Bacteriological and clinical data in respect of 113 mothers who were vaginal carriers of group-B streptococci at the time of labour, and of their 113 infants, were examined for factors that contributed to mother-infant transmission of these micro-organisms. A semi-quantitative system (1 + = 1-10, 2 + = 11-50, and 3 + = > 50 colonies) was used to record the heaviness of growth in all positive primary cultures.

Positive cultures at birth or at the time of discharge from hospital (or both) were obtained from 44/66 of the infants of mothers with 3 + vaginal cultures (67%), but from only 17/47 of the infants of mothers with 2 + or 1 + cultures (36%; p < 0-005). Of 38 infants who gave positive cultures at birth, 32 (84%) were born to mothers with 3 + vaginal cultures. At birth, infants born to heavily colonised mothers were more often positive at multiple sites and were more heavily contaminated than infants born to lightly colonised mothers. However, nearly one-half of the infants whose swabs were first positive on discharge were born to mothers with 2 + or 1 + swabs.

When the mothers had been in labour for > 6 h after rupture of the membranes, the proportion of infants yielding positive cultures at birth was higher (47%) than the proportion for infants of mothers in labour for a lesser time after membrane rupture (27%; p<0-05), but there was no difference between the two groups in the eventual isolation rate (at birth-I-at discharge).

Although the distribution of group-B streptococcal serotypes was similar for mothers and infants, mothers carrying serotype-III strains delivered infants positive at birth more frequently than did mothers carrying all other serotypes.

The most important predictive factor determining neonatal acquisition of group-B streptococci appeared to be the number of group-B streptococci in the maternal vagina at the time of labour.


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