Polymicrobial ventilator-associated pneumonia: Fighting in vitro Candida albicans-pseudomonas aeruginosa biofilms with antifungal-antibacterial combination therapy

The polymicrobial nature of ventilator-associated pneumonia (VAP) is now evident, withmixed bacterial-fungal biofilms colonizing the VAP endotracheal tube (ETT) surface. Themicrobial interplay within this infection may contribute for enhanced pathogenesis and exertimpact towards antimicrobial therapy. Consequently, the high mortality/morbidity ratesassociated to VAP and the worldwide increase in antibiotic resistance has promoted thesearch for novel therapeutic strategies to fight VAP polymicrobial infections. Under thisscope, this work aimed to assess the activity of mono- vs combinational antimicrobial therapy using one antibiotic (Polymyxin B; PolyB) and one antifungal (Amphotericin B; AmB)agent against polymicrobial biofilms of Pseudomonas aeruginosa and Candida albicans.The action of isolated antimicrobials was firstly evaluated in single- and polymicrobial cultures, with AmB being more effective against C. albicans and PolyB against P. aeruginosa.Mixed planktonic cultures required equal or higher antimicrobial concentrations. In biofilms,only PolyB at relatively high concentrations could reduce P. aeruginosa in both monospecies and polymicrobial populations, with C. albicans displaying only punctual disturbances.PolyB and AmB exhibited a synergistic effect against P. aeruginosa and C. albicans mixedplanktonic cultures, but only high doses (256 mg L-1) of PolyB were able to eradicate polymicrobial biofilms, with P. aeruginosa showing loss of cultivability (but not viability) at 2 h posttreatment, whilst C. albicans only started to be inhibited after 14 h. In conclusion, combination therapy involving an antibiotic and an antifungal agent holds an attractive therapeuticoption to treat severe bacterial-fungal polymicrobial infections. Nevertheless, optimization ofantimicrobial doses and further clinical pharmacokinetics/pharmacodynamics and toxicodynamics studies underpinning the optimal use of these drugs are urgently required to improvetherapy effectiveness and avoid reinfection