The effect of acriflavine and nalidixic acid on the growth ofListeriaspp. in enrichment media

The use of acriflavine in enrichment media forListeriaspp. has both direct and indirect effects on the isolation ofListeria monocytogenes. Increasing acriflavine concentrations affect both lag time and generation time ofL. monocytogenes, whereas hardly any effect is observed onListeria innocua. Because acriflavine binds to protein in the samples, a decrease in acriflavine activity results. This lesser activity may result in a better growth ofL. 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 ofL. monocytogenes. Because comparative studies have paid no attention to the ratio ofL. monocytogenesto otherListeriaspp., virtually nothing is known about the inferior detection ofL. 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 ofL. monocytogenes, it is worthwhile introducing an isolation medium that facilitates identification ofL. monocytogenesin the presence of high numbers of other listeriae.