Use of a dynamic in vitro attachment and invasion system (DIVAS) to determine influence of growth rate on invasion of respiratory epithelial cells bygroup B Streptococcus

Expression of capsular polysaccharide (CPS) and some surface proteins by gr oup B Streptococcus (GBS) is regulated by growth rate. We hypothesized that precise control of GBS growth, and thus surf ace-expressed components, cou ld modulate the ability of GBS to invade eukaryotic cells. To test this hyp othesis, a dynamic in vitro attachment and invasion system (DIVAS) was deve loped that combines the advantages of bacterial growth in continuous cultur e with tissue culture. Tissue culture flasks were modified with inlet and o utlet ports to permit perfusion of GBS. Encapsulated type III GBS strains M 781 and COH1 and strains COH1-11 and COH1-13 (transposon mutants of COH1 th at express an asialo CPS or are acapsular, respectively) were grown in cont inuous culture in a chemically defined medium at fast mass doubling time (t (d) = 1.8h) and slow (t(d) = 11 h) growth rates, conditions previously show n to induce and repress, respectively, type III CPS expression. Encapsulate d GBS strains invaded A549 respiratory epithelial cells 20- to 700-fold bet ter at the fast than at the slow growth rate, suggesting a role for CPS. Ho wever, unencapsulated GBS were also invasive but only when cultured at the fast growth rate, which indicates that GBS invasion is independent of CPS e xpression and can be regulated by growth rate. Growth rate-dependent invasi on occurred when GBS was grown in continuous culture under glucose-defined, thiamine-defined, and nondefined nutrient limitations. These results sugge st a growth rate-dependent regulation of GBS pathogenesis and demonstrate t he usefulness of DIVAS as a tool in studies of host-microbe interactions.