Comparative disinfectant sensitivity of methicillin-resistant and methicillin-sensitive Staphylococcus aureus cells

Cells of a Methicillin-resistant strain of Staphylococcus aureus (MRSA, ATCC 33592) were compared with a Methicillin-sensitive strain of S. aureus (ATCC 6358) for sensitivity to three different disinfectant active ingredient classes using the quantitative method (QM). The disinfectant active ingredient classes included reagent grade hydrogen peroxide, quaternary ammonium compound (QAC), and reagent grade sodium hypochlorite. Cells of both strains were grown in broth culture, mixed with a 3-part soil load [mucin, Bovine Serum Albumin (BSA) and yeast extract], dried briefly on small stainless-steel carriers, and exposed to different dosages of disinfectant classes, corresponding to low, intermediate, and high efficacy treatments. Results show consistent cell recovery from both strains in the range of 5.5-6.0-Log₁₀ of bacteria per carrier from control sets assessed on nine separate test days. Efficacy results measured as mean log reduction (LR) values show comparable sensitivity of the MRSA strain relative to the Methicillin-sensitive strain to the low treatment of all three disinfectant classes. The MRSA strain displayed enhanced sensitivity (an increase in LR of 1.5-2.0) to the three disinfectant classes at the intermediate treatment level relative to the Methicillin-sensitive strain. Interestingly, while the MRSA strain displayed higher sensitivity to hydrogen peroxide by 2-Log₁₀ relative to the Methicillin-sensitive strain at the high treatment level, the two strains displayed comparable sensitivity to high treatment levels of the other two disinfectant classes, i.e., sodium hypochlorite and QAC. Taken together, the results show comparable to enhanced sensitivity of the MRSA cells compared to Methicillin-sensitive cells to reagent grade hydrogen peroxide, QAC, and sodium hypochlorite. The results support the contention that antibiotic resistance in S. aureus is not correlated with higher resistance to these three classes of disinfectants.