Infections of the central nervous system (CNS) of animals and humans by opportunistic pathogens such as bacteria and fungi remain a global health issue, resulting in high morbidity and mortality rates. This challenge is further complicated by the rise in resistant microbial strains as well as emergence of new infectious pathogens, especially in developing countries. Plants (indigenous and alien) play a crucial role in the global health system for the treatment of opportunistic infections in animals and humans as they may possess natural products with medicinal properties. This study investigated the antimicrobial activity of leaf extracts of two alien invasive plants, namely Tithonia diversifolia (Hemsl) A.Gray (Asteraceae) and T. rotundifolia (Mill) F.S. Blake (Asteraceae) against some pathogens implicated in causing infections in the CNS. Phytochemicals that may be responsible for such activity and toxicity were also investigated. Antimicrobial activity was evaluated against six pathogenic bacterial and three fungal strains using the microdilution assay. Phytochemicals were detected through standard methods of analysis, while toxicity tests were carried out using the colorimetric MTT assay and the Ames test. Both species exhibited antimicrobial activity against the tested strains with minimum inhibitory concentration values ranging between 0.01 to 2.5 mg/ml and between 0.07 to 2.5 mg/ml for bacteria and fungi respectively. The leaf extract of both plants were rich in phenolics, flavonoids and tannins, although phenolics was significantly higher in T. diversifolia. Only T. diversifolia was cytotoxic while none of the extracts of both plants were mutagenic. The above results suggest that both species may be exploited as sources of antimicrobial treatment for opportunistic infections in animals and humans. However, T. diversifolia must be used with caution as it is a toxic plant and further toxicity screening is recommended.
