Injury of bacteria by sanitizers

Injury of test cultures was quantitated by differences in colony counts obtained with a complete medium and those obtained on conventional selective media. Staphylococcus aureus, Streptococcus faecalis, and several strains of Escherichia coli were injured when exposed to the quaternary ammonium compound methylalkyltrimethyl ammonium chloride. Representative hypochlorite sanitizers also caused injury of E. coli ML30. Sanitizer concentration appeared to be the main factor in the cause of death and injury, a higher concentration being needed to cause death. Increases in temperature did not result in substantial increases in injury; however, the lethal effect was greater at higher temperatures. Varying the cell concentration from 10(7) to 10(9) cells per ml did not change the fraction of cell population killed or injured. The inability or failure of common selective media to detect injured bacteria in food could have serious public health consequences.