Because mcr-9 often yields colistin MICs of 2-4 mg/L, labs may report "intermediate" or "susceptible, increased exposure." This is not safe. Clinical breakpoints from EUCAST and CLSI do not account for inducible resistance.
The discovery of MCR-9 has underscored the urgent need for the development of novel antibiotics and therapeutic strategies to combat resistant bacteria. Because mcr-9 often yields colistin MICs of 2-4
For the clinical microbiologist, the lesson is clear: For the infectious disease physician, the message is sobering: a "susceptible" colistin report in a carbapenem-resistant Klebsiella may be a false promise. For public health officials, the priority is surveillance—not just of mcr-1 , but of the entire MCR family. For the clinical microbiologist, the lesson is clear:
The story of mcr-9 is a masterclass in bacterial evolution. Just when clinicians learned to watch for constitutive colistin resistance, nature produced a stealth variant that hides in plain sight. It does not shout; it whispers—until colistin arrives, at which point it shouts back. Just when clinicians learned to watch for constitutive
Like its cousins, mcr-9 lives on plasmids . Think of plasmids as USB sticks that bacteria use to swap genetic code, even between different species (e.g., Salmonella swapping with E. coli ). Because mcr-9 is often linked to other resistance genes (like those that defeat carbapenems), we aren't just losing colistin. We are creating bacteria that are immune to everything .