Characteristics of Acid-Injury and Recovery of Staphylococcus aureus in a Model System

Injury and recovery characteristics of Staphylococcus aureus after exposure to acetic, hydrochloric, and lactic acids were studied. Cells of S. aureus were acid-injured at 37°C for 30 min in acidic isotonic saline solutions and were enumerated on Trypticase Soy Agar (TSA) and Trypticase Soy Agar with 7% NaCl (TSAS). A difference of at least 25% between counts on the non-selective (TSA) and selective (TSAS) media was considered evidence of acid injury. The activities of coagulase and thermonuclease were reduced in injured cells. The absence of leakage of 260/280 nm absorbing material from acid-injured cells, together with the absence of change in membrane fatty acids, indicated that membrane damage was not associated with acid injury. Inhibition of RNA synthesis was observed. During recovery, renaturation and synthesis of proteins, including enzymes and ribosomal proteins, occurred which then provided the intracellular conditions for subsequent growth.