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At least 415 records · Page 23Linked to original sources
PubMed1990

Influence of freezing-thawing and refrigeration on R-plasmid (pRPJ24) stability in Enterobacter cloacae 94R.

The effect of twenty freezing (-20 degrees C) and thawing cycles of Enterobacter cloacae 94R cells containing the R-plasmid pRPJ24 inoculated into broth and ground beef meat samples revealed no loss of resistance due to plasmid instability. In addition, low temperature storage at 4 degrees C did not produce any significant loss of the tetracycline and kanamycin resistances encoded on the pRPJ24 plasmid. The results of this study indicated that indigenous R-plasmids like pRPJ24 are stable in resident recipients like E. cloacae 94R in ground beef. However, the proportion of viable cells containing the pRPJ24 plasmid decreased significantly after 20 freezing-thawing cycles over 14 days incubation at 4 degrees C.

Animals↗
PubMed1991

Effect of temperature history on the growth of Listeria monocytogenes Scott A at refrigeration temperatures.

The effect of pre-inoculation temperature on the subsequent growth of Listeria monocytogenes Scott A at 5 degrees C was examined in microbiological medium, UHT milk, canned dog food, and raw ground beef (untreated and irradiation-sterilized). In microbiological medium, the duration of the lag phase was decreased when aerobic and anaerobic cultures were initially grown at less than or equal to 28 and less than or equal to 13 degrees C, respectively. Subsequent exponential growth rates and maximum population densities of the 5 degrees C cultures were not affected by temperature history. Differences in lag phase durations were also observed when L. monocytogenes initially cultured at 19 and 37 degrees C were grown at 5 degrees C in UHT milk and some of the canned dog food varieties. Growth of L. monocytogenes was not observed in either untreated or irradiation-sterilized raw ground beef. While temperature history can affect the growth kinetics of L. monocytogenes at 5 degrees C, it did not account for the lack of growth in raw meat, suggesting that there is an inhibitory condition or component in ground beef that is lost upon cooking.

Aerobiosis↗
PubMed1993

Five-tube most-probable-number method using the Fung-Yu tube for enumeration of Listeria monocytogenes in restructured meat products during refrigerated storage.

The survival of Listeria monocytogenes in cooked, chopped hams stored at 5 degrees C for 5 weeks was studied. Slices of chopped ham (25 g) were inoculated with a three-strain mixture of L. monocytogenes (LM 101M, LM 103M, and Scott A) at levels of < or = 350 cfu/25 g and packaged in Stomacher bags. Prior to inoculation, L. monocytogenes cells were subjected to either heat-injury (56 degrees C, 30 min) or freeze-injury (-18 degrees C, 14 days). The organisms were detected by a five-tube most-probable-number (MPN) technique using the motility enrichment Fung-Yu tube, and the direct plating method. After storage at 5 degrees C for 1 week, there was a one log10 reduction in counts. Thereafter, Listeria recovered and heat- and freeze-injured cells grew to 10(7) and 10(8) within 5 weeks, respectively. Similar results were obtained from both methods. However, direct plating could not recover L. monocytogenes at low levels (< or = 100/25 g), whereas MPN counts were obtained at these low levels. The pH (6.22 +/- 0.10) of the chopped hams remained constant throughout the study. These results indicated that low numbers of L. monocytogenes surviving sublethal heat- or freeze-injury could initiate growth after recovery in chopped hams. The five-tube MPN method using the Fung-Yu tube was effective in enumerating both low and high levels of L. monocytogenes in food.

Animals↗
PubMed1996

Listeriolysin O production and pathogenicity of non-growing Listeria monocytogenes stored at refrigeration temperature.

Three haemolytic, pathogenic strains of Listeria monocytogenes (a reference strain NCTC 7973, a food-derived strain L70 and a human strain L94) and a control strain of Listeria innocua L27 were held in phosphate-buffered saline (PBS) of pH 7.0 or 5.5 at 4 degrees C for 4 weeks. The number of viable cells did not change significantly during this storage (the cells were non-growing). Titers of Listeria listeriolysin O (LLO) activity against washed human erythrocytes and the pathogenicity of non-growing bacterial cells for 14-day-old chick embryos were determined before storage and after 1, 2, 3 and 4 weeks of storage. Prolonged storage at 4 degrees C affected both LLO production and pathogenicity of the non-growing cells, but effects were strain- and pH-dependent. At pH 7.0, all three L. monocytogenes strains had lost LLO activity after 2 weeks of storage. At pH 5.5, the reference and the food strains lost LLO activity 1 week later than when stored at neutral pH, and the human strain maintained LLO activity throughout the 4-week period. Pathogenicity of the reference strain stored at pH 7.0 and 5.5 and that of the food strain stored at pH 7.0 decreased during storage at 4 degrees C. However, the human strain stored at pH 7.0 and 5.5, and the food strain stored at pH 5.5, maintained their pathogenicity throughout the 4-week period. In all cases, non-growing L. monocytogenes cells that had ceased LLO production and/or had a reduced pathogenicity, recovered these characteristics after growth in media at 37 degrees C. This study indicates that prolonged storage of chilled-foods in which L. monocytogenes is present, but not growing may have the desirable result that the L. monocytogenes has a reduced ability to cause illness in humans. As well, pathogenicity testing involving growth of L. monocytogenes in laboratory media may not reflect the actual pathogenicity of the organism in the food as eaten.

