pH-dependent bactericidal barrier to gram-negative aerobes: its relevance to airway colonisation and prophylaxis of acid aspiration and stress ulcer syndromes--study in vitro.

pH-Dependent bactericidal activity on four gram-negative bacilli that are mainly responsible for gastric to airway colonisation has been investigated. Organisms studied were Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae and Serratia marcescens. At pH of the medium adjusted to 2.7 or lower all four organisms were killed in 1.5 h. At pH 3.1 or greater, no reduction in viable bacterial number was noted over 2 h. Even when pH of the medium was adjusted to 6.8, no increase in bacterial count was observed in 4 h. Relevance of these findings in relation to airway colonisation and prophylaxis of acid aspiration and stress ulcer syndromes is discussed. It is suggested to maintain pH of the gastric contents above 4.0 by H2 antagonists and prevent growth of gram-negative organisms in the stomach by keeping it empty.