Internalin profiling and multilocus sequence typing suggest four Listeria innocua subgroups with different evolutionary distances from Listeria monocytogenes

Abstract

AbstractBackgroundEcological, biochemical and genetic resemblance as well as clear differences of virulence betweenL. monocytogenesandL. innocuamake this bacterial clade attractive as a model to examine evolution of pathogenicity. This study was attempted to examine the population structure ofL. innocuaand the microevolution in theL. innocua-L. monocytogenesclade via profiling of 37 internalin genes and multilocus sequence typing based on the sequences of 9 unlinked genesgyrB,sigB,dapE,hisJ,ribC,purM,gap,tufandbetL.ResultsL. innocuawas genetically monophyletic compared toL. monocytogenes, and comprised four subgroups. Subgroups A and B correlated with internalin types 1 and 3 (except the strain 0063 belonging to subgroup C) and internalin types 2 and 4 respectively. The majority ofL. innocuastrains belonged to these two subgroups. Subgroup A harbored a whole set ofL. monocytogenes-L. innocuacommon andL. innocua-specific internalin genes, and displayed higher recombination rates than those of subgroup B, including the relative frequency of occurrence of recombination versus mutation (ρ/θ) and the relative effect of recombination versus point mutation (r/m). Subgroup A also exhibited a significantly smaller exterior/interior branch length ratio than expected under the coalescent model, suggesting a recent expansion of its population size. The phylogram based on the analysis with correction for recombination revealed that the time to the most recent common ancestor (TMRCA) ofL. innocuasubgroups A and B were similar. Additionally, subgroup D, which correlated with internalin type 5, branched off from the other three subgroups. AllL. innocuastrains lacked seventeen virulence genes found inL. monocytogenes(except for the subgroup D strain L43 harboringinlJand two subgroup B strains bearingbsh) and were nonpathogenic to mice.ConclusionsL. innocuarepresents a young species descending fromL. monocytogenesand comprises four subgroups: two major subgroups A and B, and one atypical subgroup D serving as a link betweenL. monocytogenesandL. innocuain the evolutionary chain. Although subgroups A and B appeared at approximately the same time, subgroup A seems to have experienced a recent expansion of the population size with higher recombination frequency and effect than those of subgroup B, and might represent the possible evolutionary direction towards adaptation to enviroments. The evolutionary history in theL. monocytogenes-L. innocuaclade represents a rare example of evolution towards reduced virulence of pathogens.