SelectListeria monocytogenesSubtypes Commonly Found in Foods Carry Distinct Nonsense Mutations ininlA, Leading to Expression of Truncated and Secreted Internalin A, and Are Associated with a Reduced Invasion Phenotype for Human Intestinal Epithelial Cells

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Abstract
The surface protein internalin A (InlA) contributes to the invasion of human intestinal epithelial cells byListeria monocytogenes. Screening ofL. monocytogenesstrains isolated from human clinical cases (n= 46), foods (n= 118), and healthy animals (n= 58) in the United States revealed mutations ininlAleading to premature stop codons (PMSCs) inL. monocytogenesribotypes DUP-1052A and DUP-16635A (PMSC mutation type 1), DUP-1025A and DUP-1031A (PMSC mutation type 2), and DUP-1046B and DUP-1062A (PMSC mutation type 3). While all DUP-1046B, DUP-1062A, DUP-16635A, and DUP-1031A isolates (n= 76) containedinlAPMSCs, ribotypes DUP-1052A and DUP-1025A (n= 72) contained isolates with and withoutinlAPMSCs. Western immunoblotting showed that all threeinlAPMSCs result in the production of truncated and secreted InlA. Searches of the Pathogen Tracker database, which contains subtype and source information for more than 5,000L. monocytogenesisolates, revealed that the six ribotypes shown to contain isolates withinlAPMSCs were overall more commonly isolated from foods than from human listeriosis cases.L. monocytogenesstrains carryinginlAPMSCs also showed significantly (P= 0.0004) reduced invasion of Caco-2 cells compared to isolates with homologous 3′inlAsequences without PMSCs. Invasion assays with an isogenic PMSC mutant further supported the observation thatinlAPMSCs lead to reduced invasion of Caco-2 cells. Our data show that specificL. monocytogenessubtypes which are common among U.S. food isolates but rare among human listeriosis isolates carryinlAmutations that are associated with, and possibly at least partially responsible for, an attenuated invasion phenotype.