An investigation into in vitro methods for the detection of chlorhexidine resistance

During an outbreak of catheter-related urinary tract infection, due to Proteus mirabilis, it was suggested that the epidemic strain was resistant to chlorhexidine. In this study, the minimum inhibitory concentrations (MICs) of chlorhexidine to the epidemic Pr. mirabilis and other laboratory isolates were tested in different media. Results were compared with killing times using 1/4 strength Ringers solution, normal human urine and the in vivo killing times in two patients' catheter bags. It was found that the MIC test was unreliable in the assessment of chlorhexidine resistance as it was dependent on the medium used, the inoculum size, and the age of the culture. The test which gave results closest to the in vivo experiments was the killing curve in normal human urine. It is concluded that chlorhexidine resistance is a complex phenomenon which is difficult to evaluate. If in vitro tests are to be used to evaluate the clinical relevance of reduced sensitivity to chlorhexidine, they must mimic the in-use conditions as closely as possible.