Tolerance of Bacteria for Quaternary Ammonium Compounds

SUMMARY– Escherichia coli and Pseudomonas fluorescens followed similar patterns of adaptation to tolerate quaternary ammonium compounds. E. coli approached a tolerance at 28 μg/ml in nutrient broth after 12 to 14 daily transfers. P. fluorescens adapted more rapidly in a similar medium, reaching a level of 120 μg/ml in 12 days. During the adaptation process, there was a gradual buildup of numbers of individual cells that tolerated the essential plateau of maximum tolerance. A reverse process was true during loss of tolerance from growth in the absence of quaternary ammonium compounds. The cells of adapted cultures were more resistant when low exposures of quaternary ammonium compounds were used to determine germicidal effectiveness. There was no apparent difference, however, between the non‐adapted and the adapted cultures, when concentrations and exposures approaching standard recommendations were used. The results indicate adaptation is an unlikely contributor to an objectionable microflora on properly cleaned food handling equipment.