The response of Gluconobacter oxydans to sorbic and benzoic acids

The minimal inhibitory concentrations (MIC) of sorbic and benzoic acids for Gluconobacter oxydans were 1000 mg/l and 900 mg/l respectively at pH 3.8. A reduction in the pH of the test medium to 3.3 reduced the MIC of both preservatives by about 300 mg/l. When G. oxydans was grown in the presence of sublethal concentrations of sorbic or benzoic acids before the MIC was determined, the MIC of both compounds increased substantially within 1 h. Growth of G. oxydans was modified in several ways by the presence of sorbic acid in the medium. The duration of the lag phase increased and there was a substantial decrease in the viable count during the lag phase in the presence of high concentrations. The generation time increased and the viable count at the end of the logarithmic phase was reduced. At 1 degree C, G. oxydans grew in the absence of sorbic acid but was inactivated by 400 mg sorbic acid/l. At 37 degrees C the viable count of suspensions of G. oxydans decreased in both the absence and presence of sorbic acid. Sorbic acid increased the death rate. Growth of G. oxydans was prevented by eliminating air from culture vessels, combined with the addition of ascorbate to the medium containing 400 mg sorbic acid/l.