Bacteria that switch antibiotic resistance on and off are going undetected. Microbiologist Karin Hjort is on a mission to find out how they do it.

For decades, scientists have been puzzled by a Mysterious type of antibiotic resistance. Called "heteroresistance," it occurs when a tiny fraction of bacteria in a population can evade antibiotics, and it is almost impossible to detect with routine clinical tests. Yet some scientists think heteroresistance could be the culprit behind many antibiotic treatment failures.

Karin Hjort, a microbiologist at Uppsala University in Sweden, is a leading expert on heteroresistance. Live Science spoke with Hjort about what heteroresistance is and what implications it has for the fight against superbugs.

(This interview has been edited for clarity and length.)

Kristel Tjandra: Can you tell me what heteroresistance is?

Karin Hjort: The definition of heteroresistance is that you have, within a mainly antibiotic-susceptible population, a subpopulation that is resistant. This subpopulation should be of a certain size. In most cases, it is 10^-7 [one ten-millionth] colony-forming unit, or viable bacterial cell, per milliliter of broth. The resistance level should also be of a certain times higher — usually eightfold — than the main population.

KT: Is heteroresistance a new phenomenon? How is this different from antibiotic resistance in a general sense?

KH: It's nothing new. Think of it as a type of antibiotic resistance. All the studies we have done so far pointed in the same direction: that heteroresistant populations become resistant in the same way that a full population could become resistant. But the difference here is that this tiny subpopulation can survive higher concentrations of antibiotics than the rest of the population. If you put an antibiotic — selection pressure — that resistant subpopulation will quickly outgrow the others. And when you take away their selection pressure, the few [susceptible cells] that survived without becoming resistant will eventually outcompete the resistant ones.