Group B streptococcus screening in pregnant women: a comparison of three media

This study compared the relative isolation rate of Group B Streptococcus (GBS) from 663 low vaginal swabs, collected from antenatal patients, on three media: horse blood agar plus neomycin (75 mg/l) (Neo), Islam agar (Islam) and Islam agar plus nalidixic acid (15 mg/l) and colistin sulphate (10 mg/l) (Islam Plus). One hundred and forty-seven (22%) GBS were isolated. At 24 hours the isolation rate was highest using Neo, but within 72 hours there was little difference. The isolation rates for Neo, Islam and Islam Plus at 24 hours were 124 (18.7%), 103 (15.6%), 109 (16.4%) (P < 0.05); at 48 hours 125 (18.9%), 116 (17.5%), 121 (18.1%) (P > 0.1); and at 72 hours 125 (18.9%), 121 (18.3%) and 127 (19.1%) (P > 0.1), respectively. Twenty-two isolates were missed on Neo, 26 on Islam and 20 on Islam Plus. Of those missed on Islam agars, 12 failed to produce pigment and were only detected on Neo. The disadvantage of Neo is the need to perform additional tests to confirm the identity as GBS. In the present study 100 suspicious colonies were identified as Group D.