Effects of alterations of zinc-to-phosphorus ratios and meconium content on group B Streptococcus growth in human amniotic fluid in vitro.

Several authors have described the bacterial growth-altering properties of amniotic fluid. We examined Group B Streptococcus growth in aseptically obtained amniotic fluid in vitro after altering its zinc, phosphorus, and meconium contents. Zinc and phosphorus levels were calculated in amniotic fluid and in meconium. Separate solutions of zinc and phosphorus were added to yield concentrations of 0.7, 7.0, 70, and 700 mumol. The solutions were incubated with Group B Streptococcus III 893 and Escherichia coli C5 strains, and 24-hour growth curves were plotted. Meconium, 0.5 mg/ml, was added to each amniotic fluid + zinc and amniotic fluid + phosphorus solution, and growth curves were plotted. The rate of proliferation of Group B Streptococcus varied directly with the zinc concentration (700 = 70 greater than 7 greater than 0.7 mumol) and inversely with the phosphorus content (700 less than 70 less than 7 = 0.7 mumol). Meconium enhanced the proliferative effect of zinc and hindered the inhibitory effect of phosphorus. Thus one possible mechanism whereby meconium enhances bacterial growth in amniotic fluid may be by alteration of zinc-to-phosphorus ratios.