Cell is something to watch out for!

iNature Enterococci are part of the human microbiome and a major cause of multidrug-resistant infections
.

On March 7, 2022, Min Dong, Jonathan Abraham and Michael S.
Gilmore of Harvard Medical School jointly published a research paper entitled "Emerging enterococcus pore-forming toxins with MHC/HLA-I as receptors" in Cell Online.
A family of enterococcal pore-forming toxins (Epxs) has been identified in isolated strains of E.
faecalis (E.
faecium), and E.
faecium (E.
hirae)
.

Structural studies have shown that Epxs form a branch of the β-barrel pore-forming toxin, with a β-barrel protrusion (called the apical region) located on top of the cap domain
.

Through a genome-wide CRISPR-Cas9 screen, this study identified the human leukocyte antigen class I (HLA-I) complex as a receptor for two members (Epx2 and Epx3), both of which recognize human, equine, MHC-I of porcine origin and bovine origin, while the recognition of MHC-I of murine origin is weak
.

Interferon exposure stimulates MHC-I expression, sensitizing human cells and intestinal organoids to Epx2 and Epx3 toxicity
.

Co-culture with Epx2-carrying E.
faecalis damages human peripheral blood mononuclear cells and intestinal organoids, and this toxicity is neutralized by Epx2 antibodies, demonstrating toxin-mediated virulence of Epx-carrying E.
faecalis
.

Since the landmark 1950s discovery that diphtheria toxin is carried by temperate bacteriophages, the transformation of harmless bacteria into pathogens by mobile elements has been described
.

Enterococci are part of commensal bacteria in the gut of terrestrial animals
.

E.
faecalis and E.
faecium have been the leading cause of multidrug-resistant (MDR) infections since the 1970s
.

Recent studies have also reported the role of E.
faecalis in alcoholic liver disease and the role of E.
niger, the third most abundant Enterococcus species in the human microbiota, in modulating immune responses to tumor antigens
.

Enterococci are known for their intrinsic and recently acquired resistance to antibiotics, resulting in high mortality from infections that are difficult to eradicate
.

Some E.
faecalis isolates express a post-translationally modified antimicrobial peptide bacteriocin called cytolysin, which lyses bacteria and eukaryotic cells and promotes pathogenesis
.

However, Enterococcus is not known to express any potent family of protein toxins with established specificity against human and animal cells
.

Pore-forming toxins (PFTs) are the most common class of bacterial toxins
.

They are produced as soluble monomers, oligomerize on the cell surface and form transmembrane pores
.

Various PFTs have evolved to disrupt epithelial barriers, disable immune cells and damage tissues
.

PFTs can be divided into α-PFTs that transmembrane pores composed of α helices and β-PFTs that transmembrane pores composed of β barrels
.

β-PFTs also include two classes of small β-barrel PFTs—the hemolysin/aerolysin family and the macropore-forming cholesterol-dependent cytolysin family
.

The well-studied Staphylococcus aureus α-hemolysin (Hla, also known as αHL or α-toxin) is the prototype of the hemolysin family
.

It is produced as a 292-residue monomer and assembles into a mushroom-shaped heptamer pore
.

Other hemolysin family members include Staphylococcus aureus leukocidin toxin, necrotizing enteritis B-like toxin (NetB), beta and delta toxins of Clostridium perfringens, and Vibrio cholerae cytolysin toxin
.

The crystal structures of these toxins show highly conserved conformations in their monomeric state and assembly pores, with one variant consisting of two-component leukocidin toxins formed from four units of each component in alternating order the heterooctameric pores
.

The prototypic aerolysin also forms a heptameric β-barrel transmembrane pore, but differs from Hla in overall domain arrangement
.

 Although small β-barrel PFTs can nonspecifically form pores at high concentrations in vitro, specific host protein receptors have been identified, establishing a critical role for the receptors in toxin host species and cell type selectivity: e.
g.
, S.
aureus leukocidin toxins Panton-Valentine leukocidin (PVL), γ-haemolysin CB (HlgCB), and leukocidin A/B (LukAB) recognize the human orthologs of their respective receptors, but not the murine orthologs thing
.

As part of a surveillance study of Enterococcus genomes collected from animals and the environment, this study identified two uncharacterized small β-barrel PFTs that are homologous to members of the hemolysin family
.

A search of public databases revealed a β-barrel PFT family in E.
faecalis, E.
faecium and E.
niger
.

Structural studies have shown that these toxins are a subclass of the hemolysin family
.

Using a genome-wide CRISPR-Cas9 screen, the study identified the human leukocyte antigen class I (HLA-I) complex as a receptor for two of the toxins, both of which can recognize human, equine, porcine, bovine source of MHC-I, but weaker recognition of mouse-derived MHC-I
.

This study further shows that toxin-carrying E.
faecalis strains induce peripheral blood mononuclear cell (PBMC) death and damage intestinal organoids in a toxin-dependent manner during co-culture, demonstrating toxin-mediated virulence
.

Dr.
Xiaozhe Xiong, Dr.
Songhai Tian of Harvard Medical School Boston Children's Hospital, Dr.
Pan Yang of Harvard Medical School, and Dr.
Francois Lebreton of Walter Reed National Military Medical Center are the co-first authors of the article, Michael S.
Gilmore, Jonathan Abraham of Harvard Medical School He and Professor Dong Min are the co-corresponding authors
.

Other collaborators include Dr.
Yin Linxiang, Dr.
Chen Pengsheng, Dr.
Zhang Jie, Boston Children's Hospital, Dr.
Qi Wanshu, Dr.
Wu Hao, Professor David T.
Breault, Professor Chen Hong, and Professor Wu Hao, and Ashlee Earl of the Broad Institute Ph.
D.
, Dr.
Kuanwei Sheng from the Weiss Institute for Biologically Inspired Engineering, Harvard University, Huan Bao, Research Fellow, Scripps Institute, and Professor Jianbin Ruan, University of Connecticut School of Medicine
.

  Reference message: https://#%20