Among 40 S. pneumoniae isolates recovered from middle ear fluid of children with AOM, 16 (40%) were serotype 19A. Serotype 19A was the most common serotype isolated in the study during 2004–2006.
Antimicrobial drug susceptibility testing was performed on the serotype 19A isolates. Eight (50%) of the 16 isolates of 19A were highly penicillin resistant (MIC >2.0 µg/mL), and all 8 were also multidrug resistant.
MLST of these16 S. pneumoniae isolates with serotype 19A showed that 6 (38%) were ST-320; 3 (19%) were ST-199; 2 (13%) were unreported STs now assigned ST- 2722 and ST-2704; 2 (13%) were ST-1673; and 1 (6%) each of the remaining 3 was ST-1451, ST-2265, and ST-63. Among the 8 multidrug-resistant isolates, 5 (63%) were ST-320, 1 (13%) was ST-1451, and 1 (13%) was the newly assigned ST-2722 (, ). The genetic distance of the Legacy strain is closest to 7 other multidrug-resistant strains with ST-320 and ST-1451 in our group of 40 recent isolates.
Genotypic characteristics of Streptococcus pneumoniae serotype 19A strains with multiple antimicrobial drug resistance*
Neighbor-joining tree of genetic relatedness among 40 Streptococcus pneumoniae isolates from a private pediatric practice, Rochester, New York, USA, 2004–2006. Scale bar indicates genetic linkage distance.
To determine the evolutionary relationship of the Legacy strain (ST-2722) to other strains, we constructed a neighbor-joining tree. ST-2722 is assumed to have genetic relatedness to another ST in the database when an ST has >5 of the same loci as ST-2722 (). ST-2722 belonged to a cluster in which all strains have 6 of 7 same loci. This cluster consisted of 59 (88%) strains with molecular type ST-156 and 1 strain each with ST-2722, ST-2128, ST-1227, ST-2616, ST-1893, ST-334, ST-1556, ST-2684, and ST-1697. The major dissimilarity of strains in this cluster was on loci recP (, ). The recP gene had 3 major variable sites at the 10th, 121st, and 368th bp among strains. For instance, the DNA at the 10th, 121st, and 368th bp are T, C, T, respectively, in ST-2722; C, T, T in ST-156; and C, T, C in ST-2128 ().
Figure 2 Genetic relatedness of Streptococcus pneumoniae ST-2722 (Legacy strain) to reported types that have 5 or 6 of the same loci as the Legacy strain. List of types available at the multilocus sequence typing database (http://spneumoniae.mlst.net). Scale bar (more ...)
Comparison of Streptococcus pneumoniae ST-2722 (Legacy strain) with the closest types that have 6 of the same alleles as ST-2722*
Figure 3 Variable sites (colors) in recP DNA sequences (allele) of ST-2722 (Legacy strain) and those in other Streptococcus pneumoniae that have the same 6 loci. List of types available at the multilocus sequence typing database (http://spneumoniae.mlst.net).
Forty-three (64%) of the 67 strains related to ST-2722 were penicillin resistant, and 28 (42%) of these strains were resistant to at least 1 other antimicrobial drug. Thirty-four (51%) of the 67 strains related to ST-2722 serotype 19A strains were serotype 9V; 18 (27%) were serotype 14; and 5 (7%) were serotype 11A. Among the 43 antimicrobial drug–resistant strains in this cluster, 24 (56%) were serotype 9V, and 14 (33%) were serotype 14; others were serotype 11, 15C, and 19F.