Stability of Serotypes during Nasopharyngeal Carriage of Streptococcus pneumoniae

Abstract

Serotype changes among natural isolates of Streptococcus pneumoniae are well documented and occur by recombinational exchanges at the capsular biosynthetic locus. However, the frequency with which this phenomenon occurs within the nasopharynx of children is not clear and is likely to be highest in the nasopharynx of children, who have high rates of pneumococcal carriage. A birth cohort of 100 infants was studied, and pneumococci were recovered from nasopharyngeal samples taken at monthly intervals during the first 6 months of life and then at 2-monthly intervals until the age of 2 years. Among the 1,353 nasopharyngeal samples were 523 that contained presumptive pneumococci, and three colonies from each were serotyped. A total of 333 isolates, including all isolates of differing serotypes from the same child, were characterized by multilocus sequence typing. Sixty-eight children carried multiple serotypes during the first 2 years of life. Two children carried a typeable and a nonserotypeable pneumococcus of identical genotype, and five children carried genetically indistinguishable isolates of serotypes 15B and 15C. These isolates were considered, respectively, to be due to loss of capsule expression and the known ability of serotype 15B and 15C pneumococci to interconvert by loss or gain of an acetyl group on the capsular polysaccharide. In all other cases, isolates from the same children that differed in serotype also differed in genotype, indicating the acquisition of a different pneumococcal strain rather than a change in capsular type. There was therefore no evidence in this study for any change of serotype due to recombinational replacements at the capsular locus among the pneumococci carried within the nasopharynges of the children.

A longitudinal household study of Streptococcus pneumoniae nasopharyngeal carriage in a UK setting

“…As has been reported previously [5,18,27], higher carriage rates are observed in children <5 years of age compared to older family members. Indeed, in this study approximately half of the children <3 years of age carried the organism for over half of the total study period, which is in agreement with a previous study conducted in Oxford [8]. This presumably reflects both longer duration of carriage in children, as has been previously reported [4,10] together with more frequent acquisition compared to adults.…”

“…The serotype distribution was broadly similar to that observed in two previous studies conducted in different geographical settings in England [8,18]. No additional protection against carriage was provided by the 9-valent formulation of the vaccine, as the two additional serotypes that are present in the latter (1 and 5) were again not found in this carriage study.…”

“…Infection with Pnc ranges from asymptomatic carriage through a variety of clinically mild, common mucosal infections (otitis media and sinusitis) to more serious, less common invasive presentations such as pneumonia, septicaemia and meningitis [1,2]. Pnc carriage rates are generally far in excess of disease rates-with marked variation by age, serotype and geographical area [3][4][5][6][7][8][9][10].…”

“…Few studies have been published in recent years on the descriptive epidemiology of pneumococcal carriage in the United Kingdom [8,18], and none on the dynamics of infection in a household setting to enable these evaluations to be undertaken.…”

Neutral microepidemic evolution of bacterial pathogens

“…To fit the predictions of the model to empirical data requires representative samples of the natural population. Here we use four samples from cross-sectional studies of carriage within a local area (9)(10)(11)(12) in which isolates were characterized by using multilocus sequence typing (MLST), a technique in which DNA sequences are obtained for seven housekeeping loci and the different sequences at each locus are assigned as different alleles (13). The samples are described in more detail in Table 3, which is published as supporting information on the PNAS web site.…”