Penicillin susceptibility and epidemiological typing of invasive pneumococcal isolates in the Republic of Ireland

Abstract

A national study was undertaken to investigate the incidence of invasive pneumococcal disease in the Republic of Ireland and to examine the associated isolates. In 1999, 144 S. pneumoniae isolates, all recovered from cases of invasive disease, were received from 12 microbiology laboratories. The incidence of invasive pneumococcal disease was estimated to be 6.6/100000 population. All isolates were analyzed for serotype, penicillin susceptibility, chromosomal relatedness (by using pulsed-field gel electrophoresis [PFGE]), and penicillin-binding protein (pbp) fingerprinting. Several findings of note were observed regarding the pneumococcal population in Ireland. First, isolates of 25 different serotypes were represented, with serotypes 14, 9V, 8, 5, 4, and 3 being the most common. This finding, together with the pbp fingerprinting and PFGE typing results, indicated the clonal spread of strains of these serotypes in Ireland. Second, 27 (18.7%) isolates had reduced susceptibility to penicillin, and 74% of these were serotype 9V. Of these, 80% appeared to belong to the same clone. This could suggest the spread of the international Spanish/French 9V penicillin-resistant clone into Ireland. Third, nine different pbp genotypes were identified, four of which were new. Two pbp genotypes accounted for the majority of isolates dividing them according to their penicillin susceptibility status but irrespective of serotype and PFGE type. This is strong evidence for the occurrence of horizontal transfer of pbp genes between strains, observed with both penicillin-susceptible and penicillin-nonsusceptible isolates. Fourth, there was evidence of serotype transformation since isolates, indistinguishable by pbp fingerprinting and PFGE typing, expressed different capsular types.

FIG. 1.

DNA fingerprint patterns of the pbp1A (n = 3), pbp2B (n = 5), and pbp2X (n = 10) genes represented among the 144 invasive pneumococcal isolates. Lane numbers indicate the pbp codes. A4, B7, and X10 are the patterns obtained with the ATCC 49619 strain for the pbp1A, pbp2B, and pbp2X genes, respectively. Lane M denotes the 100-bp DNA ladder size markers electrophoresed on each gel, the sizes of which are indicated.

FIG. 2.

Genetic relatedness of 144 isolates of S. pneumoniae and the ATCC 49619 strain (indicated by an asterisk) based on PFGE fingerprinting. Codes I to XXIV refer to clusters of pneumococcal isolates displaying PFGE types with homologies greater than 77%. The number of isolates within each cluster is also indicated.