New insight into rhodomyrtone’s antibacterial mechanism reveals disturbance of nucleoid segregation with septum misplacement in Streptococcus suis

Rhodomyrtone has been recently demonstrated to possess a novel antibiotic mechanism of action involving multiple targets resulting in interference of several bacterial biological processes including bacterial cell division. The present study aims to further elucidate the effect of rhodomyrtone on Streptococcus suis nucleoid segregation. S. suis, an important zoonotic pathogen. MIC and MBC values of rhodomyrtone against recombinant Streptococcus suis ParB-GFP, nucleoid segregation reporter strain, were 0.5 and 1 µg/ml, equivalent to the potency of vancomycin antibiotics Using live-cell imaging fluorescence, we demonstrated that the compound (2 x MIC) caused incomplete nucleoid segregation and septum misplacement. Subsequently, it led to the generation of anucleated cells. FtsZ immune-staining of rhodomyrtone (2 x MIC) treated S. suis for 30 minutes revealed that, although, large amount of FtsZ was trapped in the region of high fluidity membrane, it was able to polymerize to form the complete Z-ring. However, the Z-ring was shift away from midcell. Transmission electron micrograph further confirmed the presence of a disturbance on S. suis nucleoid segregation and septum misplacement following 120 minutes of 2 x MIC rhodomyrtone treatment. The asymmetric septum formation resulted in either generation of minicell without nucleoid or septum form over incomplete segregated nucleoid (guillotine effect) as well as formation of multi-constriction of Z-ring within a single cell. Moreover, apparent cell morphology alteration and cell lysis were found. This finding demonstrated that the disturbance of the bacterial nucleoid segregation checkpoint was part of early antibacterial mechanism of rhodomyrtone. However, more profound functional consequence on nucleoid segregation defect following rhodomyrtone treatment yet remains to be elucidated.