Evaluation of risk factors for colistin resistance among uropathogenic isolates of Escherichia coli and Klebsiella pneumoniae: a case-control study.

Introduction: The last few years have seen the emergence of multi-drug resistant (MDR) Gram-negative infections, which are associated with high morbidity and mortality. The indiscriminate use of colistin has led to the development of resistance, which can be diagnosed effectively by broth microdilution. Studies from India are limited, and this study was conducted in order to determine the prevalence and risk factors associated with colistin resistance.

Methods: Urine samples from patients admitted with urinary tract infection (UTI), growing MDR Escherichia coli and Klebsiella pneumoniae, were tested for the minimum inhibitory concentration (MIC) of colistin by broth microdilution. Isolates with an MIC >2 µg ml (resistant) were subjected to polymerase chain reaction (PCR) for the mcr1, mcr2 and mgrB genes. A case-control study with 21 cases (resistant) and 42 matched controls (sensitive) was designed to evaluate risk factors and outcomes (recurrent UTI, readmission and hospital stay >2 weeks).

Results: Two hundred and fifty MDR isolates (E. coli=142/2319 and K.pneumoniae=108/775) from 216 patients were selected from the 25 046 isolates screened. Twenty-five isolates (20 K.pneumoniae and 5 E. coli) were resistant to colistin, with a prevalence of 3.52  % in E. coli and 18.5  % in K. pneumoniae among the MDR isolates. PCR for the mcr1 and mcr2 genes was negative. Multivariate regression showed that multiple episodes of hospitalization, hospital stay >2 weeks, exposure to >three antibiotic classes and abnormality/surgery of the lower urinary tract were the significant risk factors for colistin resistance. Previous use of colistin and colistin resistance had a significant effect on all outcomes.

Conclusions: K. pneumoniae show six times higher prevalence of colistin resistance than E. coli, and the emergence of resistant organisms has led to an increase in morbidity in infected patients.