Taina Niskanen

A Widespread Outbreak of Yersinia pseudotuberculosis O:3 Infection from Iceberg Lettuce

BACKGROUND The vehicles and sources of Yersinia pseudotuberculosis infection are unknown. In Finland, clinical microbiology laboratories routinely report Y. pseudotuberculosis isolations and submit isolates for serotype analysis. In October 1998, the number of serotype O:3 infections increased markedly. METHODS Case patients with culture-confirmed Y. pseudotuberculosis O:3 infection were identified by use of laboratory-based surveillance. We conducted a population-based case-control study. Healthy community control subjects were matched by age, sex, and postal code. Isolates were subtyped by pulsed-field gel electrophoresis (PFGE). RESULTS Nationwide, 47 case patients were identified (age range, 2-77 years; median, 19 years). One patient with bacteremia died; 5 underwent appendectomies. We enrolled 38 case patients and 76 control subjects in the case-control study. Seventy-one percent of case patients and 42% of control subjects reported having eaten iceberg lettuce (matched odds ratio, 3.8; 95% confidence interval, 1.3-9.4); a dose-response relationship was found for increasing frequency of consumption. Of the 27 isolates obtained from case patients and tested in the analysis, all had indistinguishable PFGE patterns. Four lunch cafeterias that had served iceberg lettuce were associated with clusters of case patients. The lettuce was traced back to originating farms. CONCLUSIONS Iceberg lettuce was implicated as the vehicle of a widespread foodborne Y. pseudotuberculosis outbreak. Ongoing laboratory-based surveillance and serotype analysis were essential in the rapid detection of infection. Cases of yersiniosis, which appear to be sporadic, may be part of unrecognized outbreaks caused by contaminated fresh produce.

Yersinia pseudotuberculosis with limited genetic diversity is a common finding in tonsils of fattening pigs

A total of 425 pig tonsils, including 210 tonsils from fattening pigs and 215 from sows, from seven different abattoirs in Finland were studied for the occurrence of Yersinia pseudotuberculosis from 1999 to 2000. The mean prevalence of Y. pseudotuberculosis in fattening pig tonsils was 4%, varying from 0 to 10% between slaughterhouses. Y. pseudotuberculosis was not recovered from sow tonsils. All 30 Y. pseudotuberculosis isolates from eight pig tonsils were recovered after cold enrichment. Seventeen isolates from seven tonsils were found after cold enrichment for 14 days, followed by alkali treatment. Y. pseudotuberculosis was not isolated after direct plating, overnight enrichment, or selective enrichment. All 30 isolates belonged to bioserotype 2/0:3 and carried the virF gene in the virulence plasmid. The isolates exhibited calcium dependence and Congo red absorption. The pyrazinamidase test gave variable results. All isolates were characterized with pulsed-field gel electrophoresis (PFGE). Using SpeI, NotI, and XbaI enzymes, seven, five, and two different PFGE patterns were obtained, respectively. A total of 11 genotypes, gI to gXI, identified by a combination of the various SpeI, NotI, and XbaI profiles, were detected. Three pigs were found to carry more than one genotype. Overall, variations between PFGE patterns were small, indicating genetic homogeneity among pig strains of bioserotype 2/0:3.

virF-Positive Yersinia pseudotuberculosis and Yersinia enterocolitica Found in Migratory Birds in Sweden

ABSTRACT During spring and autumn migrations, 468 fecal samples from 57 different species of migratory birds were collected in Sweden. In total, Yersinia spp. were isolated from 12.8% of collected samples. The most commonly found species was Yersinia enterocolitica, which was isolated from 5.6% of all collected samples, followed by Y. intermedia (3.8%), Y. frederiksenii (3.0%), Y. kristensenii (0.9%), Y. pseudotuberculosis (0.6%), and Y. rohdei (0.4%). The pathogenic, virF-positive Y. pseudotuberculosis strains were recovered from three thrushes. These strains belonged to the same bioserotype, 1/O:2, but had two different profiles as determined by pulsed-field gel electrophoresis with NotI and SpeI enzymes. In addition, 10 Y. enterocolitica strains, all from barnacle geese, belonged to bioserotype 3/O:3, which is associated with human disease. Two of the strains were pathogenic, carrying the virF gene on their plasmids. All pathogenic Y. pseudotuberculosis and Y. enterocolitica strains were recovered during the spring, and as the birds were caught during active migration they likely became infected at an earlier stage of the migration, thus potentially transporting these bacterial pathogens over long geographical distances.

