Synergistic Effect between Usnic Acid and Polymyxin B against Resistant Clinical Isolates of <i>Pseudomonas aeruginosa</i>
Sérgio Dias da Costa Júnior, Wagner Roberto Cirilo da Silva, Adriana Maria Costa Marques da Silva, Maria Amélia Vieira Maciel, Isabella Macário Ferro Cavalcanti
Abstract
The present study aimed to characterize the susceptibility profile of Pseudomonas aeruginosa and Acinetobacter spp. clinical isolates to polymyxin B in a public hospital in Recife‐PE, Brazil, between the years of 2018 and 2019, as well as to search for the presence of the mcr ‐1 gene and evaluate the interaction between polymyxin B and usnic acid against these isolates. The strains were identified using the BD Phoenix™ automated system and the agar‐spot test was used to determine the susceptibility profile to polymyxin B. The minimum inhibitory concentrations (MICs) of usnic acid and polymyxin B were determined through the broth microdilution method according to the Clinical and Laboratory Standards Institute (CLSI). Subsequently, Polymerase Chain Reaction (PCR) was performed to detect the mcr ‐1 gene in the isolates. The interaction between usnic acid and polymyxin B was evaluated by the Checkerboard assay. Among 34 isolates of P. aeruginosa , 26.5% (9/34) were positive for the polymyxin B agar‐spot test, and 11.8% (4/34) presented an intermediate susceptibility (MIC = 4 μ g/mL), while 14.7% (5/34) presented antimicrobial resistance with MIC values ranging from 8 to 32 μ g/mL. Among 38 isolates of Acinetobacter spp., 13.2% (5/38) were positive for the polymyxin B agar‐spot test and all of them were resistant to polymyxin B with a MIC value > 32 μ g/mL. The mcr ‐1 gene was not detected in the clinical isolates. Regarding usnic acid, it presented a moderate antibacterial activity against two P. aeruginosa isolates (MIC = 250 μ g/mL) and no activity was detected against the others. A synergistic effect between usnic acid and polymyxin B was observed against three clinical isolates of P. aeruginosa which were resistant to polymyxin B (FICI ≤ 0.5). Therefore, it was possible to observe that usnic acid is a promising candidate to be used in combination with polymyxin B against infections caused by resistant P. aeruginosa .