Development and evaluation of a modified most probable number (MPN) method for enumerating rifampicin-resistant Escherichia coli in agricultural, food, and environmental samples

Low level of rifampicin-resistant E. coli is frequently used in food safety research as a model microorganism and is typically enumerated using traditional Most Probable Number (MPN) method. To simplify the process, we developed and validated a modified MPN method based on lactose peptone broth (LPB) containing rifampicin and bromocresol purple, the broth changes to a yellow color due to pH shifts induced by lactose fermentation from E. coli. E. coli TVS353 were prepared in following suspensions including healthy cells, injured cells, competitive bacteria with or without rifampicin resistance, and unknown natural microflora. E. coli TVS353 was quantified by traditional MPN method and our improved method. At a predicted level of 1 log CFU/mL, there was no significant difference in the MPN index of LPBR compared with TSB and TSBR in these suspensions (p > 0.05). Confirmation results generated from this new color change MPN method were analogous to traditional MPN methods.