Antibiotic susceptibility of 50 clinical isolates of Burkholderia pseudomallei from Singapore

Sir, Burkholderia pseudomallei is the aetiological agent of melioidosis, a potentially fatal disease in humans and animals. In Singapore, a high incidence of melioidosis cases was observed in early 2004 with a high mortality rate (40%).1 Prevention of exposure is difficult as this organism is common in the soil of many parts of south-east Asia. In the absence of a vaccine, antibiotic prophylaxis for those predisposed to melioidosis could be explored. Others have shown that oral doxycycline/ciprofloxacin could prevent melioidosis in experimentally infected mice.2 We thus examined the in vitro susceptibility of 50 clinical isolates obtained from five local hospitals in Singapore, between the years 1996 and 2004, of which 31 were from the outbreak in 2004,1 to four oral antibiotics, namely amoxicillin/clavulanic acid, doxycycline, ciprofloxacin and co-trimoxazole. MICs were determined by the Etest (AB Biodisk, Solna, Sweden) method using Mueller–Hinton (MHII) agar (Oxoid, Basingstoke, UK) and the plates were read after incubation at 37°C for 24 h. The MIC of each antibiotic (in mg/L) for each B. pseudomallei isolate was reported as susceptible, intermediate or resistant as per CLSI3 guidelines: ≤8/4, 16/8 and ≥32/16 for amoxicillin/clavulanic acid, ≤4, 8 and ≥16 for doxycycline, ≤2/38, – and ≥4/76 for co-trimoxazole and ≤1, 2 and ≥4 for ciprofloxacin. E. coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were used as susceptible controls for each antibiotic. All 50 isolates tested were susceptible to amoxicillin/clavulanic acid (MIC90 6/3 mg/L; range 2.5/0.125–6/3 mg/L) and doxycycline (MIC90 1.5 mg/L; range 0.19–4 mg/L). For co-trimoxazole (MIC90 4/76 mg/L; range 0.047/0.893–6/114 mg/L), 78% of the isolates were susceptible, 16% were intermediate and 6% were resistant. Overall, 70% of the isolates were susceptible to ciprofloxacin (MIC90 2 mg/L; range 0.064–2 mg/L). Doxycycline and amoxicillin/clavulanic acid have been shown to be effective when used alone for the treatment of localized melioidosis4 or as maintenance therapy following septicaemic melioidosis. The rate of resistance to co-trimoxazole in the present study is lower than that described in Thailand (13%),5 while Ho et al.6 reported lower susceptibility (8.5% of 71 isolates) to ciprofloxacin compared with our findings. The use of fluoroquinolones for treating melioidosis has generally been controversial because of high in vitro MICs for some strains of B. pseudomallei, which exceed levels that can be achieved in serum. However ciprofloxacin is still used in the treatment of melioidosis because it has bactericidal activity, a prolonged post-antibiotic effect and has good penetration into phagocytic cells, which might eliminate or inhibit the production of glycocalyx. Our in vitro studies indicate that of the four antibiotics tested doxycycline and amoxicillin/clavulanic acid are the preferred oral prophylaxis to be considered or further explored in our local context. We have no conflicts of interest concerning the work reported in this paper. We thank HQ Medical Corps of the Singapore Armed Forces for their support in this study.