Patient nostril microbial flora: individual-dependency and diversity precluding prediction of Staphylococcus aureus acquisition

The potential role of a patient's resident microbial flora in the risk of acquiring multiresistant bacteria (MRB) during hospitalization is unclear. We investigated this role by cross-sectional study of 103 patients at risk of acquisition of Staphylococcus aureus (SA), resistant (MRSA) or not (MSSA) to methicillin, recruited in four French hospitals. The flora was analysed by an exhaustive culture-based approach combined with molecular and/or mass-spectrometry-based identification, and SA strain typing. Forty-three of the 53 SA-negative patients at entry were followed for up to 52 weeks: 19 (44.2%) remained negative for SA and 24 (55.8%) became positive, including 19 (79%) who acquired an MSSA, four (17%) who acquired an MRSA and one who acquired both (4%). Fifty-one different species were identified among the 103 patients, of which two, Corynebacterium accolens and Staphylococcus haemolyticus (p = 0.02-0.01), were more prevalent in the absence of SA. However, the same number of patients carrying or not these two species acquired an MSSA/MRSA during follow-up, regardless of antibiotic treatment received. Clustering analysis showed that the microbial flora was highly specific to each patient, and not predictive for acquisition of MSSA/MRSA or not. Patient-specific microbial resident flora is not predictive of SA acquisition.