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Infect. Immun., Apr 1996, 1220-1226, Vol 64, No. 4
LC Paoletti, RA Ross and KD Johnson
The capsular polysaccharide (CPS) of group B streptococci (GBS) is an
important virulence factor that also serves to protect cells from
nonspecific host defense mechanisms. Expression of CPS by GBS, as with
other encapsulated bacterial pathogens, is not constitutive but varies
during growth in vitro and in primary cultures isolated from different
sites of infection. Despite this understanding, little is known about
regulation of this surface-expressed carbohydrate antigen in GBS. Here we
report that expression of type III CPS by GBS strain M781 grown in
continuous culture with a modified chemically defined medium is regulated
by growth rate. Cells in steady state at mass doubling times (tds) of 0.8,
1.4, and 1.6 h expressed an average of sixfold more cell- associated CPS
than did cells held at tds of 2.3 and 11 h. Strain M781 grown at a td of
1.4 h repeatedly produced more type III CPS than those held at a td of 11.0
h, even when limited for glucose, pyridoxamine, or thiamine. In our
studies, > or = 93% of the total CPS expressed by strain M781 was cell
associated. Strain M781 grown at a td of 11.0 h (i.e., lowered CPS
expression) was susceptible to in vitro complement- mediated
opsonophagocytosis and killing by human peripheral blood leukocytes,
whereas cells grown at a td of 1.4 h (i.e., higher CPS expression) were not
killed unless type III CPS-specific antibody was present. Factors that
allow GBS to asymptomatically colonize women yet cause invasive infection
to both mother and infant are poorly understood. Our results shed new light
on parameters that regulate the pathogenic potential of GBS and may also
serve as a way to discern more fully the genetics and biochemistry of GBS
capsule synthesis.
Copyright © 1996, American Society for Microbiology
Cell growth rate regulates expression of group B Streptococcus type III capsular polysaccharide
Channing Laboratory, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, 02115, USA.
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