Host- and tissue-specific pathogenic traits of Staphylococcus aureus

Abstract

Comparative genomics were used to assess genetic differences between Staphylococcus aureus strains derived from infected animals versus colonized or infected humans. A total of 77 veterinary isolates were genetically characterized by high-throughput amplified fragment length polymorphism (AFLP). Bacterial genotypes were introduced in a large AFLP database containing similar information for 1,056 human S. aureus strains. All S. aureus strains isolated from animals in close contact with humans (e.g., pet animals) were predominantly classified in one of the five main clusters of the AFLP database (cluster I). In essence, mastitis-associated strains from animals were categorized separately (cluster IVa) and cosegregated with bacteremia-associated strains from humans. Distribution of only 2 out of 10 different virulence genes differed across the clusters. The gene encoding the toxic shock syndrome protein (tst) was more often encountered among veterinary strains (P < 0.0001) and even more in the mastitis-related strains (P<0.0001) compared to human isolate results. The gene encoding the collagen binding protein (cna) was rarely detected among invasive human strains. The virulence potential, as indicated by the number of virulence genes per strain, did not differ significantly between the human- and animal-related strains. Our data show that invasive infections in pets and humans are usually due to S. aureus strains with the same genetic background. Mastitis-associated S. aureus isolated in diverse farm animal species form a distinct genetic cluster, characterized by an overrepresentation of the toxic shock syndrome toxin superantigen-encoding gene.

FIG. 1.

The dendrogram shows the level of similarity, expressed by the similarity coefficient, between AFLP patterns from the animal strains (n = 77). AFLP cluster identification is indicated on the right side of the figure. MLST was performed of a selection of 2, 4, 10, and 18 animal strains, representing each AFLP subcluster (defined by an arbitrarily chosen similarity-coefficient level ≥ 0.8) within AFLP clusters III, II, IVa, and I, respectively. Plus signs represent three unique AFLP patterns that were found for 0302880 (horse), 0002580 (ape [gorilla]), and 0210118 (seal).

FIG. 2.

Agglomerative two-dimensional clustering of strains (n = 1,133) and AFLP markers. AFLP fingerprints of the animal strains (n = 77) are included. The red fields in the figure represent the presence of AFLP markers; green indicates absence. Blue bars at the right side of the figure indicate the position of each animal strain in the analysis; the red bars represent the mastitis-associated strains. The AFLP database is divided in three main strain clusters (indicated as I, II, and III at the far right of the figure) and two minor clusters (indicated as IVa and IVb) (25).

FIG. 3.

A flow diagram of the virulence gene distribution in the four different strain clusters. Only significant differences are noted.