D-105. Modeling pH Effect upon Growth Parameters of Serotypes III and V Group B Streptococci with Richards Model

Y-R. Ho1, C-M. Li2, C-H. Yu1, Y-C. Tseng1, H-P. Su3, J-J. Wu1;
1Natl. Cheng Kung Univ., Tainan, TAIWAN, 2Natl. Cheng Kung Univ.; Chi Mei Med. Ctr., Tainan, TAIWAN, 3Ctr. for Disease Control, Taipei, TAIWAN.

Background: Group B streptococci (GBS; Streptococcus agalactiae) is a major pathogen causing neonatal meningitis, pneumonia, and adult infections like cellulitis and bacteremia. Serotype III GBS dominates in infections of newborns and colonization on pregnant women, while type V dominates in adult infections. Vaginal pH averages 4.5 in pregnant women, upon whom 20-40% is colonized by GBS; however, how the acidity affects GBS growth remains unclear. The aim of this study was to evaluate the pH effect on the growth of serotypes III and V GBS with Richards model for calculating growth parameters. Methods: Six colonizing GBS strains harvested from pregnant women by vaginal swab, consisting types III (WB8, WB3, and WB35) and V (WB42, WB13, and WB11), were cultivated on minimum growth medium M9 at 37°C,with pH 4.5, 5.0, 6.0, and 7.0. An aliquot was removed every one h for determining optic density at 600 nm (OD600), these were transformed to ln(N/No) and computed with Richards model in toolbox of curve fitting of Matlab software. The equation is log(N/No) = A{1+ν×exp(1+ν)×exp[μm /A(1+ν)(1+1/ν)×(λ-t)]}(-1/ν) Where t is elapsed time (h) and ν is a shape parameter. Root mean square error was chosen to evaluate the performance of model. Results: The optimal pH was 6,0 among the tested pH values. The average asymptotes of types III and V GBS were 0.949 and 0.676, and average μm were 0.235 and 0.136 at pH 4.5 (h-1), respectively. At pH 7.0, the average values of λ were 1.112 and 1.165; at pH 6.0, 0.849 and 0.950; and at pH 5.0, 0.284 and 0.494 (h) for types III and V, respectively, with small root mean square error. Conclusions: Serotypes III GBS tested possessed larger maximum specific growth rates and larger asymptotes at pH 4.5, and shorter lag times at pH 5.0-7.0. These characteristic growth features would provide serotype III a potential advantage for its dominance than serotypes V for colonizing in acidic niche like vagina of reproductive women.