Gregory Wickham

Benzalkonium chloride is a common quaternary ammonium cation-based disinfectant used as an industrial-grade biocide, but little independent work has been undertaken quantifying the concentrations required for sterilization. This study investigated relative differences in resistance between common Gram-negative and Gram-positive bacterial pathogens and determined the complete sterilization concentrations for each. A membrane filtration methodology was used to quantify an enriched isolate of deionized water, which was subjected to various concentrations of disinfectant incubated on MacConkey agar. The colony forming units at each concentration were compared to an untreated control. Three main trends, defined as ‘phases of inhibition’, were observed across all isolates studied. Phase I occurred from 0 to 1 mL disinfectant/L water and displayed a moderate, consistent rate of inhibition. Phase II occurred from 0.1% to 0.4% biocide in solution and was characterized by a dramatic increase in inhibition and a divergence of inhibition rates for each organism. Phase III occurred from 0.4% biocide in solution onward and was characterized by the gradual decline in rate of inhibition until each organism reached total inhibition. It was found that the Gram-negative group, comprising Escherichia coli and Pseudomonas aeruginosa, was generally more resistant than the Gram-positive group, comprising Enterococcus faecalis and Staphylococcus aureus, p < 0.001, with the individual Gram-negative organisms, having the highest complete sterilization concentrations. It was also observed that a variation in resistance existed between organisms of the same Gram stain group. This resulted in some organisms exhibiting resistances comparable to that of organisms of the opposite group, namely between the E. faecalis and P. aeruginosa, which exhibited no significance difference, p = 0.080. Therefore, a model is proposed in which the Gram stain groups can be generalized as being distinct in terms of intrinsic resistance, but also that the range of resistance exists as a spectrum within each group which can cause a similarity between individual organisms of different groups.