To the Editor

Group B Streptococcus (GBS) expresses a polysaccharide antigen on its surface that is used for serotype identification (serotypes Ia, Ib, II–IX). Serotyping can be performed by use of a rapid latex agglutination test or by polymerase chain reaction (PCR) analysis [Reference Kong1]. The serotype distribution of both invasive and colonizing GBS isolates is continuously evolving and demonstrates not only regional, but also temporal variation. Thus, we read with interest the article by Morozumi et al. [Reference Morozumi2]. The authors investigated the serotypes, the genetic diversity by multilocus sequence typing, and the frequency of macrolide (ML) resistance of GBS isolates responsible for invasive infections in neonates in Japan from 2006 to 2011. Although their data add important information to epidemiological studies on GBS serotype distribution worldwide, we kindly request further exploration of their results on the frequency of serotype V.

Without intrapartum antimicrobial prophylaxis, peripartum transmission to the newborn is estimated to be 50–70% [Reference Anthony3], resulting in a high frequency of early-onset GBS sepsis. In previous studies, about 20% of GBS isolates found in colonized Japanese women were designated as serotype V [Reference Kimura4, Reference Ueno5]. However, in the study by Morozumi et al. [Reference Morozumi2], the frequency of serotype V isolates in 150 GBS isolates obtained from invasive infections in neonates was zero.

We recently found a highly significant association between serotype V and ML resistance in GBS-colonized Swiss women [Reference Fröhlicher6]. This is in line with previous findings in the Asia, Europe and the United States (Table 1 [Reference Fröhlicher6–Reference Andrews19]). In the study by Morozumi et al. [Reference Morozumi2], 7/32 (21·9%) serotype Ia and 24/88 (27·3%) serotype III GBS isolates showed ML resistance. We wonder whether the lack of serotype V GBS isolates is an epidemiological variation in Japan, or alternatively, whether it can be attributed to the ability of GBS to switch capsular serotypes [Reference Luan20]. Recent studies using genome analysis confirmed capsular switching in serotype IV GBS isolates designated as clonal complex (CC)17 and its variant (i.e. ST291) [Reference Meehan, Cunney and Cafferkey15, Reference Bellais21], even though CC17 GBS isolates are typically associated with serotype III. Such a phenomenon in serotype V would be, to the best of our knowledge, novel.

Table 1. Association of macrolide resistance and serotype V

AD, Adults; AGGL, agglutination test; EEM, enzymatic extraction method; ID, immunodiffusion, INF, infants; NR, not reported; NN, neonates; PCR, polymerase chain reaction; PNW, pregnant women; RFLP, restriction fragment length polymorphism.

Countries in Europe are presented in alphabetical order. The list is not exhaustive.