Comparison of Listeria monocytogenes isolates across the island of Ireland.

Building a comprehensive knowledge base of the association of Listeria monocytogenes isolates across national food chains, clinical cases, and environments can play a key role in helping control the incidence of listeriosis. Today, many food chains cross national borders and are often shared by neighboring countries. This study characterized L. monocytogenes isolated from food samples in Northern Ireland and investigated whether similarities in the population and associations of L. monocytogenes strains exist in the neighboring countries of Northern Ireland and the Republic of Ireland, which together constitute the island of Ireland. Listeria monocytogenes isolates were characterized using serotyping and pulsed-field gel electrophoresis subtyping. This data was then interrogated against existing data for the Republic of Ireland, to identify any shared trends in the ecology and contamination patterns of L. monocytogenes strains. The results of this study indicated that contaminated food products often shared L. monocytogenes strains with other products. A total of six different strain subtypes were identified among 18 contaminated products. Overall strain diversity in positive samples was low, with no sample yielding more than one L. monocytogenes strain, as determined by pulsed-field gel electrophoresis subtyping. When comparisons against an Irish strain database were performed, many related strain subtypes were also shared by a variety of sources in the Republic of Ireland. This study highlights the potential benefits that a whole-island surveillance approach may present to food safety and public health in both Northern Ireland and the Republic of Ireland.

Sensitive detection of Escherichia coli O157:H7 by conventional plating techniques.

The direct detection and estimation of concentration of Escherichia coli O157:H7 down to 1 CFU/g of cheese was achieved by conventional plating techniques. Cheese was manufactured with unpasteurized milk inoculated with E. coli O157: H7 at 34 +/- 3 CFU/ml. The numbers of E. coli O157:H7 were monitored during cheese ripening by plating on sorbitol MacConkey agar supplemented with cefixime and potassium tellurite (CT-SMAC) and on CT-O157:H7 ID medium. Using the pour plate method, E. coli O157:H7 colonies could easily be distinguished from non-O157:H7 colonies on CT-O157:H7 ID medium but not on CT-SMAC. Higher numbers of E. coli O157:H7 were detectable with O157:H7 ID medium. Latex agglutination and PCR were used to confirm the identification of typical E. coli O157:H7 colonies, and nontypical colonies as not being E. coli O157:H7. As few as 1 CFU/g of cheese could be detected. E. coli O157:H7 also was detected in deliberately contaminated milk at concentrations as low as 4 CFU/10 ml.

The effect of different media on the survival and induction of stress responses by Campylobacter jejuni.

The survival kinetics of Campylobacter jejuni strain CI 120 to a challenge of pH 4.5 was studied in seven different media. A medium effect was observed, showing up to a 5-log difference in stress resistance of cells. Strain variation in survival of C. jejuni was observed in Brucella broth (BBL). The ability of C. jejuni CI 120 to respond to a stress after growth in seven different media was also examined. An Adaptive Tolerance Response (ATR) was induced in only three of the seven media tested. The degree of resistance induced by the ATR varied between the different media. The production, during growth, of an extracellular component that confers stress resistance against subsequent acid challenge was observed in only four of seven media tested. Due to the direct effect of medium on stress/survival of C. jejuni, the results suggest that studies using different media may not be comparable.

Induction of an adaptive tolerance response in the foodborne pathogen, Campylobacter jejuni.

In this study we aimed to determine if Campylobacter had the ability to induce an adaptive tolerance response (ATR) to acid and/or aerobic conditions. Campylobacter jejuni CI 120 was grown to the appropriate phase in Brucella broth under microaerobic conditions. Cells were initially adapted to a mild stress (pH 5.5) for 5 h prior to challenge at pH 4.5, a lethal pH. Survival was examined by determining the numbers of viable cells on Campylobacter blood free selective agar base. Stationary phase cells adapted at pH 5.5 induced an ATR that enabled a 100-fold greater survival compared to an uninduced culture. Aerobic adaptation also protected the cells against acid challenge. The cross protection provided a 500-fold increase in survival when compared to unadapted cells. The incorporation of chloramphenicol during the induction period eliminated the ATR and resulted in death kinetics similar to an uninduced culture. These data suggest that Campylobacter spp. have the ability to induce an ATR to sublethal treatments, which increased their ability to withstand subsequent stresses.