The in-vitro activity of colistin in gram-negative bacteria.
BACKGROUND
Colistin is a polypeptide antibiotic belonging to the polymyxins, and has been increasingly used for the treatment of multiresistant gram-negative infections. There is little current available data on the susceptibility of gram-negative bacilli to colistin, in part because susceptibility testing for colistin remains problematic, and also because the use of colistin is not widespread. This study tested clinical isolates of gram-negative bacilli for susceptibility to colistin using the reference susceptibility testing method of agar dilution.


METHODS
102 strains of gram-negative bacilli were collected over a one-year period. Antibiotic susceptibility profiles were derived from disc susceptibility testing, and organisms were identified by standard microbiological methods. Isolates were selected for inclusion in the study using susceptibility profiles and epidemiological data. Minimum inhibitory concentrations to colistin were obtained by performing agar dilution according to a standardised method.


RESULTS
30 percent of tested isolates were resistant to colistin. All Acinetobacter spp. and Escherichia coli were susceptible to colistin. Colistin resistance was detected predominantly in Stenotrophomonas maltophilia and Pseudomonas aeruginosa, but was also present in Enterobacter spp. and Klebsiella spp.


CONCLUSIONS
Colistin resistance is uncommon in the Enterobacteriaceae, but present in a significant proportion of S. maltophilia and P. aeruginosa isolates. From the results of this study, we recommend that susceptibility testing be performed whenever the clinical use of the polymyxins is considered.
Tan
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Laboratory Medicine Services, Changi General Hospital, 2 Simei Street 3, Singapore. thean_yen_tan@cgh.com.sg
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D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003091:Colistin / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D006090:Gram-Negative Bacteria / Q000187:drug effects* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests
D000900:Anti-Bacterial Agents; D003091:Colistin