Antimicrobial Susceptibility of Western Hemisphere Isolates of Burkholderia pseudomallei: Phenotypic and Genomic Analyses

Abstract

Current antimicrobial treatment recommendations for melioidosis, the disease caused by Burkholderia pseudomallei, are largely based on studies of strains isolated from the Eastern Hemisphere (EH), where most human cases are identified and reported. In this study, we evaluated the antimicrobial susceptibility of 26 strains in the CDC (Centers for Diseases Control and Prevention) collection from the Western Hemisphere (WH) isolated from 1960 to 2015. Minimal inhibitory concentration (MIC) values were measured by standard broth microdilution for 16 antimicrobials following Clinical and Laboratory Standards Institute (CLSI) guidelines. Twenty-four of the 26 WH strains were susceptible to the six antimicrobials with CLSI-defined MIC susceptibility interpretive criteria for B. pseudomallei: amoxicillin/clavulanate, ceftazidime, imipenem, doxycycline, tetracycline, and trimethoprim/sulfamethoxazole. One WH strain demonstrated intermediate amoxicillin/clavulanate resistance and another strain had intermediate resistance to tetracycline. For all antimicrobials tested, the susceptibility profiles of WH isolates were comparable with previously reported MIC results of EH strains. The overall similarities suggest that the same antimicrobials are useful for melioidosis treatment in both the WH and EH. Using in silico analyses of WH genomes, we identified a novel amino acid substitution P258S in the beta-lactamase PenA, which may contribute to decreased susceptibility to amoxicillin/clavulanate in B. pseudomallei.

Keywords: Burkholderia pseudomallei; Western Hemisphere; antimicrobial susceptibility.

FIG. 1.

The distribution of MIC values measured for 26 Burkholderia pseudomallei strains from the WH, as well as for two susceptible and two nonsusceptible B. pseudomallei strains from the EH. EH strains were used as “query” strains for comparison of susceptibility and DNA sequences. Y axis depict number of strains, X axis show MIC values (in μg/mL). Blue bars indicate low-level MICs or MICs in the susceptible range, orange bars indicate high-level MICs or MICs in the nonsusceptible range. The colors were assigned based on MIC interpretive criteria described in CLSI M45, M100, and EUCAST documents. *There are CLSI breakpoints for testing B. pseudomallei against AMC, CAZ, IPM, DOX, TET, and SXT. Because no CLSI breakpoints are currently defined for B. pseudomallei for other antibiotics tested, for interpretation of CHL results, B. pseudomallei breakpoints published by EUCAST were used; CLSI breakpoints for IPM were used for interpretations of MEM results; for interpretations of AMX and CLA alone, the individual components of the CLSI AMC drug combination breakpoints for B. pseudomallei were used; CLSI breakpoints for Enterobacteriaceae were used for ATM, CAZ/AVI, TMP, SMX, and GEN for interpretation of B. pseudomallei results; and CLSI Enterobacteriaceae ATM breakpoints were used for interpretation of ATM/AVI susceptibility, using the ATM value in ATM/AVI for this combination drug because there are no ATM/AVI breakpoints published by CLSI for any organism at this time. AMC, amoxicillin/clavulanate; AMX, amoxicillin; ATM, aztreonam; AVI, avibactam; CAZ, ceftazidime; CHL, chloramphenicol; CLA, clavulanate; CLSI, Clinical and Laboratory Standards Institute; DOX, doxycycline; EH, Eastern Hemisphere; EUCAST, European Committee on Antimicrobial Susceptibility Testing; GEN, gentamicin; IPM, imipenem; MEM, meropenem; MIC, minimal inhibitory concentration; SMX, sulfamethoxazole; SXT, trimethoprim/sulfamethoxazole; TET, tetracycline; TMP, trimethoprim; WH, Western Hemisphere.

FIG. 2.

Diagram representing Burkholderia pseudomallei PenA protein. The positions of conserved regions are shown as red bars with labels on top of the diagram. The positions of known mutations for ceftazidime resistance—C69Y,^, P167S^, and D240G, and for clavulanic acid resistance—S72F,^, are shown with red arrows at the bottom of the diagram. The position of the newly identified substitution is shown with a black arrow at the bottom of the diagram. Amino acids are numbered according to the Ambler scheme.