In vitro antimicrobial photodynamic inactivation of multidrug-resistant Acinetobacter baumannii biofilm using Protoporphyrin IX and Methylene blue

Background: Acinetobacter baumannii is a challenging pathogen due to the rapid development of antimicrobial resistance and biofilm formation. The objective of this study was to evaluate the effect of antimicrobial photodynamic inactivation against biofilms of multidrug-resistant A. baumannii isolated from clinical, abattoir and aquatic sources. Methods: The isolates were tested for susceptibility to imipenem, meropenem, tigecycline and colistin using autoSCAN-4 automated system and rechecked by the E-test. Methylene blue, Protoporphyrin IX, and a halogen lamp were used in the in vitro assay against biofilms of the isolates. The antimicrobial photodynamic inactivation was assessed by counting colony-forming units (CFU). Results: The isolates from abattoir and aquatic sources were resistant to carbapenems ( > 64 mu g/mL) but susceptible to tigecycline (2 mu g/mL) and colistin (Abattoir, 0.35 mu g/mL and Aquatic, 0.24 mu g/mL), whereas the clinical isolate was susceptible to only colistin (0.5 mu g/mL) using the E-test. The log survival percentages of the control group at a concentration of 20 mu M were 5 x 10(-6) % for Protoporphyrin IX and 2 x 10(-6) % for Methylene blue. Therefore, Methylene blue showed higher bacterial reduction of 7.0 log(10) colony forming units than 6.0 log(10) for Protoporphyrin IX. No significant difference was observed with respect to the origin of isolates and the minimum inhibitory concentrations. Conclusion: The results indicate that antimicrobial photodynamic inactivation could be an alternative strategy for the control of infections caused by multi-drug resistant A. baumannii by significantly reducing biofilm growth at a sub-lethal concentrations.