For serogroup 35, definition of serotype 35A occurs if reactions are interpreted as negative using sera specific for 35B, 35C and 35F.
From 2015-2019, a random subset of pneumococci serogroup 35 nasopharyngeal and middle ear fluid isolates from Rochester NY children were analyzed using Quellung and whole-genome sequencing (WGS).
23 isolates positive for serogroup 35 by Quellung analyzed by WGS showed that 18 strains had been identified as serotype 35A by Quelling that were 35B by WGS. The 18 strains were retested by Quellung and 14 were reconsidered as positive with serotype 35B typing sera. 4 isolates were repeatedly negative for 35B by Quellung. We analyzed the genetic features of these 4 atypical 35B isolates. One isolate had a mutation on wciB (Y to C). Analysis of noncapsular region identified one region showing low similarity to the typical 35B (35B in Quelling reaction and WGS). That region was about 35K bp, coding 31 proteins that showed less than 75% similarity with typical 35B strains in DNA.
Caution is essential during interpretation of Quellung reactions for serogroup 35. We identified 4 atypical strains of serotype 35B that may have differential capsule expression. Further analysis is ongoing.