LOSS OF VIABILITY BY STAPHYLOCOCCUS AUREUS IN ACIDIFIED MEDIA

Abstract

Survival of Staphylococcus aureus (108 cells per milliliter) after 24 hr of incubation at 37 C in Trypticase Soy broth acidified with acetic, citric, hydrochloric, lactic, and phosphoric acids was investigated. When the organism was exposed to the medium adjusted with hydrochloric acid to pH values of 5.2–3.6, 90–99.99% of the cells were inactivated. Acetic, lactic, and phosphoric acids were more active against S. aureus than was hydrochloric, whereas citric was equivalent to hydrochloric. Mixtures of lactic and hydrochloric acids inactivated more cells than did either acid alone but mixtures of other acids with hydrochloric, while superior to hydrochloric acid itself, offered no apparent advantage over use of single acids. The undissociated acid molecule was responsible for enhanced inactivation of cells by partially dissociated acids since anions of these acids had no effect on cell survival. Cells were more susceptible to inactivation by hydrogens ions at high incubation temperatures (45 C) and when the number of bacteria was low. Cells of S. aureus were most sensitive to the effects of hydrogen ions between the 12th and 24th hr during a 120-hr incubation.