , and Bacillus coagulans ) prefer tryptic soy agar .

You also need a source for bacteria, and this is not hard to find! You can swab your mouth or skin, pets, soil, or household surfaces like the kitchen sink or toilet bowl. If you want to study a particular type of bacteria, you can also purchase live cultures . Keep reading to see four experiments using bacteria, and many more ideas for science projects (also consider this hands-on Bacteria Growing Kit )! Adult supervision is recommended when working with bacteria.

How Can Bacteria Help Us?

Where would we be without bacteria? Well, we might not be getting bacterial diseases, but we would still be a lot worse off! Bacteria perform all sorts of very important functions, both in our bodies and in the world around us. Here are just a few.

Digestion. Our large intestines are full of beneficial bacteria that break down food that our bodies can’t digest on their own. Once the bacteria break it down, our intestines are able to absorb it, giving us more nutrients from our food.

Vitamins. Bacteria in our intestines actually produce and secrete vitamins that are important for our health! For example, E. coli bacteria in our intestines are a major source of vitamin K. (Most E. coli is good for us, but there is a harmful type that causes food poisoning.)

Food. Bacteria are used to turn milk into yogurt, cheese, and other dairy products.

Oxygen. Cyanobacteria (which used to be called blue-green algae) live in water and perform photosynthesis, which results in the production of much of the oxygen we need to breathe.

Cleanup. Oil spills, sewage, industrial waste — bacteria can help us clean all of these up! They ‘eat’ the oil or toxins and convert them into less harmful substances.

Bacteria are amazing creatures, aren’t they? They can be so dangerous and yet so important at the same time. Keep reading to see an experiment that uses good bacteria!

How Can Bacteria Harm Us?

Some types of bacteria cause disease and sickness. These kinds of bacteria are called pathogens. They reproduce very rapidly, like all bacteria. These come in many forms and can cause illnesses from an ear infection to strep throat to cholera. They can get into our bodies via our mouth and nose, or through cuts and scrapes. Some are airborne, others are found in food, resulting in food poisoning. Bacteria are also the cause of plaque buildup on our teeth, which can lead to cavities and gum disease.

Before the discovery of antibiotics, many severe bacterial diseases had no cure and usually resulted in death. Antibiotics work by destroying bacteria or inhibiting their reproduction while leaving the body’s own cells unharmed. After a time, some bacteria develop resistance to an antibiotic, and it will no longer be effective against them. Because of this, scientists are always researching new antibiotics. (Many diseases, such as chicken pox, hepatitis, or polio, are caused by viruses rather than bacteria. Antibiotics have no effect against these diseases.)

Bacterial infections are common, but many of them can be avoided by good cooking, cleaning, and hand-washing practices.

What Are Antibacterial Agents?

How do people stop bacteria from growing and spreading? They control it in two ways: by killing the bacteria cells, and by stopping the bacteria from reproducing. An agent is a solution or method which either kills or stops reproduction. Bactericides are agents that kill bacteria cells. Static agents inhibit cell growth and reproduction.

There are a variety of ways to kill bacteria or keep it from reproducing.

Physical methods:

• Sterilization. The application of heat to kill bacteria. Includes incineration (burning), boiling, and cooking.
• Pasteurization. The use of mild heat to reduce the number of bacteria in a food.
• Cold temperatures. Refrigeration and freezing are two of the most common methods used in homes, for preserving food’s life span.

Chemical methods:

• Antiseptics. These agents can be applied directly to living tissues, including human skin.
• Disinfectants. These agents are not safe for live tissues. Disinfectants are used to clean toilets, sinks, floors, etc.
• Some food preservatives are: sodium benzoate, monosodium glutamate (MSG), sulfur dioxide, salts, sugar, and wood smoke.
• Amoxycillin and Ampicillin—inhibit steps in cell wall synthesis (building)
• Penicillin—inhibits steps in cell wall synthesis
• Erythromycin—inhibits RNA translation for protein synthesis

SAFETY NOTE

While most environmental bacteria are not harmful to healthy individuals, once concentrated in colonies, they can be hazardous.

To minimize risk, wear disposable gloves while handling bacteria, and thoroughly wash your hands before and after. Never eat or drink during bacteria studies, nor inhale or ingest growing cultures. Work in a draft-free room and reduce airflow as much as possible. Keep petri dishes with cultured mediums closed—preferably taped shut—unless sampling or disinfecting. Even then, remove the petri dish only enough to insert your implement or cover medium with bleach or 70% isopropyl alcohol.