Low prevalence of yadA-positive Yersinia enterocolitica in sows.

Yersinia enterocolitica 4/O:3 is commonly isolated from fattening pigs in Finland, but its prevalence in sows is relatively unknown. The current study determined the prevalence of yadA-positive Y. enterocolitica in sows and fattening pigs with polymerase chain reaction (PCR) and culture methods. The strains were characterized with pulsed-field gel electrophoresis (PFGE) using NotI DNA-restriction enzyme. Pathogenic Y. enterocolitica was detected in only 14% of sow tonsils compared with 56% of tonsils of fattening pigs. The prevalence varied between seven slaughterhouses from 0% to 30% in sows and from 30% to 87% in fattening pigs. A total of 37 NotI profiles were obtained from 148 Y. enterocolitica 4/O:3 strains isolated from 134 tonsil samples: eight profiles were obtained from 26 sow strains and 34 from 122 fattening pig strains. Two types predominated in both sows and fattening pigs. The prevalence of yadA-positive Y. enterocolitica in fattening pigs increased from 1995 versus 1999; the mean prevalence in five slaughterhouses for the 2 years was 33% and 64%, respectively. Seven NotI profiles, including the two common types, were found both years. In conclusion, pathogenic Y. enterocolitica was detected at a significantly lower rate in sows than in fattening pigs. Moreover, the prevalence of this pathogen in fattening pigs was significantly higher in 1999 than in 1995. Some Y. enterocolitica 4/O:3 strains persisting among slaughter swine were demonstrated to be very stable genetically. This study shows that fattening pigs are an important reservoir of different genotypes of Y. enterocolitica 4/O:3 strains.

Yersinia pekkanenii sp. nov.

The taxonomic position of three strains from water, soil and lettuce samples was studied by using a polyphasic taxonomic approach. The strains were reported to lack the virulence-encoding genes inv and virF in a previous study. Controversially, API 20 E and some other phenotypic tests suggested that the strains belong to Yersinia pseudotuberculosis, which prompted this polyphasic taxonomic study. In both the phylogenetic analyses of four housekeeping genes (glnA, gyrB, recA and HSP60) and numerical analyses of HindIII and EcoRI ribopatterns, the strains formed a separate group within the genus Yersinia. Analysis of the 16S rRNA gene sequences showed that the strains were related to Yersinia aldovae and Yersinia mollaretii, but DNA-DNA hybridization analysis differentiated them from these species. Based on the results of the phylogenetic and DNA-DNA hybridization analyses, a novel species, Yersinia pekkanenii sp. nov., is proposed. The type strain is ÅYV7.1KOH2(T) ( = DSM 22769(T)  = LMG 25369(T)).

In search of human-associated bacterial pathogens in antarctic wildlife: Report from six penguin colonies regularly visited by tourists

Abstract We investigated the potential role of Antarctic tourism in the introduction of human-associated pathogens into Antarctic wildlife. We collected and analyzed 233 fecal samples from eight bird species. The samples were collected at six localities on the Antarctic Peninsula, which often is visited by tourists. Every sample was investigated for pathogens of potential human origin: Campylobacter jejuni, Salmonella spp., and Yersina spp. None of these bacteria was found. Our data suggest that the tourism industry so far has achieved its goal of not introducing pathogens into the Antarctic region. There is, however, an urgent need to further investigate the situation in areas closer to permanent Antarctic settlements.

