Antimicrobial efficacy of chlorhexidine-treated surfaces against clinical isolates implicated in nosocomial infections

<b>Introduction.</b> Bacterial infections and antimicrobial resistance are significant threats to global public health, both of which spread through contamination of solid surfaces. We have previously developed an antimicrobial surface technology that directly bonds the broad-spectrum biocide chlorhexidine to steel surfaces. These surfaces were shown to kill bacteria within minutes of contact and to be effective against bacteria evolved in the laboratory for resistance to chlorhexidine in solution.<b>Hypothesis/Gap Statement.</b> We hypothesized that resistance to these surfaces could exist outside of the naive and chlorhexidine-resistant laboratory strains tested previously. We also sought to test whether strains that were resistant to chlorhexidine in solution were also resistant to chlorhexidine-based antimicrobial surfaces.<b>Aim.</b> To test the efficacy of these surfaces against a range of bacteria isolated from the hospital environment and to compare this to the resistance of these bacteria to chlorhexidine in solution or when dissolved in solid media.<b>Methodology.</b> Ninety-one isolates of mixed bacterial species were obtained from Queen Elizabeth Hospital Birmingham. The isolates, along with laboratory strains of <i>Escherichia coli</i>, <i>Pseudomonas aeruginosa</i> and <i>Staphylococcus aureus</i>, were tested for sensitivity to chlorhexidine-coated steel surfaces in a 30-min exposure simulated splash assay. Resistance to chlorhexidine in solution was also assayed by solid and broth media MIC assays.<b>Results.</b> We demonstrate that within 30 min of incubation, the surfaces reduced the survival of all 91 isolates. Over 85% of these isolates were killed (exhibiting a 7-8 log reduction compared with control surfaces), whilst 12% experienced a 3-4 log reduction. We also show that resistance to the surfaces did not correlate with resistance to freely diffusible chlorhexidine in liquid or solid media.<b>Conclusion.</b> The results demonstrate the efficacy of chlorhexidine-coated surfaces against a broad range of bacterial isolates from the hospital environment and imply the potential for a mode of exposure to dictate the effectiveness of different antimicrobial resistance mechanisms. Future studies should investigate the genetic mechanisms providing resistance to chlorhexidine-coated surfaces and whether these differ in the capacity to provide resistance to chlorhexidine in different modes of exposure.