Antibodies to capsule polysaccharide promote selection of capsule-negative and small colony variants of Staphylococcus aureus in lactating mice.

TUCHSCHERR, LORENA P.N.; BUZZOLA FERNANDA R.; LEE, JEAN C.; SORDELLI, DANIEL O.

Les Diablerets, Suiza

Conferencia; Gordon Research Conference 2007 on Staphylococcal infections; 2007

Gordon Research Conferences

Capsular polysaccharide (CP) serotypes 5 (CP5) and 8 (CP8) play a role in the pathogenesis of different S. aureus infections in diverse hosts [O’Riordan, 2004, Lee, 2006].  S. aureus that do not react to anti-CP5 and anti-CP8 antibodies are currently defined as non-typeable (NT) [Cocchiaro, 2006].  The reason why NT S. aureus is positively selected during long term, persistent infection remains undisclosed. The present study was designed to establish whether antibody to capsule would be a selection factor not only for NT S. aureus [Tuchscherr 2006] but also SCVs during infection, which are known to be related to persistent infection. Based upon the murine model of mastitis, we utilized a discontinuous experimental system of NT S. aureus enrichment [Tuchscherr, 2006] to attempt retrieval of NT and SCV S. aureus by applying selective pressure exerted by rabbit anti-CP5 or anti-CP8 antiserum. All NT derivatives obtained from repeated experiments at any of their cycles were stable and did not revert to the wt, CP+ phenotype, but started to revert after three cycles of intraperitoneal passage and retrieval from blood.  SCVs were detected from the sixth cycle of the experiment and were also subsequently isolated thereafter. The SCV phenotype of the S. aureus isolated at the sixth or seventh cycles were unstable and reverted to the wild type phenotype upon 2 or 3 in vitro subcultures on Columbia blood agar (CBA).  As the number of cycles increased, stability of the SCV phenotype increased too.  No NT or SCV S. aureus were recovered from mice treated with non-immune serum up to the fifteenth cycle of any of the experiments.  Examination of the mammary glands after challenge of mice treated with anti-CP5 or non-immune serum revealed histopathological differences according to the cycle of the experiment.  These differences demonstrated that after 10 intramammary passages through mice treated with anti-CP5 antibodies, S. aureus (ca. 70% NT) was able to produce only a very mild inflammatory response with minimal alteration of the gland secretory function. Conversely, mice treated with non-immune serum exhibited severe damage to the mammary gland with considerable loss of its functional capacity. Experiments performed with mice treated with anti-CP8 antiserum and challenged with CP8+ S. aureus yielded comparable results, although the timeframe for appearance of NT and SCV S. aureus was delayed and the severity of the histopathological changes induced by the CP8+ S. aureus was decreased when compared with that induced by CP5+ S. aureus. The results of our experiments led us to conclude that: a) antibodies to CP5(8) significantly reduces the intramammary  bacterial load after intramammary challenge with CP5(8)+ S. aureus. b) antibodies to CP5(8) are able to select NT S. aureus and SCVs in the discontinuous animal model of mammary infection utilized.  In other words, the presence of antibodies to CP5(8) may cause a shift in the bacterial phenotype to one that is more efficient to persist in the host. c) Selection of stable NT S. aureus is not a requisite for further selection of SCVs. d) Selection of NT and SCV during infection correlated with a tissue damage milder than that produced by the CP5(8) wild-type strains. These findings indicate that anti-CP antibodies may promote the selection of staphylococcal phenotypes related with persistence of the microorganism and chronic infection.