Abstract
S-layer proteins (SLPs), which are present in the external layer of certain strains of lactic acid bacteria isolated from the intestinal tract, are known to recognize and bind to specific proteins and glycan structures and contribute to adsorption to the host intestinal mucosa. The binding properties of certain SLPs are considered to exert a competitive inhibitory effect on infection because similar properties are involved in the infection mechanisms of several viruses. However, little is known regarding whether SLPs directly inhibit viral infection. In the present study, we investigated the effect of an SLP of the Lactobacillus crispatus KT-11 strain, a probiotic strain isolated from a healthy human infant, on human rotavirus infection. The impact of KT-11 lithium chloride extract (KT-11 LE), which contains SLP, on the infection of the P[4] genotype human rotavirus strain DS-1 was evaluated by monitoring the amplification of viral protein 6 (VP6) expression in human intestinal epithelial Caco-2 cells by quantitative reverse transcription-polymerase chain reaction assay after infection. KT-11 LE showed a significant suppressive effect on DS-1 infection in a dose-dependent manner with pre-infection treatment, whereas post-infection treatment was not effective. A 45 KDa protein isolated from KT-11 LE was investigated for homology using the BLAST database and was found to be a novel SLP. KT-11 SLP concentrate (KT-11 SLP) significantly inhibited the proliferative process of the DS-1 strain but not that of the P[8] genotype human rotavirus strain Wa. KT-11 SLP exerted significant inhibitory effect on DS-1 infection by pre-infection treatment even after digestion with gastric juice up to 2 h. Our results provided crucial evidence that SLPs from certain Lactobacillus strains can inhibit human rotavirus infection of intestinal epithelial cells.
Highlights
Lactic acid bacteria, including the genus Lactobacillus play a crucial role in the production of fermented dairy products such as cheese, yoghurt, and fermented milk
We focused on the antiviral effects of the S-layer protein (SLP) of Lactobacillus crispatus strain KT-11, a probiotic strain isolated from a healthy infant (Tobita et al, 2009, 2010, 2018)
The nine partial amino acid sequences of the 45 kDa peptide identified by LC-MS/MS were GTAVTDFSK, GSVNVTAGLNGR, LNVSTVASEILK, NFAAYQVSNGK, AVVADVNAALK, YNSVTVSPK, AYYEVVENGK, FINADNIDGTER, and VVTYGGTYTFK. Mascot search of these results revealed that all the sequences were included in the translated product of the SLP gene (LOCUS_02350) of KT-11 (Figure 3C)
Summary
Lactic acid bacteria, including the genus Lactobacillus play a crucial role in the production of fermented dairy products such as cheese, yoghurt, and fermented milk. There has been considerable focus on the action of the S-layer protein (SLP) of lactic acid bacteria as an antiviral component. The SLP from L. acidophilus ATCC 4356 has been reported to block Junin virus, H9N2 avian influenza virus, alphavirus, and flavivirus infections by competitively binding to dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN) (Martínez et al, 2012; Prado Acosta et al, 2019). It is unclear whether the SLP is efficacious in preventing other viruses
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