Listeria monocytogenes is often present in meat and meat products that are sold in the area of northeast Bosnia and Herzegovina. The major objective of this study was to examine the virulence of L. monocytogenes strains isolated from these types of food in that geographic area. Polymerase chain reaction was used to detect eight genes responsible for virulence of this pathogen, namely, prfA, inlA, inlB, hly, plcA, plcB, actA, and mpl. All examined isolates were confirmed to possess the eight virulence genes. Ten different pulsed-field gel electrophoresis (PFGE) macrorestriction profiles were recognized among 19 L. monocytogenes strains after restriction with two different endonucleases (ApaI and AscI). The pathogenicity of three different PFGE types of L. monocytogenes was confirmed through in vivo tests, which were performed on female white mice (Pasteur strain), and it ranged from 3.55 × 10(8) LD50 to 1.58 × 10(10) LD50. All of the three different PFGE types of L. monocytogenes were regarded as moderately virulent in relation to the reference strain L. monocytogenes Scott A. This result might be one of the reasons for the absence of reported listeriosis in northeast Bosnia and Herzegovina, despite the high degree of food contamination with this pathogen.

Recent genome sequencing of isolates of Listeria monocytogenes serotype 4b implicated in some major outbreaks of foodborne listeriosis has revealed unique genetic markers in these isolates. The isolates were grouped into two distinct epidemic clones, ECI and ECII. In the present study, selected ECI- and ECII-specific genetic markers were detected in 16 and 15 of 89 L. monocytogenes 4b isolates, respectively. The ECI markers were found in 6 of 34 clinical isolates, 9 of 50 food isolates, and 1 of 5 environmental isolates, and the ECII markers were detected in 7 of 34 clinical isolates, 7 of 50 food isolates, and 1 of 5 environmental isolates. Hence, of the isolates with the epidemic clonal genetic markers, 38% (13 of 34) were of clinical origin, 32% (16 of 50) were of food origin, and 40% (2 of 5) were of environmental origin. The predominance of the epidemic clonal markers among the clinical and environmental isolates supports the hypothesis that these markers are correlated with the pathogenic potential of strains and with their environmental persistence. Several isolates had only one epidemic clonal marker, either the ECI-specific marker 133 or the ECII-specific marker 4bSF18. Pulsed-field gel electrophoresis analysis revealed higher genomic diversity among the strains with ECII-like characteristics than among those strains carrying the ECI-specific genetic markers.

A range of Gram-negative and Gram-positive bacteria (Escherichia coli, Salmonella typhimurium, S. enteritidis, Staphylococcus aureus and Listeria monocytogenes) was subjected to high hydrostatic pressures of 400 and 600 MPa at room temperature in broth adjusted at two pH values (5.3 and 6.5). The effects on viability of compression followed immediately by decompression and of different lengths of treatment (1, 5 and 10 minutes) were determined. L. monocytogenes was resistant to a compression/decompression cycle up to 400 MPa, while the other test organisms exhibited loss of viability ranging over three orders of magnitude. When the pressure was raised to 600 MPa, the viability of microbial suspensions was reduced between 2.5 and 5.9 logs. At both pressures a varying proportion of the cells exhibited sublethal injury. Experiments in which the cells were held at pressure for 1, 5 and 10 minutes showed that no Gram-negative bacteria from a population of 10 6 to 10 7 cells/ml survived 5 minutes at 400 MPa. In contrast, L. monocytogenes was not eliminated by treatment at 600 MPa for 10 minutes, although its survival fells by 5 logs from 10 7 to 10 2 cfu/ml.