Recovery of different Listeria ribotypes from naturally contaminated, raw refrigerated meat and poultry products with two primary enrichment media

Abstract

Isolation rates for Listeria monocytogenes and the other Listeria spp. typically improve when samples are enriched in more than one primary enrichment medium. This study evaluated the abilities of two primary enrichment media, University of Vermont-modified Listeria enrichment broth (UVM) and Listeria repair broth (LRB), to recover different ribotypes of Listeria spp. from raw meat and poultry samples. Forty-five paired 25-g retail samples of ground beef, pork sausage, ground turkey, and chicken (160 samples) underwent primary enrichment in UVM and LRB (30؇C for 24 h) followed by secondary enrichment in Fraser broth (35؇C for 24 and 40 h) and plating on modified Oxford agar. After 24 h of incubation at 35؇C, 608 Listeria colonies from selected positive samples were biochemically confirmed as L. monocytogenes (245 isolates), L. innocua (276 isolates), and L. welshimeri (89 isolates) and then ribotyped with the automated Riboprinter microbial characterization system (E. I. du Pont de Nemours & Co., Inc.). Thirty-six different Listeria strains comprising 16 L. monocytogenes (including four known clinical ribotypes), 12 L. innocua, and 8 L. welshimeri ribotypes were identified from selected positive samples (15 samples of each product type; two UVM and two LRB isolates per sample). Twenty-six of 36 (13 L. monocytogenes) ribotypes were detected with both UVM and LRB, whereas 3 of 36 (1 L. monocytogenes) and 7 of 36 (3 L. monocytogenes) Listeria ribotypes were observed with only UVM or LRB, respectively. Ground beef, pork sausage, ground turkey, and chicken yielded 22 (8 L. monocytogenes), 21 (12 L. monocytogenes), 20 (9 L. monocytogenes), and 19 (11 L. monocytogenes) different Listeria ribotypes, respectively, with some Listeria ribotypes confined to a particular product. More importantly, major differences in both the number and distribution of Listeria ribotypes, including previously recognized clinical and nonclinical ribotypes of L. monocytogenes, were observed when 10 UVM and 10 LRB isolates from five samples of each product were ribotyped. When a third set of six samples per product type was examined from which two Listeria isolates were obtained by using only one of the two primary enrichment media, UVM and LRB failed to detect L. monocytogenes (both clinical and nonclinical ribotypes) in two and four samples, respectively. These findings stress the importance of using more than one primary enrichment medium and picking a sufficient number of colonies per sample when attempting to isolate specific L. monocytogenes strains during investigations of food-borne listeriosis.

Prevalence of Listeria sp. in droppings from urban rooks (Corvus frugilegus)

“…Indeed, different strains can be isolated from the same sample by an enrichment procedure and a direct plating ( P h i and Gilbert 1988;Loncarevic rt al. 1996) or by different enrichment media (Ryser et al 1996).…”

“…These different strains have been isolated from foods by two methods, an enrichment procedure and a direct plating procedure. Molecular typing methods including restriction enzyme analysis, pulsed-field gel electrophoresis and ribotyping, have recently been used to characterize and compare L. monoqytogenes strains isolated from the same food sample (Loncarevic et al 1996;Ryser et al 1996). T h e results suggest that the ability to recover specific Listeria strains is at least partially dependent on the enrichment procedure and the picking of a sufficient number of colonies per sample.…”

Evaluation of an rDNA Listeria probe for Listeria monocytogenes typing

“…However, ribotyping, regardless of the probe used, cannot accurately distinguish between serovar 4b strains, a serovar responsible for major foodborne outbreaks of listeriosis, as previously observed (4,16,25,26). Despite this, a major advantage of this method is the automation of the procedure (1,7,17,30).…”

“…For example, DI increases if ribotyping is combined with RAPD (4), restriction enzyme electrophoresis (REA) or multilocus enzyme electrophoresis (MEE) (26). Ribotyping is appropriate for ecological studies (1,21,30), but should be replaced by DNA macrorestriction pattern analysis for epidemiological investigations of human foodborne listeriosis, in which it is essential to identify the epidemic clone as accurately as possible so as to identify food vehicles, in conjunction with a case-control study.…”

High pressure response of fruit jams contaminated withListeria monocytogenes

“…It can also survive freezing (Gola et al 1990). These micro-organisms, which are generally associated with dairy products, have been detected in a wide variety of foods, such as raw meat (Donnelly 1994;Ryser et al 1996), cheese (Szczawinski et al 1997), different pasteurized products (Fernández Garayzabal et al 1986;Yen et al 1994), and even in vegetable products (Heisick et al 1989). Conner et al (1986) found L. monocytogenes in marrow and showed that this could be transmitted to humans.…”