Practical Identification of Bacteria

In all cases it is absolutely essential that the tests are carried out on pure cultures. Tests carried out on mixed cultures are of no significance whatsoever. 

Inoculation of the sample onto selective media is bad practice, as this does not produce a pure culture but only suppresses the growth of unwanted species. Although experience suggests that the same problem is usually caused by the same or similar organisms, occasionally some unusual organism may be present. This may survive the selective medium, and start to grow once the organism is subcultured into various test media. 

Once the bacteria have grown on the non-selective medium, cultures should be examined for their colonial morphology, and also to determine whether the organism has had any effect on the medium. Generally cultural characteristics on simple media are too variable to be of diagnostic value and, although great 

In numerical classification and identification, all the tests are carried out simultaneously, and the same weighting is given to each character. One major problem is that not all members of the species or genus may give the same result. It is not unusual to find that only 80 or 90% of the organisms tested give a positive result. This can cause considerable problems in programming computers and lead to considerable confusion. 

The use of rapid systems, such as the API system, relies on a series of biochemical tests carried out simultaneously. The results of these tests are pooled 

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The use of various characteristics carries no guarantee that the tests being used are significant. The classification of bacteria therefore tends to be heavily biased towards the practical aspects of microbiology, such as the identification of medically and industrially important species. 

To control the microbiological situation, sometimes a full count of microbe species at different locations is required, rather than determination of the presence of certain microbial groups like sHme-forming species, fungi, yeasts, anaerobic bacteria, etc. Conventional methods for identification of microbes include enumeration and screening of individual species isolated from count plates on selective media. Ready-made plates are available for the selection of different types of microbes. Commercial identification systems for industry and medical purposes, based on numerous biochemical tests or other characteristics, are available, including a database that covers practically aU groups of bacteria. 

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