The science of bacteria provides one instance of how quickly our knowledge can change. Our understanding about the importance of bacteria has changed dramatically since 1975. A standard college-level biology textbook written in 1975 says that there are about 3,000 species of bacteria, and presents material about bacteria on 25 of its 1,000 pages. Currently, using advanced molecular and genetic techniques, there are about 9,000 known species of bacteria, but we also know that that is just the proverbial tip of the iceberg. Current estimates of bacterial abundance range from a million to a billion species.

Bacteria are much more abundant and beneficial than we used to think. The approximately five nonillion (5x1030) bacteria on Earth is more biomass that that of all plants and animals combined! Indeed, bacteria can live almost anywhere on the planet, even deep within solid rock.

The bacterial concentration in seawater is about one million per milliliter, and they have a critical role decomposing organic matter, making it available to other marine organisms. The biomass of these oceanic bacteria exceeds that of all the marine multicellular organisms! Also, one particular group of bacteria, the cyanobacteria, account for almost one third of all photosynthetic activity and about half of the photosynthesis that takes place in the open ocean. We depend on the oxygen produced by these single celled organisms.

Similarly, bacteria are much more abundant in the soil than we used to think. A typical gram of soil contains 40 million bacterial cells. These bacteria accomplish many beneficial things in the soil, which are all part of the nitrogen cycle. Different species of bacteria can:

• Break down organic materials into compounds useful to other organisms (decomposition),

• Convert atmospheric nitrogen gas into forms useful to plants (nitrogen fixation, especially in legumes),

• Convert ammonium into nitrites and nitrites into nitrates (nitrification), which can be used by growing plants, and

• Convert nitrates into atmospheric nitrogen gas (denitrification).

Some decomposers and denitrifying bacteria can survive in the absence of oxygen. These bacteria are useful in agriculture and in sewage treatment plants.

Even in people, bacterial cells outnumber human cells by a factor of ten. They live throughout our bodies. Perhaps 1,000 species live on our skin, where they can prevent disease-causing organisms from colonizing our body. A few species live on our eyes and eyelids, although tears contain chemicals that can kill bacteria. Perhaps 100 million bacterial cells live in each milliliter of saliva, and there are an estimated 600 species living in our mouths. Most of these species are particular about where they live in your mouth: on your cheek, teeth, gums, tongue or in your saliva.

There are anywhere from 500 to 1,000 species of bacteria living in our gut, where some of them help us by breaking down our food into smaller compounds that our body can utilize. Most of these species are known only by their DNA and cannot be cultured in the laboratory, and so we do not know what they are doing there; they have no currently known beneficial or harmful effects.

Two of the most abundant bacteria helping us digest food in our guts are enterococcus and escherichia coli. They are also present in many other animals, including birds and mammals. They are easily cultured in the laboratory, and,although they themselves are not harmful, their presence in the water at swimming beaches is correlated to increased risk of disease, especially gastrointestinal disease and ear infections.

The vast majority of the bacteria in the body are harmless. But our immune system is able to recognize some of the dangerous bacteria, attacking them and removing them from our body. The fevers we get are evidence of our immune system working to protect us from these pathogens.

Modern medicine uses antibiotics, some of which are produced by other fungi and other microbes, to help our bodies fight bacterial infections like pneumonia, tuberculosis and diarrhea. One hundred years ago they were the three leading causes of death in the United States; now they are controlled and we live longer and healthier lives. Antibiotics are not a cure-all, however, as they are not effective against viruses (like colds). And their overuse has helped some bacteria develop resistance to the antibiotics.

So, we apparently have between 2,000 and 3,000 bacterial species living in our bodies. Remember 1975, when it was thought that there were about 3,000 bacterial species in the entire world?


Robert Culbert leads guided birding tours and is an ecological consultant living in Vineyard Haven.