The Antibacterial Activity of Thymol Against Drug-Resistant Streptococcus iniae and Its Protective Effect on Channel Catfish (Ictalurus punctatus)

Lizi Yin; Chao Liang; Wenyan Wei; Shuanghui Huang; Yongqiang Ren; Yi Geng; Xiaoli Huang; Defang Chen; Hongrui Guo; Jing Fang; Huidan Deng; Weimin Lai; Shumin Yu; Ping Ouyang

doi:10.3389/fmicb.2022.914868

The Antibacterial Activity of Thymol Against Drug-Resistant Streptococcus iniae and Its Protective Effect on Channel Catfish (Ictalurus punctatus)

Front Microbiol. 2022 Jun 6:13:914868. doi: 10.3389/fmicb.2022.914868. eCollection 2022.

Authors

Affiliations

¹ Department of Basic Veterinary, College of Veterinary Medicine, Sichuan Agricultural University, Chengdu, China.
² Aquaculture Research Institute, Chengdu Academy of Agriculture and Forestry Sciences, Chengdu, China.
³ Department of Aquaculture, College of Animal Science and Technology, Sichuan Agricultural University, Chengdu, China.

Abstract

Streptococcus iniae is a zoonotic pathogen, which seriously threatens aquaculture and human health worldwide. Antibiotics are the preferred way to treat S. iniae infection. However, the unreasonable use of antibiotics leads to the enhancement of bacterial resistance, which is not conducive to the prevention and treatment of this disease. Therefore, it is urgent to find new efficient and environmentally friendly antibacterial agents to replace traditional antibiotics. In this study, the antibacterial activity and potential mechanism of thymol against S. iniae were evaluated by electron microscopy, lactate dehydrogenase, DNA and protein leakage and transcriptomic analysis. Thymol exhibited potent antibacterial activity against S. iniae in vitro, and the MIC and MBC were 128 and 256μg/mL, respectively. SEM and TEM images showed that the cell membrane and cell wall were damaged, and the cells were abnormally enlarged and divided. 2MIC thymol disrupted the integrity of cell walls and membranes, resulting in the release of intracellular macromolecules including nucleotides, proteins and inorganic ions. The results of transcriptomic analysis indicated that thymol interfered with energy metabolism and membrane transport, affected DNA replication, repair and transcription in S. iniae. In vivo studies showed that thymol had a protective effect on experimental S. iniae infection in channel catfish. It could reduce the cumulative mortality of channel catfish and the number of S. iniae colonization in tissues, and increase the activities of non-specific immune enzymes in serum, including catalase, superoxide dismutase, lysozyme and acid phosphatase. Taken together, these findings suggested that thymol may be a candidate plant agent to replace traditional antibiotics for the prevention and treatment of S. iniae infection.

Keywords: Streptococcus iniae; antibacterial mechanism; channel catfish (Ictalurus punctatus); protective effect; thymol; transcriptomic.