“Three is a crowd….”

I have a personal rule with regards to bacteriology cultures. This is the “three species rule”. If there are more than three different bacterial species on a plate then I do not support each individual species being identified and reported.

The reasons for this are as follows:

  • When multiple bacterial species are found on one plate, the end result hardly ever affects patient management.
  • Who knows which bacterial species, if any on the plate is causing the problem?
  • Multiple bacterial species on one plate often indicates contamination of one sort or another.
  • Releasing antimicrobial susceptiblities on multiple different bacteria is likely to lead to overtreatment and select out for bacterial resistance.

Far better to group them together and report as mixed skin flora, mixed enteric flora, mixed environmental organisms, or whatever is most appropriate. This applies for sterile site samples also. It saves a huge amount of laboratory time and consumables and simplifies the report for the clinician.

It is only a rule, and rules can of course be broken occasionally, as I am well used to doing myself…..

The one routine exception I would have for this rule is for sputum cultures from cystic fibrosis patients. In this rather unique cohort it is important to detail all the potential pathogens, regardless of number. When interpreting CF sputum cultures, it is just as important to look at previous culture results, to see what organisms have appeared and what has disappeared.

It is always good to step back, have a look at the culture result you are putting out and think to yourself, “What clinical impact is this result going to have for the patient?” or put more simply “Am I helping the patient here?”



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