Microbiology For Dummies
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Because you usually can’t see the microorganisms all around you every day, it’s easy to overlook them. But following are ten ways that microbes affect your life in important ways.

Making delicious foods

The yeast Saccharomyces cerevisiae has been used for millennia because it ferments sugars and makes carbon dioxide, which causes bread to rise.

The lactic acid bacteria, such as Lactobacillus and Bifidobacteria, are used in making fermented milk products. Buttermilk, sour cream, cheese, and yogurt are made by letting these bacteria ferment milk to different degrees.

Microorganisms are also responsible for the pickling process. Species of Lactobacillus are happy in very acidic conditions, so they’re almost always there at the final stages of pickling.

Lactic acid bacteria are also used to cure meats such as salami, pepperoni, and summer sausage that are then dried and/or smoked after fermentation.

Soy sauce is a fermentation product of soybean and wheat paste. The lactic acid bacteria and fungi such as Aspergillus produce the distinctive taste of soy sauce.

Vinegar is made not by lactic acid but by the acetic acid bacteria from wine, apple cider, or dilute ethanol.

Growing legumes

Leguminous plants, such as soybeans, peas, clover, alfalfa, and beans, form intimate associations with nitrogen-fixing bacteria like Rhizobium. The bacteria essentially move into the root cells and form little homes for themselves called nodules. Within these nodules, the bacteria change completely from their free-living form and produce nitrogen compounds that are used by the plant in exchange for shelter and organic compounds made by the plant.

Brewing beer, liquor, and wine

Along with bread-making, the yeast Saccharomyces cerevisiae is probably best known for its use in making alcoholic beverages. This process involves mixing a sugar source — barley for beer, fruit juice for wine, or rice or potato starch for spirits — with yeast and water.

The yeast first consumes the sugars aerobically until the oxygen runs out; then it ferments the sugars anaerobically, producing ethanol and carbon dioxide as waste. When the concentration of ethanol gets to be around 20 percent, the yeast stops fermenting, so distilling is used to make liquor with a higher alcohol concentration.

Killing insect pests

The bacterium Bacillus thuringiensis (also known as Bt) produces a toxin that is specific to certain types of insects, many of which are pests encountered by gardeners and farmers. The toxin is so effective at killing the target insect that for years it has been used as an insecticide.

Another group of insect pathogens uses small worms called nematodes to get inside of their insect host. Members of the bacterial genera Photorhabdus and Xenorhabdus hitch a ride inside soil-dwelling nematodes without harming them. It’s only when the nematode crawls inside of insect grubs that the bacteria emerge and produce toxins that kill the grub, giving the nematode a lot of nutrients to use for reproduction.

Treating sewage

Microorganisms are an important part of wastewater treatment during a few different steps of the process. After the sewage has been physically filtered to remove solid waste, called primary treatment, microbes break down the insoluble organic matter during secondary treatment.

Microorganisms in sewage treatment plants also convert nitrate in the wastewater to nitrogen gas that is lost to the air. This process, called denitrification, is important because water with a high nitrate content can stimulate huge blooms of algae that foul lakes, rivers, and streams.

Contributing to medicine

Microbes have evolved over the last 3.5 billion years to compete with one another mostly through chemical means. It’s not surprising, then, that the vast majority of the thousands of antibiotics known to us are made by microorganisms.

Species of the bacterial genus Streptomyces alone produce more than 500 different antibiotics, including cycloheximide, cycloserine, erythromycin, kanamycin, lincomycin, neomycin, nystatin, streptomycin, and tetracycline. Other antibiotics are made by Bacillus bacteria and fungi, such as Penecillium.

Because of this long history of competition and survival in the face of these chemicals, bacteria are naturally suited to develop antibiotic resistance. This means that despite having such an impressive arsenal of antibiotics, we’re still struggling to control the growth of many strains of bacteria that have become resistant to them.

Setting up your aquarium

In nature, fish live in complex environments filled with plants, insects, and microorganisms. Because of their biological complexity, natural habitats are good at absorbing and recycling fish waste, such as ammonia, so that it doesn’t build up. In a closed container, however, like an aquarium, ammonia released in fish waste can build up quickly, making the water inhospitable and toxic.

To stop this buildup of ammonia from happening, aquarium owners have to do two things:

  • About once a week, change one-quarter of the water in the tank.

  • Encourage the establishment in the tank of ammonia-oxidizing bacteria and archaea that live in the filter and the tank sediment and convert ammonia to less harmful products in a process called nitrification.

Making and breaking down biodegradable plastics

Most of the plastic used today is synthetically made from petroleum and is extremely resistant to degradation. The best kind of biodegradable plastics are the ones made by bacteria because they can also be broken down by bacteria. These contain either polylactide (PLA), made as a product of fermentation, or polyhydroxyalkanoates (PHAs), made by bacteria as storage compounds.

Unfortunately, biodegradable bacterially produced plastics haven’t yet been able to compete with the synthetically made ones because oil is still cheaper than the sugar needed to feed the bacteria.

Turning over compostable waste

The ability of the compost pile in your backyard or in your municipal waste treatment center to turn kitchen and yard waste into what looks like dirt depends directly on microorganisms. The process of composting starts with the initial breakdown of simple organic matter by microorganisms that produce, among other things, carbon dioxide, heat, and humus (broken down organic matter).

Next, heat-tolerant bacteria called thermophiles continue breaking things down and increasing the temperature up to between 140 degrees F and 158 degrees F (60 degrees C to 70 degrees C). This phase is important because the increased temperature helps break down complex organic matter and kills most animal and plant pathogens.

It’s also at this point that the pile is mixed to provide oxygen and to keep the temperature from going too high and killing all the composting microbes.

Finally, the compost pile cools and matures to include a complex community of microorganisms and insects that are beneficial to the environment.

Maintaining a balance

The complex microbial communities on and in the human body can sometimes get out of balance. Probiotics have been designed to administer bacterial species that are beneficial to the human body. They’re thought to help the body’s microbial community return to a balanced state.

Fecal biotherapy (sometimes called fecal transplant) is the insertion of feces from a healthy person (the donor) into the colon of a person suffering from a gastrointestinal disorder or disease. The idea is that the patient’s microbial community is unbalanced and the healthy community of microbes from the donor sample can help restore a healthy balanced microbial community.

About This Article

This article is from the book:

About the book authors:

Jennifer C. Stearns, PhD, is an Assistant Professor in the Department of Medicine at McMaster University. She studies how we get our gut microbiome in early life and how it can keep us healthy over time. Michael G. Surette, PhD, is a Professor in the Department of Medicine at McMaster University, where he pushes the boundaries of microbial research. Julienne C. Kaiser, PhD, is a doctoral career educator.

Jennifer C. Stearns, PhD, is an Assistant Professor in the Department of Medicine at McMaster University. She studies how we get our gut microbiome in early life and how it can keep us healthy over time. Michael G. Surette, PhD, is a Professor in the Department of Medicine at McMaster University, where he pushes the boundaries of microbial research. Julienne C. Kaiser, PhD, is a doctoral career educator.

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