Bacteria come in quite a variety of different shapes even square, but the main ones are spheres, rods, commas and spirals.

By far the most common shapes are rods and spheres and when the early microbiologists first saw them down a microscope they called them bacilli, from the Latin word "baculus" meaning "rod" or "stick" and cocci, from the Greek word "kokkus" meaning "grain" or "berry".

There are few variations on this shape. Some are short and fat, others longer. Some are bigger and some smaller. This and the obvious different locations where the bacteria were found was really all early microbiologists had to go on. They assumed that all rod-shaped bacteria were closely related and used the word Bacillus to name the genus. All examples of rods were given different species names. Eg the rod in the gut was Bacillus coli, the rod causing typhoid was Bacillus typhi and the rod found in rotting hay was Bacillus subtilis.

We now know that lumping all these very different species together in the same genus is incorrect and since the days of the Gram stain we know they have very different cell wall structures. These days we have retained the genus Bacillus for some Gram positive microbes and placed other rod-shaped bacteria in a variety of different genera such as Escherichia, Salmonella, Clostridia etc. All this can be very confusing especially since the term "bacillus" is still used, quite rightly, to describe the shape of the microbe and is used in some trivial names eg the typhoid bacillus for the bacterium Salmonella typhi.

When cocci were first being observed their organisation was just about all microbiologists had to go on to identify them. Thus, basic divisions between diplococci, streptococci and staphylococci were made and these names used for the different genera. We now know that diplococci are really streptococci but the distinction between streps and staphs remains because there is a good scientific basis for it.

The genus Streptococcus contains a large number of species which are widely distributed in the animal kingdom. Many species are normally present, forming part of the normal flora but others are the aetiological agents of some serious diseases such as pneumonia, scarlet fever and meningitis, especially neonatal meningitis. They also frequently cause infections of the upper respiratory tract which, although less serious today because of improved control measures, often lead to more life-threatening conditions such as acute glomerulonephritis and rheumatic fever.

The genus Streptococcus is defined mainly in terms of the morphology of the cells: Gram-positive cells mainly in chains with some in pairs.

Historically, the most important streptococci were those causing major diseases such as pneumonia (Streptococcus pneumoniae), rheumatic fever (Streptococcus pyogenes) and life-threatening infections in newborns (Streptococcus agalactiae), so it's not surprising that this is where most of the early research on identification and taxonomy was performed.

By far the most significant contribution was that by Rebecca Lancefield who concentrated on β-haemolytic streptococci because these were associated with important diseases. She was able to classify many of these pathological streptococci by means of the presence of cell surface antigens (C substance) which she called "Group Antigens" and which were designated as Group A-E. Later studies on other streptococci lead to the discovery of further Group Antigens, H,K, M and D.

Other schemes aimed at classifying streptococci used a variety of biochemical characteristics and it became apparent that streptococci which were quite similar biochemically could possess a different Group Antigen. Understandably things got rather confused.

With the advent of modern molecular biology techniques it has become possible to examine the genetic make-up of organisms in a variety of different ways. One of these which has proved to be particularly useful is to look at species relationships based on an analysis of their 16S rRNA.

The diagram below shows how streptococci group together based on this analysis, note that it takes no account of any other characteristic such as haemolysis, Lancefield Antigen or biochemical test. Interestingly, these 16S rRNA groups are remarkably similar to the groups produced by other means.

For further details see: Kawamura, Hou, Sultana, Miura and Ezaki. (1995) Int. J. Syst. Bact. 45:406-408

Early oral microbiologists who cultivated samples from the mouth on blood agar noticed that the agar plates took on a distinctive green colouration after incubation. See Kissplates. Lacking any kind of detailed knowledge of the bacterial species growing in the mouth they assumed there was only a single species present and named it Streptococcus viridans (viridans because the Latin word for green is viridis). The name has stuck and when it became apparent that many different species were present they were grouped together as the "viridans streptococci". This is unfortunate because the "greening", also called "alpha-haemolysis", is not characteristic of all oral streptococci. We now know that this "greening" is due to the haemoglobin being bleached by the hydrogen peroxide produced by some species. So it is not really "haemolysis" at all.

The oral streptococci are not really a "group" in the sense normally used because the members share little in the way of common characteristics. In fact the only characteristic they share is that they inhabit the mouth.

No members of the "Pyogenic" or "Bovis" are included in the oral streptococci. Among the other groups shown above, Streptococcus pneumoniae and Streptococcus thermophilus are not normally included either. Nor are Streptococcus suis or Streptococcus acidominimus.

All the rest belong to the oral streptococci, although only two members of the "Mutans Group", Streptococcus mutans and Streptococcus sobrinus are found in human mouths. The others are found in various animals.

The "oral streptococci", therefore, are a very heterogeneous group with little in common other than they are streptococci and inhabit the mouth. Although they are considered commensals most are also opportunistic pathogens and have been linked with a variety of diseases especially brain and liver abscesses and infective endocarditis.

Streptococcus mutans was first described by JK Clark in 1924 after he isolated it from a carious lesion but it wasn't until the 1960s that real interest in this microbe was generated when researchers began studying dental caries in ernest. Many strains were isolated which were biochemically very similar but carried different antigenic markers. Altogether, 7 serotypes designated a, b, c, d, e, f and g were described. Group e were so-called because they cross-reacted with Lancefield Group E antigen. Note that the other serotypes did not have any relation with the Lancefield antigen system.

Later studies which looked at the protein profiles of strains, their cell wall structures and gross DNA composition confirmed the serological findings that there was considerable variation amongst the large number of isolates identified as Streptococcus mutans.

Based on a whole raft of studies, the Streptococcus mutans isolates were sub-divided into a number of distinct species some of which were of animal and some human sources. Thus the "Mutans streptococci" were born and the name Streptococcus mutans was retained to describe the most common of the two main human strains, the other being called Streptococcus sobrinus. The retention of the name Streptococcus mutans has lead to some confusion but was necessary to comply with the rules governing scientific nomenclature.

For a detailed description of the current classification of oral streptococci see:

"Current Classification of the Oral Streptococci" by RA Whiley & D Beighton Oral Microbiol. Immunol. (1998) 13: 195-216.

For further information on mutans streptococci see also Streptococcus mutans and mutans streptococci

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