Influence of pH and NaCl on monolaurin inactivation of Streptococcus iniae Original Research Article

Streptococcus iniae, a Gram-positive bacterium, has been recognized as a cause of human and fish streptococcosis. Aquacultured fish are susceptible to infection and have been epidemiologically linked with wound infections of fish handlers. Therefore, there is a need to develop methods of prevention and/or control of this pathogen. This study determined the minimum inhibitory concentration of monolaurin (glycerol monolaurate) against S. iniae strains PW and BU and utilized gradient plates at 37°C to determine the combined influence of pH and NaCl on monolaurin inactivation of S. iniae. The minimum inhibitory concentration of monolaurin against strain PW was 12.4±0.3 μg ml−1 and against strain BU was 12.1±0 μg ml−1. Compared to growth in the absence of monolaurin, the incorporation of 6 μg ml−1 monolaurin into the salt (1.12–6.95%)–pH (4.26–8.88) gradient plates prevented growth of strain PW at pH values <6.00 and strain BU at pH values <6.20, regardless of the salt concentration. The combination of monolaurin with environmental variables such as pH and salt proved to be an effective tool for in vitro control of S. iniae.