Infect. Immun., May 1996, 1495-1501, Vol 64, No. 5
JB Dale, RG Washburn, MB Marques and MR Wessels
The major virulence determinant of group A streptococci is the ability to
resist opsonization and phagocytic ingestion. The present studies were
performed to compare the mechanisms of resistance to opsonization of type
18 and type 24 streptococci and to determine the relative roles of M
protein-fibrinogen interaction and the hyaluronate capsule in preventing
phagocytic ingestion and killing. By use of parent strains and acapsular
transposon mutants in the presence and absence of fibrinogen, we show that
type 18 and type 24 streptococci rely on somewhat different mechanisms for
resistance to opsonization. Type 24 streptococci bound fibrinogen avidly to
their surfaces, and encapsulated organisms were completely resistant to
opsonization only in the presence of fibrinogen. In contrast, type 18
streptococci bound 10-fold less fibrinogen than type 24 streptococci and
were fully resistant to phagocytosis only when they expressed capsule. The
general structural characteristics of the amino-terminal halves of type 18
and type 24 M proteins differed in that type 18 M protein contained only
one complete B repeat, whereas type 24 M protein contained five complete B
repeats, a structural difference which could potentially be related to the
differences in fibrinogen binding between the two serotypes.
Immunofluorescence assays of complement deposition were used in combination
with 125I-C3 binding assays to show that encapsulated type 24 streptococci
were fully resistant to opsonization by C3 only in the presence of plasma.
Encapsulated and unencapsulated type 18 streptococci were equally opsonized
by C3 in either plasma or serum, yet only encapsulated organisms resisted
phagocytic killing in blood. The results of this study indicate that
opsonization by C3 does not necessarily lead to phagocytic ingestion and
that the hyaluronate capsule and M proteins are variably important in
resistance to different group A streptococci to opsonization and phagocytic
killing.
Copyright © 1996, American Society for Microbiology
Hyaluronate capsule and surface M protein in resistance to opsonization of group A streptococci
VA Medical Center, Memphis, TN 38104, USA.
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