Methicillin-resistant and -susceptible Staphylococcus aureus sequence type 398 in pigs and humans

Abstract

Methicillin-resistant Staphylococcus aureus sequence type 398 (ST398 MRSA) was identified in Dutch pigs and pig farmers. ST398 methicillin-susceptible S. aureus circulates among humans at low frequency (0.2%) but was isolated in 3 human cases of bacteremia (2.1%; p = 0.026). Although its natural host is probably porcine, ST398 MRSA likely causes infections in humans.

Figure 1

A) Meta-analysis of the amplified fragment length polymorphism data obtained for the pig-associated methicillin-resistant Staphylococcus aureus sequence type 398 (ST398 MRSA) and its closely related methicillin-susceptible S. aureus (MSSA) strains, carriage MSSA isolates from healthy children and elderly persons, invasive MSSA from hospitalized children and elderly persons, and invasive animal S. aureus isolates (including 2 MRSA isolates) (10,11). Green and red represent 161,700 binary outcomes generated by high throughput restriction fragment length polymorphism with 147 marker fragments. Marker absence corresponds with green, marker presence corresponds with red, and gray represents ambiguous positions (i.e., weak bands), scored as marker absence in the mathematical analyses. ST398 MRSA strains are boxed. The dendrogram on the left shows the phylogenetic strain clustering; the dendrogram on the x-axis shows marker clustering. Marker groups are cluster specific. Markers on the right are defined as follows: blue, carriage isolates (n = 829); black, bacteremia isolates (n = 146); yellow, animal isolates (n = 77); red, ST398 MRSA isolates (n = 46); pink, reference strains (Mu50/N315). B) Detail highlighting the ST398 isolates. Markers and lanes on the right are defined as follows: black, carriage isolate; red, clinical isolate; 1, ST398 MRSA isolated from humans; 2, ST398 MRSA isolated from pigs; 3, ST398 MSSA human carriage isolates; 4, ST398 MSSA human bacteremia isolates; 5, ST398 MSSA animal clinical isolate.

Figure 2

Principal component analysis analysis of the amplified fragment length polymorphism data obtained for the pig-associated methicillin-resistant Staphylococcus aureus sequence type 398 (ST398 MRSA) and its closely related methicillin-susceptible S. aureus (MSSA) strains, carriage MSSA isolates from healthy children and elderly persons, invasive MSSA from hospitalized children and elderly persons, and invasive animal S. aureus isolates (including 2 MRSA isolates). The cubes, plotted in 3-dimensional space, represent all of the strains displayed in Figure 1, panel A. Each axis represents the score calculated for that strain on each principal component. The distribution is shown from 2 different angles. ST398 strains are circled. Blue, carriage isolates (n = 829); black, bacteremia isolates (n = 146); yellow, animal isolates (n = 77); red, ST398 MRSA isolates (n = 46); pink, reference strains (Mu50/N315).