You are probably familiar with the scenario… A blood culture takes a couple of days to become positive. A Gram stain shows Gram negative rods. The plates are subbed but after another couple of days there is still no growth on the plates. The clinicians are getting impatient and are phoning the lab looking for an identification…

It is certainly a frustrating situation but one that occurs not infrequently.

Is there anything that can be done to help the clinician, and more importantly the patient, whilst we are waiting for the organism to grow?

There are several organisms to consider in this sort of scenario. We should be thinking about HACEK organisms, anaerobes, Pasteurella spp., oxidative non-fermentors like Burkholderia spp., Capnocytophaga spp. Consider micro-aerophilic bacteria such as Campylobacter spp. and Helicobacter spp.  And don’t forget about exotic organisms such as Brucella spp.  Even then, this list is by no means exhaustive, and I am sure there are others that you have come across that I have forgotten about!

What can the lab/clinical microbiologist do to narrow the differential down and manage accordingly pending plate growth?

A few things come to mind:

Aerobic or anaerobic bottle positive?: If the aerobic bottle only is positive it can point to non-fermentors like Burkholderia spp. If the anaerobic bottle only is positive, then one must think about anaerobes (e.g. Bacteroides spp. Fusobacterium spp. )

Gram stain appearance: A cocco-bacillary appearance should make one think of Haemophilus and Brucella. Longish, pleomorphic spindle-shaped organism on Gram point towards Capnocytophaga. “Seagulls” or “squiggles” should send you in the direction of Campylobacter/Helicobacter

Patient History: A history of dog bites/exposure should make one think of Capnocytophaga. A prosthetic valve or other valve disease, or clinical stigmata of endocarditis can indicate a HACEK organism. A history of injecting drug use makes one suspicious of Burkholderia cepacia. A travel history to an endemic area could make one think of Brucella spp. or Burkholderia pseudomallei. In a patient with neck pain and swelling, you don’t want to miss a Fusobacterium necrophorum. If the patient is frankly septic, you want to make sure they are getting covered for Capnocytophaga and Pasteurella.

Of course, all this is speculation, and our educated guesses may be completely wrong in the end. It is however important speculation… We want to make sure that the patient is being covered for the most likely and the most serious possibilities.

Taking another, but no less important angle, from a lab point of view it is essential that any slow growing Gram-negative organisms are worked up in a biohazard cabinet. Laboratory exposure incidents for organisms such as Brucella spp.  and Burkholderia pseudomallei are resource-intensive, stressful for the staff, and for the most part avoidable.

And sometimes the bugs just like to mock us, even make fools of us. Just recently, I was convinced a wavy Gram negative rod in (multiple) positive blood cultures from a patient was going to turn out to be a Campylobacter, only for it to finally be identified as a Helicobacter cinaedi… Wrong again!

Michael