A synthetic oligonucleotide probe and a cloned polynucleotide probe based on the yopA gene for detection and enumeration of virulent Yersinia enterocolitica

PhoP and OxyR transcriptional regulators contribute to Yersinia pestis virulence and survival within Galleria mellonella

The virulence of Yersinia pestis KIM6+ was compared with multiple isolates of Yersinia pseudotuberculosis and Yersinia enterocolitica toward larvae of the greater wax moth Galleria mellonella. Although Y. pestis and Y. pseudotuberculosis were able to cause lethal infection in G. mellonella, these species appeared less virulent than the majority of Y. enterocolitica strains tested. Y. pestis survived primarily within hemocytes of G. mellonella, and induced a strong antibacterial peptide response that lasted for at least 3 days in surviving larvae. Immunization with dead bacteria to induce an antibacterial response led to increased survival of the larvae following infection. Mutant strains lacking the either phoP or oxyR, which were less resistant to antibacterial peptides and hydrogen peroxide respectively, were attenuated and restoration of the wild-type genes on plasmids restored virulence. Our results indicate that the Y. pseudotuberculosis-Y. pestis lineage is not as virulent toward G. mellonella as are the majority of Y. enterocolitica isolates. Further, we have shown that G. mellonella is a useful infection model for analyzing Y. pestis host-pathogen interactions, and antibacterial peptide resistance mediated by phoP and reactive oxygen defense mediated by oxyR are important for Y. pestis infection of this insect.

Low occurrence of pathogenic Yersinia enterocolitica in clinical, food, and environmental samples: a methodological problem

While Yersinia enterocolitica is an important pathogen, which can cause yersiniosis in humans and animals, its epidemiology remains obscure. The pig is the major reservoir of pathogenic Y. enterocolitica of bioserotype 4/O:3, the most common type found in humans. Y. enterocolitica is thought to be a significant food-borne pathogen, although pathogenic isolates have seldom been recovered from foods. The low isolation rate of this pathogenic bacterium in natural samples, including clinical, food, and environmental samples, may be due to the limited sensitivity of culture methods. During the last decade, numerous DNA-based methods, such as PCR and colony hybridization assays, have been designed to detect pathogenic Y. enterocolitica in natural samples more rapidly and with better sensitivity than can be achieved by culture methods. In addition, the occurrence of pathogenic Y. enterocolitica in natural samples is clearly higher with PCR than with culture methods. The methods available for detection of pathogenic Y. enterocolitica in natural samples are reviewed in this article.

Occurrence of Yersinia enterocolitica and Campylobacter spp. in slaughter pigs and consequences for meat inspection, slaughtering, and dressing procedures

The purpose of the present investigation was to assess the occurrence of Yersinia enterocolitica and Campylobacter spp. in the lymphoid tissues and intestinal tract in pigs and the risk for contamination during the compulsory meat inspection procedures and the procedures during slaughtering and dressing. Another objective of the investigation was to compare traditional isolation methods, the use of a polymerase chain reaction (PCR) method (BUGS'n BEADS bacterial DNA isolation kit) and an ELISA method (VIDAS CAM) as tools in risk management in the slaughterhouse. The results indicate that the compulsory procedure for the incision of the submaxillary lymph nodes represents a cross-contamination risk for virulent Yersinia. In the screening of 97 animals in 1999, 5.2% of the samples were positive, and by the sampling of 24 samples in 2000-2001, 12.5% of the samples were positive. In the last case, Y. enterocolitica O:3 was found in the kidney region in one of the subsequent carcasses that was only touched by the meat inspection personnel before sampling. In addition, incision of the mesenteric lymph nodes might represent a cross-contamination risk since 8.3% of the samples were positive. The association between antibody titres and the occurrence of virulent yersiniae in the tonsils (21-18) was striking, with virulent yersiniae found in the tonsils in most pigs with high titres. The contents of the stomach, ileum, caecum, and colon also represent contamination risks for Y. enterocolitica O:3 if the slaughterhouse personnel cuts into the viscera with their knives by accident; the frequency of virulent Yersinia varied from 4.2% to 16.7% within these sections. Campylobacter was detected in the gastrointestinal tract of all pigs, and the high contamination of tonsils (66.7%) and intestinal tract (100%) might represent an occupational health hazard. There was no statistical difference between the traditional method for isolation of Y. enterocolitica [International Organization for Standardization, 1994. Microbiology-General Guidance for the Detection of Presumptive Pathogenic Yersinia enterocolitica (ISO 10273). International Organization for Standardization, Genève, Switzerland (16 pp.)] and the BUGS'n BEADS detection method for virulent Y. enterocolitica. Likewise, there was no statistical difference between the traditional method for isolation of Campylobacter spp. [Nordic Committee on Food Analysis, 1990. Campylobacter jejuni/coli. Detection in Food. Method No. 119, 2nd ed. Nordic Committee on Food Analysis, Esbo (7 pp.)] and the BUGS'n BEADS detection method or the VIDAS CAM method for detection of Campylobacter spp.