When finished experimenting, seal dishes in a plastic bag and dispose. Cover accidental breaks or spills with bleach or alcohol for 10 minutes, then carefully sweep up, seal in a plastic bag, and discard.

Preparing Culture Dishes

Before you can grow bacteria, you’ll need to prepare sterile culture dishes. A 125ml bottle of nutrient agar contains enough to fill about 10 petri dishes.

Water Bath Method – Loosen the agar bottle cap, but do not remove it completely. Place the bottle in hot water at 170-190 °F until all of the agar is liquid. To prevent the bottle from tipping, keep the water level even with the agar level.

• Let the agar cool to 110-120 °F (when the bottle still feels warm but not too hot to touch) before pouring into petri dishes.
• Slide open the cover of the petri dish just enough to pour agar into the dish. Pour enough agar to cover 1/2 to 2/3 of the bottom of the dish (about 10-13ml). Don’t let the bottle mouth touch the dish. Cover the dish immediately to prevent contamination and tilt it back and forth gently until the agar coats the entire bottom of the dish. (Fill as many dishes as you have agar for: you can store extras upside down until you’re ready to use them.)
• Let the petri dishes stand one hour for the agar to solidify before using them.

Experiment #1: Cheek Cell Swab

Make a culture dish using the instructions above. Once the culture dish is prepared, use a sterile cotton swab or inoculating needle and swab the inside of your cheek. Very gently rub the swab over the agar in a few zigzag strokes and replace the lid on the dish. You’ll need to let the dish sit in a warm area for 3-7 days before bacteria growth appears. Record the growth each day with a drawing and a written description. The individual bacteria are too tiny to see without a high-power microscope, but you can see bacteria colonies. Distinguish between different types of bacteria by the color and shape of the colonies.

Experiment #2: Testing Antibacterial Agents

Preparing Sensitivity Squares

One method for testing the antibacterial effectiveness of a substance is to use ‘sensitivity squares.’ Cut small squares of blotter paper (or other absorbent paper) and then soak them in whatever substance you want to test: iodine, ethyl alcohol, antibacterial soap, antiseptics, garlic, etc. Use clean tweezers to handle the squares so you don’t contaminate them. Label them with permanent ink, soak them in the chosen substance, and blot the excess liquid with a paper towel.

Collecting Bacteria

Decide on a source for collecting bacteria. For using sensitivity squares, make sure there is just one source, and keep each dish as consistent as possible. Sources could include a kitchen sink, bathroom counter, cell phone, or another surface you would like to test. Rub a sterile swab across the chosen surface, and then lightly rub it across the prepared agar dish in a zigzag pattern. Rotate the dish and repeat.

Setting Up an Experiment

Each experiment should have a control dish that shows bacteria growth under normal conditions and one or more test dishes in which you change certain variables and examine the results. Examples of variables to test are temperature or the presence of antiseptics. How do these affect bacteria growth?

• Label one dish ‘Control.’ Then in your test dish, use tweezers to add the sensitivity squares that have been soaked in a substance you wish to test for antibacterial properties. It’s a good idea to add a plain square of blotter paper to see if the paper by itself has any effect on bacteria growth. For best results, use multiple test dishes and control the variables so the conditions are identical for each dish: bacteria collected from the same place, exposed to the same amount of antibacterial substance, stored at the same temperature, etc. The more tests you perform, the more data you will collect, and the more confident you can be about your conclusions.
• Place all the dishes in a dark, room-temperature place like a closet.

Wait 3-7 days and examine the bacteria growth in the dishes, without removing the lids. You will see multiple round dots of growth; these are bacteria colonies. Depending on where you collected your bacteria samples, you may have several types of bacteria (and even some mold!) growing in your dishes. Different types of colonies will have different colors and textures. If you have a compound or stereo microscope, try looking at the colonies up close to see more of the differences.

Compare the amount of bacteria in the control dish to the amount in the test dishes. Next, compare the amount of bacteria growth around each paper square. Which one has bacteria growing closest to it? Which one has the least amount of bacteria growing near it? If you did more than one test dish, are the results similar in all the test dishes? If not, what variables do you think might have caused the results to be different? How does this affect your conclusions?

