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Bacteria is properly a plural noun. The singular is bacterium. As these are single-celled organisms, it is on the whole only microbiologists (and others who use microscopes) that talk about 'one bacterium'. In academic English, be more careful than in everyday English.

Sometimes the singular can be used as the name of a particular species, for example 'the bacterium Leptospira', as we say 'œman is a tool-using ape', and not 'œmen are tool-using apes' (unless for a specific purpose - e.g. feminist propaganda).

Bacterion was originally a Greek word and meant 'a little staff'. The Romans took the word over, but changed it into bacterium to fit Latin better. (Both Greek and Latin form neuter plurals in -a; see -a in Latin and -a in Greek.). Scientists, in the days when all academics spoke Latin and most spoke Greek, applied the word to micro-organisms, because the first ones originally observed under the microscope had the form of little sticks.