A secret 'switch' that lets bacteria resist antibiotics and it's been evading Lab tests for decades.

[Grim Reaper 6]

For decades, microbiologists like Weiss thought of antibiotic resistance as something a bacterial species either had or didn't have. But "now, we are realizing that that's not always the case," he said.

Normally, genes determine how bacteria resist certain antibiotics. For example, bacteria could gain a gene mutation that enables them to chemically disable antibiotics. In other cases, genes may code for proteins that prevent the drugs from crossing bacterial cell walls. But that is not the case for heteroresistant bacteria; they defeat drugs designed to kill them without bona fide resistance genes. When they're not exposed to an antibiotic, these bacteria look like any other bacteria.

When typical antibiotic-resistant bacteria propagate, they pass resistance genes to the next generation, creating a legion of antibiotic-resistant populations that collectively counter the treatment. In contrast, bacteria in a heteroresistant population are sensitive to antibiotics. But at certain doses of an antibiotic, a small proportion of this population — as few as 1 in a million — may turn resistant and survive the drugs while the rest of the population stops growing and dies..

https://www.livescience.com/health/medicine-drugs/scientists-have-found-a-secret-switch-that-lets-bacteria-resist-antibiotics-and-it-s-been-evading-lab-tests-for-decades