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Typing Bacteria

Introduction

Bacterial "typing" is a very important part of modern microbiology. Briefly, it entails using a test of some sort to distinguish different strains of a particular bacterial species from each other. This ability has important implications in many aspects of microbiology, not least medical microbiology.

What are bacterial strains?

In every-day language we refer to different species by their "trivial" names but each also has a formal, scientific name which is much more precise and which is used by scientists when they need to avoid any confusion.

So a chimpanzee for example is Pan troglodytes, earwigs all belong to the order Dermaptera of which Forficula auricularia is just one example, and mankind is Homo sapiens. You will, however, be very aware that not all the individual organisms within a species are the same. If this comes as a surprise, look at your colleagues and recognise that within any species there is considerable genetic variation.

Sometimes these differences are used by biologists to subdivide a species into subspecies or varieties. For example, Sleeping sickness (trypanosomiasis) is caused in West Africa by Trypanosoma brucei. However, not all variants of this species cause the disease in man. So the pathogenic version is called T. brucei var. gambiense and the non-pathogenic version is called T. brucei var. brucei. In this case, but not all, the differences are not big enough to allow biologists to create a new species.

Before we get too far off the point, the same is true for bacteria. If a species of bacteria is isolated and cultivated in the laboratory it is known as a strain and is assumed to be different from other strains of the same species unless we can prove it is identical. These strains are often given distinguishing numbers or acronyms. For example we have Streptococcus mutans LT11, Streptococcus sanguis NCTC 7863 etc etc. Think of these strain names as personal names (Streptococcus mutans Fred Bloggs) and you will have it about right.

Medical significance of strains

Sometimes strains of a particular bacterial species are very much more pathogenic than other strains. In other species, different strains may be equally pathogenic. Whichever is the case, it is important that we know which strain is causing an outbreak of disease because this allows us to trace the source. By tracing the source of an outbreak we can prevent further infection. This is not true for all infections, of course, but it is the case for many serious infections which can give rise to severe epidemics.

A good example is an outbreak of Salmonella-related food-poisoning.

More than 2000 different types of Salmonella enterica have been recognised. A means by which these can be differentiated has been developed based on the presence or absence of particular antigens. These antigens are detected using antibodies which are found in serum hence the different types are known as "serotypes". If medical microbiologists are presented with an outbreak of food-poisoning and they can isolate Salmonella enterica from the faeces of every patient this is not necessarily significant. If, however, they all have the same strain then it is highly likely that they were infected from a common source. Knowledge of the serotype causing the general outbreak will enable epidemiologists to trace the source and prevent further infection.

Although strains of Salmonellae can be recognised by "serotyping" this method is not always practical for other bacteria. However, a number of different schemes have been devised, one or more of which is applicable to a particular bacterial species.

Examples of typing schemes

Biotyping

Biotyping relies on picking up differences in biochemical reactions. Strains are referred to as "biotypes". The particular biochemical reactions exploited are legion. The more familiar tests include detecting the ability of an isolate to ferment a variety of different sugars. In practice, little reliance is placed on just one aspect of an organisms metabolism and sugar-fermentaion patterns may be used in conjunction with a panel of other tests which detect quite different metabolic feats. Different biotyping schemes have been devised to be used with particular microbial species and many are now available in easy to use kits, if not completely automated.

Bacteriocin-typing

Bacteriocin typing detects different antibacterials produced by a particular strain which are usually only active against other strains of the same species.

Protein-typing

Protein typing relies on differences, some major, others minor, in the range of proteins made by different strains. The proteins are extracted from a culture of the strain, separated by electrophoresis and compared with those of other strains.

Phage-typing

Phage typing detects different susceptibilities to attack by certain bacteriophages (bacterial viruses). This relies on the presence or absence of particular receptors on the bacterial surface which are used by the virus to bind to the bacterial wall.This test is particularly useful in detecting different strains of staphylococci.

In the test, the isolated staphylococcus is inoculated over the whole surface of a growth plate. The plate is then inoculated in discrete areas with each of 23 different phages which are known to be capable of attacking staphylococci and to which staphylococci show a strain-dependent susceptibility. The plate is then incubated overnight and the staphylococci multiply to create a visible confluent growth (known as a lawn) over the whole plate. If, however, the staphylococci are attacked by a particular phage they are lysed (broken down) which causes spaces to appear in the lawn. The pattern of spaces in the lawn reveals the different susceptibilities of strains to the 23 different phages which are always inoculated onto the plate in the same sequence.

Modern developments

The variety of typing schemes available and their limited applicability can be very confusing, not to say difficult to put into practice because of all the different methods which must be kept available in the medical microbiology laboratory.

However, in addition to the more traditional methods of typing bacteria there are now a range of methods available which detect differences in either the DNA or ribosomal RNA of strains as well as methods relying on mass-spectrometry and gas-chromatography.

These methods have a broad applicability and are likely to prove useful across a wide range of species not just in the microbial world. This means that the range of expertise and equipment which previously had to be kept available in a medical microbiology laboratory could be a reduced. This is one of the reasons why these modern methods are likely to prove particularly popular notwithstanding their speed and ability to detect quite small differences.

The relentless quest for ever more reliable, fast, accurate and cheap methods to identify bacterial strains reflects the importance of being able to do it.

SUMMARY
1.	Within any bacterial species there is a lot of genetic variability. Sometimes this affects pathogenicity but not always. This variability is important because it enables us to track down a particular strain of a species which might be the cause of an outbreak of infection. A large range of tests are available to identify different strains but not all methods are equally applicable to every species.
2.	Examples of tests include biotyping, bacteriocin-typing, protein-typing and phage-typing. More recently tests to distinguish strains based on differences in DNA and RNA are becoming available. The main advantages of these are reliability, sensitivity and a common methodology.

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