I am an avid proponent of multi-culturalism. However I am not such a big fan of mixed cultures when it comes to agar plates in the laboratory. Here are a few Q&As offering my personal views on the topic.

Do mixed cultures always represent contamination?

Not necessarily. The organisms within a mixed culture may or may not be contaminants. A mixed culture does not imply that no infection is present, nor that all the bacteria present are harmless. It just means that with the cocktail of organisms we have on the plate in front of us there may not be a lot that we can do to help the clinician by working up individual organisms.

What defines a mixed culture?

I generally regard a mixed culture as one containing three or more organisms. However if one micro-organism is particularly predominant (or clearly a recognised pathogen over and above the others present) then I may make an exception depending on the clinical scenario.

Do we still over-work mixed cultures in the laboratory?

Undoubtedly. Sometimes we just try too hard. Sometimes we just need to take a step back and have the courage to say “This is mixed, lets report it as such”. Some people have much lower thresholds than others for working up mixed cultures, so there is definite heterogeneity in how these culture plates are dealt with. I also anecdotally get the impression that if staff are quiet, the threshold for working up mixed cultures is lower. If staff are very busy, then there is more of a tendency to “cut to the chase” and just report it out as mixed. I believe that some staff feel they are not doing their job properly if they don’t work up individual colonies in a mixed culture.

Should mixed cultures from sterile sites be worked up?

Because they are sterile sites and thus more critical samples, the threshold for working mixed cultures up should naturally be a bit lower. However the same fundamental principles still apply; “ Can I help the clinician by working these colonies up?” Mixed enteric organisms from peritoneal fluid post perforated appendix, mixed Coagulase Negative Staphylococci from blood cultures, and mixed respiratory organisms from BAL fluid generally should be all reported as and for what they are. An example of where I would have a lower threshold for working up individual colonies in a mixed culture is in something like a liver abscess.

Sometimes I feel a tinge of guilt about changing a report to mixed flora where someone has spent time and effort over the previous few days diligently working up individual organisms. However it is only by doing this that behaviour and philosophies change…..

Michael