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

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

Les Diablerets

Conferencia; 2007 Staphylococcal Gordon Research Conferences; 2007

We showed previously that capsule-negative (nontypeable; NT) Staphylococcus aureus persisted in the infected mammary glands of lactating mice in greater numbers than did isogenic encapsulated strains.  Factors related to persistence of NT S. aureus in the host are ill defined. Experiments were conducted using a mouse model of discontinuous mammary infection: lactating mothers were passively immunized 24 h before challenge with rabbit antibodies specific for capsular polysaccharide serotype 5 (CP5) or non-immune rabbit serum. On day 0 the animals received an intramammary inoculum of 106 CFU of a serotype 5 S. aureus strain.  Homogenates of the infected mammary glands were plated quantitatively on day 4, and staphylococci recovered from the mammary glands were injected to another group of lactating mice. This cycle was repeated up to 15 times.  Passive immunization with anti-CP5 antibodies resulted in a dramatic 4-log10 reduction in the CFU S. aureus per gland compared with mice given non-immune serum. By the fourth cycle of the experiment NT colonies were identified by a colony immunoblot method from mice given CP5 antibodies.  Likewise, small colony variants (SCVs) were detected from the sixth cycle of the experiment, and by the tenth cycle some of the SCVs were stable in vitro.  In contrast, NT or SCV S. aureus were not recovered from any of the mice treated with non-immune serum up to the fifteenth cycle of the experiments.  NT colonies from passively immunized mice were stable and did not revert to the CP5+ phenotype in vitro. However, reversion to the CP5+ phenotype was observed after three cycles of intraperitoneal passage and retrieval of S. aureus colonies from blood.  After 10 intramammary passages through mice treated with anti-CP5 antibodies, S. aureus (ca. 80% NT) induced a very mild inflammatory response with minimal alteration of the gland secretory function.  In contrast, mice treated with non-immune serum and infected with CP5+ S. aureus showed severe histopathologic damage to the mammary gland and considerable loss of its functional capacity. In conclusion: 1) expression of NT and SCV phenotypes may be a bacterial strategy to evade the host immune response. 2) Whereas a CP5-based vaccine may be partially protective in preventing acute S. aureus infection, CP5 antibodies alone may select for NT S. aureus and SCVs that contribute to persistent staphylococcal infection.