Detection of a novel repeated sequence useful for epidemiological typing of pathogenic Yersinia enterocolitica

Strains (n = 203) of Yersinia species were used in genotyping and PCR experiments in order to evaluate the genotyping potential of the YeO:3RS probe. This probe comprises a 12.5 kb genomic fragment of the Y. enterocolitica O:3 lipopolysaccharide O-antigen gene cluster cloned into plasmid pBR322. The genotyping potential of YeO:3RS was shown to reside in the region upstream of the O-antigen gene cluster, i.e., in the first 1.65 kb of the cloned genomic fragment that contains a repeated sequence (RS) present in multiple copies in the genome. In genotyping, the YeO:3RS probe was hybridised to DNA of Yersinia enterocolitica isolates (n = 112) from humans, animals and food, along with strains of other Yersinia species (n = 5) and Salmonella enterica strains (n = 3). The YeO:3RS probe efficiently detected and subtyped all European pathogenic Yersinia enterocolitica isolates of the serobiotypes O:3/4, O:9/2 and O:5,27/2 studied (n = 87), whereas it hybridised only weakly or not at all with the other strains. Within Yersinia enterocolitica serobiotype O:3/4 strains, YeO:3RS genotyping was as discriminatory as genotyping by pulsed-field gel electrophoresis (PFGE) of XbaI-NotI digested genomic DNA. When these two methods were combined, YeO:3RS genotyping divided both of the two predominant PFGE types into six subtypes, thus increasing the discrimination. In PCR screening of additional 86 Yersinia strains, the 1.65 kb region was detected in European pathogenic serotypes O:1 and O:2 in addition to serotypes O:3, O:5,27 and O:9, indicating that it can be exploited in detecting and typing of European pathogenic serotypes in general.

Evaluation of a 5'-nuclease (TaqMan) assay for the detection of virulent strains of Yersinia enterocolitica in raw meat and tofu samples

Culture methods for detecting virulent Yersinia enterocolitica require selective enrichment and a series of confirmatory tests that are time-consuming, costly, and laborious. The objective of this study was to evaluate a fluorogenic 5'-nuclease assay for detecting the enterotoxin yst gene of virulent Y. enterocolitica in pure cultures, inoculated ground pork samples, and naturally contaminated food samples. These results were then compared with "gold standard" methods recommended by the U.S. Food and Drug Administration in the Bacteriological Analytical Manual for detecting pathogenic Y. enterocolitica. The 5'-nuclease assay was able to identify the organism in 100% of the repetitions when 10(2) CFU/ml or more organisms were present in pure cultures and 10(3) CFU/g or more organisms were present in ground pork. Similar recovery efficiency on cefsulodin-irgasan-novobiocin (CIN) agar plates was only evident when 10(5) CFU/ml or more organisms were present in pure culture and 10(6) CFU/g or more organisms were present in inoculated ground pork. The 5'-nuclease assay indicated a contamination rate of 35.5% (94/265) in various meats and tofu, whereas the CIN plating method indicated a contamination rate of 28.3% (75/265). This resulted in 100% sensitivity and 64.5% specificity for the 5'-nuclease assay when compared with the standard culture recovery method. Only 75% (60/80) of the Yersinia spp. isolated on CIN was identified as containing a virulence plasmid by autoagglutination and crystal violet binding tests. These results indicate that the true rate of contamination of virulent Y. enterocolitica in pork and other processed meats and foods is being underestimated using current detection methods. This study demonstrates the potential of the 5'-nuclease assay for rapidly and specifically detecting virulent Y. enterocolitica in processed foods with the added advantage of being an automated detection system with high-throughput capability.

