Bacteria live nearly everywhere. They're so common, in fact, that according to an estimate cited by the National Institutes of Health, bacteria in your body outnumber your cells by a factor of ten to one. While we often think of bacteria as hostile invaders, pathogenic bacteria--bacteria that harm their hosts--are actually only a small minority of the bacteria we encounter. Many important non-pathogenic bacteria call humans home.
In news articles and the popular press, the words E. coli and food poisoning often come linked together. This bad reputation is almost entirely undeserved. While some strains of E. coli found in cattle are toxic to humans, E. coli bacteria are actually normal residents of your large intestine that colonised the colon shortly after your birth. E. coli is the best-studied of all bacteria. Its abundance, the relative ease with which it can be cultured and our extensive knowledge of its peculiarities have made it a favourite with molecular biologists and genetic engineers around the world.
Bacteroides is a genus of bacteria comprising a number of species that are far and away the most plentiful inhabitants of your intestines. Each gram of your faeces contains bacteria numbering 10 to the 11th power (that's a ten followed by 11 zeroes), and over a fifth of these are Bacteroides. Together with E. coli and your other gut bacteria, Bacteroides breaks down nutrients, making them available to you, consumes some of the nutrients you don't use and helps prevent harmful bacteria from colonising your intestines. A study in 2005 in "Proceedings of the National Academy of Sciences" suggests that intentionally altering the type and population of gut bacteria might someday be a viable way to treat obesity as well.
Lactobacillus is a genus of bacteria that converts lactose and some other sugars to lactic acid. Lactobacilli are best known for the pleasant sour taste they lend cheeses like Emmental and foods like yoghurt. Some Lactobacilli live in the human intestine; lactobacilli are especially plentiful in the human vagina, a warm moist environment where they can flourish. It is believed that Lactobacilli, together with other "friendly bacteria," may play a important role in the vagina by preventing colonisation by harmful species of bacteria like those that cause bacterial vaginosis (a common bacterial infection of the vagina). According to a study in 2000 by Imperial College of London researchers, published in the "Journal of Medical Microbiology," recolonising the vagina with lactobacilli following an infection by harmful bacteria has been suggested as one possible way to prevent recurring infections.
Staphylococcus is a genus that includes more than 30 species, many of which nestle in places like the crevices on your skin, the waxy surface of your ear canal and your sweat glands. For the most part they're relatively good neighbours; some Staphylococcus species, however, may occasionally turn nasty, causing skin infections like impetigo or colonising wounds in hospitalised patients. According to a paper in the "British Medical Journal" in May 2010, antibiotic-resistant strains of Staphylococcus aureus are a fast-growing problem in many countries worldwide.
B. linens is one bacterium that isn't nearly as well known as it deserves to be. It may not be harmful, but it certainly is a unwelcome guest--it's responsible for the cheesy smell of socks and feet. Curiously enough, B. linens is valued by some for its culinary contributions; certain unusually pungent cheeses like Limburger are produced with the aid of B. linens bacteria.
Innumerable other bacteria live on us or inside us. The National Institutes of Health is currently sponsoring the Human Microbiome Project, an ambitious program to learn more about the genetics of these close friends of our species.