Iceberg lettuce as suggested source of a nationwide outbreak caused by two Salmonella serotypes, Newport and Reading, in Finland in 2008

A nationwide outbreak of Salmonella enterica serotypes Newport and Reading occurred between 17 October and 28 November 2008 in Finland. A total of 77 culture-confirmed Salmonella Newport and 30 Salmonella Reading cases, including one case with a double infection, were reported. All strains isolated from the patients were subtyped using serotyping, microbial resistance profiling, and pulsed-field gel electrophoresis (PFGE). Here, the PFGE patterns of the studied Salmonella Newport strains were identical, whereas four different PFGE profiles were found among the Salmonella Reading strains. Two elderly patients died within 2 weeks of the onset of symptoms. Three geographical clusters of cases with an epidemiological link were identified. The traceback investigation suggested that the factor connecting the cases was ready-chopped iceberg lettuce available for mass catering use. However, none of the tested food, environmental samples, or the samples taken from the staff of the processing plant contained Salmonella bacteria. Tracing back to outbreak sources with a short shelf life can be complex.

Different Yersinia enterocolitica 4:O3 genotypes found in pig tonsils in Southern Germany and Finland.

The distribution of different genotypes of Y. enterocolitica 4:O3 strains recovered from pig tonsils in Southern Germany and Finland in 1999-2000 was investigated. A total of 96 and 207 Y. enterococolitica 4:O3 isolates recovered from 47 and 66 tonsils of finishing pigs in Germany and Finland, respectively, were characterised with PFGE using NotI enzyme. In all, 39 different NotI profiles were obtained, only one of which, NB1, was found in both Germany and Finland. All strains were further characterised with ApaI and XhoI enzymes. When the 54 German and 74 Finnish strains were characterised with all three enzymes, 51 genotypes were obtained. The 23 genotypes found in German strains differed from the 28 found in Finnish strains. These results indicate that Y. enterocolitica 4:O3 genotypes have a differential geographical distribution and thus can be used in epidemiological studies.

Prevalence and Genetic Diversity of Listeria monocytogenes in the Tonsils of Pigs

This study was set up to establish the prevalence of Listeria monocytogenes in the tonsils of sows and fattening pigs from five Finnish slaughterhouses and to evaluate the genetic similarity of L. monocytogenes strains isolated from the tonsils. A total of 271 pig tonsils (132 tonsils from fattening pigs and 139 from sows) from five different slaughterhouses in various parts of Finland were studied from June 1999 to March 2000. Overall, 14 and 4% of pig tonsils harbored L. monocytogenes and Listeria innocua, respectively. The prevalence of L. monocytogenes in tonsils of fattening pigs (22%) was significantly higher than in sows (6%). The isolates (n = 38) recovered from tonsils showed a wide genetic diversity by means of 24 different pulsed-field gel electrophoresis (PFGE) types presented by the strains. Moreover, in numerical analyses of restriction patterns, no association was found between the clustering of strains and the slaughterhouses, and strains showing a similar PFGE type were recovered from pigs of different slaughterhouses. The high prevalence of L. monocytogenes showing various PFGE types in the tonsils of pigs could indicate a potential source of contamination of pluck sets, carcasses, and the slaughterhouse environment and of subsequent processing steps.

Characterisation of non-pathogenic Yersinia pseudotuberculosis-like strains isolated from food and environmental samples

Non-pathogenic Yersinia pseudotuberculosis-like strains were recovered from Finnish food and environmental samples. These strains could not be differentiated from Y. pseudotuberculosis strains using API 20E or other phenotypical tests. However, all of the strains were inv-, and virF-negative with polymerase chain reaction (PCR), while all Y. pseudotuberculosis strains used as controls were inv-positive and fresh Y. pseudotuberculosis strains were also virF-positive, indicating that the Y. pseudotuberculosis-like strains were non-pathogenic. Using pulsed-field gel electrophoresis (PFGE) with NotI enzyme and ribotyping with EcoRI and HindIII enzymes, the Y. pseudotuberculosis-like strains, which grouped genetically together, could be differentiated from true Y. pseudotuberculosis strains and from strains belonging to other sucrose-negative Yersinia species. In addition, the O-antigen gene cluster of one Y. pseudotuberculosis-like strain was characterized, and it differed from those of known Y. pseudotuberculosis serotypes. This study demonstrates that identification of Y. pseudotuberculosis from food and environmental sources using solely biochemical reactions can be incorrect, and when a strain cannot be serotyped to known Y. pseudotuberculosis serotypes, the pathogenic potential of isolates should be determined.