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 gener ation time of L. monocytogenes, whereas hardly any effect is observed on Listeria innocua. Because acriflavine binds to protein in the sampl es, a decrease in acriflavine activity results. This lesser activity m ay result in a better growth of L. monocytogenes. At low pH-values (pH <5.8) more acriflavine is bound, but growth promoting effects are limi ted because growth of this pathogen is restricted at low pH. On accoun t of this, one may expect that enrichment protocols employing low acri flavine concentrations with an adequate buffer, favour the isolation o f L. monocytogenes. Because comparative studies have paid no attention to the ratio of L. monocytogenes to other Listeria spp., virtually no thing is known about the inferior detection of L. monocytogenes. Previ ous comparative studies combined with the results of this work indicat e strongly that during enrichment procedures, other listeriae or compe titive micro-organisms may mask the presence of this pathogen. For tha t reason, in enrichment protocols for the detection of L. monocytogene s, it is worthwhile introducing an isolation medium that facilitates i dentification of L. monocytogenes in the presence of high numbers of o ther listeriae. (C) 1996 Academic Press Limited