The dairy industry suffers massive economic losses due to staphylococcal
mastitis in cattle. The Staphaureux
latex agglutination test (Oxoid, Basel, Switzerland) was reported to lead to negative results in 54% of bovine Staphylococcus aureus strains, and
latex-negative strains are thought to be less virulent than Staphaurex
latex-positive strains. However, comparative information on virulence and resistance profiles of these 2 groups of Staph. aureus is scarce. Our objective was to associate the
latex agglutination phenotype of Staph. aureus strains isolated from
bovine mastitis milk with data on clonal complexes, virulence genes, and antibiotic resistance to (1) determine the virulence profiles of the Staphaureux test positive and Staphaurex test negative groups, and (2) provide data needed to improve treatment of
bovine mastitis and to identify potential
vaccine targets. Seventy-eight Staph. aureus strains isolated from 78 cows on 57 Swiss farms were characterized.
Latex agglutination was tested by Staphaureux kit, and resistance profiles were generated by disk diffusion.
A DNA microarray was used to assign clonal complexes (CC) and to determine virulence and resistance gene profiles. By the Staphaureux test, 49% of the isolates were
latex-positive and 51% were
latex-negative. All
latex-negative strains were assigned to CC151, whereas
latex-positive strains were assigned to various clonal complexes, including CC97 (n=16), CC8 (n=10), CC479 (n=5), CC20 (n=4), CC7 (n=1), CC9 (n=1), and CC45 (n=1). Although the
latex-negative isolates were susceptible to all
antimicrobial agents tested, 24% of
latex-positive isolates were classified as intermediate with regard to
cefalexin-
kanamycin and 13% were resistant to both
ampicillin and
penicillin. Microarray profiles of
latex-negative isolates were highly similar, but differed largely from those of
latex-positive isolates. Although the
latex-negative group lacked several
enterotoxin genes and sak, it exhibited significantly higher prevalence rates of genes encoding
enterotoxin C,
toxic shock syndrome toxin, and
leukocidins (lukM/lukF-P83, lukD). Our findings suggest that
latex-negative isolates represent a group of closely related strains with specific resistance and virulence gene patterns.