Expanded Gram-Negative Activity of Marinopyrrole A

Abstract

The rise of bacterial infections is a global health issue that calls for the development and availability of additional antimicrobial agents. Known for its in vitro effects on Gram-positive organisms, the drug-like small molecule marinopyrrole A was re-examined for the potential of broader efficacy against a wider array of microbes. We uncovered selective efficacy against an important subset of Gram-negative bacteria from three genera: Neisseria, Moraxella, and Campylobacter. This susceptibility is correlated with the absence of canonical LPS in these specific Gram-negative species, a phenomenon observed with other hydrophobic anti-microbial compounds. Further, when exposed to molecules which inhibit the LpxC enzyme of the LPS synthesis pathway, previously resistant LPS-producing Gram-negative bacteria showed increased susceptibility to marinopyrrole A. These results demonstrate marinopyrrole A's efficacy against a broader range of Gram-negative bacteria than previously known, including N. gonorrhea, a species identified as a priority pathogen by the WHO.

Keywords: antibacterial; bacterial infections; marinopyrrole A; susceptibility screening.

Figure 1

Susceptibility (IC₅₀) of gram-negative bacteria P. mirabilis to marinopyrrole A and gentamicin when co-treated with CHIR-090 or PF-04753299, both LpxC enzyme inhibitors. CHIR-090 and PF-04753299 are both competitive inhibitors for LpxC, an enzyme essential to the production of LPS. These inhibitors can cause reductions in LPS synthesis in Gram-negative organisms. When marinopyrrole A was co-treated with CHIR-090 (A) or PF-04753299 (B), the inhibitors caused a dose-dependent reduction in LPS synthesis which led to an increase in marinopyrrole A susceptibility of P. mirabilis (canonical LPS-producing Gram-negative bacteria). The potency of the positive control compound gentamicin is not affected in the presence of LpxC inhibitors (C,D). Average percent bacterial density was calculated using OD₆₀₀ measurements after 24 h of incubation. IC₅₀s were calculated with hill-slope modeling by normalizing to the average percent optical density of solvent-only controls exposed to the corresponding concentration of LpxC inhibitor.

Figure 2

Inhibition of bacteria Proteus mirabilis treated with LpxC enzyme inhibitors alone. Inhibition of bacteria is not impacted by treatment with LpxC inhibitors alone: optical density remains above 90% for 0.0025 and 0.025 mg/L of CHIR-090, and above 80% for 0.02 and 0.2 mg/L of PF-04753299. Average percent bacterial density calculated using OD600 measurements after 24 h of incubation. Statistical analysis done by unpaired student’s t-test, * denotes p-values < 0.001 when compared to DMSO only.