Type VI Secretion System (T6SS) Structure, Mechanism, and Functional Diversity in Bacteria

 

Table of Content

• Introduction
• Mechanism Of T6SS Effector Delivery
• Antibacterial Effectors Delivered By T6SS
• Roles Of Antibacterial T6SS In Bacterial Communities
• Anti-host T6SS Effectors
• The T6SS As A Weapon Against Microbial Competitors
• A Contact-independent Role For T6SS In Metal Uptake
• Conclusion
• Reference

Ready Made Presentation

Introduction

• T6SS is a protein nanomachine used by Gram-negative bacteria to translocate effector proteins into target cells.
• Initially seen as a virulence factor, it's primarily used for inter-bacterial competition, delivering toxic antibacterial effectors.
• Expanding functions, including action against microbial fungi and scavenging scarce metal ions.
• Plays a crucial role in polymicrobial communities.
• Estimated >25% of proteobacteria encode at least one T6SS in their genome.

T6SS Variability

Core Components:

• T6SS relies on a set of core components, typically 13 to 14 in number. Vital Parts are TssA through TssM and the PAAR protein, for assembly, regulation, and effective delivery of effector proteins.

Categorization:

• Exhibit significant genetic and functional diversity and grouped into six sub-families (1, 2, 3, 4a, 4b, 5) which differ in their component proteins and may fulfill distinct roles across bacterial species.

Divergent T6SS-like Systems:

• T6SSii (Francisella pathogenicity island) and T6SSiii (phylum Bacteroidetes) have own unique component proteins but share an mode of action with the canonical T6SSi.

Proteobacterial T6SS (T6SSi):

• The most widespread and extensively studied variant of T6SS. However, its genetic composition and function may vary among different proteobacterial species.

Varies in Bacterial Species and Strains:

• T6SS varies widely among bacteria, from one to multiple systems. Even within a species, strains can differ in their T6SS sets, showing adaptability to various niches and strategies.

Core Components of Type VI Secretion System (T6SS)

1. Type VI Secretion System Component A (TssA)

• Plays a key role in the assembly and overall function of the T6SS
• Stabilizes the baseplate structure
• Assists in the proper formation of the T6SS apparatus

2. Type VI Secretion System Component M (TssM)

• Functions as a regulatory protein
• Interacts with multiple T6SS components
• Coordinates the assembly and disassembly of the secretion system

3. Proline-Alanine-Alanine-Arginine Repeat (PAAR) Protein

• Located at the tip of the T6SS spike complex
• Responsible for puncturing the target cell membrane
• Facilitates the delivery of effector proteins into target cells
• Enables direct translocation of effectors

T6SS Functional Diversity

• T6SSs are highly adaptable, with some capable of serving dual roles – targeting both bacteria and eukaryotic host cells.

Specialized Functions:

• Others have distinct roles, such as antibacterial or anti-host functions, finely tuned to the organism's niche.
• Example 1: Vibrio cholerae - Antibacterial and anti-eukaryotic T6SS functions.
• Example 2: Burkholderia thailandensis - Separate T6SSs for antibacterial and anti-host activities.
• Adaptive Strategies: T6SSs adapt their usage based on the organism's niche and survival strategy, making them versatile and effective tools.
• Diverse Effector Arsenal: T6SSs deploy a variety of effectors, facilitating interactions with different cell types and enabling competition or manipulation.
• Customized Gene Regulation: Gene expression regulation for T6SS is tailored to the organism's specific needs, ensuring precise functionality.

Mechanism of T6SS Effector Delivery

T6SS Propulsion Mechanism:

• The T6SS is a dynamic nanomachine that utilizes a contraction mechanism to expel an extracellular puncturing structure from the secreting cell. This mechanism is responsible for the delivery of effector proteins into neighboring target cells.
• The contraction-based propulsion system is similar to the injection mechanism of contractile bacteriophages.
• The T6SS is like a microscopic harpoon, where a puncturing structure, loaded with effector proteins, is forcibly propelled out of the secreting cell.
• This puncturing structure is designed to breach the target cell's membrane, allowing the translocation of effectors into adjacent cells.