Biotechnical use of polymerase chain reaction for microbiological analysis of biological samples

Since its introduction in the mid-80s, polymerase chain reaction (PCR) technology has been recognised as a rapid, sensitive and specific molecular diagnostic tool for the analysis of micro-organisms in clinical, environmental and food samples. Although this technique can be extremely effective with pure solutions of nucleic acids, it's sensitivity may be reduced dramatically when applied directly to biological samples. This review describes PCR technology as a microbial detection method, PCR inhibitors in biological samples and various sample preparation techniques that can be used to facilitate PCR detection, by either separating the micro-organisms from PCR inhibitors and/or by concentrating the micro-organisms to detectable concentrations. Parts of this review are updated and based on a doctoral thesis by Lantz [1] and on a review discussing methods to overcome PCR inhibition in foods [2].

Rapid 5' nuclease (TaqMan) assay for detection of virulent strains of Yersinia enterocolitica

We have developed a rapid procedure for the detection of virulent Yersinia enterocolitica in ground pork by combining a previously described PCR with fluorescent dye technologies. The detection method, known as the fluorogenic 5' nuclease assay (TaqMan), produces results by measuring the fluorescence produced during PCR amplification, requiring no post-PCR processing. The specificity of the chromosomal yst gene-based assay was tested with 28 bacterial isolates that included 7 pathogenic and 7 nonpathogenic serotypes of Y. enterocolitica, other species of Yersinia (Y. aldovae, Y. pseudotuberculosis, Y. mollaretti, Y. intermedia, Y. bercovieri, Y. ruckeri, Y. frederiksenii, and Y. kristensenii), and other enteric bacteria (Escherichia, Salmonella, Citrobacter, and Flavobacterium). The assay was 100% specific in identifying the pathogenic strains of Y. enterocolitica. The sensitivity of the assay was found to be >/=10(2) CFU/ml in pure cultures and >/=10(3) CFU/g in spiked ground pork samples. Results of the assay with food enrichments prespiked with Y. enterocolitica serotypes O:3 and O:9 were comparable to standard culture results. Of the 100 field samples (ground pork) tested, 35 were positive for virulent Y. enterocolitica with both 5' nuclease assay and conventional virulence tests. After overnight enrichment the entire assay, including DNA extraction, amplification, and detection, could be completed within 5 h.

Detection of pathogenic Yersinia enterocolitica in enrichment media and pork by a multiplex PCR: a study of sample preparation and PCR-inhibitory components

A multiplex PCR assay including sample preparation was developed to detect viable pathogenic strains of Yersinia enterocolitica in PCR-inhibitory samples, such as pork and enrichment media. The method developed was used to simultaneously detect the plasmid-borne virulence gene yadA and a Yersinia-specific region of the 16S rRNA gene. According to an auto-agglutination test for virulence-plasmid-bearing strains of Y. enterocolitica, all potential pathogenic strains tested were detected by the assay. A DNA extraction procedure, an aqueous two-phase system composed of polyethylene glycol 4000 and dextran 40 and a buoyant density centrifugation method, based on Percoll, were compared with regard to their efficiency in separating Yersinia enterocolitica from PCR inhibitors originating from enrichment media and pork. Using the density gradient centrifugation method resulted in a detection level of 4.0 x 10(2) CFU Y. enterocolitica per ml enrichment media. To ensure detection of viable bacteria a short enrichment step was included in the sample preparation together with the density gradient centrifugation. When this sample treatment method was evaluated with a selective enrichment medium together with a background flora inoculated with approximately 1.0 x 10(1) CFU per ml of Y. enterocolitica and incubated at 25 degrees C, a positive PCR result was obtained after 6 to 8 h. Our results indicate that selective enrichment followed by buoyant density gradient centrifugation provides a convenient and user-friendly sample preparation method prior to PCR.

