Evaluation of in-vitro colistin susceptibility and clinical profile of carbapenem resistant Enterobacteriaceae related invasive infections.

Purpose: To determine the colistin susceptibility. To compare E-test vs broth-microdilution (BMD) method for invasive carbapenem resistant Enterobacteriaceae (CRE) infections. To study treatment options for the CRE. To analyze the clinical profile and outcome of CRE infections.

Methods: Antimicrobial susceptibility testing was performed for 100 invasive CRE isolates. Gradient diffusion and BMD methods were performed to determine colistin MICs. Essential agreement (EA), categorical agreement (CA), very major error (VME), and major error (ME) were worked out between BMD method and E-test. The clinical profile of patients was analyzed.

Results: The majority of the patients suffered from bacteremia [47(47%)]. Klebsiella pneumoniae was the most common organism isolated overall as well as among bacteremic isolates. 9(9%) CRE isolates were colistin resistant by BMD of which six were Klebsiella pneumoniae. There was 97% CA between E-test and BMD. EA was 68%. VME was found in three out of nine colistin resistant isolates. No ME was found. Among the other antibiotics tested for CRE isolates, the highest susceptibility was seen to tigecycline [43(43%)] followed by amikacin [19 (19%)]. The most common underlying condition was post solid organ transplantation [36(36%)]. A higher survival rate was seen among non-bacteremic CRE infections (58.49%) than bacteremic CRE infections (42.6%). Four out of nine patients with colistin resistant CRE infections survived and had a satisfactory outcome.

Conclusion: Klebsiella pneumoniae was the most common organism causing invasive infection. Survival rates were higher in non-bacteremic CRE infections than bacteremic infections. Good CA was seen between E-test and BMD for colistin susceptibility, but the EA was poor. VME was more common than ME when E-tests were used for colistin susceptibility testing resulting in false susceptibility. Tigecycline and aminoglycosides are possible adjunct drugs for the treatment of invasive CRE infections.