There are some bacteria out there that usually don’t cause disease. They tend to just hang out and not cause any harm. When we see them in the bloodstream, they often are contaminants, meaning they were introduced during collection of the sample, and are not truly in the bloodstream. Figuring out which ones are contaminants and which ones are not can be tricky. Some are almost always (see * below) contaminants while others are NEVER contaminants. Some can be a contaminant or an infection, like streptococcus viridans group.

*Important to note: any organism can cause an infection in the right context, even those who are usually deemed as contaminants. If you are unsure whether a true infection is present, it’s always best to call an infectious disease specialist to assist with management.

Contaminant =  growth of bacteria in the blood culture bottle that were not present in the patient’s bloodstream and thus introduced during the collection of the sample (Dawson et al.)

NEVER assumed to be contaminants:

Staphylococcus aureus
Streptococcus pneumoniae
Group A/B streptococcus
Listeria monocytogenes
Neisseria meningitidis
Gram negative bacilli
Fungus (yeast or mold)
…Amongst many other numerous organisms that will take too long to mention in a list

Common blood contaminants:

Coagulase-negative staphylococci
Propionibacterium acnes
Micrococcus spp.
Corynebacterium spp.
Bacillus spp. (*except Bacillus anthracis which causes anthrax!!)

What if multiple bacteria are growing?!

• multiple growth of bacteria can suggest contamination.
• however, it again depends on the pathogens involved. If all are skin flora, then probably contaminant. If any of the aforementioned ‘USUALLY NOT CONTAMINANTS’ are present, then it is not a contaminant!

Where do contaminants come from:

1. Patient’s skin
2. Equipment used to obtain sample and transfer it to culture bottle
3. Provider’s skin
4. General environment

Factors to consider when deciding whether culture
is contaminant or not:

1. Patient’s clinical status
   • are there any signs or symptoms to suggest infection with this microbe?
   • Example:
     • You would not expect a patient with pneumonia to grow CoNS in their blood.
     • However, you would take CoNS seriously in a patient with recent valve replacement and fevers.

2. Microbiology of the species – see above
3. Time to positivity of blood culture – studies have shown that the average time of positivity of true infections is ~12 hours vs. ~24 hours for contaminants
4. Inoculum of the isolate – how much growth is there?
   • If only 1 out of 4 blood culture bottles are positive –> MORE LIKELY contaminant
   • If >1 out of 4 blood culture bottles are positive –> true pathogen

5. Any foreign material? – Contaminants become pathogens when they infect hardware and prosthetic valves/grafts. If those are present and there is a possibility they are infected, would not disregard any pathogen as a contaminant.
6. Patient’s response to antibiotics and isolate’s susceptibility pattern – clues to whether an isolate is a contaminant or a true infection
   • Contaminant: patient does not respond to antibiotics that treat the blood culture isolate
   • True pathogen: patient DOES respond to antibiotics that treat the blood culture isolate
   • Contaminant: patient responds to antibiotics despite blood culture isolate resistant
   • True pathogen: patient does not respond to antibiotics and isolate is resistant

FUN FACT: A retrospective review looked at 626 blood cultures and discovered that by 48 hours, 98% of aerobic gram-positive and gram-negative bloodstream infections were identified.

*This shows that unless there is a high suspicion for anaerobe growth, antibiotics can be de-escalated at 48 hours if there is no growth. (Pardo, J., Klinker, K.P, et al. 2014. Time to positivity of blood cultures supports antibiotic de-escalation at 48 hours. Annals of Pharmacology. 48 (1): 33-40).)

TAKE HOME POINTS:

• Microbes known to be common contaminants CAN cause disease in certain circumstances
• Always repeat blood culture when first one is positive for microbes
• If you’re not sure if it’s a contaminant or not ⇒ call ID
  (it’s always better to double check rather than to miss a true infection)

References:

1. Dawson, S. 2014. Blood culture contaminants. Journal of Hospital Infection; 87, 1-10. DOI: 10.1016/j.jhin.2014.02.009
2. Pardo, J., Klinker, K.P, et al. 2014. Time to positivity of blood cultures supports antibiotic de-escalation at 48 hours. Annals of Pharmacology. 48 (1): 33-40).
DOI: 10.1177/1060028013511229
3. Hossain, B., Islam, M.S., et al. 2016. Understanding bacterial isolates in blood culture and approaches used to define bacteria as contaminants: a literature review. Pediatric Infectious Disease Journal. 35(5): S45-51.
DOI: 10.1097/INF.0000000000001106
4. Pien, B.C., Sundaram, P., and et al. 2010. The clinical and prognostic importance of positive blood cultures in adults. American Journal of Medicine. 123 (819-828).
DOI: 10.1016/j.amjmed.2010.03.021