Tolerance to acid in pH 5.0-grown organisms of potentially pathogenic gram-negative bacteria

A wide range of potentially pathogenic species of Gram-negative bacteria were far more resistant to extreme acidity (pH 2.0-3.5) when cultured at pH 5.0 (habituated to acid) than after pH 7.0 culture. The differences were particularly great for Citrobacter spp., Enterobacter spp., Klebsiella spp. and for Vibrio parahaemolyticus; substantial habituation was also observed for Proteus mirabilis and Aeromonas formicans but the effect was less marked for Serratia marcescens and Acinetobacter calcoaceticus. Growth at pH 5.0 was substantially poorer than at pH 7.0 for most of the above species and also for Salmonella typhimurium and Salm. enteritidis but phosphate markedly enhanced growth at pH 5.0 for many of these species without affecting growth at pH 7.0.