Coagulase-Negative Staphylococci Isolated from Blood Cultures: Contamination or Infection? Atherapeutic and Diagnosticchallenge

The possibility of contamination of the blood cultures, particularly by Coagulase-Negative Staphylococci (CoNS), is one of the challenges in microbiology. We performed a study from 2010 to 2015 to evaluate the percentage of CoNS arising from probable contamination and the percentage of CoNS probably responsible for sepsis. Additionally, we analysed susceptibility to antibiotics of all found microorganisms. In the observation period, we performed 18065 blood cultures with blood drawn from 4199 patients (averaging 4.3 blood cultures per patient). We used Tokar’s algorithm to layer positive blood cultures. Identification and antibiogram were performed with the Vitek2 Compact system (BioMèrieux). From 2010 to 2015 we found 1558 positive patients: bacteremia was considered very probable in 279 patients (6.6% of total patients, 17.9% of positive samples), bacteremia was considered as arising from contaminants in 510 patients (12.1% of total patients, 32.7% of positive samples), bacteremia was considered undetermined in 12 patients (0.3% of total patients, 0.8% of positive samples). Out of all the found strains, sensitivity to antibiotics was as follows: Penicillin, 3.6%; Oxacillin, 15.1%; Eritromicin, 22.3%; Vancomicin, 100%; Daptomicin, 98.5; Teicoplanin (except S. epidermidis and S. haemolyticus), 94.9%. In our laboratory, we execute identification and antibiogram also in the case of probable contamination. In this case, the positive bottle is reported with a note that defers the treatment decision to the clinician. The analysis of the SIR index showed that CoNS can represent a therapeutic challenge because CoNS have high methicillin resistance (84.9%). On the other hand, sensitivity to Vancomicin, Daptomicin, and Teicoplanin offers the possibility of an alternative therapeutic route.