Persistence of nonencapsulated Staphylococcus aureus in mouse mammary glands after intramammary challenge.

TUCHSCHERR, LPN; BUZZOLA, FR; LEE, JC; SORDELLI, DO

New Orleans, Louisiana, EEUU de NA

Congreso; 104th ASM General Meeting; 2004

American Society for Microbiology

S. aureus capsular polysaccharide (CP) types 5 (CP5) or 8 (CP8) contributes to pathogenesis of infection in several animal models.  In bovines with subclinical mastitis a significant proportion of S. aureus isolates express neither CP5 nor CP8 (nontypeable, NT).  This study was designed to analyze whether the lack of CP5 or CP8 expression contributes to lengthened persistence of staphylococci in the mammary gland.  To such purpose, mice were injected by the intramammary route with S. aureus in doses ranging from 1x105  to 5x106 CFU of three S. aureus strains: a) jl278 (a streptomycin-resistant derivative of S. aureus Reynolds that expresses CP5). b) Isogenic strain jl812 (it is erithromycin-resistant jl278 with those cap cluster genes relevant to capsular type coding replaced to express CP8).  And c) isogenic strain jl801, an NT mutant derived from jl278 which does not express CP.  Number 4 left (L4) and right (R4) mammary glands of lactating female CF1 mice were injected and 2 h later pups were returned to their mothers.  At different times thereafter (days 1, 4, 8, 12 and 16) mice were sacrificed and L4 and R4 glands surgically removed, and homogenized.  Homogenates were quantitatively plated on trypticase soy agar, and the number of CFU per gland were determined.  An increase in the number of CFU of the three strains was observed by 24 h after injection, followed by a steady decrease.  No differences were found in the number of CFU recovered at days 1, 4 and 8.   Significant differences were noted thereafter.  For instance, by day 12 the CFU counts (log) of NT jl801 was significantly higher (3.39±0.69) than those of jl278 (0.89±0.40) and jl812 (1.24±0.40) (p=0.0058).  All animals exhibited up to the last day of the experiment signs of lactation.  Kidneys of none of the animals at any time point revealed signs of disease, suggesting lack of significant dissemination from the mammary gland.   Our results support the hypothesis that non-capsulated S. aureus may have an advantage over CP5 and CP8 to persist over longer periods of time in the mammary gland of lactating animals.