Carla Raggi, Perla Filippini, Monica Monaco, Annalisa Pantosti, Roberta Creti and Lucilla Baldassarri
Purpose: To examine the resistance to benzalkonium chloride (BKC) and the distribution of biocide-resistance genes in S.aureus clinical isolates and to determine whether any correlation may exist with antibiotic resistance pattern and biofilm formation.
Methods: MICs to BKC were determined in a collection of S.aureus (HA-MRSA, CA-MRSA and MSSA) both in suspension and on biofilm-embedded cells. Characteristic of the isolates (qac genes and biofilm formation) were determined by PCR and a plate assay, respectively.
Results: MICs to BKC were higher among MRSA than MSSA, where the CA-MRSA showed MIC levels closer to the MSSA group. qacA/B genes were found only among HA-MRSA and conferred higher resistance to the disinfectant while smr gene did not. MBC, but not MIC, were higher for biofilm embedded vs. planktonic cells, but no correlation was found with the ability to form biofilm.
Conclusion: We confirmed that presence of qacA/B but not smr confers higher resistance to BKC; MICs among MRSA were more spread compared to MSSA, suggesting that factors associated to the MR phenotype may confer resistance to BKC. Interestingly, MSSA showed higher biocide tolerance in both the planktonic and biofilm form according to the MIC/MBC fold change values. Although no correlation could be observed between biofilm thickness and biocide resistance, biofilm-embedded cells responded differently to disinfectants suggesting the current practices for efficacy testing of biocides may not be relevant in the evaluation of disinfectant efficacy against biofilm-embedded
microorganisms.