Time course of the serological response to Yersinia enterocolitica O:3 in experimentally infected pigs

A total of 25 pigs inoculated with Yersinia enterocolitica serovar O:3 and 25 un-inoculated controls were followed weekly by sampling blood and faeces for 70 days post infection (p.i.). All inoculated pigs were faeces culture positive from day 5 to 21 p.i., whereafter shedding of bacteria declined to < 10% of the pigs at day 49 p.i. and to 0% at day 68 p.i. All control pigs remained Y. enterocolitica O:3 culture negative. When examined in an indirect ELISA using purified LPS from Y. enterocolitica 0:3, sera from all inoculated pigs showed significantly higher optical densities (OD) as compared to the control group. All inoculated pigs had seroconverted at day 19 p.i. and remained seropositive until slaughter at day 70 p.i. The maximum mean anti-LPS response was observed at day 33 p.i. with a positive/negative ratio of 780. No cross-reactions were observed with sera from 21 pigs, infected with Salmonella typhimurium. At necropsy at day 70 p.i., Y. enterocolitica O:3 was isolated from the tonsils of 20 inoculated pigs, whereas the rest of the gastrointestinal tract and associated lymph nodes were culture negative. The remaining inoculated pigs and all control pigs were culture negative at necropsy at day 70 p.i. The ELISA seems to be a promising alternative to bacteriological culture for detection of Y. enterocolitica O:3 infection in pig herds.

Probes and polymerase chain reaction for detection of food-borne bacterial pathogens

DNA-hybridization and the polymerase chain reaction (PCR) are techniques commonly used to detect pathogenic bacteria. In this paper, the use of these techniques for detection of Salmonella, E. coli, V. cholerae, non-O1 Vibrio, Yersinia enterocolitica, Campylobacter, Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and C. botulinum is reviewed with emphasis on application in food microbiology. In food control, DNA-techniques have most often been used in a 'culture confirmation' fashion, i.e. bacteria are enriched and sometimes even purified by traditional culture procedures and thereafter identified by the use of DNA-based methods. The most desirable approach is, however, to detect organisms directly in the food, but major problems remain to be solved before this can be routinely performed.

Characterization of Yersinia enterocolitica isolated from the oral cavity of swine in Slovakia

Yersinia enterocolitica was detected in 160 of 2760 examined samples that were prepared from dental cavity of 920 pigs. From the isolates 49 strains were characterized with respect to their genetic and phenotypic markers of virulence. All strains were belonging to serogroup 0:3, biovar 4. Forty-one isolates (84%) harboured the virulence plasmid detected by DNA colony hybridization. Of the other assays of virulence--autoagglutination, binding of crystal violet, pyrazinamidase activity, calcium dependence and Congo red binding--the latter two exhibited the best correlation with the detected presence of plasmid DNA.

Molecular characterization of Yersinia enterocolitica by pulsed-field gel electrophoresis and hybridization of DNA fragments to ail and pYV probes

Sixty strains of Yersinia enterocolitica from five serogroups (O:3; O:9; O:8; O:5; and O:5,27) and eight non-Y. enterocolitica strains, recovered from diverse sources (humans, animals, food, and the environment) in Europe, Argentina, and the United States, were examined by the pulsed-field gel electrophoresis (PFGE) technique of contour clamped homogeneous electric field electrophoresis (CHEF) by using NotI and XbaI as restriction enzymes. NotI and XbaI generated 36 and 33 restriction endonuclease digestion profiles (REDP), respectively. By combining the results of both enzymes, 42 unique genomic groups were differentiated. DNA fragments were transferred to nylon membranes and hybridized with digoxigenin-labelled oligonucleotide probes to the ail gene and virulence plasmid to determine hybridization patterns and the potential virulence of the strains. The strains were tested for the presence of the plasmid by PFGE-CHEF and phenotypic characteristics encoded for by the virulence plasmid. Thirty of the 60 Y. enterocolitica strains tested harbored the virulence plasmid. The specificity of the ail and pYV probes was 100% when tested with 68 Yersinia strains and 19 different non-Yersinia strains. Sixteen selected Y. enterocolitica strains were tested for their virulence by lethality in iron- and desferrioxamine-sensitized mice. No correlation between REDP and the virulence of the strains was observed. The observed REDP and the hybridization patterns were very homogeneous within a serogroup and independent of the source of isolation. In addition, PFGE-CHEF was shown to be valuable in identifying and confirming serogroups. Principal component analysis of Dice similarity indices from REDP was an excellent tool for determining genetic relatedness among strains.

