Taxonomy is not every man's meat, but neither is it everyone's poison. It can be likened to a cocktail: a skilful blend in which it is not easy to discern the individual ingredients. In taxonomy the ingredients are (i) classification, or the orderly arrangement of units, (ii) nomenclature, the naming or labelling of the units, and (iii) identification of the unknown with a unit defined and named by (i) and (ii). The subdivisions should be taken in the order indicated, for without adequate classification it is impossible to name rationally, and without a system of labelled units it is impossible to identify others with them or to communicate the results.

Classification

Before discussing the identification of bacteria, the principles of classification and nomenclature must first be dealt with briefly. Since this book is essentially a practical manual, theoretical speculations about the validity of bacterial species (Lwoff, 1958; Cowan, 1962a; Lapage et al., 1975) are not considered.

For this Manual, the concept of bacterial species is therefore accepted as a convenient unit. However, as it so obviously has different values in different groups of bacteria, no attempt is made to define it, or to analyse the qualities that distinguish one species from another. Nor is any attempt made to determine whether a taxonomic group (taxon) is a species, a variety or a subspecies; or to estimate the value or importance of different kinds of bacterial characters (Cowan, 1968, 1970b).