Bacterial Growth: The SagA of E. faecium
An enzyme that remodels the cell wall of Enterococcus faecium helps these gut bacteria to divide and generate peptide fragments that enhance the immune response against cancer.
-
Download
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Bacterial Growth: The SagA of E. faeciumeLife 13:e97277.https://doi.org/10.7554/eLife.97277 - Cite
- CommentOpen annotations (there are currently 0 annotations on this page).
- Department of Genomic Medicine, Division of Cancer Medicine, The University of Texas MD Anderson Cancer Center, United States
Bacteria and other microbes residing in our intestinal tract – known as gut microbiota – are important for maintaining health and normal immunity. Among these is a group of lactic acid-producing bacteria called Enterococcus that inhibit the growth of harmful pathogens and aid digestion. As a result, these species are routinely found in probiotic supplements and fermented foods that attempt to improve gut health.
Enterococci can also become opportunistic pathogens capable of causing infections. Indeed, two of the most frequently identified species of Enterococci – E. faecium and E. faecalis – are both known to acquire antibiotic resistance. However, some strains of E. faecium have also been found to positively impact human health. For example, enrichment of E. faecium has been associated with an improved response to various types of cancer immunotherapy (Matson et al., 2018; Routy et al., 2018). Presence of these bacteria has also been shown to help prevent infections in the gut, and E. faecium are widely used as safe probiotics (Bhardwaj et al., 2010; Ishibashi et al., 2012).
E. faecium protects the gut from infections by releasing a hydrolase enzyme called secreted antigen A (SagA), which is involved in remodeling its cell wall. SagA breaks down peptidoglycans, the main component of the E. faecium cell wall, to produce small fragments called muramyldipeptides (MDPs; Kim et al., 2019). These MDPs activate receptors known as NOD2 in the host’s immune cells, leading to improved immunity in the gut.
Recent studies in mouse models showed that the MDPs generated by SagA could also increase anti-tumor immunity and improve cancer immunotherapy outcomes (Griffin et al., 2021). However, the role SagA plays in the bacteria itself remained unknown as it was believed that E. faecium needed this protein to survive. Now, in eLife, Howard Hang and colleagues – including Steven Klupt, Kyong Tkhe Fam and Xing Zhang as joint first authors – report that SagA does not affect the viability of E. faecium, but is required for cell wall remodeling and cell separation during replication (Klupt et al., 2024).
The team (who are based at Scripps Research) genetically modified E. faecium to generate a strain in which the gene for SagA was deleted. The growth of this bacterial strain was then compared to: (i) a wild-type strain, (ii) a mutant strain with inactive SagA, and (iii) a ‘complementation’ strain in which the gene for SagA had first been deleted and then re-expressed. Bacteria that lacked the gene for SagA or had an inactive version of the enzyme grew more slowly in liquid culture than wild-type E. faecium; but this was restored in the complementation strain. Experiments also showed that deleting the gene for SagA made E. faecium more sensitive to various antibiotics that target the bacterial cell wall. These exciting results raise the possibility of targeting SagA and other peptidoglycan hydrolase enzymes to overcome antibiotic resistance.
Transmission electron microscopy (TEM) – which uses a beam of electrons to generate images with ultrahigh resolution – revealed that E. faecium strains lacking the gene for SagA had more difficulty separating during replication. This caused the bacteria to cluster together, which could impair their growth. Cryo-electron tomography – a modification of TEM which can create three dimensional images of cells – was then used to quantify cell morphology parameters such as thickness of the cell wall and septum (a transient structure which helps to separate dividing cells). This revealed that deleting the gene for SagA alters the placement and projection angle of the new cell wall; however, this morphology was restored in the complementation strain.
To investigate the functional implications of deleting the gene for SagA, Klupt et al. used mass spectrometry to analyze specific components of the cell wall. They found that the strain in which the gene for SagA has been deleted generated fewer MDPs than wild-type E. faecium, which led to poor NOD2 signaling. Mouse models of cancer also did not respond to immunotherapy when they were colonized with the deficient strain. Finally, Klupt et al. demonstrated that deleting the gene for SagA reduced the population of cancer-targeting immune cells within the tumor.
Taken together, the findings show that catalytically active SagA is required for cell wall remodeling and cell separation in E. faecium, and its production of MDPs is required to mount an effective anti-tumor immune response (Figure 1). Furthermore, deleting the gene that codes for SagA impairs bacterial growth and increases sensitivity to antibiotics. Similar observations have been made in other bacteria that express peptidoglycan hydrolase enzymes, albeit via different mechanisms (Frirdich and Gaynor, 2013). This suggests that these enzymes could potentially be important antibacterial targets.
Figure 1
Reduced remodeling of bacterial cell walls impacts the immune response to cancer.
(A) SagA is an enzyme that helps to remodel the cell wall of E. faecium by breaking down its main component, peptidoglycan. This makes the bacteria more likely to become resistant to antibiotics …
Notably, hydrolases from other immunotherapy-promoting Enterococcus species share more than 90% sequence homology in their catalytic domain. Future studies investigating how bacteria regulate the activity of these hydrolases could lead to better treatments for cancer and combating antibiotic resistance.
References
-
Safety assessment and evaluation of probiotic potential of bacteriocinogenic Enterococcus faecium KH 24 strain under in vitro and in vivo conditionsInternational Journal of Food Microbiology 141:156–164.https://doi.org/10.1016/j.ijfoodmicro.2010.05.001
-
Peptidoglycan hydrolases, bacterial shape, and pathogenesisCurrent Opinion in Microbiology 16:767–778.https://doi.org/10.1016/j.mib.2013.09.005
-
Purification and characterization of multiple bacteriocins and an inducing peptide produced by Enterococcus faecium NKR-5-3 from Thai fermented fishBioscience, Biotechnology, and Biochemistry 76:947–953.https://doi.org/10.1271/bbb.110972
Article and author information
Author details
Publication history
Copyright
© 2024, Prasad and Jenq
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Download links
Further reading
-
- Immunology and Inflammation
- Microbiology and Infectious Disease
The members of the Mycobacterium tuberculosis complex (MTBC) causing human tuberculosis comprise 10 phylogenetic lineages that differ in their geographical distribution. The human consequences of this phylogenetic diversity remain poorly understood. Here, we assessed the phenotypic properties at the host-pathogen interface of 14 clinical strains representing five major MTBC lineages. Using a human in vitro granuloma model combined with bacterial load assessment, microscopy, flow cytometry, and multiplexed-bead arrays, we observed considerable intra-lineage diversity. Yet, modern lineages were overall associated with increased growth rate and more pronounced granulomatous responses. MTBC lineages exhibited distinct propensities to accumulate triglyceride lipid droplets—a phenotype associated with dormancy—that was particularly pronounced in lineage 2 and reduced in lineage 3 strains. The most favorable granuloma responses were associated with strong CD4 and CD8 T cell activation as well as inflammatory responses mediated by CXCL9, granzyme B, and TNF. Both of which showed consistent negative correlation with bacterial proliferation across genetically distant MTBC strains of different lineages. Taken together, our data indicate that different virulence strategies and protective immune traits associate with MTBC genetic diversity at lineage and strain level.
-
- Immunology and Inflammation
Macrophages control intracellular pathogens like Salmonella by using two caspase enzymes at different times during infection.
