Evaluation of Epidemiological Cut-Off Values Indicates that Biocide Resistant Subpopulations Are Uncommon in Natural Isolates of Clinically-Relevant Microorganisms

Abstract

To date there are no clear criteria to determine whether a microbe is susceptible to biocides or not. As a starting point for distinguishing between wild-type and resistant organisms, we set out to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) distributions for four common biocides; triclosan, benzalkonium chloride, chlorhexidine and sodium hypochlorite for 3319 clinical isolates, with a particular focus on Staphylococcus aureus (N = 1635) and Salmonella spp. (N = 901) but also including Escherichia coli (N = 368), Candida albicans (N = 200), Klebsiella pneumoniae (N = 60), Enterobacter spp. (N = 54), Enterococcus faecium (N = 53), and Enterococcus faecalis (N = 56). From these data epidemiological cut-off values (ECOFFs) are proposed. As would be expected, MBCs were higher than MICs for all biocides. In most cases both values followed a normal distribution. Bimodal distributions, indicating the existence of biocide resistant subpopulations were observed for Enterobacter chlorhexidine susceptibility (both MICs and MBCs) and the susceptibility to triclosan of Enterobacter (MBC), E. coli (MBC and MIC) and S. aureus (MBC and MIC). There is a concern on the potential selection of antibiotic resistance by biocides. Our results indicate however that resistance to biocides and, hence any potential association with antibiotic resistance, is uncommon in natural populations of clinically relevant microorganisms.

Chlorhexidine and Mupirocin Susceptibility of Methicillin-Resistant Staphylococcus aureus Isolates in the REDUCE-MRSA Trial

“…We used protocols endorsed by the Clinical and Laboratory Standards Institute that are widely accepted as reference methods for categorizing bacterial susceptibility to antibiotics and that have been used often in other studies of staphylococcal susceptibility to CHG, allowing our results to be compared to those in published reports. The susceptibility breakpoint for CHG applied in this study was derived from the MIC epidemiologic cutoff, a standard approach used when validated breakpoints are not available (31,48). As in other reports (15,19,20), qacA was not a specific predictor of CHG resistance: three of five qacA-positive isolates were susceptible to CHG, and MBCs were always within 2 doubling dilutions of MICs.…”

“…Starting with a 20% (wt/vol) CHG solution (SigmaAldrich, St. Louis, MO), a 2-fold dilution series (from 32 to 0.0625 g/ml) was prepared daily. An isolate was classified as nonsusceptible to CHG if the MIC was Ͼ4 g/ml, which is outside the wild-type distribution of CHG MICs for S. aureus (epidemiologic cutoff) (31,48). Susceptibility to mupirocin was determined by the Etest method (bioMérieux, Durham, NC) according to the manufacturer's instructions.…”

Antimicrobial activity of octenidine against multidrug-resistant Gram-negative pathogens

“…Noteworthy, to date, there is no breakpoint consensus to define biocide-reduced susceptibility, including for CHG. Based on the epidemiological cutoff (ECOFF) proposed by Morrissey et al [17], we might consider most of our isolates as non-susceptible to CHG.…”

The effects of essential oil, povidone-iodine, and chlorhexidine mouthwash on salivary nitrate/nitrite and nitrate-reducing bacteria

“…The minimum inhibitory concentrations of chlorhexidine for Staphylococcus aureus, Salmonella spp., and Escherichia coli are 8 mg/L (0.0008%), 32 mg/L (0.0032%), and 64 mg/L (0.0064%), respectively (24). Exposure to 0.0025% chlorhexidine for 30 min suppressed the adhesion of oral Candia species to buccal epithelial cells by 50.89% (25).…”