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Mixture of soil bacteria on agar plate (M2 medium with cycloheximide for inhibition of soil fungi). Cultivation 5 days, 28°C. Soil sample was treated with 1.5 M NaOH so predominant colonies belong to Bacillus spp.(and related aerobic, endospore-forming genera), Mycobacterium spp. and other spore-forming Actinobacteria.
Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, water, and deep in the Earth's crust, as well as in organic matter and the live bodies of plants and animals. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water;
in all, there are approximately five nonillion (5×1030) bacteria on Earth, forming a biomass on Earth, which exceeds that of all plants and animals. However, most bacteria have not been characterised, and only about half of the phyla of bacteria have species that can be grown in the laboratory.
There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, with large numbers of bacteria on the skin and as gut flora. The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, and a few are beneficial.
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