Emergence of Multidrug-Resistant Pneumococcal Serotype 35B among Children in the United States

Abstract

Streptococcus pneumoniae serotype 35B is a nonvaccine serotype associated with high rates of penicillin nonsusceptibility. An increase in the proportion of multidrug-resistant (MDR) 35B isolates has recently been reported. The genetic events contributing to the emergence of MDR serotype 35B are unknown. The sequence type (ST) composition of 78 serotype 35B isolates obtained from pediatric patients with invasive pneumococcal disease from 1994 to 2014 and 48 isolates from pediatric patients with otitis media (noninvasive) from 2011 to 2014 was characterized by multilocus sequence typing (MLST). The most common STs were ST558 (69.2%), ST156 (10.3%), and ST452 (3.8%). Two major clonal complexes (CC), CC558 and CC156, were identified by eBURST analysis. Overall, 91% (71/78) of isolates were penicillin nonsusceptible and 16.7% (13/78) were MDR. Among all invasive serotype 35B isolates, MDR isolates increased significantly, from 2.9% (1/35) to 27.9% (12/43) (P = 0.004), after the 13-valent pneumococcal conjugate vaccine (PCV13) was introduced. All CC156 isolates were identified after the introduction of PCV13 (0/35 [0%] before versus 9/43 [20.9%] after; P = 0.003) and were MDR. All CC156 isolates had similar antimicrobial susceptibility patterns; in contrast, high variability in antimicrobial susceptibility was observed among CC558 isolates. The distributions of CC558 and CC156 among invasive and noninvasive isolates were not different. The increased prevalence of MDR serotype 35B after the introduction of PCV13 was directly associated with the emergence of ST156. Genotyping suggests that capsular switching has occurred between MDR vaccine serotypes belonging to ST156 (e.g., 9V, 14, and 19A) and serotype 35B.

Keywords: Streptococcus pneumoniae; multidrug resistance; otitis media; pneumococcal disease; pneumococcal vaccine; pneumococcus; serotype 35B.

FIG 1

Number of serotype 35B invasive isolates by year from 1994 to 2014.

FIG 2

eBURST analysis of 78 invasive serotype 35B isolates. Each sequence type (ST) is represented by a circle or node. The size of the circle corresponds to the number of isolates belonging to that ST. STs sharing 6 alleles (single-locus variant [SLV]) or 5 alleles (double-locus variant [DLV]) are connected by a black line. New ST designations are pending.

FIG 3

Sequence type (ST) distribution before and after the introduction of PCV13. *, category includes a double-locus variant.

FIG 4

eBURST analysis of penicillin MIC distribution and multidrug-resistant serotype 35B isolates. (A) Penicillin MIC distribution among serotype 35B sequence types (STs). Yellow, 0.008 to 0.06 μg/ml; orange, 0.12 to 0.5 μg/ml; fuchsia, 1 to 2 μg/ml; brown, ≥4 μg/ml. (B) Distribution of multidrug-resistant (MDR) isolates among serotype 35B STs. Blue, non-MDR isolates; red, MDR isolates. New ST designations are pending.