Abstract
Autotransporter proteins are widely present in Gram-negative bacteria. They play a pivotal role in processes related to bacterial pathogenesis, including adhesion, invasion, colonization, biofilm formation, and cellular toxicity. Bioinformatics analysis revealed that Orientia tsutsugamushi, the causative agent of scrub typhus, encodes six different autotransporter genes (scaA–scaF). Although four of these genes (scaA, scaC, scaD, and scaE) are present in diverse strains, scaB and scaF have been detected in only a limited number of strains. Previous studies have demonstrated that ScaA and ScaC are involved in the adherence of host cells. However, the putative function of other O. tsutsugamushi Sca proteins has not been studied yet. In this study, we show that scaB is transcribed and expressed on the surface of O. tsutsugamushi Boryong strain. Using a heterologous Escherichia coli expression system, we demonstrated that ScaB-expressing E. coli can successfully mediate adherence to and invasion into non-phagocytic cells, including epithelial and endothelial cells. In addition, pretreatment with a recombinant ScaB polypeptide inhibits the entry of O. tsutsugamushi into cultured mammalian cells. Finally, we also identified the scaB gene in the Kuroki and TA686 strains and observed high levels of sequence variation in the passenger domains. Here, we propose that the ScaB protein of O. tsutsugamushi can mediate both adhesion to and invasion into host cells in the absence of other O. tsutsugamushi genes and may play important roles in bacterial pathogenesis.
Highlights
Scrub typhus is a re-emerging vector-borne disease prevalent in the Asia-Pacific region (Kelly et al, 2009; Kala et al, 2020; Richards and Jiang, 2020)
We newly detected the presence of the scaB gene in the Kuroki and TA686 strains and observed high levels of sequence variation in the passenger domains. These results demonstrate that O. tsutsugamushi utilizes ScaB as a ligand protein for the adhesion and invasion of host cells and suggest that ScaB may play an important role in bacterial pathogenesis
To determine whether scaB is actively transcribed in O. tsutsugamushi Boryong during interaction with host cells, we purified total RNA from bacteria-infected L929 cells, removed any contaminating bacterial or host genomic DNA, and performed RT-PCR to amplify the scaB transcripts
Summary
Scrub typhus is a re-emerging vector-borne disease prevalent in the Asia-Pacific region (Kelly et al, 2009; Kala et al, 2020; Richards and Jiang, 2020). Rodents infected with bacterial strains closely related to O. tsutsugamushi have been found in Africa and Southern Europe (Izzard et al, 2010; Cosson et al, 2015; Kolo et al, 2016; Masakhwe et al, 2018) These reports suggest that the disease is no longer restricted to its endemic territory and is widely distributed. We newly detected the presence of the scaB gene in the Kuroki and TA686 strains and observed high levels of sequence variation in the passenger domains Taken together, these results demonstrate that O. tsutsugamushi utilizes ScaB as a ligand protein for the adhesion and invasion of host cells and suggest that ScaB may play an important role in bacterial pathogenesis
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