For a variation on this experiment, test the effect of temperature on bacterial growth instead of using sensitivity squares. Put a control dish at room temperature, and place other dishes in dark areas with different temperatures.

Experiment #3: Soap Survey

Every time you touch something you are probably picking up new bacteria and leaving some behind. This is how many infectious diseases spread — we share our bacteria with everyone around us! Even bacteria that lives safely on our skin can make us sick if it gets inside our bodies through our mouths or cuts and scrapes. This is one reason why it is so important that we wash our hands frequently and well.

What kind of soap works best for cutting down on the bacteria on our hands? You can test this by growing some bacteria cultures using agar and petri dishes.

• Two (or more)  petri dishes
• Sterile swabs
• Blotter paper  or other absorbent paper
• Forceps  or tweezers
• Different kinds of hand cleaners: regular soap, antibacterial soap, dish soap, hand sanitizer

1. Prepare the agar according to the directions on the label, then pour enough to cover the bottom of each petri dish. Cover the dishes and let them stand for about an hour until the agar has solidified again. (If you aren’t going to use them right away after they have cooled, store them upside down in the refrigerator.)

2. When your petri dishes are ready, collect some bacteria from your hand or the hand of a volunteer. (Make sure the person hasn’t washed his or her hands too recently!) Do this by rubbing the sterile swab over the palm in a zigzag pattern.

3. Remove the cover from the petri dish and lightly rub the swab back and forth in a zigzag pattern on the agar. Turn the dish a quarter turn and zigzag again. Cover the dish and repeat steps two and three for the other dish, using a new sterile swab. Label the dishes “Test” and “Control.” (You may want to do more than one test dish, so you can compare the results.)

4. Cut the blotter paper into small “sensitivity squares.” Use permanent ink to label the squares for the different types of hand cleaners you are going to test, e.g., “R” for regular soap, “A” for antibacterial soap, and “S” for hand sanitizer. Using tweezers, dip each square into the appropriate cleaner. Blot the excess cleaner on a paper towel and then place the squares on the agar in the “Test” dish. (Spread the squares out so there is distance between them.) Add one square of plain blotter paper to test if blotter paper by itself has any effect. Don’t put any squares in the “Control” dish – this one will show you what the bacterial growth will look like without any soap.

5. Put the dishes in a dark, room-temperature place like a closet and leave them undisturbed for a few days.

What Happened

The rate of bacteria growth in your dishes will depend on temperature and other factors. Check your cultures after a couple of days, but you’ll probably want to wait 5-7 days before recording your data. You will see multiple round dots of growth; these are bacteria colonies. There may be several types of bacteria growing in the dishes. Different types of colonies will have different colors and textures.

For each soap test, count and record the number of bacteria colonies in each dish. To see how effective each soap was, divide the number of colonies in the test dish by the number of colonies in the control dish, then subtract the result from 1 and write the answer as a percentage. For example, if your control dish had 100 colonies and your soap test dish had 30, the soap eliminated 70% of the bacteria: 1 — (30 ÷ 100) = .7 = 70%

According to your results, which type of soap was the most effective at eliminating bacteria?  Does “antibacterial” soap really work better than regular soap? How well did washing hands in water without soap work? What further tests could you do to determine which soaps and hand washing methods are most effective at eliminating bacteria?

Experiment #4: Bacteria in the Air

You need two culture dishes for this experiment, in which you’ll demonstrate how antibacterial agents (such as antibiotics and household cleaners) affect bacteria growth.

Leave the dishes with their lids off in a room-temperature location. Leave the culture dishes exposed for about an hour.

While you wait, cut small squares of paper (blotter paper works well), label them with the names of the antibacterials you’re going to test (e.g. ‘L’ for Lysol, ‘A’ for alcohol, etc.), and soak each in a different household chemical that you wish to test for antibacterial properties. If you have time, you might also experiment with natural antibacterial agents, such as tea tree oil or red pepper. Wipe off any excess liquid and use tweezers to set each of the squares on a different spot in one of the culture dishes. The second culture dish is your ‘control.’ It will show you what an air bacteria culture looks like without any chemical agents.

