Liv Marit Rørvik

Contamination pattern of Listeria monocytogenes and other Listeria spp. in a salmon slaughterhouse and smoked salmon processing plant

A smoked salmon processing plant including a smokehouse and a slaughterhouse was examined for the occurrence of Listeria monocytogenes and other Listeria spp. From a total of 475 samples the overall frequency of L. monocytogenes was 16%, while other Listeria spp. were found in 22% of the samples. L. monocytogenes was most often detected in samples from the smokehouse, where 29% of the environmental and 26% of the fish samples during processing contained the bacteria. 17% of the fish raw material to the smokehouse were contaminated, while 11% of the samples from vacuum-packed smoked salmon were positive for L. monocytogenes. The slaughterhouse was sporadically contaminated, but L. monocytogenes was not found in 50 samples of slaughtered fish. L. monocytogenes was found in the seawater outside the slaughterhouse. Multilocus enzyme electrophoresis divided the isolated L. monocytogenes strains into 11 electrophoretic types (ETs). One ET, ET-6, which is the most common ET in Norway, seemed to have colonized the smokehouse. Isolates from the seawater, from the slaughterhouse and from fish coming into the smokehouse, before filleting, were other ETs.

Occurrence of and a possible mechanism for resistance to a quaternary ammonium compound in Listeria monocytogenes.

In a study of 200 Listeria monocytogenes isolates, 10% were determined to be resistant to benzalkonium chloride (BC). Serial subcultivation of initially BC sensitive (BC(S)) and BC resistant (BC(R)) isolates in sublethal concentrations of BC resulted in enhanced and approximately equal resistance of all strains to the compound. Fifty per cent of the BC(R) isolates showed resistance to ethidium bromide (EB) as well. A proton motive force (pmf)-dependent efflux of EB was demonstrated in BC(R) isolates, and in originally sensitive strains adapted to grow in BC. This efflux was not found in BC(S) strains. The result indicate that BC can induce a broad resistance mechanism based on a pmf-driven efflux pump. There was no indication that this type of resistance was related to resistance to antibiotics.

Enterotoxigenic Staphylococcus aureus in bulk milk in Norway

Aims:  To investigate the presence of enterotoxigenic Staphylococcus aureus in bulk milk and in a selection of raw milk products.

Listeria monocytogenes in the smoked salmon industry

Smoked salmon is sporadically contaminated with Listerial monocytogenes. Contamination levels are normally low and consumers are probably seldom exposed to risk concentrations. No clones of L. monocytogenes seem to be specific to smoked salmon, some clones found in smoked salmon having been isolated from several sources, including patients. Cold-smoking has been shown to eliminate L. monocytogenes in challenge tests at temperatures from 17.1 to 21.1 degrees C, while from 22.2 to 30 degrees C the bacteria survived. Under natural cold-smoking conditions (19 to 22 degrees C) the frequency and level of L. monocytogenes seems to decrease. Hot-smoking seems to eliminate the bacteria when smoke is applied during the whole heating process. The prevention of recontamination of both cold-smoked and hot-smoked salmon is therefore of great importance. L. monocytogenes multiply considerably in smoked salmon during storage. Growth is faster in challenge tests than in naturally-contaminated smoked salmon. The declared shelf-life under refrigeration should be shorter than that customarily stipulated by many producers. While the sources of L. monocytogenes in smoked salmon processing plants have still to be determined, raw salmon does not seem to be an important source. The main issue for producers is to prevent colonization of the processing environment and spread of the bacteria to products. This should be achieved by the systemic implementation of hygienic measures, including the HACCP approach.

