Exploring the use of carbon monoxide-releasing molecules as antimicrobials against Pseudomonas aeruginosa

Antimicrobial resistance is a major public health risk that currently contributes to hundreds of thousands of deaths every year and is predicted to continue rising if the issue is not addressed. One method of addressing this problem is through the development of new antimicrobials. Carbon monoxide-releasing molecules (CORMs) have emerged as a potential new class of antimicrobial that have shown to be effective at reducing the growth of bacterial pathogens and have synergistic effects with some currently used antibiotics. This study explored the use of two novel CORMs, namely the photo-activated Trypto-CORM and the water-triggered Ebor-CORM, against Pseudomonas aeruginosa strain PAO1. This study demonstrates that the time at which photoirradiation of Trypto-CORM occurs, relative to the bacterial growth phase, can significantly alter its antimicrobial effects. Photoirradiation of Trypto-CORM during the mid-exponential growth phase resulted in the greatest reduction in bacterial densities. While Ebor-CORM was shown to effectively reduce densities of PAO1 alone and in combination with sub-inhibitory concentrations of colistin, in the presence of 4 μg/ml colistin, Ebor-CORM provided a cytoprotective effect and facilitated growth. Additionally, Ebor-CORM was shown to be effective against a colistin-resistant mutant of P. aeruginosa with no cytoprotective effects being observed. Interestingly, both CORMs were less effective against PAO1 growing under oxygen limiting conditions. This is hypothesised to result from the expression of cytochromes with a lower affinity for carbon monoxide. Finally, both CORMs significantly altered the virulence of PAO1 and increased production of pyocyanin, however, opposing effects were observed for biofilm and pyoverdine production with Ebor-CORM significantly reducing the production of these virulence factors. It is hypothesised that the CORMs are interacting with different pathways that regulate virulence. Together these results suggest that CORMs could be effective antimicrobials, however, their interactions with bacterial pathways are complex and the mechanisms by which they induce cell death remain unknown.