Genotypic survey of recent beta-lactam-resistant pneumococcal nasopharyngeal isolates from asymptomatic children in Chile

Abstract

To assess pneumococcal strain variability among young asymptomatic carriers in Chile, we used serotyping, antibiotic susceptibility testing, and genotyping to analyze 68 multidrug-resistant pneumococcal isolates recovered from 54 asymptomatic children 6 to 48 months of age. The isolates represented capsular serotypes 19F (43 isolates), 14 (14 isolates), 23F (7 isolates), 6B (3 isolates), and 6A (1 isolate). Genotypic analysis, which included pulsed-field gel electrophoresis (PFGE) of chromosomal digests, penicillin binding protein (PBP) gene fingerprinting, and dhf gene fingerprinting, revealed that the isolates represented six different genetic lineages. Clear circumstantial evidence of capsular switching was seen within each of four of the genetically related sets. The majority of the isolates, consisting of the 43 19F isolates and 2 type 6B isolates, appeared to represent a genetically highly related set distinct from previously characterized pneumococcal strains. Each of three other genetically defined lineages was closely related to one of the previously characterized clones Spain(6B)-2, France(9V)-3, or Spain(23F)-1. A fifth lineage was comprised of four type 23F isolates that, by the techniques used for this study, were genetically indistinguishable from three recent type 19F sterile-site isolates from the United States. Finally, a sixth lineage was represented by a single type 23F isolate which had a unique PFGE type and unique PBP and dhf gene fingerprints.

FIG. 1

PFGE types A, B, G, K, and V encountered among the 68 NP isolates from children in Chile. Lanes: 2 and 3, PFGE subtypes V1 and V2 from Chile isolates; 4 to 8, PFGE subtypes B1 (from France^9V-1), B2 (from a 1997 U.S. project [15]), B2 (Chilean isolate), B23 (Chilean isolate), and B24 (Chilean isolate) (note that subtype B25 is not shown but differs from subtype B1 by only four bands); 9 and 10, PFGE subtypes K1 and K6 from Spain^6B-1 and a Chilean isolate, respectively; 11 and 12, subtype G1 from the 1997 U.S. study (15) and a Chilean isolate, respectively; 13, unique PFGE subtype W1; 15 and 16, PFGE subtype A1 from Spain^23F-1 and a Chilean isolate, respectively; 17, PFGE subtype A7 from a Chilean isolate. Lanes 1 and 14 contain molecular size markers.

FIG. 2

RFLP profiles for dhf, pbp1A, pbp2B, and pbp2X encountered among 68 NP isolates from children in Chile. Lanes: 2 to 4, dhf profiles 2, 1, and 9, respectively; 6 to 10, pbp1A profiles 8, 6, 12, 13, and 14, respectively; 11 to 17, pbp2B profiles 5, 3, 6, 14, 25, 22, and 26, respectively; 19 to 21, pbp2X profiles 2, 25, and 12, respectively.