Isolation of Lactococcus lactis subsp. cremoris from nature by colony hybridization with rRNA probes

Lactococcus lactis subsp. cremoris is widely used in the manufacture of fermented milk products. Despite numerous attempts, efforts to isolate new strains by traditional plating and identification methods have not been successful. Previously, we described oligonucleotide probes for 16S rRNAs which could be used to discriminate L. lactis subsp. cremoris from related strains. These probes were used in colony hybridization experiments to screen large numbers of colonies obtained from enrichment cultures. A total of 170 strains of L. lactis were isolated from six milk samples, two colostrum samples, and one corn sample by using oligonucleotide probe 212RLa specific for the species L. lactis. Fifty-nine of these isolates also hybridized to L. lactis subsp. cremoris-specific probe 68RCa, and 26 of the strains which hybridized to the L. lactis subsp. cremoris-specific probe had the L. lactis subsp. cremoris phenotype.

Recognition of pathogenic Yersinia enterocolitica by crystal violet binding and polymerase chain reaction

Cefsulodin-Irgasan-Novobiocin (CIN) agar is used for the selective isolation and enumeration of Yersinia enterocolitica from clinical specimens and food. The medium contains crystal violet and about 1 mmol l-1 calcium and can be used for the phenotypic characterization of strains that carry a virulence plasmid. At 32 degrees C, irrespective of pathogenicity, colonies are translucent with a pale pink centre surrounded by a transparent border ('bullseye'), while at 37 degrees C pathogenic strains grow as calcium-dependent microcolonies which, because of crystal violet binding, are intensely coloured. These results were confirmed by the polymerase chain reaction with primers directed at the virF gene, which is present only in pathogenic strains of Y. enterocolitica. Pathogenic strains of Y. enterocolitica can be recognized by growth at 37 degrees C on Yersinia selective agar.

Digoxigenin-labelled inv- and ail-probes for the detection and identification of pathogenic Yersinia enterocolitica in clinical specimens and naturally contaminated pig samples

A non-radioactive colony hybridization method was developed for the rapid detection of Yersinia enterocolitica in primary isolates and for differentiation between pathogenic and non-pathogenic strains. The method is based on, respectively, the presence of the inv-locus in all Yersinia spp. and the presence of the ail-gene in pathogenic Y. enterocolitica only. Hybridization results with ail-probes of 132 strains of Y. enterocolitica were in good agreement with pathogenicity phenotypes as indicated by a tissue culture invasion (TCI) assay and by serotyping. All TCI+ strains and only two TCI- strains were positive by hybridization with ail. Hybridization results with inv- or ail-probes of 150 primary isolates of human, animal or slaughterhouse origin were compared with those of conventional methods to detect and identify Y. enterocolitica. All samples that were positive for Yersinia spp. by cultivation (four of 66) or were positive for pathogenic Y. enterocolitica by cultivation and serotyping (six of 84) were also positive by hybridization with, respectively, the inv- or ail-probe. In three slaughterhouse swab samples, in which Yersinia spp. were not detected by cultivation (2%), strong positive hybridization signals were obtained with the inv- and/or ail-probe. Four other swab samples which were negative by cultivation produced weak positive signals by hybridization with inv- and/or ail-probes. These results indicate that the method can be used for (1) the identification of pathogenic Y. enterocolitica isolates and (2) the detection of Yersinia spp. in primary isolates of naturally contaminated samples.

Rapid identification of Actinobacillus actinomycetemcomitans based on analysis of 23S ribosomal RNA

Actinobacillus actinomycetemcomitans is a key microorganism in the pathogenesis of several different forms of periodontal diseases. Identification of this bacterium from clinical specimens may often be complicated by the fact that the colony morphology on TSBV selective medium closely resembles that of Haemophilus aphrophilus and a key differentiating characteristic, catalase reaction, may be variable. Recent genetic studies have shown that the 23S ribosomal RNA molecule is split into two smaller forms in A. actinomycetemcomitans, but is intact in H. aphrophilus. Based on this finding, we describe a new, rapid method for identifying A. actinomycetemcomitans in which single colonies isolated from culture on TSBV agar in 5% CO2 in air are lysed, electrophoresed on 1.5% submarine agarose gels and visualized by staining with ethidium bromide. Using this assay, A. actinomycetemcomitans can be easily distinguished from morphologically similar colonies such as H. aphrophilus strains by differences in 23S rRNA within 2 h.

