The majority of genotypes of the virulence gene inlA are intact among natural watershed isolates of Listeria monocytogenes from the Central California Coast

Authors: Gorski, Lisa; Parker, Craig; Liang, Anita; Walker, Samarpita; ROMANOLA, KELLY - Former ARS Employee

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2016
Publication Date: 12/1/2016
Citation: Gorski, L.A., Parker, C., Liang, A.S., Walker, S., Romanola, K. 2016. The majority of genotypes of the virulence gene inlA are intact among natural watershed isolates of Listeria monocytogenes from the Central California Coast. PLoS One. 11(12):e0167566. doi:org/10.1371/journal.pone.0167566.

Interpretive Summary: Internalin A is an essential virulence gene involved in the uptake of the foodborne pathogen Listeria monocytogenes into host cells. It is intact in clinical strains and mutated in isolates from processed foods and processing facilities. An intact copy of the gene is necessary for virulence. The genotypes found among environmental isolates just beginning to be addressed. We sequenced the inlA gene from 112 different L. monocytogenes strains isolated over a 3-year period from naturally contaminated watersheds near a leafy green growing area in Central California. Intact genes were found in 101 of the isolates, and 23 different inlA sequences were among them. Seven strains contained an inlA gene that had a small deletion near the end of the gene; however, this variant has been found in clinical isolates of L. monocytogenes, so it is likely active in virulence. Three strains had truncated inlA genes. The functional types of inlA were found in strains isolated over the entire 3-year sampling period, but the truncated inlA type was found only during the first 3 months of sampling. Truncated inlA genes may not be fit in the environment, and intact inlA genes may relate to strain fitness in the environment. 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.

Technical Abstract: Internalin A is an essential virulence gene involved in the uptake of the foodborne pathogen Listeria monocytogenes into host cells. It is intact in clinical strains and often truncated due to Premature Stop Codons (PMSCs) in isolates from processed foods and processing facilities. The genotypes found among environmental isolates just beginning to be addressed. We sequenced the inlA alleles from 112 L. monocytogenes strains isolated over a 3-year period from naturally contaminated watersheds near a leafy green growing area in Central California. This strain collection contained 14 strains of serotype 1/2a, 12 of serotype 1/2b, and 86 of serotype 4b). Twenty-seven different inlA alleles were found. Twenty-three of the alleles are predicted to encode intact copies of InlA, while three contain PMSCs. Another allele has a 9 nucleotide (3 amino acid), previously described for a clinical strain, indicating that it is still functional. Ninety-eight of these strains isolated from the environment contained intact inlA genes, and eight strains contained the allele predicted to contain the 3 amino acid deletion. Both of these alleles were found throughout the 3-year sampling period. Three strains contained inlA alleles with PMSCs, and these were found only during the first 3 months of the study. SNP analysis of the intact alleles indicated clustering of alleles based on serotype and lineage with serotypes 1/2b and 4b (lineage I strains) clustering together, and serotype 1/2a (lineage II strains) clustering separately. The finding that 90% of environmental L. monocytogenes isolates contain intact inlA alleles varies significantly from isolates found in processing plants, and indicates the potential virulence of the majority of L. monocytogenes strains endemic to this region. 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.