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Previous research involving internalin A (inlA) and internalin B (inlB) single-and double-knockout mutants of Listeria monocytogenes has suggested the involvement of two surface proteins, InlA and InlB, in the adherence of the cells to a glass surface. This phenomenon was further investigated with a larger number (n=27) of L. monocytogenes wild-type strains that were isolated from catfish processing plants and catfish fillets, in addition to internal controls, one ATCC 7644 strain, and L. monocytogenes EGDe strain. Of the wild-type strains, three were shown to produce truncated forms of InlA protein. A blot succession method was used to measure the ease of detachment of sessile L. monocytogenes from a glass surface after attachment at 4°C for 8 h. Real-time reverse transcriptase polymerase chain reaction was used to quantitate mRNA levels of inlA and inlB in L. monocytogenes strains after the cells were incubated at 4°C for 8 h. An inverse relationship between the ease of cell removal from glass surface and the relative inlA and inlB mRNA levels with R 2 value of 0.664 and 0.431, respectively, was observed. This suggests that the attachment strength of L. monocytogenes on glass surface is positively correlated with inlA and inlB expression. There were no differences (p>0.05) in attachment strength among serotypes. These results suggest that L. monocytogenes InlA and InlB proteins play a role in adherence to a glass surface at low temperature, and the attachment ability is independent of serotype.

Azotobacter vinelandii contains an iron-regulatory small RNA ArrF whose expression is dependent upon the levels of iron and ferric uptake regulator. The deletion of this ArrF-encoding gene resulted in a 300-fold increase in the production of poly-beta-hydroxybutyrate (PHB), a polymer of industrial importance. This arrF mutant exhibited wild-type growth and growth-associated PHB production. Limited iron and aeration elevated the PHB production in the mutant as well as wild type. Real-time RT-PCR revealed that phbB, phbA, and phbC were upregulated approximately 61-, 18-, and eightfold, respectively, in the mutant. The phbR transcript of the activator PhbR for this operon was also approximately 11 times more abundant. The analysis of phbR transcript predicted a region of complementarity near its Shine-Dalgarno sequence that could potentially basepair with the conserved region of ArrF. These results suggest that ArrF represses the expression of PhbR in an antisense manner and derepression of this activator in the mutant elevates the expression of phbB, phbA, and phbC, resulting in the PHB overproduction.

Incidence of Listeria spp. in whole raw catfish, catfish fillets, and processing environments from two catfish processing facilities was determined in August 2008 and August 2009. Thirty-nine (18.4%) of 212 samples collected in August 2008 were positive for Listeria monocytogenes. Prevalences of Listeria species L. innocua and L. seeligeri-L. welshimeri-L. ivanovii were 11.3 and 23.6%, respectively. Of 209 samples collected in August 2009, 12.4% were positive for L. monocytogenes, 11% for L. innocua, and 19.6% for L. seeligeri-L. welshimeri-L. ivanovii. No Listeria grayi was detected in any of the samples. L. monocytogenes was not found in catfish skins and intestines, but was detected in catfish fillets, on food contact surfaces, and on non-food contact surfaces with frequencies of 45.0, 12.0, and 11.1%, respectively. In August 2008 isolates, serotypes 1/2b (62.2%) and 3b (15.6%) were frequently isolated, whereas the majority of the August 2009 isolates (92.3%) were serotype 1/2b. Genotyping analyses revealed that some genotypes of L. monocytogenes isolates were detected in one facility even after a year, but no persistence of L. monocytogenes was observed in the other facility. In addition, some L. monocytogenes isolates from fresh fillets showed genotypes that were either identical, or more than 90% similar, to those of L. monocytogenes isolates from food contact surfaces in the processing lines. The results of this study suggest that processing environment rather than whole raw catfish is an important source of L. monocytogenes contamination in the catfish fillets. These results should assist the catfish industry to develop better control and prevention strategies for L. monocytogenes.