In vitro method to differentiate isolates of type III Streptococcus agalactiae from symptomatic and asymptomatic patients

Abstract

Streptococcus agalactiae (group B streptococci) isolates from infected infants have been demonstrated to have threeto fourfold or higher levels of cell-associated lipoteichoic acid than isolates from asymptomatically colonized infants, suggesting a role for this cell surface polymer in the relative virulence of these organisms. The present study indicates that symptomatic isolates of type III group B streptococci can be readily differentiated from asymptomatic strains by their response to various levels of phosphate in a chemically defined medium (FMC). Both classes of isolates had the same doubling time (TD of 30 to 35 min) in FMC containing 65 mM sodium phosphate. However, levels of phosphate >125 mM distinguished the two classes of strains. Asymptomatic strains pregrown in 65 mM phosphate to the stationary phase rapidly initiated growth at elevated phosphate levels, while symptomatic strains initiated growth only after a prolonged incubation period (>400 min). These results suggest that the physiological growth response of clinical isolates of group B streptococci to phosphate can serve as a diagnostic aid in screening potentially virulent strains in pregnant women and newborn infants.

Sialic acid levels and lag time of growth in chemically defined medium containing 200 mM phosphate among strains of various serotypes of Streptococcus agalactiae

“…category A was the same as that of the low-level strains for LTA (normal growth strains), as previously reported (17). For category B, although their doubling time was slightly longer than that of category A, they were concluded to be normal growth strains, since they finally, after 10 h of incubation, reached the stationary phase.…”

“…Category C included two strains of type Ia, one of type Ib, and five of type III; category D included one type III strains and two control strains. These two categories did not exhibit the longer lag phase (400 min) shown by the high-LTA-level strains (17). However, since the doubling time was much longer during the initial stage of incubation, whereas after 18 to 24 h of incubation growth attained nearly the same level as that in strains with normal growth, these categories were identified as strains exhibiting a lag time of growth.…”

“…In the present study, vaginal isolates exhibiting a lag time of growth in FMC medium containing 200 mM phosphate were all found among strains with low or moderate levels of sialic acid. Previous studies on type III isolates from infected neonates have demonstrated that those strains have high sialic acid levels as well as high LTA levels (17,19,20,27). However, in our unpublished data on a few isolates (four strains) from neonates with late-onset infection, two strains of type la had high levels of sialic acid (12.1 and 13.3 ,ug/mg of cell dry weight) but did not exhibit a lag time of growth in the above medium, and two strains of type III that were used in the present study as control strains exhibited a lag time of growth and had moderate levels of sialic acid (9.7 and 10.8 ptg/mg of cell dry weight).…”

“…Detection of strains exhibiting a prolonged lag phase on FMC medium containing 200 mM phosphate. The method of Maurer and Mattingly (17) with chemically defined medium (FMC) containing either 65 or 200 mM phosphate was used. The medium was prepared as described previously (24) without adding guanine.…”

“…In the present study, we determined sialic acid levels in 100 strains isolated from vaginal specimens and detected strains exhibiting a prolonged lag phase of growth in a chemically defined medium (FMC) containing 200 mM phosphate. This phenomenon was suggested to be a characteristic of high-LTA-level strains (17).…”