Purpose: We sequenced the inlA alleles from 103 L. monocytogenes strains isolated over a 3-year period from naturally contaminated watersheds near a leafy green growing area in Central California to assess the ecology of this essential virulence gene in the environment.
Methods: The inlA genes from each strain were amplified by PCR and sequenced using standard techniques. The sequences and other strain information including (serotype, date of isolation, and the watershed) were analyzed.
Results: Twenty-six different inlA alleles were found, some of which were unique. Twenty-three of the alleles are predicted to encode intact copies of InlA, while two are predicted to encode inactive versions due to mutations resulting in Premature Stop Codons. Another allele encodes has a 9 nucleotide (3 amino acid) deletion, an allele that was previously described for a clinical strain, indicating that it is still functional. Of the 103 strains analyzed, 93 encode intact copies of inlA, 8 encode the allele with the 9 nucleotide deletion, and 2 encode shortened, truncated copies of inlA. The intact and 9-nucleotide shortened alleles were persistent in the watersheds. Further analysis of the strains is ongoing.
Significance: The finding that 90% of environmental L. monocytogenes isolates contain intact inlA alleles varies significantly from isolates found in processing plants. It indicates the potential virulence of the majority of L. monocytogenes strains near active produce fields. This information is important to public health labs and growers as to the varieties of L. monocytogenes that could potentially contaminate fresh produce in the field by various means.