Enumeration of salmonellae in crocodile pond water by direct plate counts and by the mpn technique

Abstract

The use of Rappaport-Vassiliadis (RV) selective enrichment medium for the enumeration of salmonellae in pond water from four crocodile farms by the most probable number (MPN) method was compared to direct plating on xylose Lysine Desoxycholate agar. Direct plate counts (DPC) were consistently higher than MPN estimations (means of DPC 4.6 log 10 viable bacteria (v.b.) per 100 ml, as opposed to MPN 2.6 log 10 v.b. per 100 ml). It is suggested that the difference may be caused by differing viability of Salmonella subspecies or serovars in the RV medium, as four laboratory strains of S. enterica subspp. enterica and salamae were enumerated equally well by both DPC and MPN determinations, whereas the MPN failed to detect two strains of S. enterica subsp. diarizona at a concentration level of approximately 2 log 10 v.b. per 100 ml. It is concluded that the MPN determination method may not be sufficient and reliable when applied to samples containing Salmonella subspecies and serovars of reptilian origin.