Bacterial Toxins↗
PubMed2006

Comparison of pathogenic and spoilage bacterial levels on refrigerated poultry parts following treatment with trisodium phosphate.

This study was undertaken to determine whether trisodium phosphate decontamination of poultry could give a competitive advantage to pathogens and increase microbiological risk to consumers. Chicken legs were co-inoculated with similar concentrations of pathogenic (Salmonella Enteritidis or Listeria monocytogenes) and spoilage (Pseudomonas fluorescens or Brochothrix thermosphacta) bacteria. Samples were dipped in TSP (12%, 15 min) or were non-treated (control). Microbiological analyses were carried out at 0, 1, 3 and 5 days of storage (3 degrees C). Levels of spoilage bacteria were higher than those of S. Enteritidis on both treated and non-treated legs. Similar bacterial loads were observed for L. monocytogenes and B. thermosphacta. However, P. fluorescens counts on TSP-treated samples were significantly lower than those of L. monocytogenes at all sampling times. Our results found that P. fluorescens (a spoilage organism) was more susceptible to TSP treatment than L. monocytogenes when inoculated at 10(6) cfu g(-1).

Animals↗
PubMed2006

Effect of preincubation temperature and pH on the individual cell lag phase of Listeria monocytogenes, cultured at refrigeration temperatures.

The impact of precultural temperature and pH on the distribution of the lag phase of individual Listeria monocytogenes cells was assessed during preincubation at 7 degrees C, using a dilution protocol to obtain single cells, and optical density measurements to estimate the individual lag phase. Firstly, the pure temperature effect (37, 15, 10, 7, 4 and 2 degrees C) was investigated on a subsequent growth at 7 degrees C and pH 7.4. Secondly, low precultural temperatures (10, 7 and 4 degrees C) were combined with a controlled pH at 7.4 and 5.7 with a subsequent growth at 7 degrees C and at different pH values (7.4, 6.0 and 5.5). For all temperature-pH combinations, the individual cell lag phase was determined using a three-phase linear growth model. It was observed that at low precultural temperatures (2, 4 and 7 degrees C), a high proportion of L. monocytogenes cells were able to grow at 7 degrees C with almost no lag phase, consequently, the resulting distributions were positively skewed. Beside this, the variability observed was lower than at higher precultural temperatures. Regarding the precultural pH effect, at pH 7.4 the mean values of the lag phases were shorter at lower preincubation temperatures; while at pH 5.7 small pH transitions produced shorter individual lag phases at all precultural temperatures. The quantification of the effect of precultural conditions on the individual cell lag phase duration would improve the accuracy of the existing growth models, especially when a series of processing and storage steps are linked together in a process model or exposure assessment. Distributions will be fitted to the data for every set of conditions, generating useful tools for further risk assessment purposes.

Colony Count, Microbial↗
PubMed2005

Detection of the apr gene in proteolytic psychrotrophic bacteria isolated from refrigerated raw milk.

Bacteria of the genus Pseudomonas have been associated with the spoilage of raw milk and dairy products due to the production of thermostable proteolytic enzymes. The apr gene encodes for alkaline metalloprotease in Pseudomonas and other related bacteria. Its presence in psychrotrophic proteolytic bacteria isolated from raw milk collected from cooling tanks was verified. A polymerase chain reaction (PCR) technique was used with degenerate primers. Total DNA from 112 isolates was pooled in different groups and then used as template for the amplification reactions. Controls consisted of DNA extracted from 26 cultures. An expected DNA fragment of 194 bp was detected in groups that contained bacteria identified as Pseudomonas. The PCR product was observed only when DNA from control cultures of Pseudomonas aeruginosa, Pseudomonas fluorescens, Serratia marcescens and Aeromonas hydrophila were used. A detection limit assay indicated that the apr gene could be directly amplified from pasteurized milk inoculated with 10(8) CFU/ml of P. fluorescens. With this method it was possible to detect proteolytic bacteria at 10(5) CFU/ml in reconstituted skim milk powder if cells were recovered for DNA extraction before amplification.

Animals↗