Store the dishes (with lids on) in a dark place like a closet where they will be undisturbed for a few days. After 3-7 days, take both culture dishes and carefully observe the bacteria growth in each dish, leaving the lids on. The bacteria will be visible in small, colored clusters. Take notes of your observations and make drawings. You could also answer the following questions. In the control culture, How much of the dish is covered with bacteria? In the sensitivity square test culture, Have the bacteria covered this dish to the same extent as the control culture? What effect have each of the chemicals had on the bacteria growth? Did a particular chemical kill the bacteria or just inhibit its growth?

• For further study you could use an  antibiotic disc set  to see what different antibiotics can do against bacteria.
• For a  more advanced project , learn how gram staining relates to the use of antibiotics.

Experiment #5: Homemade Yogurt

Generally when people think of ‘bacteria,’ they think of harmful germs. However, not all forms of bacteria are bad! You can enjoy a tasty product of good bacteria by making a batch of yogurt at home.

You’ll need to use a starter (available at grocery or health food stores), or else one cup of plain, unflavored yogurt that has live cultures in it. (If it contains live cultures, it will say so on the container.)

Slowly heat four cups of milk until it is hot, but not boiling or scalding. The temperature should be around 95-120 degrees to kill some of the harmful bacteria. Cool slightly, until milk is warm, and then add one cup of active yogurt or the starter.

Put the mixture in a large bowl (or glass jars) and cover. Make sure that the bowl or jars are sterilized before using by either running them through the dishwasher or washing them with very hot water.

There are two different methods for culturing the yogurt mixture: You can put the covered bowl or jars into a clean plastic cooler, and fill the cooler with hot water to just below the top of the culture containers. With this method, you will need to occasionally refill the cooler with hot water, so that the temperature of the yogurt stays consistent. The other method is to wrap the containers in a heating pad and towels, setting the heating pad on low to medium heat.

Check the mixture after heating for 3 1/2 to 4 hours. It should be ‘set up,’ having a smooth, creamy consistency similar to store-bought yogurt. If the mixture is not set up yet, heat it for another 1-2 hours. When it is the right consistency, add some flavoring—such as vanilla extract, chocolate syrup, or berries—and store the yogurt in the refrigerator. It should keep for a couple of weeks. For safety, we suggest that you do not eat any yogurt that has separated or has a non-typical consistency.

More Bacteria Experiment Ideas

Here are some other project ideas for you to try on your own or use as a basis for a bacteria science fair project:

• Mouthwash . Swab your teeth and gums and see how well toothpaste or mouthwash work against the plaque-causing bacteria on your teeth.
• Dog’s mouth : Have you heard people say that dogs’ mouths are cleaner than humans’? Design an experiment to test whether this is really true!
• Band-aid . Some band-aids are advertised as being antibacterial. Test to see if they really work better than regular band-aids at inhibiting bacteria.
• Water bottle . Is it safe to keep refilling a water bottle without washing it? Test a sample of water from the bottom of a water bottle that has been used for a couple days and compare it to a sample from a freshly-opened, clean water bottle. You can also test to see if a bottle gets more bacteria in it if you drink with your mouth or with a straw.
• Shoes . Do bacteria grow in your shoes? Is there a difference in bacteria growth between fabric shoes and leather? Do foot powders work to cut down on bacteria?
• Toothbrush . Do bacteria grow on your toothbrush? What are some ways you could try to keep it clean? Mouthwash? Hot water?
• Makeup . Some people recommend getting new mascara every six weeks because bacteria can grow in the tube. Test this by comparing bacteria growth from old mascara and new, unused mascara. You can also test how much bacteria is on other kinds of makeup.

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Yogurt Production

Series: Methods In Molecular Biology > Book: Lactic Acid Bacteria

Protocol | DOI: 10.1007/978-1-4939-8907-2_5

• Fermented Milk Development Department, Food Development Laboratories, R&D Division, Meiji Co., Ltd., Hachiouji, Tokyo, Japan

Yogurt is a popular fermented dairy product produced by lactic acid bacteria, including Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus . During yogurt production, these bacteria produce lactic acid, decreasing pH and

Yogurt is a popular fermented dairy product produced by lactic acid bacteria, including Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus . During yogurt production, these bacteria produce lactic acid, decreasing pH and causing milk protein to coagulate. Their metabolites, such as carbonyl compounds, nonvolatile or volatile acids, and exopolysaccharides, strongly affect the quality of yogurt. In this chapter, the general methods for yogurt production are summarized.

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