Bacteria are everywhere, from the bottom of the ocean to the outer atmosphere. Scientists estimate there are about five million trillion trillion of the little guys (That's a five with 30 zeros after it!). The numbers of species listed here are just the ones that have been identified so far; scientists think there may be more than a million.

Lots of bacteria live on (and in) us. There are about ten times as many bacterial cells on and inside your body as there are human cells, but luckily most of them don't do us any harm. In fact, some are extremely helpful! For example, about 1,000 species live in our guts and help break down milk, produce vitamins, and do lots of other clever things.

The cells of Kingdom Monera are different from those of the other four kingdoms in one big way: they're prokaryotic. This means that unlike eukaryotes, they have no nuclei to contain their DNA, and they usually don't have any other membrane-bound internal structures either. That doesn't mean they're all alike, though! Some scientists think that this kingdom should be split in two, because of recent genetic research showing that some bacteria are as different from each other as they are from plants, fungi or animals. So, don't be suprised if you see some people using systems with six kingdoms. They use two new ones to replace Kingdom Monera: Kingdom Bacteria and Kingdom Archaea.

Major Phyla (groups)	Description	Masks	Approx. # of species
Chlorobia	These "green sulfur bacteria" can be shaped like rods, cones, or spirals. They're anaerobic, meaning that they live in places without oxygen. Like plants, they have chlorophyll, and can make their food using light— and not just sunlight. There's at least one species living on the sea floor, at depths that no sunlight can penetrate, which lives off the glow of lava seeping from the Earth's crust!		20
Proteobacteria	Some of the bacteria in this phylum are helpful to us, like the ones that live on the roots of certain plants and fertilize the soil by adding nitrogen to it from the air. Other members of this phylum, like Salmonella, which causes some of the most serious cases of food poisoning, can make us really ill. This phylum also includes E. coli, which has more recently become famous for causing food poisoning. Most strains, however, are harmless.		1,600
Archaebacteria	This phylum is very different genetically from other types of bacteria, and may even be less closely related to them than it is to eukaryotes like plants, animals, fungi, and protists. As a result, many people now place these organisms into a new kingdom, called Archaea.		300
Firmicutes	This phylum contains a wide variety of bacteria with thick cell walls. The most notorious member of the phylum is Bacillus anthracis, which causes anthrax.		2,500
Cyanobacteria	Cyanobacteria contain chlorophyll and can make their own food using photosynthesis. When they're finished, there's some oxygen left over as a waste product. 2.4 billion years ago there was almost no oxygen in the Earth's atmosphere, and some scientists think huge mats of cyanobacteria floating on the oceans produced enough to bring the atmospheric levels up to what they are today— about 20% oxygen. This would have choked out all the old life forms for whom oxygen was toxic, and opened things up for new ones like us who need it to live.		80
Aquificae	Most of these bacteria live in extreme environments, like hot springs or thermal vents on the ocean floor, and are autotrophic, meaning they make their own food. The other autotrophic organisms you might have come across are plants, which rely on sunlight to perform photosythesis. These bacteria, on the other hand, make their food by breaking down various chemical compounds in a process called chemosynthesis, so they don't need light. As a result, they're often the first step in food chains in places where there isn't any available, like at the bottom of the ocean.		30
Spirochaetes	Most of the spiral-shaped bacteria in this phylum are harmless, but a few of them can cause serious diseases in humans. Syphilis and Lyme Disease, both caused by spirochaetes, can be deadly if not treated.		90
Bacteroides	Lots of these rod-shaped bacteria live in our digestive tracts and those of other mammals. They help us break down complex molecules so that we can digest them, and they keep other harmful bacteria from colonising our intestines.		130
Fusobacteria	These bacteria tend to be found in our mouths and throats. Most of them are potentially pathogenic (harmful). They can cause sore throats and gum disease.		30
Fibrobacteres	Some bacteria in this phylum live in a special pouch in the stomachs of cows, goats, sheep, and other animals called a rumen. They can digest cellulose, the main ingredient in grass and leaves, into a form that the animal's own digestive system can handle. That's why you or I couldn't survive by eating raw grass the way a cow can— we haven't got a rumen full of helpful Fibrobacteres!		5