Action of disinfectant quaternary ammonium compounds against Staphylococcus aureus

Mode-of-action studies concluded that alkyldimethylbenzylammonium chloride (ADBAC) (a blend of C(12), C(14) and C(16) alkyl homologues) and didecyldimethylammonium chloride (DDAC) are both membrane-active agents, possessing subtly different modes of action reflecting early cell interactions against Staphylococcus aureus. ADBAC and DDAC exhibited similar MIC behaviors from 0.4 ppm to 1.8 ppm over an inoculum range of 1 x 10(5) to 1 x 10(9) CFU/ml at 35 degrees C. For ADBAC and DDAC, an increased rapidity of killing against S. aureus (final concentration, 2 x 10(9) CFU/ml) was observed at 35 degrees C compared to 25 degrees C. Concentration exponents (eta) for killing were <2.5 for both agents, and temperature influenced the eta value. Examination of leakage and kill data suggested that a single leakage marker was not indicative of cell death. ADBAC and DDAC possessed Langmuir (L4) and high-affinity (H3/4) uptake isotherms, respectively. ADBAC molecules formed a single monolayer of coverage of cells at the end of primary uptake, and DDAC formed a double monolayer. Rapid cell leakage occurred at bactericidal concentrations, with total depletion of the intracellular potassium and 260-nm-absorbing pools released in this strict order. Autolysis was observed for ADBAC and DDAC at concentrations of 9 mug/ml (0.0278 mM and 0.0276 mM, respectively) and above, together with the depletion of approximately 30% of the internal potassium pool. Autolysis contributed to ADBAC and DDAC lethality, although high biocide concentrations may have inhibited autolytic enzyme activity.