Getting the best from Microbiology

We think of microbiology as having miraculous qualities, where putting infective material on agar will always grow a pathogen with no interference by other organisms.  Sadly, this is not the case. 

To start with, there are many types of agar, some called “permissive” allowing every Tom, Dick and Harry bacterium to grow whereas others are called “selective” because they contain antibiotics or other inhibitors, specific nutrients or have acidic or alkaline pHs to encourage growth of a particular pathogen and discourage growth of other organisms.

Then there are “indicator” agar plates which show a colour change when a specific pathogen grows. 

You may think that using permissive agar would be the place to start, however, aggressive contaminants and commensals can easily overgrow a pathogen on these plates. Choosing the right selective medium depends on the pathogen(s) which in turn depends on sampling location, type of lesion, duration etc.  Consequently, to get the best out of microbiology, a lab needs you to provide a great history so that the most appropriate agar(s) can be chosen, or to give a list of the pathogens that you are interested in.

There is more to come on getting the best from Microbiology