Spectrum of Pneumococcal Serotype 11A Variants Results from Incomplete Loss of Capsule O-Acetylation

Streptococcus pneumoniae is a significant bacterial pathogen that expresses >90 capsule serotypes. Conventional serotyping methods assume that each serotype is a genetically and antigenically distinct entity; however, recent investigations have revealed pneumococcal isolates that cannot be unambiguously serotyped because they share the properties of more than one serotype. Here, we employed a novel serotyping method and NMR spectroscopy to examine clinical isolates sharing properties of serotypes 11A and 11E. These ambiguous clinical isolates were provisionally named 11A variant (11Av) isolates. Serotype 11A pneumococci characteristically express capsule beta-galactose-6-O-acetylation (beta Gal6OAc) mediated by the capsule synthesis gene wcjE, while 11E strains contain loss-of-function mutations in wcjE and completely lack the expression of beta Gal6OAc. Although 11Av isolates also contained mutated wcjE alleles, 11Av clinical isolates were composed of antigenically homogeneous bacteria expressing reduced amounts of 11A-specific capsule antigen. NMR data confirmed reduced but detectable amounts of beta Gal6OAc on 11Av capsule polysaccharide. Furthermore, the transformation of strains with wcjE alleles from 11Av strains was sufficient to restore partial beta Gal6OAc in an 11E background. We conclude that, instead of being distinct entities, serotypes 11A and 11E represent two extremes of an antigenic spectrum resulting from variable capsule O-acetylation secondary to heterologous wcjE mutations. These findings challenge whether all clinically relevant pneumococci can be definitively categorized into distinct serotypes.