# Zinc-enhanced activity of an antimicrobial halogenated phenazine against Streptococcus mutans and other gram-positive bacteria

## TL;DR

A halogenated phenazine compound, HP-29, shows strong antimicrobial activity against Gram-positive bacteria, and its effect is greatly enhanced when combined with zinc.

## Contribution

The discovery that zinc enhances the antimicrobial activity of HP-29 against Gram-positive bacteria, including Streptococcus mutans, is novel.

## Key findings

- HP-29 inhibits the growth of Streptococcus mutans and other Gram-positive bacteria in a dose-dependent manner.
- Zinc supplementation significantly enhances the antimicrobial effect of HP-29, while iron, cobalt, and manganese supplementation reduce it.
- Transcriptome analysis reveals that HP-29 disrupts metal ion regulation, with zinc amplifying this disruption.

## Abstract

Halogenated phenazine (HP) compounds have shown promise as antimicrobial agents, particularly against biofilm-associated Gram-positive pathogens. Among these compounds, HP-29 demonstrates potent activity against methicillin-resistant Staphylococcus aureus by inducing rapid iron starvation. As maintenance of trace metals homeostasis is critical for the survival of Streptococcus mutans, this study investigated the antimicrobial efficacy of HP-29 and the impact of metal supplementation on this major oral and occasional systemic pathogen. As anticipated, HP-29 inhibited S. mutans growth in a dose-dependent manner, with iron supplementation alleviating the antimicrobial effect. Cobalt, manganese, or nickel supplementation also mitigated the inhibitory activity of HP-29, but, unexpectedly, the addition of zinc greatly enhanced HP-29 antimicrobial activity. This zinc-driven potentiation of HP-29 extended to other Gram-positive pathogens, including Enterococcus faecalis and S. aureus. Inductively coupled plasma mass spectrometry analysis revealed that intracellular iron content decreased significantly following exposure to HP-29. When combined with zinc, HP-29 triggered a 5-fold increase in intracellular zinc and reduced manganese levels by ~50%. Transcriptome analysis showed that HP-29 treatment, with or without zinc, altered expression of genes linked to iron and manganese uptake as well as zinc efflux, suggesting broad disruption of metal ion regulation. These findings highlight HP-29 as a potent antimicrobial that broadly impairs metal homeostasis. The unexpected synergy of HP-29 with zinc points toward a promising dual-agent therapeutic strategy against Gram-positive pathogens.

Widespread development of antibiotic resistance has created a constantly moving target when combating infectious microbes. Here, we further explore an antimicrobial halogenated phenazine, HP-29, which is effective against Gram-positive bacteria through disruption of intracellular trace metal equilibrium. We showed that HP-29 inhibits growth of the oral and systemic pathogen Streptococcus mutans and that its antimicrobial effect is greatly potentiated by the addition of zinc. The zinc-mediated enhancement of HP-29’s efficacy was also observed in other Gram-positive pathogens, including Enterococcus faecalis and Staphylococcus aureus. Intracellular trace metal quantifications and transcriptome analysis confirmed that HP-29 treatment impairs trace metal homeostasis, an outcome that is exacerbated when S. mutans is treated with both HP-29 and zinc. The observed synergy of HP-29 with zinc supports the development of a dual-agent therapeutic strategy against Gram-positive pathogens.