Improvement of transport condition of swabs for group B streptococcal (GBS) screening

BACKGROUND For the screening-based strategy for prevention of perinatal GBS disease, CDC Guidelines as many others recommend use of appropriate transport media (Amies, Stuart, e.g.) and processing of specimen as soon as possible within 1 to 4 days. False negative cultures occur for several causes including lost of GBS viability during transport. Could Lim broth, recommended for the selective enrichment, and Granada tubes be used as transport media for swab? Simulating conditions of routine practice, Lim broth and Granada tubes, were evaluated in vitro as transport media.
METHODS Tubes of 3 brands of Lim broth (Becton Dickinson, bioMérieux, Copan) and Granada tubes (bioMérieux) were inoculated with low inocula of 10-100 CFU of GBS. Each type of tubes was incubated at 4°C, room T° (RT) and 35°C. GBS were enumerated from each tube by subculture on blood agar after 1, 2, 3 and 4 days of storage at the different T°. All tests were processed in triplicates with 3 strains of GBS belonging to serotype Ia, III and V.
RESULTS No difference of survival was observed between the 3 strains. T° had significant impact on GBS recovery for each type of tubes. At 4°C the viability was hardly sustained along the 4 days. At RT and 35°C, an increase >6 log of the inocula was observed. The increase of GBS density was sustained at least 4 days for the 3 brands of Lim broth. For the Granada broth, such increase was also observed but at day 3 for tubes incubated at 35°C, viability decreased and for some tubes, GBS subcultures were negative at day 3 or 4.

CONCLUSION To improve sensitivity of GBS screening cultures, Lim broth could be recommended as a strong transport media and the advisable storage condition would be RT to 35°C up to 4 days. In this way, initiating selective enrichment culture at the time of collection of specimen would provide higher sensitivity even for low density of colonization. Transport at 4°C should be avoided in favour with RT to 35°C. Studies in clinical setting are expected. For Granada tubes, storage at RT was fine but improvement seemed restricted in time at 35°C as there was a loss of viability after 3 days. For Granada tubes, extended evaluation and delimitation of use are needed.