Design and Optimization of a yst-PCR to Detect Yersinia enterocolitica in Meat Food.

In this study, a polymerase chain reaction (PCR) directed to the yst chromosomal gene (yst-PCR) was used as a rapid, sensitive, and specific method to detect Yersinia enterocolitica strains belonging to different biotypes in foods; a competitive Internal Amplification Control (cIAC) is also developed. The cIAC had a molecular weight of 417 bp and was detected until a concentration of 0.85 ng/µL. No other strains of other Yersinia species, nor Enterobacteriales order were detected by this PCR. In pure culture, the detection limit (DL) of the yst-PCR was lower for ystA+ strain (10 colony-forming unit [CFU]/mL) than for ystB+ strain (1 × 102 CFU/mL); which was the concentration detected in Y. enterocolitica inoculated minced meat. The proposed protocol included an enrichment step in peptone sorbitol bile (PSB) broth at 25°C for 24 h followed by isolation on Mac Conkey agar and chromogenic medium. An aliquot of the PSB broth homogenate and a loopful from the confluent zone of solid media were collected to perform DNA extraction for yst-PCR, and typical colonies were characterized by biochemical assays. Among 30 non-contaminated food samples, 4 samples were yst-positive and no Y. enterocolitica isolates were obtained. It is suggested that this yst-PCR could be used in the investigation of Y. enterocolitica in foods.

Genotypic and phenotypic characteristics of Yersinia enterocolitica isolated from the surface of chicken eggshells obtained in Argentina.

The prevalence of Yersinia enterocolitica on chicken eggshell surfaces in San Luis, Argentina, was investigated. The pathogenic potential of recovered isolates was assessed by means of phenotypic virulence tests and the presence of the 72-kb pYV plasmid. Antimicrobial susceptibility was determined by the agar diffusion method. DNA digested with XbaI was analyzed by pulsed-field gel electrophoresis (PFGE), and relationships between genomic DNA profiles were established. Eight Y. enterocolitica B2 O:9 strains were recovered after enrichment, for a prevalence of 2.27%. All strains harbored the virulence pYV plasmid, bound Congo red, grew in a low-calcium medium, and autoagglutinated at 37 degrees C. They lacked pyrazinamidase activity and did not hydrolyze esculin. These Y. enterocolitica strains were susceptible to amikacin, ciprofloxacin, chloramphenicol, and trimethoprim-sulfamethoxazole and were resistant to rifampin. According to the genomic DNA patterns obtained by PFGE, the isolates clustered into two groups, I and II. The highest similarity coefficient observed between Y. enterocolitica strains was 0.947. Microbiological controls on production stages of eggs and good culinary practices are necessary to reduce the risk of Y. enterocolitica infection for consumers.

Survival of Aeromonas hydrophila in Fresh Tomatoes ( Lycopersicum esculentum Mili) Stored at Different Temperatures and Treated with Chlorine.

This study examines the survival of two Aeromonas hydrophila strains ( A. hydrophila ATCC 7965 [strain A] and A. hydrophila isolated from food [strain B]) on the surface and core tissue of fresh tomatoes stored at different temperatures and the efficacy of chlorine treatment on their survival. Counts of A. hydrophila on the surface of tomatoes stored at 25 and 35 °C were significantly higher between days 1 and 4 for both strains as compared to results obtained at 6°C. Core tissue counts of A. hydrophila cells on tomatoes dipped in a cellular suspension at 25°C and stored at 25°C were significantly higher (P < 0.05) than counts obtained with dip suspensions at 6 or 35°C. In chopped tomatoes stored at 25 and 35°C, populations of aerobic mesophiles showed significant increases after 96 and 70 h, respectively. The populations of both A. hydrophila strains in chopped tomatoes stored at 6°C increased significantly after 96 h, while at 25 and 35°C the counts increased in the first hours of incubation. Viable counts of A. hydrophila on the surface and central tissue of tomatoes significantly decreased (P < 0.05) when the samples were dipped for 2 min in chlorine at a concentration of 50 ppm (50 µg/ml). The results suggest that tomatoes should be kept at low temperatures during storage, shipping, and retail stocking and that chlorine at a concentration of 50 ppm should be used to reduce the levels of A. hydrophila .

Prevalence of Yersinia enterocolitica in Hake (Merluccius hubbsi) Fillets.

Yersinia enterocolitica is a human pathogenic bacterium. The prevalence of Y. enterocolitica in refrigerated hake fillets sold for human consumption in retail stores was investigated in order to determine the degree of pathogenicity. Three hundred samples were enriched in 0.067 M phosphate-buffered saline, pH 7.6, with 1% sorbitol and 0.15% biliary salts, postenriched in 0.5% KOH, and isolated in salmonella-shigella agar and MacConkey agar. Twelve strains of Yersinia were isolated from the whole group of samples, 11 (91.6%) of which corresponded to Y. enterocolitica and 1 (8.3%) to Y. intermedia . The Yersinia strains recovered were Y. enterocolitica B1 O:5 Lis Xz (1 strain), Y. enterocolitica B3 O:5 (1 strain), Y. enterocolitica B1 O:6,30-6,31 (1 strain), Y. enterocolitica B1 O:7,8-8-8,19 (1 strain), Y. enterocolitica B1 O:7,8-8-8,19 Lis Xz (7 strains) and Y. intermedia B1 O:40 Lis Xo (1 strain). Of the 12 strains isolated, 8 (66.6%) were recovered by alkaline postenrichment. The first two strains were positive for virulence tests (autoagglutination and calcium dependence for growth at 37°C). The antibiotic susceptibility of the isolated strains was studied by the agar-diffusion method according to Bauer-Kirby, modified by Barry. Some of the isolated strains were potentially pathogenic, representing a risk for human health.

Biovars, Serovars, and Phagovars of Yersinia enterocolitica Isolated From 450 Samples of Cold Food in San Luis, Argentina.

A search of Yersinia enterocolitica in foods of animal origin has been carried out. Isolates were obtained from 450 samples of cold foods: 100 samples of cooked ham, 150 samples of salami, 100 samples of porcine cheese (artisan cold foods), and 100 samples of mortadella. Enrichments were performed in 0.067 M phosphate buffered saline solutions, pH 7. 6, containing 1% sorbitol and 0.15% biliary salts. The samples were postenriched in 0.5% KOH. Subcultures were done in Salmonella-Shigella agar and MacConkey agar. Isolates were identified through biochemical, serological, and phagotyping methods. The following biovars (B), serovars (O), and phagovars (Lis) were isolated from cooked ham B2 0:9 Lis X3 (1%), from salami B1 0:5 Lis Xz and B2 0:9 Lis X3 (1.33%), from porcine cheese B2 0:9 Lis X3 (2%), and from mortadella (0%). Virulence tests (calcium dependent growth at 37°C and autoagglutination activity at 37°C) were always negative. Serovar B2 0:9 Lis X3 associated with human disease was isolated. It is concluded from the results of this study that Y. enterocolitica isolates from cold foods lack of pathogenic importance.