When working on sterile site isolates or blood cultures, our gut instinct is to save and store the isolates that are well recognised pathogens. However in my experience it is the putative pathogens for which we are just as likely to need to go back to and do further work.
Take the following hypothetical situations:
Multiple specimens are sent on a joint revision to exclude low grade infection. A couple of colonies of Staphylococcus epidermidis is grown from one of the samples, (rightly) thought to be of uncertain significance. It is decided not to treat, but the surgeon comes back to the laboratory a couple of months later saying that the patient’s symptoms have not improved, and is wondering whether that coagulase negative staphylococcus may have been significant after all. “Has the isolate been stored?”
or for another example..
A Propionibacterium acnes is grown from a blood culture after 4 days incubation. No clinical details were provided on the request form and the isolate is reported as a likely contaminant. However a couple of weeks later it transpires that the patient has a pacemaker in situ which is showing signs of chronic low grade infection and the cardiologist would like to do further work-up on the P. acnes. “Has the isolate been stored?”
Putative pathogens are exactly the ones that the clinicians start looking towards when things are not going quite right from a clinical point of view, or something has been missed in the initial clinical assessment (like the presence of prosthetic material). The laboratory needs to be ready for such scenarios, because not everything happens the way we expect them to.
I am not saying that we should be doing extensive antibiograms on such isolates, “just in case” one of them turns out to be significant, but I do believe in storing them, at least until the patient is clinically improving. Once the cultures have gone in the bin, there is no going back….