Two other men in the same hospital ward also were infected with drug-resistant E. coli. The researchers don’t say whether they think the pet shop worker infected them, but it’s a possibility.

The mcr-1 gene can be passed from one bacteria to another if it sits on a piece of material called a plasmid, which makes it easy for one species of bacteria to pass it along to another species of bacteria.

Mcr-1 protects bacteria against an antibiotic called colistin, which isn’t used much because it damages the kidneys and has to be injected. But colistin is often the only antibiotic that will work against a type of superbug called CRE, for carbapenem-resistant Enterobacteriaceae. Doctors call it an antibiotic of last resort.