Evidence for clonal expansion after antibiotic selection pressure: pneumococcal multilocus sequence types before and after mass azithromycin treatments

Abstract

Background: A clinical trial of mass azithromycin distributions for trachoma created a convenient experiment to test the hypothesis that antibiotic use selects for clonal expansion of preexisting resistant bacterial strains.

Methods: Twelve communities in Ethiopia received mass azithromycin distributions every 3 months for 1 year. A random sample of 10 children aged 0-9 years from each community was monitored by means of nasopharyngeal swab sampling before mass azithromycin distribution and after 4 mass treatments. Swab specimens were tested for Streptococcus pneumoniae, and isolates underwent multilocus sequence typing.

Results: Of 82 pneumococcal isolates identified before treatment, 4 (5%) exhibited azithromycin resistance, representing 3 different sequence types (STs): 177, 6449, and 6494. The proportion of isolates that were classified as one of these 3 STs and were resistant to azithromycin increased after 4 mass azithromycin treatments (14 of 96 isolates [15%]; P = .04). Using a classification index, we found evidence for a relationship between ST and macrolide resistance after mass treatments (P < .0001). The diversity of STs-as calculated by the unbiased Simpson index-decreased significantly after mass azithromycin treatment (P = .045).

Conclusions: Resistant clones present before mass azithromycin treatments increased in frequency after treatment, consistent with the theory that antibiotic selection pressure results in clonal expansion of existing resistant strains.

Keywords: Africa; MLST; Streptococcus pneumoniae; clonal expansion; multilocus sequence typing.

Figure 1.

Multilocus sequence type (ST) and antibiotic resistance before and after repeated mass azithromycin distributions. For each class of antibiotic resistance, STs are listed in the center, and the frequency of each ST is displayed in a bar. Pretreatment frequencies are listed on the left of the ST, and posttreatment frequencies are listed on the right. An empty bar indicates a susceptible isolate, and a colored bar indicates a nonsusceptible isolate. In the case of azithromycin, the ermB genetic resistance determinant is represented by light blue, and mefA/E is represented by dark blue; isolates with both ermB and mefA/E are represented by light and dark blue stripes, and isolates with neither resistance determinant are represented by gray.