The Classification and Identification
of Bacteria of Medical Importance

There are two major groups of Gram-positive cocci that are of medical importance: the staphylococci and the streptococci . When viewed microscopically, staphylococci appear in clumps, like bunches of grapes. Staphule is Greek for grapes. Streptococci form chains, and are named after streptos, the Greek word for twisted. These groups of bacteria can be distinguished because staphylococci produce an enzyme, catalase and streptococci do not. Catalase causes the conversion of hydrogen peroxide to water with the concomitant release of oxygen gas, seen as bubbles in the reaction tube. The catalase test is a more reliable test to differentiate staphylococci from streptococci than microscopic observation.

The staphylococcus that causes most of the serious clinical problems associated with this group is Staphylococcus aureus. This is so named because the classical description of the species is that its colonies appear golden yellow (aurum - Latin for gold). However, clinically significant isolates are made that do not conform to this description. To differentiate Staphylococcus aureus from other staphylococci, the coagulase test is used. Coagulase is an enzyme that causes plasma to clot, and is elaborated by Staphylococcus aureus but not by the coagulase-negative staphylococci such as Staphylococcus epidermidis, Staphylococcus capitis and Staphylococcus saprophyticus. An alternative test to differentiate Staphylococcus aureus from the coagulase-negative staphylococci is the DNase test. Staphylococcus aureus produces a DNase that can diffuse from a colony and hydrolyse DNA within a plate.

Coagulase-negative staphylococci are resistant to drying, but Staphylococcus aureus is less so. A significant proportion of hospital staff, and a smaller number of the general population carry Staphylococcus aureus< as part of their commensal flora. It particularly favours moist sites such as the axilla and perineum. In older books, the coagulase-negative staphylococci were lumped together and referred to as "Staphylococcus albus" (albus is Latin for white). This is because the tend to grow as whitish grey colonies in contrast to the golden colonies of Staphylococcus aureus.

Streptococci are classified according to their ability to break down blood in fresh blood agar plates. Some streptococci have no effect on blood. These are the non-haemolytic streptococci (see below). The a-haemolytic streptococci cause partial breakdown of blood, and their colonies are surrounded by a greenish halo. The green pigment is thought to comprise the metabolic degradation products of haem. Because of the colour of halo that surrounds a-haemolytic streptococci, they are often referred to as "viridans" streptococci (viridis is Latin for green).

There is one a-haemolytic streptococcus that must be differentiated from the others. This is Streptococcus pneumoniae. This is the cause of pneumococcal pneumonia and meningitis, as well as less serious infections. Streptococcus pneumoniae is sensitive to optochin, an antimicrobial agent, and lyses when suspended in a solution of bile salts. All other viridans streptococci are resistant to optochin and are also insoluble in bile salts.

The viridans streptococci are a large and heterogeneous group of bacteria that are poorly differentiated, but they include organisms that play a role in tooth decay and those that can cause endocarditis and brain abscesses. The viridans streptococci are not often differentiated in diagnostic laboratories.

The b-haemolytic streptococci cause the complete breakdown of blood in fresh blood agar plates. The colonies are surrounded by haloes that are completely clear. Clinically, the most important of the b-haemolytic streptococci is Streptococcus pyogenes. This belongs to the "Lancefield Group A" based upon its antigenic structure. Streptococcus pyogenes may be differentiated from other b-haemolytic streptococci on the basis of its sensitivity to the antibiotic bacitracin.

The most important of the non-haemolytic streptococci are the enterococci such as Enterococcus faecalis and Enterococcus faecium. Until the early 1990's these bacteria were classified in the genus Streptococcus but molecular biological techniques have shown that they are sufficiently distant from other streptococci to warrant being placed in their own genus. As their names imply, these bacteria can be found in the gut, and can grow in the presence of bile salts.

The re-classification of these bacteria provide a good example of how molecular biology is revolutionising older classification systems, but it is not the most extreme example. The unicellular eukaryotes of the genus Cryptosporidium for many years was classified with the protozoa on the basis of its microscopic appearance and its other natural characteristics. Molecular studies have shown that this classification may not be entirely appropriate, since from an evolutionary point of view, these organisms are much more closely related to fungi than they are to protozoa, and so cryptosporidia have been classified in the wrong Kingdom!