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Bacteria are the next-smallest microbes, single cells ranging from about 0.2 microns to 10 microns in size. They are the smallest, simplest, and most ancient fully-functional life form. They have only one chromosome, and the genetic material is not even organized in a nucleus. Bacteria are the most numerous and most varied life form on Earth, accounting for more total biomass than any other. Every gram of good, productive topsoil contains some 10 billion bacteria, comprised of thousands of colonies with millions of organisms in each colony. They occupy every known habitat. Most of them are harmless-indeed, most have no interaction with people at all; e few cause inconvenience in the form of bad taste and odor or a tendency to cause clogging of fine-filters; and a very few can cause disease. The most common odor-causing types merit special mention: they are filamentous bacteria resembling mold under the microscope, except that they are much thinner than molds, and are called actinomycetes.

Many of them produce antibiotics of the “-mycin” type in addition to some of the same “earthy-musty-mildewy-moldy-fishy” odor compounds produced by molds and algae. Bacteria are so varied, it is difficult to know how to categorize them, but the following types are important to know about:

  • Autotrophs: those that make their own food by photosynthesis or oxidation
  • Heterotrophs: those that must consume organic matter from the environment to live
  • HPC organisms: those detected by the Heterotrophic Plate Count procedure
  • Pathogens: those capable of causing disease
  • Opportunistic pathogens: those capable of causing disease only if given an unusual “opportunity”-open wounds, burns, defective immunity
  • Coliforms: those that “resemble” (biochemically) Escherichia coli, the predominant commensal intestinal organism of mammals-indicator of sewage contamination
  • Total Coliforms: an important public health screening test to identify all coliform organisms, both fecal and environmental, belonging to the genera Escherichia, Klebsiella, Enterobacter, Citrobacter, and Serratia
  • Fecal Coliforms: an important public health test to identify E. coli, specifically
  • Anaerobic bacteria: those that prefer or require the absence of dissolved oxygen; they often produce “septic” smells, and some produce “rotten egg” smell
  • iron bacteria:” partial autotrophs that oxidize dissolved iron to rust
  • sulfur bacteria:” anaerobic autotrophs that chemically reduce sulfate ion or meta bonze organics containing sulfur to produce the stink of rotten eggs, or hydrogen sulfide, H2S, and also organic sulfides, all with offensive T&O.