Induction of gentamicin resistance by visible light

Recent studies have demonstrated the ability of visible light or phototherapy to modify the intracellular DNA of prokaryotic and eukaryotic cells. The present study was undertaken to determine the effect of light used in phototherapy on antibiotic resistance in prokaryotic cells using tester strains of gentamicin-sensitive Escherichia coli and Staphylococcus aureus. A growing population of the tester microorganisms was inoculated on plates containing nutrient medium and gentamicin. Experiments were performed to determine the effect of blue light on the induction of gentamicin-resistant mutants. The plates were divided into two populations, one of which was illuminated, while the other was kept in the dark to serve as a control. During photoirradiation, the plates were protected from direct sunlight and air coded to maintain a temperature of 27 degrees C. The sample distance from the light source was adjusted to maintain a fluence rate (450 nm) of 141 uW/cm2. Control experiments were performed to investigate the effect of photoirradiation on the media and gentamicin. An increased frequency of mutation to gentamicin resistance was seen in the irradiated population of bacteria. The mutagenic effect was observed over a wide range of gentamicin concentrations and correlated in a linear fashion with increasing duration of photoirradiation. There was an inverse correlation between the size of the bacterial inoculum and the recovery of mutant bacteria.