The effect of acriflavine and nalidixic acid on the growth of Listeria spp in enrichment media

The use of acriflavine in enrichment media for Listeria spp. has both direct and indirect effects on the isolation of Listeria monocytogenes. Increasing acriflavine concentrations affect both lag time and generation time of L. monocytogenes, whereas hardly any effect is observed on Listeria innocua. Because acriflavine binds to protein in the samples, a decrease in acriflavine activity results. This lesser activity may result in a better growth of L. monocytogenes. At low pH-values (pH<5.8) more acriflavine is bound, but growth promoting effects are limited because growth of this pathogen is restricted at low pH. On account of this, one may expect that enrichment protocols employing low acriflavine concentrations with an adequate buffer, favour the isolation of L. monocytogenes. Because comparative studies have paid no attention to the ratio of L. monocytogenes to other Listeria spp., virtually nothing is known about the inferior detection of L. monocytogenes. Previous comparative studies combined with the results of this work indicate strongly that during enrichment procedures, other listeriae or competitive micro-organisms may mask the presence of this pathogen. For that reason, in enrichment protocols for the detection of L. monocytogenes, it is worthwhile introducing an isolation medium that facilitates identification of L. monocytogenes in the presence of high numbers of other listeriae. (C) 1996 Academic Press Limited