Prolonged survival of Serratia marcescens in chlorhexidine

During an outbreak of Serratia marcescens infections at our hospital, we discovered widespread contamination of the 2% chlorhexidine hand-washing solution by S. marcescens. Examination by electron microscopy of the sides of bottles in which this solution was stored revealed that microorganisms were embedded in a fibrous matrix. Bacteria, free in the liquid, were morphologically abnormal, showing cell wall disruption or cytoplasmic changes. Furthermore, bacteria adherent to the walls of the storage jugs and embedded in this fibrous matrix also had morphologically abnormal cytoplasm. Despite these changes, viable S. marcescens organisms were recovered from the fluid during a storage period of 27 months. The concentration of chlorhexidine required to inhibit these strains of Serratia was 1,024 microgram/ml; however, the organism could survive in concentrations of up to 20,000 micrograms/ml. Additional studies are needed to define the mechanism(s) that allows such bacteria to contaminate and survive in disinfectants.