Detection and Characterization of Colistin Heteroresistance in an Enterobacter cloacae ST48 Strain Isolated From Raw Milk in Algeria.

An Enterobacter cloacae ST48 isolate exhibiting a colistin-heteroresistant phenotype was obtained from raw cow milk in Algeria. Two subpopulations were detected, both carrying the blaTEM-1 gene. While one subpopulation (E. cloacae TS34-a) was highly susceptible to colistin (MIC 0.06 µg/mL), the other (E. cloacae TS34-b) was resistant (MIC 8 µg/mL). Population analysis profiling (PAP) confirmed heteroresistance. PCR amplification revealed the absence of mcr genes in both subpopulations. However, multiple non-synonymous mutations, including a stop-codon mutation in phoP gene, were found in the resistant subpopulation. This is the first report of colistin-heteroresistant E. cloacae isolated from raw milk, underscoring the importance of vigilance in the dairy industry and food safety.