The Relationship Between Heavy Metal Concentration and Antibacterial Efficacy of Crude Guava (Psidium guajava L.) Leaf Extracts

Abstract

This study aimed to investigate the impact of heavy metals, specifically iron (Fe), lead (Pb), and zinc (Zn), on the antimicrobial efficacy of guava leaves collected from both polluted and non-polluted areas. Topsoil samples were collected from both sites and analyzed for heavy metal content using atomic absorption spectroscopy. The results revealed significantly higher levels of iron in the polluted site than the non-polluted site. In contrast, zinc concentrations were found to be higher in the non-polluted site. Mature guava leaves were also collected from each site, from which crude extracts of guava leaves were prepared by 48-hr maceration extraction in 95% ethanol. Intriguingly, the crude extract from the polluted site exhibited larger zones of inhibition against S. aureus and B. subtilis isolates. Linear regression models were then constructed to assess the effects of heavy metal concentrations on the inhibitory activity of the extracts. The analysis revealed a direct and significant effect of Fe on B. subtilis inhibition. In contrast, Zn exhibited a significant inverse effect on the inhibition of B. subtilis. These findings suggest that heavy metals have individual effects on the inhibitory activity of guava leaves. Moderate levels of heavy metal-induced stress is known to enhance the production of plant secondary metabolites, hence, strengthening antibacterial activity. Meanwhile, metal deficiency or toxicity creates the opposite effect. Therefore, this warrants further investigation on the heavy metal profile of other local medicinal plants to elucidate the effects of pollution on their safety and antibacterial efficacy.