Bacteria are living organisms and, as such, grow and divide over time. Most bacterial species will double in number approximately every 20 minutes (this time, called the generation time, will depend on the species concerned and incubation conditions). Therefore it will take about 18 hours for the number
of bacteria on a culture plate to produce visible bacterial colonies (at this point each colony will consist of several million bacterial cells). Thus bacterial culture of clinical samples takes at least 18-24 hours before results may be available. In practice, most clinical laboratories incubate their cultures for a minimum of 48 hours and sometimes for longer periods (often several days) in case there are very slow-growing organisms present in the sample.
Bacterial identification and antibiotic sensitivity testing
Following incubation, the colonies growing on the various culture media need to be identified. The colonies will vary in appearance, including size, colour, texture, and smell. Each bacterial species has a characteristic appearance on culture media and an experienced microbiologist can rapidly identify many bacterial species based on their colonial appearance.
Bacterial identification and antibiotic sensitivity tests are performed on a pure culture; therefore a single well-separated colony is selected from the culture plate; a single bacterial colony can be assumed to consist only of a single bacterial species. Most swabs from acute wounds show no bacterial growth or growth of one predominant organism. This could be the potential pathogen and is processed further to include identification and antibiotic sensitivities.
- KEY POINT
It is important to note that the recognition of possible pathogens, or other organisms of significance in the culture, depends entirely on the experience and expertise of the microbiologist. There is no automated process yet able to replace or mimic this human interpretation.
The isolate is identified using a variety of methods and assigned a scientific name; examples are Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. Occasionally an isolate will be reported with a familiar name rather than the strict scientific one, for example, group A Streptococcus rather than Streptococcus pyogenes. Only significant bacterial isolates will appear in the final report and they will have associated antibiotic sensitivities performed. Bacterial growth that is not considered significant (e.g. skin organisms such as diphtheroid-like organisms and coagulase-negative staphylococci) is either reported as normal skin flora or the final report will state No significant pathogens isolated. Similarly, if human intestinal
organisms are present then this would be reported as mixed coliforms isolated or mixed faecal flora isolated. Of course these organisms may also be present with recognized pathogens. This approach is taken because the body is covered with hundreds of different bacteria and most of them are part of the normal flora and non-pathogenic.