Isolation and characterization of Shiga toxin-producing Escherichia coli O157:H7 and non-O157 from beef carcasses at a slaughter plant in Mexico.

The contamination of beef carcasses with Shiga toxin-producing O157:H7 and non-O157 Escherichia coli (STEC) obtained from a slaughter plant in Guadalajara, Mexico was investigated. A total of 258 beef carcasses were sampled during a 12-month period. All samples were assayed for STEC by selective enrichment in modified tryptone soy broth supplemented with cefixime, cefsulodin and vancomycin, followed by plating on Sorbitol MacConkey Agar supplemented with cefixime and tellurite (CT-SMAC). Simultaneously, all samples were assayed by immunomagnetic separation (IMS) and plated on CT-SMAC and CHROMagar. The presence of the stx1, stx2, eaeA and hly933 genes, recognized as major virulence factors of STEC, was tested for O157:H7 and non-O157 E. coli isolates by multiplex polymerase chain reaction (PCR). STEC was detected in two (0.8%) samples. One of these STEC isolates corresponded to the serotype O157:H7 showing stx2, eaeA and hyl933 genes. The other isolate corresponded to non-O157 STEC and only had the stx1 gene. Thirteen carcasses (5%) were positive for nonmotile E. coli O157 and 7 (2.7%) were positive for E. coli O157:H7. The presence of O157:H7 and non-O157 STEC on beef carcasses in this slaughter plant in Guadalajara, Mexico, emphasizes the importance of implementing the Hazard Analysis and Critical Control Point (HACCP) system, as well as the need for implementing, evaluating, and validating antimicrobial interventions to reduce the presence of potential pathogenic microorganisms.

Survival of Vibrio cholerae O1 in ceviche and its reduction by heat pretreatment of raw ingredients.

The survival of Vibrio cholerae O1 serotypes Inaba and Ogawa was determined in ceviche prepared from inoculated ground fish. Ground mackerel purchased from a seafood distribution center was inoculated with V. cholerae and stored at 8 or 20 degrees C. Counts of V. cholerae decreased in 2.6 to 2.7 log10 CFU/g during 96 h of storage at 8 degrees C or 2.5 to 2.6 log10 CFU/g during 24 h at 20 degrees C. Survival studies indicated that serotype Inaba decreased its number following a linear or retarded trend, whereas serotype Ogawa followed an accelerated death trend. No effect of the initial level of inoculum was observed. Odor scores of ceviche indicated that this food became marginally acceptable within as little as 48 h of storage at 8 degrees C or 3 h at 20 degrees C and were related to total volatile nitrogen values but not to aerobic plate counts, pH, or coliform counts. A heat pretreatment that consisted of stirring 100 g of inoculated ground fish into 40 ml of boiling water produced an 8-log reduction of V. cholerae within 3 min without affecting the color, odor, or flavor of ceviche prepared with such pretreated fish. According to this study, V. cholerae present in contaminated ceviche will likely survive longer than the shelf life of this food. Preheating the ground raw fish used for preparing ceviche for 3 min should effectively eliminate V. cholerae O1, providing science-based conditions for implementing a critical control point if a hazard analysis critical control point plan were to be developed for preparation of ceviche.