Detection of pathogenic Yersinia enterocolitica by polymerase chain reaction and digoxigenin-labeled polynucleotide probes

Yersinia enterocolitica is widespread in nature, but only a few bioserotypes are involved in human infections. Pigs are considered to be the major reservoirs of pathogenic strains. It is essential to have an accurate and rapid method for the detection of pathogenic yersiniae. To achieve this objective, 19-base synthetic oligonucleotide primers were used in a polymerase chain reaction (PCR) to detect the ail gene (which is conserved only in pathogenic strains) in strains of Y. enterocolitica and related species originating from pigs or pork products. Digoxigenin-labeled probes derived from the ail, inv, and yst genes were also evaluated on these strains. The PCR amplified a 273-bp fragment of the ail gene involved in eukaryotic cell invasion and serum resistance. The PCR detected template DNA only in strains of Y. enterocolitica traditionally classified as human pathogens but not in biotype 1A strains and related species. Other members of the family Enterobacteriaceae were also negative for the target gene. The digoxigenin-labeled ail probe gave identical results to the PCR. By use of this nonisotopic method, inv-homologous DNA was detected only among yersiniae, except for Y. ruckeri. Although all pathogenic serotypes of Y. enterocolitica were positive for the heat-stable enterotoxin yst gene, two strains of biotype 1A, one Y. intermedia strain, and six other species of the Enterobacteriaceae were also positive. Our results support the notion that pigs constitute an important reservoir of pathogenic Y. enterocolitica and that the inv-homologous sequence is Yersinia specific.

Polymerase chain reaction-gene probe detection system specific for pathogenic strains of Yersinia enterocolitica

The polymerase chain reaction technique was used to develop a rapid diagnostic assay for detection of pathogenic Yersinia enterocolitica strains. The assay targeted a stretch of 163 bp of the yst gene and could be applied to both pure cultures and crude DNA extracted from feces. The defined primer pair amplified the targeted sequence from only pathogenic strains and fecal samples seeded with the serotype O:3 strain of Y. enterocolitica, whereas neither nonpathogenic strains nor normal stools yielded any amplified fragments. Of the other Yersinia species and non-Yersinia species tested, only two strains of Y. kristensenii yielded the same amplified product. A 20-mer oligonucleotide probe specifically hybridized within the amplified yst fragment of Y. enterocolitica but did not hybridize with the amplified yst fragment of Y. kristensenii by Southern and dot blot hybridizations. This confirms the reliability of this diagnostic assay in both clinical and epidemiological studies. The availability of the extracted DNA for the polymerase chain reaction was checked by simultaneous amplification of a part of the 16S rDNA and the yst gene. The entire diagnostic assay, including a simplified technique for DNA extraction, the amplification process, and gel electrophoresis, could be completed within 1 working day, which is better than the time required for the time-consuming traditional techniques used in clinical laboratories.

Differentiation between pathogenic and non-pathogenic Yersinia enterocolitica strains by colony hybridization with a PCR-mediated digoxigenin-dUTP-labelled probe

The Polymerase Chain Reaction (PCR) method was used to generate a vector-free digoxigenin-dUTP labelled probe that targets the Yersinia enterocolitica gene encoding the heat stable enterotoxin (yst). The probe was used in DNA-DNA colony hybridization to screen 113 strains of Y. enterocolitica and related species for the presence of the enterotoxin gene. In Y. enterocolitica, the probe clearly discriminated between pathogenic and non-pathogenic strains even those belonging to the same serotype. Of the other Yersinia species, only three strains of Y. kristensenii possessed DNA sequences homologous to the yst gene. The probe was further checked for its specificity in artificially inoculated fecal samples and could easily detect the target sequence of the yst gene. The digoxigenin-labelled probe proved to be a reliable epidemiological tool to discriminate between pathogenic and non-pathogenic strains in pure and mixed culture, thus offering the advantage of using a non-radioactive detection system in clinical laboratories with the possibility of reusing the same hybridization solution several times and obtaining results within a relatively short time.

