Susceptibility of bacterial species commonly found in abdominal abscesses to low-dose photodynamic therapy: Effects of methylene blue concentration, fluence rate, and fluence

Abstract

AbstractObjectivesThe objective of this study was to determine the effects of methylene blue (MB) concentration, laser fluence rate, and laser fluence on efficacy ofin vitrophotodynamic therapy (PDT) for four bacteria commonly found in human abscesses.Materials and MethodsPDT experiments were performed with four of the most common bacteria found in abdominal abscesses:Escherichia coli(E. coli),Enterococcus faecalis(E. faecalis),Staphylococcus aureus(S. aureus), andPseudomonas aeruginosa(P. aeruginosa). MB concentration was varied from 50-300 µg/mL, and laser fluence rate was varied from 1-4 mW/cm2at a fluence of 7.2 J/cm2. Higher fluence rates and fluences were explored forP. aeruginosa. Primary outcomes were reduction in colony forming units (CFU) following PDT, and measured MB uptake following drug incubation.ResultsGram-positive bacteria (E. faecalisandS. aureus) were eradicated at all MB concentrations and laser fluence rates tested. Efficacy was reduced forE. coli, but still resulted in >6 log10reduction in CFU when MB concentration was at least 100 µg/mL.P. aeruginosarequired higher fluence (28.8 J/cm2) to achieve comparable efficacy, while increasing fluence rate did not have a significant effect on PDT efficacy. MB uptake was reduced in Gram-negative species compared to Gram-positive, particularlyP. aeruginosa, although uptake was not significantly correlated with CFU reduction.ConclusionsGram-positive bacteria can be eradicatedin vitrowith low levels of MB (50 µg/mL), laser fluence (7.2 J/cm2), and laser fluence rate (1 mW/cm2).E. colishowed substantial cell killing (>6 log10CFU reduction) with these same parameters. Low MB uptake and PDT efficacy inP. aeruginosacould be overcome by increasing the laser fluence, while increasing fluence rate did not have an effect.