Identification of Listeria species by microarray-based assay

We have developed a rapid microarray-based assay for the reliable detection and discrimination of six species of the Listeria genus: L. monocytogenes, L. ivanovii, L. innocua, L. welshimeri, L. seeligeri, and L. grayi. The approach used in this study involves one-tube multiplex PCR amplification of six target bacterial virulence factor genes (iap, hly, inlB, plcA, plcB, and clpE), synthesis of fluorescently labeled single-stranded DNA, and hybridization to the multiple individual oligonucleotide probes specific for each Listeria species and immobilized on a glass surface. Results of the microarray analysis of 53 reference and clinical isolates of Listeria spp. demonstrated that this method allowed unambiguous identification of all six Listeria species based on sequence differences in the iap gene. Another virulence factor gene, hly, was used for detection and genotyping all L. monocytogenes, all L. ivanovii, and 8 of 11 L. seeligeri isolates. Other members of the genus Listeria and three L. seeligeri isolates did not contain the hly gene. There was complete agreement between the results of genotyping based on the hly and iap gene sequences. All L. monocytogenes isolates were found to be positive for the inlB, plcA, plcB, and clpE virulence genes specific only to this species. Our data on Listeria species analysis demonstrated that this microarray technique is a simple, rapid, and robust genotyping method that is also a potentially valuable tool for identification and characterization of bacterial pathogens in general.

Genetic homogeneity among Listeria monocytogenes strains from infected patients and meat products from two geographic locations determined by phenotyping, ribotyping and PCR analysis of virulence genes

Thirty Listeria monocytogenes isolates from human patients and foods originated from two different geographic locations without any epidemiological relations were analyzed for their genotypic and phenotypic virulence gene expressions and genetic relatedness. All strains contained virulence genes, inlA, inlB, actA, hlyA, plcA and plcB, with expected product size in PCR assay except for the actA gene. Some strains produced actA gene product of 268 and others 385 bp. Phenotypically, all were hemolytic but showed variable expressions of phospholipase activity. Ribotyping classified isolates into 12 different groups based on the similarity to DuPont Identification numbers (DID), which consisted primarily of clinical or food isolates or both. Cluster analysis also indicated possible existence of clones of L. monocytogenes that are found in food or human hosts or are evenly distributed between these two. Two isolates (F1 from food and CHL1250 from patient) had unique ribotype patterns that were not previously reported in the RiboPrinter database. This study indicates distribution of diverse L. monocytogenes strains in clinical and food environments. The isolates showed 92-99% genetic homogeneity, in spite of their origins from two different geographic locations and environments.

A phylogenetic analysis of Bacillus thuringiensis serovars by RFLP-based ribotyping

AIMS

To determine the 23S and 5S rRNA gene fingerprints in order to reveal phylogenetic relationships among Bacillus thuringiensis strains.

METHODS AND RESULTS

Eighty-six B. thuringiensis strains which include 80 serovar type strains, five intraserovar strains and a non-serotypeable strain, wuhanensis, were tested. Total DNA was digested with EcoRI and HindIII. The 23S and 5S rRNA gene restriction fragment length polymorphisms showed 82 distinctive ribopatterns. The dendrogram generated by numerical analysis showed 10 phylogenetic groups and six ungrouped serovars at the 95.5% DNA relatedness rate. A second dendrogram was constructed using a combination of the data from this study and from a previous study on 16S rRNA gene fingerprinting. It revealed eight distinct phylogenetic groups and three ungrouped serovars at the 94% DNA relatedness rate.

CONCLUSION

This method permitted the classification and positioning of a wide variety of B. thuringiensis strains on a phylogenetic tree. Bacillus thuringiensis strains appear to be relatively homogeneous and to share a high degree of DNA relatedness.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study contributes a further step to the definition of valid taxonomic sublevels for the B. thuringiensis species.