Abstract
BackgroundMore than 94 serotypes of Streptococcus pneumoniae have been described to date, however the majority of disease is caused by approximately 20 serotypes. Some pneumococci do not react with commercially available antisera used for serotyping and are thus regarded as non-serotypeable (NT). These pneumococci are commonly isolated during carriage studies and very rarely cause invasive disease. Colonization may occur with more than one serotype however disease with more than one serotype is rarely detected. Thus there are limited data describing cases of pneumococcal disease caused by more than one isolate.ResultsIn two cases of invasive pneumococcal disease in South Africa, a non-serotypeable and a serotypeable isolate were co-detected during routine serotyping. A serotype 1 and 18C isolate were each co-detected with a non-serotypeable isolate in 2009 (case A) and 2010 (case B), from cerebrospinal fluid and blood, respectively. Both patients were 10–14 years old. For case A, the serotypeable isolate could not be obtained due to low representation in the mixed culture. Using electron microscopy we confirmed lack of capsule for the non-serotypeable isolates. Comparison of the case A non-serotypeable isolate with a serotype 1 genome revealed only the presence of the rhamnose biosynthesis genes (rmlA, B, C and D) in the capsular locus, all other capsular genes were absent. Nonetheless it had a multilocus sequence type (ST) associated with serotype 1 (ST217 and ribosomal ST3462) and its core genome clustered with other ST217 isolates. The case B non-serotypeable isolate had all serotype 18C capsular genes except for variation in the wchA and wze genes, compared to the 18C isolate. Both case B isolates were ST9817 and their core genomes were identical.ConclusionsThe ability of pneumococci to alter capsule production is a potential vaccine escape mechanism and therefore non-serotypeable pneumococci should be monitored as such organisms may increase under vaccine pressure.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-016-0745-0) contains supplementary material, which is available to authorized users.
Highlights
More than 94 serotypes of Streptococcus pneumoniae have been described to date, the majority of disease is caused by approximately 20 serotypes
While reviewing national surveillance data to identify episodes of mixed invasive pneumococcal disease (IPD) caused by two or more serotypes, we identified two cases whereby one of the co-detected isolates was non-serotypeable (NT)
Our genotypic results confirm that the NT isolate is a serotype 1 variant as both the sequence type and ribosomal multilocus sequence typing (MLST) profile are exclusively associated with serotype 1 (ST217 and rST3462)
Summary
More than 94 serotypes of Streptococcus pneumoniae have been described to date, the majority of disease is caused by approximately 20 serotypes. Some pneumococci do not react with commercially available antisera used for serotyping and are regarded as non-serotypeable (NT). These pneumococci are commonly isolated during carriage studies and very rarely cause invasive disease. Streptococcus pneumoniae (pneumococcus) is a commensal of the human nasopharynx. The polysaccharide capsule is known to be a major virulence factor of the pneumococcus. It is immunogenic and can be detected by specific antisera, and forms the basis for serotyping and current vaccines against pneumococcal disease. In South Africa, PCV-7 was introduced into the Expanded Programme for Immunisation in 2009 and replaced by PCV-13 in 2011 in a 2 + 1 schedule at 6, 14 and 40 weeks of age [2]
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