Background: Usage of colistin as a growth promoter in livestock farms is a well-known factor contributing to colistin resistance. An earlier single publication from Chennai, India reported the presence of colistin resistant bacteria in raw food samples. Aim of the current study was to analyse the extent of the problem in a larger sample size.
Materials/methods: Food samples like chicken, fish, meat and vegetables were collected from multiple sources such as households, supermarkets and outlets in Chennai, between November 2018 and October 2019. Approximately one gram of minced food sample was inoculated in 9ml of Brain heart infusion broth. After overnight incubation, 1ml of suspension was enriched overnight in BHIB containing colistin (10 µg colistin disk/colistin sulfate). 50 µl of sample was inoculated in ChromID Colistin R agar plates and incubated for 24 to 48 hours. Species with intrinsic resistance to polymyxins were discarded. Colonies recovered were identified by MALDI-TOF MS and antimicrobial susceptibility testing was performed using Vitek-2 system. Colistin MIC was determined using broth microdilution method.
Results: A total of 381 food samples (251 vegetables, 52 fruits, 69 chicken, 8 mutton and 1 fish) were analysed. 93/381 food samples (39 chicken, 4 mutton, 41 vegetables and 9 fruits) carry colistin resistant isolates. 121 colR isolates were identified including 82 K. pneumoniae, 1 K. aerogenes, 8 E. coli, 7 Enterobacter spp., 2 Cronobacter spp.,2 Salmonella enterica, 19 P. aeruginosa. Klebsiella species showed high colistin MIC ranging from 4 to ≥256µg/ml and 51% of these are sensitive to all the tested antibiotics. E. coli shows colistin MIC of 4 to 16µg/ml with resistance to cotrimoxazole and fluroquinolone. All Enterobacter isolates had colistin MIC of ≥256µg/ml and were susceptible to other tested antibiotics. Other species (Cronobacter, Salmonella) shows colistin MIC 4 or 8 µg/ml. Colistin MIC of P. aeruginosa ranges from 4 to 16 µg/ml.
Conclusions: Presence of colistin resistant bacteria in a large number of community food samples is extremely worrying. The recent Indian ban on growth promotional usage of colistin is encouraging but the implementation must be systematically monitored.