Biochemical Testing and Rapid Identification of Organisms of Pharmaceutical and Medical Significance

BIOCHEMICAL TESTING AND RAPID IDENTIFICATION OF ORGANISMS OF PHARMACEUTICAL AND MEDICAL SIGNIFICANCE

The differing ability of bacteria to ferment sugars, glycosides and polyhydric alcohols is widely used to differentiate the Enterobacteriaceae and in diagnostic bacteriology generally. Fermentation can be indicated by pH changes in the medium with or without gas production visualized by the collection of bubbles in inverted tubes. More specialized media examine the ability of certain strains to oxidize or reduce particular substrates. There are many hundreds of individual biochemical tests available that each separately seek the presence of a particular enzyme or physiological activity. Taxonomic studies have led to the recognition that certain of these tests in combination characterize particular species of bacteria. Various manuals such as Bergey’s Manual (Holt, 1994) and Cowan and Steel’s Manual for the Identification of Medically Important Bacteria (Barrow & Feltham, 2004) provide a logical and sequential framework for the conduct of such tests. Identification of particular species and genera by such processes is time-consuming, expensive and may require numerous media and reagents.

This process has become simplified in recent years by the development of rapid identification methods and kits. The latter often use multi-well micro-titration plates that can be inoculated in a single operation either with an inoculated wire or with a suspension of a pure culture. Each individual well contains the medium and reagents for the conduct of a single biochemical test. Identification kits vary in their complexity and also in the precision of the identification made. Simple kits may perform only 8–15 tests, more complex ones are capable of performing 96 simultaneous biochemical evaluations. Scoring of each test and entry into a computer database then allows the pattern of test results to be compared with a large panel of organisms and a probability of identity calculated. As different sets of tests will be required for different classes of bacteria, guidance as to the initial choice of kit is given on the basis of the Gram stain reaction, and the results of oxidase and catalase tests performed directly on isolated colonies. In large diagnostic laboratories and in quality assurance laboratories automated systems are deployed that can inoculate, incubate and analyse hundreds of individual samples at a time.