Pseudomonas aeruginosa (PA) biofilm infection is clinically prevalent and difficult to eradicate. In the present work, we aimed to evaluate the in vitro and in vivo efficacy of colistin (COL)-based combinations against PA biofilm. MICs and fractional inhibitory concentration indexes (FICIs) of four antibiotics (COL, amikacin, levofloxacin, and meropenem) to bioluminescent strain PAO1, carbapenem-resistant PAO1 (CRPAO1), and clinically isolated strains were assessed. Minimal biofilm eradication concentrations (MBECs) of monotherapy and combinations were examined by counting the live bacteria in biofilm, accompanied by visual confirmation using confocal laser-scanning microscopy. An animal biofilm infection model was established by implanting biofilm subcutaneously, and the therapeutic effect was evaluated according to the change in luminescence through a live animal bio-photonic imaging system. In vitro, even combined with 4 or 8 mg/L COL, meropenem needed to reach 128 or 256 mg/L to eradicate the biofilm. Moreover, 2 mg/L COL combined with 32 mg/L amikacin or 4-8 mg/L levofloxacin could kill the PAO1 and CRPAO1 in biofilm within 24 h. In vivo, COL combined with amikacin or levofloxacin could shorten the eradication time of biofilm than monotherapy. For PAO1 biofilm, combination therapy could eradicate the biofilm in all mice on the 5th day, whereas monotherapy only eradicated biofilms in almost half of the mice. For CRPAO1 biofilm, the biofilm eradication rate on the 6th day in the COL+ amikacin, amikacin, or COL alone regimen was 90%, 10%, or 40%, respectively. COL combined with levofloxacin did not show a better effect than each individual antibiotic. COL-based combinations containing levofloxacin or amikacin were promising choices for treating PA biofilm infection. IMPORTANCE Infections associated with PA biofilm formation are extremely challenging. When monotherapy fails to achieve optimal efficacy, combination therapy becomes the last option. After evaluating multiple drug combinations through a series of experiments in vitro and in vivo, we confirmed that colistin-based combinations containing levofloxacin or amikacin were promising choices for treating PA biofilm infection. The efficacy of these combinations derives from the different bactericidal mechanisms and the bacterial susceptibility to each antibiotic. This study provided a new regimen to solve the incurable problem of biofilm by using COL combined with other antibiotics.
Keywords: Pseudomonas aeruginosa; amikacin; biofilm; colistin; levofloxacin; meropenem.