Antibacterial mechanism of rhodomyrtone involves the disruption of nucleoid segregation checkpoint in Streptococcus suis

Abstract

Rhodomyrtone has been recently demonstrated to possess a novel antibiotic mechanism of action against Gram-positive bacteria which involves multiple targets, resulting in the interference of several bacterial biological processes including cell division. The present study aims to closely look at the downstream effect of rhodomyrtone treatment on nucleoid segregation in Streptococcus suis, an important zoonotic pathogen. Minimum inhibition concentration (MIC) and minimum bactericidal concentration (MBC) values of rhodomyrtone against recombinant S. suis ParB-GFP, a nucleoid segregation reporter strain, were 0.5 and 1 µg/ml, respectively, equivalent to the potency of vancomycin. Using fluorescence live-cell imaging, we demonstrated that rhodomyrtone at 2 × MIC caused incomplete nucleoid segregation and septum misplacement, leading to the generation of anucleated cells. FtsZ immune-staining of rhodomyrtone-treated S. suis for 30 min revealed that although large amount of FtsZ was trapped in the region of high fluidity membrane, it appeared to be able to polymerize to form a complete Z-ring. However, the Z-ring was shifted away from midcell. Transmission electron micrograph further confirmed disruption of nucleoid segregation and septum misplacement at 120 min following rhodomyrtone treatment. Asymmetric septum formation resulted in either generation of minicells without nucleoid, septum formed over incomplete segregated nucleoid (guillotine effect), or formation of multi-constriction of Z-ring within a single cell. This finding spotlights on antibacterial mechanism of rhodomyrtone involves the early stage in bacterial cell division process.