Potential Uptake of Escherichia coli O157:H7 from Organic Manure into Crisphead Lettuce

ABSTRACT To investigate the potential transfer of Escherichia coli O157:H7 from contaminated manure to fresh produce, lettuce seedlings were transplanted into soil fertilized with bovine manure which had been inoculated with approximately 104 CFU g−1E. coli O157:H7. The lettuce was grown for approximately 50 days in beds in climate-controlled rooms in a greenhouse. As the bacterium was not detected in the edible parts of the lettuce, the outer leaves of the lettuce, or the lettuce roots at harvest it was concluded that transmission of E. coli O157:H7 from contaminated soil to lettuce did not occur. The pathogen persisted in the soil for at least 8 weeks after fertilizing but was not detected after 12 weeks. Indigenous E. coli was detected only sporadically on the lettuce at harvest, and enterococci were not detected at all. The numbers of enterococci declined more rapidly than those of E. coli in the soil. Pseudomonas fluorescens, which inhibited growth of E. coli O157:H7 in vitro, was isolated from the rhizosphere.

Genetic Variation among Staphylococcus aureus Strains from Norwegian Bulk Milk

ABSTRACT Strains of Staphylococcus aureus obtained from bovine (n = 117) and caprine (n = 114) bulk milk were characterized and compared with S. aureus strains from raw-milk products (n = 27), bovine mastitis specimens (n = 9), and human blood cultures (n = 39). All isolates were typed by pulsed-field gel electrophoresis (PFGE). In addition, subsets of isolates were characterized using multilocus sequence typing (MLST), multiplex PCR (m-PCR) for genes encoding nine of the staphylococcal enterotoxins (SE), and the cloverleaf method for penicillin resistance. A variety of genotypes were observed, and greater genetic diversity was found among bovine than caprine bulk milk isolates. Certain genotypes, with a wide geographic distribution, were common to bovine and caprine bulk milk and may represent ruminant-specialized S. aureus. Isolates with genotypes indistinguishable from those of strains from ruminant mastitis were frequently found in bulk milk, and strains with genotypes indistinguishable from those from bulk milk were observed in raw-milk products. This indicates that S. aureus from infected udders may contaminate bulk milk and, subsequently, raw-milk products. Human blood culture isolates were diverse and differed from isolates from other sources. Genotyping by PFGE, MLST, and m-PCR for SE genes largely corresponded. In general, isolates with indistinguishable PFGE banding patterns had the same SE gene profile and isolates with identical SE gene profiles were placed together in PFGE clusters. Phylogenetic analyses agreed with the division of MLST sequence types into clonal complexes, and isolates within the same clonal complex had the same SE gene profile. Furthermore, isolates within PFGE clusters generally belonged to the same clonal complex.

Listeria monocytogenes in foods in Norway

Three-hundred-and-eighty-two samples of different retail food items in Norway (imported soft cheese, raw chicken, minced meat, fermented sausages, vacuum-packed processed meat products, smoked salmon, peeled shrimps, raw minced fish) and 78 carcass samples (sheep, pig, cattle), were screened for Listeria monocytogenes. Of the 460 samples investigated, 78 were found to contain L. monocytogenes. Five of these contained greater than 10(3) cfu/g, four greater than 10(2) cfu/g, while the remainder were shown to contain L. monocytogenes only after enrichment. L. monocytogenes was isolated most frequently from raw chicken, sporadically from soft cheese, shrimps, processed meat products and smoked salmon, and not at all from carcasses and fermented sausages.

Bacteriological quality of organically grown leaf lettuce in Norway

Aim:  To investigate bacteriological quality in organically grown leaf lettuce, including the presence of selected pathogenic bacteria, and to obtain information about organic lettuce production, including fertilizing regimes.

Influence of bovine manure as fertilizer on the bacteriological quality of organic Iceberg lettuce

Aim:  To investigate the bacteriological quality, and the occurrence of selected pathogenic bacteria from organically grown Iceberg lettuce fertilized with bovine manure in the form of compost, firm manure and slurry in a 2‐year field trial.

Inhibition of Listeria monocytogenes in chicken cold cuts by addition of sakacin P and sakacin P-producing Lactobacillus sakei

Aims: To evaluate the potential of sakacin P and sakacin P‐producing Lactobacillus sakei for the inhibition of growth of Listeria monocytogenes in chicken cold cuts, by answering the following questions. (i) Is sakacin P actually produced in food? (ii) Is sakacin P produced in situ responsible for the inhibiting effect? (iii) How stable is sakacin P in food?