Where is typing going?

The basic premise in any bacterial typing scheme is that epidemiologically related isolates are derived from the clonal expansion of a single precursor. In simple terms this means that a certain characteristic is more useful than others on the basis that it is conserved within a strain but diverse with a species. Such a definition is appropriate when considering the spread of a species within a ward hospital or even a community. However, as we have developed more and more refined molecular techniques for analysis of the bacterial genome, such a definition has become stressed when considering the evolution that is occurring especially amongst drug-resistant bacteria whose genetic make-up is changing due to the selective pressure induced by drug usage in the hospital or community. Increased sophistication does not necessarily bring improved typing of bacterial strains which might only be variants of the same clonal type. Molecular epidemiology is perhaps too exacting to be practical in certain circumstances and one should be cautious in its interpretation and extrapolation. It is open to debate whether we shall see in the next century unanimity in the choice of pheno- or genotypic tests to be used for typing of bacterial strains.