AUTHOR=Li Shiyu , Li Wei , Liang Qiancai , Cao Jizhen , Li Han , Li Zhicheng , Li Anxing 

TITLE=Characterization and virulence of Streptococcus agalactiae deficient in SaeRS of the two-component system

JOURNAL=Frontiers in Microbiology

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2023.1121621

DOI=10.3389/fmicb.2023.1121621

ISSN=1664-302X

ABSTRACT=<p>There are a variety of regulatory systems in bacteria, among which the two-component system (TCS) can sense external environmental changes and make a series of physiological and biochemical reactions, which is crucial for the life activities of bacteria. As a member of TCS, <italic>SaeRS</italic> is considered to be an important virulence factor in <italic>Staphylococcus aureu</italic>s, but its function in tilapia (<italic>Oreochromis niloticus</italic>)-derived <italic>Streptococcus agalactiae</italic> remains unknown. To explore the role of <italic>SaeRS</italic> in regulating virulence in the two-component system (TCS) of <italic>S. agalactiae</italic> from tilapia, Δ<italic>SaeRS</italic> mutant strain and CΔ<italic>SaeRS</italic> complementary strain were constructed by homologous recombination. The results showed that the abilities of growth and biofilm formation of Δ<italic>SaeRS</italic> strain were significantly decreased when cultured in a brain heart infusion (BHI) medium (<italic>P</italic> &lt; 0.01). Also, the survival rate of the Δ<italic>SaeRS</italic> strain in blood was decreased when compared with the wild strain <italic>S. agalactiae</italic> THN0901. Under the higher infection dose, the accumulative mortality of tilapia caused by the Δ<italic>SaeRS</italic> strain was significantly decreased (23.3%), of which THN0901 and CΔ<italic>SaeRS</italic> strains were 73.3%. The results of competition experiments in tilapia showed that the invasion and colonization abilities of the Δ<italic>SaeRS</italic> strain were also dramatically lower than those of the wild strain (<italic>P</italic> &lt; 0.01). Compared with the THN0901, the mRNA expression levels of virulence factors (<italic>fbsB</italic>, <italic>sip</italic>, <italic>cylE</italic>, <italic>bca</italic>, etc.) in the Δ<italic>SaeRS</italic> strain were significantly down-regulated (<italic>P</italic> &lt; 0.01). <italic>SaeRS</italic> is one of the virulence factors of <italic>S. agalactiae</italic>. It plays a role in promoting host colonization and achieving immune evasion during the infection of tilapia, which provides a basis for exploring the pathogenic mechanism of <italic>S. agalactiae</italic> infected with tilapia.</p>