In this article we will discuss about the need for pure cultures for identification of bacteria.
Despite the fact that some bacteria may be having remarkable characteristics for identification, their isolation and preparation of pure cultures is a necessity. Modern techniques of identification using gene problems often with enhancement by a polymerase chain reaction PCR can be performed using mixed populations.
However, these techniques are limited to a small number of clinically important species. ‘The Nature of Bacterial Identification Schemes’ the single selection of a colony from a plate does not assure purity. This particularly is true while selective media are used.
Live but non-growing contaminants may often be present in or near a colony and can be sub-cultured along with the chosen organisms. That is why non-selective media are preferred for final isolation because they allow such contaminates to develop into visible colonies. In non-selective media too, some well isolated colonies may not grow soon.
Inoculants may be slow growing and may appear on the plate only after a longer incubation period. In case of bacteria that form extracellular slime the filaments become firmly embedded and are difficult to separate. It is difficult to obtain pure cultures in cyanobacteria too as the contaminants get embedded in the gelatinous sheaths surrounding the cells.
In general, however, the colonies obtained as a result of streaking for pure cultures are similar to one another providing evidence of purity. Although this may be true, there may exist variations, e.g., capsular variations, pigmented or non-pigmented variants that are selected by certain media, temperature or other growth conditions. Another criterion of purity is morphology. The organisms from a pure culture usually exhibit a high degree of morphological similarity in stains or wet mounts.
There are exceptions depending upon the age of the culture, the medium used and other growth conditions, e.g., coccoid body formation, cyst formation, spore formation and pleomorphism. A good example is that the both culture of marine spirillum after 2 or 3 days may lead one to conclude that the culture is contaminated with cocci. But experienced microbiologists are aware that such spirilla often develop into thin walled coccoid forms after a period of active growth.