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How They Eat
"Eat" is really a figure of speech when it comes to bacteria. They don't have mouths or digestive systems, so they can't eat in the same way that animals do. Rather, they absorb molecules directly into their cells via channels in their cell walls and membranes. They then break the molecules down and convert them into the materials they need. When bacteria encounter molecules that are bound together -- such as human tissues -- the bacteria secrete enzymes that break the tissues down into small parts.
Almost Anything
Species of bacteria exist that eat almost anything on Earth. Not only organic substances, such as sugars, proteins, soybean oil and wood, but even inorganic -- sulfur, iron and gasoline. Termites and other "wood eating" creatures have bacteria in their stomachs that actually eat and break down the wood. One of the main ways bacteria are classified is according to what they eat.
Diagnostic Eating
One of the ways that doctors and scientists differentiate between harmful and harmless bacteria in the human body is by observing what they eat. For instance, Clostridium is a species of bacteria often found in the human body. Some Clostridium eat only carbohydrates, while others eat protein, which presents a danger to bodily wounds. Still others can digest both carbohydrates and proteins. By culturing the Clostridium on both protein and carbohydrates, a scientist can determine if it poses a danger to human health.
Helpful Eating
While some bacteria are detrimental to human health, many others have proved helpful to human life by eating things that would otherwise pose a hazard. Fermented foods such as beer, cheese and sourdough bread, for instance, are naturally preserved by bacteria that eat other organisms that would otherwise eat the food. Bacteria that feed on decaying plants and animals play a vital role in the balance of nature. Bacteria that eat oil are often used to clean up oil spills, and water treatment plants use bacteria to consume human waste.