Abstract
Staphylococcus aureus causing persistent, recurrent bovine intramammary infections are still a major challenge to dairy farming. Generally, one or a few clonal lineages are predominant in dairy herds, indicating animal-to-animal transfers and the existence of distinct pathotypic traits. The aim of this study was to determine if long term persistence and spreading of S. aureus are associated with specific phenotypic traits, including cellular invasion, cytotoxicity and biofilm formation. Mastitis isolates were collected over a 3-years period from a single dairy herd, resulting in two persistent subtypes, the high within-herd prevalent subtype ST9 (CC9)-methicillin-susceptible S. aureus (MSSA), designated HP/ST9, and the low within-herd prevalent subtype ST504 (CC705)-MSSA, designated LP/ST504. Characterization of the two different coexisting persistent subtypes showed that the following phenotypic traits are particularly associated with high within-herd prevalence: lack of capsular polysaccharide expression, high cellular invasiveness, low cytotoxicity and high biofilm/ poly-N-acetylglucosamine (PNAG) production, which may concomitantly contribute to the spreading of HP/ST9 within the herd. By contrast to HP/ST9, LP/ST504 is characterized by the formation of colony dendrites, which may help the bacteria to access deeper tissues as niches for persistence in single animals. Thus, within a single herd, two different types of persistence can be found in parallel, allowing longtime persistence of S. aureus in dairy cattle. Furthermore, this study indicates that ST9 (CC9)-MSSA strains, which are currently thought to have their primary reservoir in swine and humans, can also successfully spread to new hosts and persist in dairy herds for years.
Highlights
Staphylococcus aureus frequently causes intramammary infections (IMI) in dairy cattle
The latter biotype, which was isolated from the udder of two cows over a period of more than two years was assigned to spa-type t529, ST504 (CC705, former CC151) and agr-II
This study provides evidence for the existence of specific phenotypic traits linked to persistence of S. aureus in dairy cattle
Summary
Staphylococcus aureus frequently causes intramammary infections (IMI) in dairy cattle. Since several studies have shown a high prevalence of non-encapsulated strains in persistent bovine IMI (up to 86%)[4,11] as well as human chronic infections[12], loss of capsular polysaccharide (CP) expression may be a key feature associated with chronicity of S. aureus. A recent study of Bardiau et al revealed a correlation of biofilm formation, low CP expression and cellular invasion, fostering the hypothesis that certain S. aureus strains might be associated with specific intracellular and extracellular niches within the host[15]. Genotypic and phenotypic properties associated with persistence, including cellular invasion, biofilm formation and cytotoxicity of these two S. aureus subtypes were investigated to unravel the mechanisms contributing to persistence and spread of S. aureus in herds of dairy cattle
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