Lethal Effect of Photodynamic Treatment on Persister Bacteria

Persister bacteria tolerate bactericidal antibiotics due to transient and reversible phenotypic changes. As these bacteria can limit the effectiveness of antibiotics to eradicate certain infections, their elimination is a relevant issue. Photodynamic therapy seems suitable for this purpose, but phenotypic tolerance to it has also been reported for Pseudomonas aeruginosa. To test whether any phenotypic feature could confer tolerance against both antibiotics and photoinactivation, survivors from exposures to light in the presence of methylene blue were treated with ofloxacin, an antibiotic effective on nongrowing bacteria. Susceptibility to ofloxacin was normal in these bacteria in spite of their increased ability to survive photodynamic inactivation, suggesting the absence of cross-tolerance. It thus seemed possible to use one of these treatments to eliminate bacteria which had phenotypic tolerance to the other. To test this strategy, persister bacteria emerging from ofloxacin treatments were submitted to the action of light and methylene blue while the antibiotic remained in the bacterial suspension. Persisters lost their clonogenic ability under these conditions and the effects of the treatments seemed to be synergistic. These observations suggest that photodynamic antimicrobial therapy could be used as a complement to antibiotic treatments to eliminate persister bacteria from localized infections.