Bacteria (细菌) , like people, can be divided into friend and foe. 81. Inspired by evidence that the friendly sort may help with a range of ailments (疾病) , many people consume bacteria in the form of vogurts and dietary supplements. Such a smattering of artificial additions, however, represents but a drop in the ocean. There are at least 800 types of bacteria living in the human gut. And research by Steven Gill of the Institute for Genomic Research in Rockville, Maryland, and his colleagues, published in this week’s Science, suggests that the collective genome of these organisms is so large that it contains 100 times as many genes as the human genome itself. Dr Gill and his team were able to come to this conclusion by extracting bacterial DNA from the faeces (粪便) of two volunteers. 82. Because of the complexiW of the samples, they were not able to reconstruct the entire genomes (基因组) of each of the gut bacteria, just the individual genes. But that allowed them to make an estimate of numbers. What all these bacteria are doing is tricky to identify -- the bacteria themselves are difficult to cultivate. 83. So the researchers guessed at what they might be up to by comparing the genes they discovered with published databases of genes whose functions are already known. 84. This comparison helped Dr Gill identify for the first time the probable enzymatic (酶的) processes by which bacteria help humans to digest the complex carbohydrates in plants. The bacteria also contain a plentiful supply of genes involved in the synthesis of chemicals essential to human life -- including two B vitamins and certain essential amino acids -- although the team merely showed that these metabolic pathways exist rather than proving that they are used. Nevertheless, the pathways they found leave humans looking more like ruminants: animals such as goats and sheep that use bacteria to break down otherwise indigestible matter in the plants they eat. The broader conclusion Dr Gill draws is that people are superorganisms whose metabolism represents an amalgamation of human and microbial attributes. The notion of a superorganism has emerged before, as researchers in other fields have come to view humans as having a diverse internal ecosystem 85. This, suggest some, will be crucial to the success of personalized medicine, as different people will have different responses to drugs, depending, on their microbial flora. (微生 物群落) Accordingly, the next step, says Dr Gill, is to see how microbial population vary between people of different ages, backgrounds and diets. Another area of research is the process by which these helpful bacteria first colonize the digestive tract. Babies acquire their gut flora as they pass down the birth c and take a gene-filled gulp of their mother’s vaginal and faecal flora. It might not be the most delicious of first meals, but it could well be an important one.