Chapter 6 – Cultural methods

Publisher Summary The usual microbiological techniques of plating material on selective or differential media and subculturing to obtain pure cultures cannot be applied to tuberculosis bacteriology. Cultures are usually made in bottles rather than in Petri dishes because mycobacteria, especially tubercle bacilli, are present in very small numbers in most specimens; this necessitates comparatively large inocula that are spread over the surface of the medium without looping out. The bottles are tightly stoppered to prevent drying of the cultures, which would certainly happen in Petri dishes. Most material submitted for culture contains many other organisms that grow in one or two days, and, within a week, they would overgrow the entire surface of the medium and probably digest it before the mycobacteria started to grow. Pathological material that is known to contain other organisms must be treated in an attempt to destroy them but to preserve the mycobacteria. The reagents used must also reduce the specific gravity of the specimen so that it can be centrifuged with a reasonable certainty that the majority of the mycobacteria will be deposited.