The Classification and Identification
of Bacteria of Medical Importance

The Enterobacteriaceae are a large family of medically important Gram-negative bacilli. They can grow in the presence or absence of oxygen, and are frequently found in the guts of humans and other animals, and hence their name. They are differentiated from one another largely on the basis of their metabolic behaviour and on their antigenic structure. Some, like Escherichia coli and members of the genus Klebsiella can ferment lactose to produce acid, whereas others including salmonellas , shigellas and proteeae cannot and are thus known as non-lactose fermenters (NLF's). There are almost 2,000 different varieties of salmonella recognised on the basis of differences in their surface antigens. Members of the genus Proteus are so highly motile that a single colony can grow to swarm over the entire surface of a Petri dish after overnight incubation. This is how the genus acquired its name, from Proteus, the Greek sea god who had the miraculous ability to change his shape. The family Enterobacteriaceae include Yersinia pestis, the cause of plague, Salmonella typhi, the cause of typhoid, Shigella dysenteriae, the cause of bacillary dysentery, and Salmonella enteritidis implicated in many cases of food poisoning.

Members of the family Enterobacteriaceae do not elaborate the enzyme complex known as "oxidase", whereas many Gram-negative bacteria do. Pseudomonas aeruginosa is an oxidase-positive Gram-negative bacillus that is an obligate aerobe. It cannot be grown in the absence of oxygen. It is responsible for wound infections, and the bacteria in this species produce a soluble pigment.

The vibrios and campylobacters are Gram-negative rods that appear curved or spiral in shape. These bacteria are commonly found in natural waters, both fresh-water and marine. Vibrio cholerae causes cholera, a waterborne infection. Campylobacters have only been recognised as human pathogens since the late 1970's, although they have been long considered to be animal pathogens. Campylobacters are now responsible for more cases of bacterial enteritis annually than salmonellas.

The bacterium Helicobacter pylori, found associated with stomach ulcers, is a good example of a bacterium that has undergone a radical change in taxonomy. When they were first observed, they could not be cultured and were called "campylobacter-like organisms". Conditions were then discovered that allowed these bacteria to be grown in artificial culture, and they were first called Campylobacter pyloridis. This was then found to be linguistically incorrect, and the name was corrected to Campylobacter pylori. Later, molecular studies showed that it was not very closely related to the other campylobacters, and they then became the type species of a new genus, Helicobacter. This entire process of taxonomic change took less than five years.

Some Gram-negative bacilli appear so short that they resemble cocci in the light microscope. Because of this they are sometimes called cocco-bacilli. These include members of the genus Moraxella, related to the neisserias, and also members of the genus Acinetobacter. Members of this genus are increasingly associated with hospital-acquired infection.

Some Gram-negative bacteria are very fastidious (fussy) in their nutritional requirements. Members of the recently recognised genus Legionella, some of which cause atypical pneumonias like Legionnaires' disease, require higher levels of iron and cysteine than are usually present in bacteriological media, and they grow best in media that incorporate activated charcoal to adsorb their toxic metabolic products. Similarly bacteria of the genus Bordetella also generate toxic metabolic products that inhibit their own artificial culture. These bacteria also grow best on media that contain activated charcoal. Bordetella pertussis, the cause of whooping cough, is an important member of this genus. At one time, species of the genus Bordetella were classified in the genus Haemophilus, but they were re-classified. This is partly because they require neither the X- nor the V-factorfor growth such as required by members of the genus Haemophilus. The X-factor has now been identified as haem, and the V-factor is nicotinamide adenine dinucleotide or NAD. Haemophilus influenzae requires both X- and V-factors for growth whereas Haemophilus parainfluenzae requires just the V-factor to support its growth, since it can elaborate its own supply of haem.

The most important group of obligately anaerobic Gram-negative bacilli are the bacteroides. This is a heterogeneous group that form part of the human commensal flora, and that are also implicated in anaerobic infections. The taxonomy of the anaerobic Gram-negative rods is currently undergoing radical revision.