Use of pulsed field electrophoresis of DNA restriction fragments for comparing Listeria monocytogenes strains isolated from human infections and food in Austria

Use of molecular typing methods to trace the dissemination of Listeria monocytogenes in a shrimp processing plant

Molecular typing of bacteria has been widely used in epidemiological studies but not as extensively for tracing the transmission of pathogenic bacteria in food plants. This study was conducted to examine the potential use of two molecular typing methods, random amplified polymorphic DNA (RAPD) analysis and pulsed-field gel electrophoresis (PFGE), to trace Listeria monocytogenes contamination in a shrimp processing plant. Ribotyping and phase typing were also performed on a select number of strains. One hundred fifteen strains of L. monocytogenes collected in different areas of a shrimp processing plant were first serotyped and then subtyped by molecular typing. RAPD and PFGE showed great promise for typing L. monocytogenes isolates since distinguishable and reproducible DNA polymorphisms were obtained. When the composite profile from both (RAPD and PFGE) methods was generated, there was an increase in the discriminatory power to discern differences between strains of L. monocytogenes. The results indicated that environmental strains all fell into composite profile groupings unique to the environment, while strains from both water and utensils shared another composite profile group. L. monocytogenes fresh shrimp isolates belonging to one profile group were found in different areas of the processing line. This same profile group was also present in food handlers from the processing and packaging areas of the plant.

The use of PCR ribotyping for typing strains of Listeria spp

The potential of PCR ribotyping for discriminating between and within various species of Listeria, as well as strains of Listeria monocytogenes was examined. In total, 49 strains of Listeria monocytogenes and 12 isolates of Listeria spp. were analyzed. The genomic DNA isolated from these strains was subjected to PCR amplification in the regions between 16S and 5S rRNA. Amplifications were performed with both low and high concentrations of Taq polymerase. Length polymorphisms in the amplified DNA products enabled distinction between various strains of Listeria spp. and between various serotypes of L. monocytogenes. Six composite profiles for serotype 4b strains, 8 for 1/2a strains and 11 for 1/2b strains, were observed. In addition, several different PCR ribotyping strategies were evaluated. Restriction fragment length polymorphisms of the spacer region between the 16S and 23S rRNA genes of 16 strains of L. monocytogenes were not observed, except for two isolates. PCR ribotyping analysis displayed promise as an alternative to traditional L. monocytogenes molecular typing methods.

RAPD typing for distinguishing species and strains in the genus Listeria

The randomly amplified polymorphic DNA (RAPD) technique was employed in the development of a typing protocol for Listeria isolates, particularly Listeria monocytogenes strains. A single strain of L. monocytogenes was used and 200 random decamer primers were screened for their discriminatory abilities by visualizing the amplification products electrophoretically. Three candidate primers displaying potentially useful banding patterns were selected and tested against 52 L. monocytogenes strains, encompassing 11 serotypes, and 12 other strains representing five other Listeria spp. Thirty-four banding profiles were obtained with one particular primer. RAPD analysis allowed differentiation between Listeria spp. and was found to further subdivide strains of the same serotype. Where only one primer was used strains from different serotypes were occasionally found to produce identical banding profiles. RAPD analysis, which in our hands proved to be reproducible, shows much promise as a molecular alternative to traditional L. monocytogenes typing protocols.