Comparative genetic characterization of Listeria monocytogenes isolates from human and animal listeriosis cases

Listeria monocytogenes isolates from human sporadic and epidemic cases (n = 119) and from animal cases (n = 76) were characterized by automated riboty ping and PCR-restriction fragment length polymorphism (PCR-RFLP) typing of the virulence genes actA and hly. This combination of typing methods differ entiated 39 distinctive strains, each reflecting a unique combination of ri botypes, hly and actA alleles. Simpson's index of discrimination indicated a high discriminatory ability of ribotyping for both animal (0.867) and hum an isolates (0.857), which was further increased by the addition of hly and actA typing (0.916 and 0.904. respectively). Ribotype and hly allele data were further used to group isolates into three genetically distinct lineage s. Each lineage is composed of several ribotype fragment subsets, each of w hich contains multiple ribotypes characterized by common ribotype fragments . To determine whether certain clones of L. monocytogenes show indications for unique pathogenic potential or host specificity, frequency distribution s for five genetic characteristics (i.e. lineage, ribotype, ribotype fragme nt subset and hly and actA allele) were calculated for isolates from animal cases, human epidemic cases and human sporadic cases. Lineage III isolates were found less frequently in human cases (1 of 119 isolates) than in anim al cases (8 of 76 isolates; P = 0.003). These results suggest the possibili ty of host specificity for non-primate mammals among lineage III strains. I n addition, lineage I strains were found more frequently among human cases than among animal cases (P < 0.001). Among the eight hly alleles observed, hly allele 1 was more common among human isolates as compared to animal iso lates (P = 0.002). We also identified one ribotype (DUP-1030) which was sig nificantly more common among animal isolates (P = 0.005) and one ribotype ( DUP-1038; lineage I) which was significantly more common among human epidem ic isolates as compared to human sporadic isolates (P < 0.001). These findi ngs confirm the presence of clonal groups of L, monocytogenes, which appear to be characterized by unique virulence or host specificity patterns. This study also establishes baseline data describing the genetic diversity of h uman and animal L. monocytogenes isolates which can be utilized in future s urveillance programmes to track the emergence of new strains.