Development and application of oligonucleotide probes for identification of Lactococcus lactis subsp. cremoris

Lactococcus lactis subsp. cremoris is of considerable interest to the dairy industry, which relies upon the few available strains for the manufacture of cheddar cheese free of fermented and fruity flavors. The subspecies cremoris differs from related subspecies by the lack of a few phenotypic traits. Our purpose was to identify unique rRNA sequences that could be used to discriminate L. lactis subsp. cremoris from related subspecies. The 16S rRNAs from 13 Lactococcus strains were partially sequenced by using reverse transcriptase to identify domains unique to L. lactis subsp. cremoris. All five strains of the subspecies cremoris had a unique base sequence in a hypervariable region located 70 to 100 bases from the 5' terminus. In this region, all L. lactis subsp. lactis biovar diacetylactis strains examined had a sequence identical to that of L. lactis subsp. lactis 7962, which was different from other strains of the subspecies lactis by only one nucleotide at position 90 (Escherichia coli 16S rRNA structural model) (J. Brosius, J. L. Palmer, J. P. Kennedy, and H. F. Noller, Proc. Natl. Acad. Sci. USA 75:4801-4805, 1978). Oligonucleotide probes specific for the genus Lactococcus (212RLa) and for the subspecies cremoris (68RCa) were synthesized and evaluated by hybridization to known rRNAs as well as fixed whole cells. Efficient and specific hybridization to the genus-specific probe was observed for the 13 Lactococcus strains tested. No hybridization was seen with the control species. All five strains of the subspecies cremoris hybridized to the subspecies-specific probe.

Comparative study of a DNA hybridization method and two isolation procedures for detection of Yersinia enterocolitica O:3 in naturally contaminated pork products

We compared a DNA-DNA hybridization assay, using a synthetically produced oligonucleotide probe, and two conventional isolation procedures (methods A and B) with regard to their relative efficiency in detecting Yersinia enterocolitica O:3 in naturally contaminated pork products. Method A was as described by Wauters et al. (Appl. Environ. Microbiol. 54:851-854, 1988). Method B has been recommended by the Nordic Committee on Food Analysis (method no. 117, 1987). The genetic probe was used in a colony hybridization assay to detect virulent yersiniae at each of the isolation steps with composed methods A and B. A total of 50 samples of raw pork products obtained from 13 meat-processing factories in Norway were examined. Y. enterocolitica serogroup O:3, biovar 4, was isolated from altogether 9 (18.0%) of the samples by using the two isolation procedures. In contrast, colony hybridization using the genetic probe indicated that 30 (60.0%) of the samples contained virulent yersiniae. All samples which were positive on cultivation were also positive by hybridization. The results indicate that the occurrence of pathogenic Y. enterocolitica in Norwegian pork products is substantially higher than previously demonstrated and, therefore, reinforce our suggestion that pork products represent an important potential source of human infection in Norway. The results also indicate that the use of conventional isolation procedures may lead to considerable underestimation of pathogenic Y. enterocolitica in pork products.

Yersinia enterocolitica in food hygiene

Yersinia enterocolitica and Yersinia enterocolitica-like bacteria constitute a fairly heterogenous group of bacteria which includes both well-established pathogens and a range of environmental strains which are ubiquitous in terrestrial and freshwater ecosystems. Pathogenic significance in man is mainly associated with a few serogroups (O:3, O:9, O:8, O:5,27). The pathogenic serogroups show different geographical distributions. The development of isolation procedures which clearly differentiate pathogenic from non-pathogenic variants has been difficult. Of special significance in food hygiene is the ability of Y. enterocolitica to grow in refrigerated foods. There is strong indirect evidence that pigs and food products of porcine origin are the major sources for human infection with Y. enterocolitica serogroups O:3 and O:9, the dominant human pathogens in most parts of the world. The reservoir(s) for serogroup O:8, which prevails in the U.S.A., is uncertain. The pig is the only animal consumed by man which regularly harbours pathogenic Y. enterocolitica. Improved isolation methods and DNA colony hybridization using genetic probes has indicated that the prevalence of pathogenic Y. enterocolitica in pork products is substantially higher than previously suggested. Prevention and control measures should focus on information of people involved in food processing and preparation and on the improvement of hygiene during slaughtering of swine. Important critical control points at the stage of slaughter are: (i) circumanal incision and removal of intestines, (ii) excision of the tongue, pharynx, and particularly the tonsils, (iii) post-mortem meat inspection procedures which involve incision of the mandibular lymph nodes, and (iv) deboning of head meat.