A successful, diverse disease-associated lineage of nontypeable pneumococci that has lost the capsular biosynthesis locus

Abstract

Streptococcus pneumoniae strains which fail to produce a polysaccharide capsule are commonly isolated from carriage and disease contexts. Here we use a multilocus approach to distinguish genuine nontypeable pneumococci from closely related nontypeable streptococcal isolates in a data set of 121 untypeable pneumococci from nasopharyngeal swabs and middle ear fluid of Finnish children and demonstrate that 70 of these belong to a pneumococcal lineage which has lost its capsular locus. Strains of this relatively old lineage include sequence types 344, 448, and 449. Comparison with the multilocus sequence typing database shows that strains of this lineage have spread intercontinentally and have been isolated from carriage, mucosal, and invasive disease. Furthermore we note a particular association of this nontypeable lineage with outbreaks of conjunctivitis. The diversification and geographic spread of this lineage suggest that loss of capsule is not inconsistent with long-term persistence and raise questions about the capsule's role in pneumococcal transmission.

FIG. 1.

ME tree of nontypeable pneumococci. The concatenated sequences of the STs identified among the 70 nontypeable pneumococcal isolates from Finland and those of the serotypeable strains of the pneumococcal reference set (10) were used to construct an ME tree, as described in Materials and Methods. Bootstrap confidence levels greater than 60% are shown next to the node in question. The positions of nontypeable pneumococcal STs are indicated with circles. Solid circles are nontypeable STs in the Finnish collection; open circles were STs in the Finnish nontypeable collection that also were represented in the pneumococcal reference set. A related S. mitis group isolate that has been shown to be closely related to but distinct from authentic pneumococci (10) was used as an outgroup.

FIG. 2.

ME tree of all STs in the multilocus sequence typing database and the nontypeable pneumococci. The concatenated sequences of the nontypeable pneumococcal STs and all STs in the multilocus sequence typing databases were used to construct an ME tree. The tree of all STs present in the database as of 31 March 2005 is shown at left, and the part of the large tree that includes STs within cluster A is shown together with its sister clade. Those STs which have been reported as nontypeable in the multilocus sequence typing database are indicated with solid circles. The association of nontypeable strains with the clade containing cluster A strains is clear and this group of strains is shown at right with the allelic profiles of STs shown after the ST number. The serotypes associated with those STs expressing capsule were as follows: STs 52 and 1217, 19F; ST 922, 17F; and ST 1117, 13. Some isolates are identical on the ME tree but differ in allelic profile at ddl as this locus is not used in the sequence concatenation.