Liquid Granada Medium for Detection of Group B Streptococci

Abstract

Heelan et al. (5) recently presented data on the sensitivity of a liquid Granada medium (9) (GBS broth) for detection of Streptococcus agalactiae. We have tested this medium, and it contains, as Granada medium, starch, proteose peptone 3, methotrexate, a Good's buffer, serum, glucose, and selective agents. So, we agree with the authors that detection of orangered colonies in GBS broth is 100% specific for the presence of beta-hemolytic group B streptococci (GBS). Nevertheless, some statements in this paper need clarification.The authors claim that sometimes Granada agar lacks sensitivity for GBS detection, but no comparison between GBS broth and Granada agar is reported. In the two references cited to support this claim (3, 7), it is acknowledged that the batches of Granada agar used were probably spoiled (10). Moreover, the flurry of references (1,2,4,6,8,9,14,15) that support a very good sensitivity of Granada agar to detect GBS are not mentioned.Regarding sensitivity of GBS broth, the figures reported are somewhat confusing. One-hundred fifty-eight GBS strains were recovered from 580 swabs, but only in 108 cases were red colonies detected, so GBS broth sensitivity is 68.3% instead of the 87.8% or 90.3% reported. GBS pigment was not observed in 35 tubes that later (after subculture) proved to be positive (25% of all positives), and this figure makes the usefulness of this medium for direct GBS detection doubtful. When (study 1) GBS broth was used as a detection medium, 9 of 75 GBS strains (12%) were completely lost (positive in subculture from LIM broth but negative in subculture from GBS broth). When (study 2) GBS broth was used as a detection and transport medium, 6 of 83 GBS (7%) were also completely lost. This failure to detect (even after subculture) some GBS-positive vaginorectal samples can be an important drawback of GBS broth. Another fact is the dramatic failure of LIM broth to recover GBS (21 of 83 positive samples were lost in study 2) found in Heelan's work. This finding needs further evaluation, as enrichment in LIM broth is an accepted procedure for GBS detection in the Centers for Disease Control and Prevention guidelines (13).Poor performance in detecting GBS could be explained by the fact that when liquid Granada media are kept refrigerated the starch precipitates (retrogradation) (16), producing an opaque white mass, where GBS colonies are sometimes barely noticeable. However, if not kept refrigerated the medium deteriorates in a few days (12). We have observed that when media containing folate antagonists (as methotretaxate) are used as transport media and swabs are kept in these media for several hours before starting incubation, the number of GBS CFU nosedives. This renders the use of such media as transport media unsafe. For these reasons, in our view, the use of liquid selective Granada media as transport media for detection of GBS colonization in pregnant women cannot be recommended.

Evaluation of liquid biphasic Granada medium and instant liquid biphasic Granada medium for group B streptococcus detection

“…The ILB medium proved to be less sensitive than the detection rate described by De la Rosa et al 6 . The fact that some of the ILB tubes were less concentrated (less amount of powder for the same 3 mL of distilled H 2 O) and/or errors in the lyophilization process could explain the lower detection rate found.…”

“…Specimens should then be incubated for 18h, subcultured onto blood agar plates and suspected colonies of GBS identified through antigen detection, genetic probes or by CAMP test. Evaluación del medio Granada líquido bifásico y del medio Granada líquido bifásico instantáneo para la detección de estreptococo del grupo B Granada agar is a selective and differential medium for rapid detection of beta-hemolytic GBS and has been recommended by several authors for GBS detection [6][7][8] . Other Granada media have been recently commercialized, such as liquid biphasic Granada (LB) and instant liquid biphasic Granada (ILB), which seem to have some advantages, such as shorter time requirements (4 h, 10 h) for identifying GBS (both LB and ILB), easy reading, and a longer shelf life (ILB) 9 .…”

Group B streptococcal colonisation in pregnant women: turnaround time of three culture methods

“…Differing only in the proportion of agar 3 and 10 g/L, there are two types of media: Granada LB and Granada agar, respectively. 2 In Portugal, there are no official recommendations for screening pregnant women. At Hospital São Marcos e Braga, in 2005, the prevalence of colonisation of pregnant women who came to the hospital for delivery was 34.9%; in the same period, the incidence of neonatal infection was 9/1000 live births.…”

Use of Unmodified Starches and Partial Removal of Serum To Improve Granada Medium Stability

“…Moreover, ␣-amylase hydrolysis of starch produces maltose, from which GBS produces acid (19). This acid can overpower the buffering capacity of the medium and hinder GBS pigment production, which is impaired at pH values below 7 (4,6,13,21). The starch hydrolysis is related to the amylase content of the serum used, and this can help explain the differences in stability observed among different batches of GM (14).…”