Introduction
Staphylococcus aureus is a Gram-positive bacterium, which often exists on human skin and the surface of medical equipment. The pathogen can also be found in the nose of healthy individuals (Grundmann et al., 2006). In the last several decades, the excessive use of antibiotics has led to multidrug resistance of the strain, which causes serious challenges to clinical medicine and disease treatment (Gatadi et al., 2019; Liu et al., 2021; Singh et al., 2021).
Extracellular matrices (EMs) are complex growth structures formed by many cells gathered together (Roy et al., 2018). This mechanism can make microorganisms adsorb more firmly on the surface of objects (Satpathy et al., 2016). The inhibitory effect of plant-derived bioactives onS. aureus has been widely studied (Cui et al., 2019; Shi et al., 2020). Cui et al. found that phenolic acids derived from rice straw can effectively inhibit the formation of EMs of S. aureus8. The combination of phenolic acid and antibiotics can make it easier for antibiotics to enter the cell, which can destroy the redox and generate peroxides in S. aureus cells, thus, achieving the lethal effect of the pathogen (Wei et al., 2022).
Strobilanthes cusia (Nees) Bremek is one of the important raw materials of traditional Chinese medicine widely distributed in Asia, commonly called “Malan,” which mostly grows in Southwest China (Liau et al., 2007). S. cusia plays an important role in maintaining people’s health (Zhang et al., 2021). As a traditional Chinese medicine,S. cusia contains a large number of alkaloids, such as indigo and indirubin (Li et al., 2021), which have anti-inflammatory, antivirus and other effects (Li & Peng, 2013; Zhou et al., 2017). Indigo showed a significant antibacterial effect in an in vitro antibacterial test. Indirubin could activate the CYP3A4 gene in the human body and plays an important role in detoxification (Kumagai et al., 2016). However, the solubility of indirubin and indigo in water is small, which limits their clinical application (Wang et al., 2021).
Indican (indoxyl-β-d-glucosidase) is the major alkaloid inS. cusia (Yu et al., 2021). It is the precursor of indigo and indirubin. Indoxyl is an intermediate product in the formation of indigo and indirubin. When plants are subjected to environmental stress, indican in vacuoles could be hydrolyzed by β-glucosidase to generate indoxyl, which could protect plants from environmental disturbance. During this process, parts of indoxyl can be oxidized to form indigo and indirubin (Zhang et al., 2020). Because indoxyl is very unstable, previous studies were mainly focused on the final products, indigo and indirubin, whereas there have been few investigations on the influence of indican and indoxyl on microbial growth.
In this study, the extraction of bioactives from the leaves of S. cusia was carried out to investigate the influence of temperature on the component bioactives. Then, the effects of different bioactive substances, especially indoxyl, on the growth and metabolism of S. aureus were evaluated using microbial growth, metabolites, intracellular redox and observing the morphological changes of cells. The results of this study can provide new ideas for improving the medicinal properties of Chinese herbal medicines such as Banlangen and put forward new directions for improving the extraction methods and conditions of their biologically active substances.