Despite the frequent and long-term use of biocides, there are still no clear criteria to determine whether a microbe is susceptible to biocides or not. In our research, we tackled this problem by determining the minimum inhibitory concentration (MIC) for six biocides; triclosan, benzalkonium chloride, chlorhexidine digluconate, sodium hypochlorite, isopropanol and glutaraldehyde for 454 isolates of select gram-negative bacilli (142 Escherichia coli, 86 Klebsiella pneumoniae, 44 Enterobacter cloacae complex, 29 Acinetobacter baumannii in 153 Pseudomonas aeruginosa). Based on this data, we determined the tentative epidemiological cut-off values (tECOFF). In most cases, a normal distribution of MIC was demonstrated, but in some bacteria–biocide combinations we observed a bimodal distribution, indicating the existence of subpopulations with reduced susceptibility to biocides. We have analysed the influence of resistance to multiple antibiotic classes (multi-drug resistant, MDR) on biocide susceptibility by comparing the MIC distribution of non-MDR and MDR strains. We did not observe significant differences in the distribution of MIC values. Our results therefore indicate that resistance to biocides and consequently a possible connection with MDR in select clinically important gram-negative bacilli, is rare.
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