It's often difficult to completely eliminate a bacterial infection with antibiotics; part of the population usually manages to survive. We've known about this phenomenon for quite some time, dating back nearly to the discovery of penicillin. For more than 50 years, scientists have believed that the resistant bacteria were individuals that had stopped growing and dividing.
Up to now, in fact, it hasn't been possible to track the growth of cells before and after their exposure to antibiotics, which makes any analysis of the phenomenon quite imprecise. "Using microfluidics, we can now observe every bacterium individually, instead of having to count a population," says John McKinney, director of EPFL's Microbiology and Microsystems Laboratory (LMIC).
This new tool has revealed quite a few surprises. "We thought that surviving bacteria made up a fixed population that stopped dividing, but instead we found that some of them continued to divide and others died. The persistent population is thus very dynamic, and the cells that constitute it are constantly changing - even though the total number of cells remains the same. Because they're dividing, the bacteria can mutate and thus develop resistance in the presence of the antibiotic," explains LMIC scientist Neeraj Dhar.
This point is extremely important. "We were able to eliminate a purely genetic explanation of the phenomenon," continues Dhar. In other words, "a population of genetically identical bacteria consists of individuals with widely varying behavior. Some of them can adapt to stressors that they have not previously encountered, thanks to the selection of persistent individuals. This could lead to a revision of the entire theory of adaptation," says McKinney.