The scope of the bacterial world once seemed much simpler than it does now. During the mid to late 1800s, most known bacteria could be neatly covered by a few generic names. Spherical bacteria were placed in one of three genera: Micrococcus, Streptococcus, or Staphylococcus. Rod-shaped bacteria were assigned to either the genus Bacterium or the genus Bacillus. If bacteria had the shape of a curved rod, they were put in the genus Vibrio, and if they were spiral, they were placed in either the genus Spirillum or Spirochaeta. Because very little was known then about the biochemical characteristics of these bacteria, and the Gram stain had not yet been applied as a general classification tool, their morphology was the primary method of identification. As a result, bacteria that we now know are very different often had the same generic name. Consider that Escherichia coli was once called Bacterium coli; Pseudomonasaeruginosa was Bacterium aeruginosa, and Streptococcus lactis was Bacterium lactis, even though the first two are rods and the last is a coccus.
Present classification schemes include over 1,000 genera, and bacterial identification is based on hundreds of characteristics. Although more than 5,000 species are presently known, new ones are discovered every year. The recent increase in new species is probably due in part to an improvement in identification techniques. Most bacteriologists feel that we have only scratched the surface and that myriad bacteria remain to be discovered.
You will notice that certain generic names appear both capitalized and uncapitalized. For example, the word Staphylococcus as shown here refers to a particular genus of coccus-shaped bacteria, but when the word appears uncapitalized, unitalicized, or in the plural (staphylococci), it is a general way of designating the members of that genus or of describing an arrangement or cell type. This same usage is true of the use of streptococci, micrococci, mycobacteria, pseudomonads, and clostridia. Likewise, Bacillus and Spirillum refer to genera, and bacillus and spirillum to shapes.
It is well worth stating that it makes little difference to bacteria what we call them. The plague bacillus, whether named Yersinia pestis or Pasteurella pestis (its former name), is still quite capable of causing bubonic plague. In the medical microbiology laboratory, the public health laboratory, or the hospital, a scheme of classification becomes something much more than an orderly cataloguing of tongue-twisting names. It presents a foundation of characteristics to be used in identification. With it, not only can technicians identify Y. pestisand scores of other pathogens, but they can also distinguish these pathogens from their less injurious relatives.