Expression of ActA, Ami, InlB, and Listeriolysin O in Listeria monocytogenes of Human and Food Origin

Expression of proteins involved in the adhesion of Listeria monocytogenes to mammalian cells or in the intracellular life cycle of this bacterium, including listeriolysin O (LLO), ActA, Ami, and InlB, was used to compare two populations of L. monocytogenes strains. One of the populations comprised 300 clinical strains, and the other comprised 150 food strains. All strains expressed LLO, InlB, and ActA. No polymorphism was observed for LLO and InlB. Ami was detected in 283 of 300 human strains and in 149 of 150 food strains. The strains in which Ami was not detected were serovar 4b strains. Based on the molecular weights of the proteins detected, the strains were divided into two groups with Ami (groups Ami1 [75% of the strains] and Ami2 [21%]) and into four groups with ActA (groups ActA1 [52% of the strains], ActA2 [18%], ActA3 [30%], and ActA4 [one strain isolated from food]). Logistic regression showed that food strains were more likely to belong to group ActA3 than human strains (odds ratio [OR] = 2.90; P = 1 × 10⁻⁴). Of the strains isolated from patients with non-pregnancy-related cases of listeriosis, bacteremia was predominantly associated with group Ami1 strains (OR = 1.89; P = 1 × 10⁻²) and central nervous system infections were associated with group ActA2 strains (OR = 3.04; P = 1 × 10⁻³) and group ActA3 strains (OR = 3.91; P = 1 × 10⁻³).