T6SS Assembly Components:

• The T6SS consists of 14 core components (TssA-M, PAAR) and sub-assemblies, which include:
• A membrane complex,
• Cytoplasmic baseplate,
• Cytoplasmic contractile sheath,
• The expelled puncturing structure adorned with effectors

Puncturing Structure Assembly:

• Assembly begins with the formation of the bell-shaped membrane complex, composed of TssJ and TssLM proteins. The baseplate docks with the cytoplasmic face of the membrane complex.
• The Hcp tube assembles onto the base of VgrG, and a sheath structure, consisting of TssBC subunits, polymerizes around the Hcp tube in an extended, high-energy 'primed' conformation.
• Rapid contraction of the TssBC sheath propels the puncturing structure, including Hcp, VgrG, and effectors, through the baseplate and membrane complex, ultimately out of the secreting cell.

Inter-Cellular Effector Delivery:

• The force generated by the contracted sheath is sufficient to breach the target cell's membrane, achieving direct contact-dependent translocation of effector proteins into neighboring cells.

Depolymerization and Effector Release:

• Following contraction, the contracted sheath is specifically depolymerized by the ATPase TssH. However, the exact mechanism of how effectors are released inside the target cell is still an area of ongoing investigation.

Reusability:

• In contrast to bacteriophages, the T6SS can be reused for multiple firing events by the same cell. Dynamic cycles of assembly, contraction, and disassembly have been observed, making it a versatile and efficient nanomachine.

Accessory Components:

• T6SSs can incorporate additional structural or regulatory components, like SciZ/TagL, TagA, and TagJ, which further enhance their functionality.
• T6SSs can include additional structural or regulatory components to enhance their functionality.
• SciZ/TagL: These components may provide peptidoglycan-binding functionality, which is typically present in TssL. This peptidoglycan interaction can assist in stabilizing the T6SS's interaction with the cell wall.
• TagA: In some T6SSs, TagA, a membrane-associated TssA-family protein, interacts with TssA. It plays a role in catching, stopping, and stabilizing the extending TssBC sheath when it reaches the opposite side of the cell.
• TagJ: TagJ may contribute to the recruitment of TssH to the contracted sheath in certain systems.

Regulatory Components:

• Many T6SSs are equipped with conserved post-translational regulatory components that modulate T6SS activity.
• PpkA: This membrane-bound protein threonine kinase phosphorylates the T6SS-associated Fha protein. It overcomes inhibition mediated by a negative regulator, such as TagF, which allows the assembly of an active T6SS. PpkA can be activated in response to incoming T6SS-mediated attacks from neighboring cells, making the T6SS act as a "defensive" system in some cases.
• PppA: This antagonistic phosphatase promotes spatial relocation of the T6SS machinery between firing events. The presence of PpkA and PppA allows for specific mechanisms to increase the efficiency of T6SS antibacterial activity.

Chaperone Proteins:

Certain effector proteins require chaperone or adaptor proteins to

facilitate their recruitment and loading onto the T6SS machinery before

secretion.

• EagR/EagT: Chaperone proteins like EagR/EagT are involved in binding to the N-terminal PAAR-containing domains of Rhs and Tse6-like effectors. They stabilize transmembrane regions, enabling effectors to cross the recipient cell's inner membrane. These chaperones also assist in loading effectors onto the cognate VgrG.
• Tap-1/TecL: The Tap-1/TecL family proteins act as modular adaptors for VgrG-dependent cargo effectors. The C-terminal half of these proteins varies, allowing them to interact with different effectors. This flexibility permits the horizontal acquisition of new effectors and their interaction with existing VgrG homologues.
• TecT: TecT is a chaperone that facilitates the interaction of a PAARdependent cargo effector with the cognate PAAR protein. It competes with a co-chaperone for access to the chaperone, and its role is to protect and load specific effectors onto the machinery.

T6SS Effector Coupling & Assembly

• Structure: Spike = PAAR (1) + VgrG trimer → + Hcp tube; both spike & Hcp carry effectors; flexible, non-restrictive assembly
• Effector Loading: Direct / cargo / specialized domains; chaperones (Eag, Tec, Tap) assist loading but are not secreted
• Variability:
• Hcp: structural | effector-fused | chaperone role
• VgrG: structural | specialized | cargo-bound | homo/heterotrimers | co-effector systems
• PAAR: structural | specialized | cargo-bound | adaptor-dependent | Rhs effectors
• Outcome: Diverse effector delivery (extracellular or into target cells) → synergy + increased targeting success

Antibacterial Effectors Delivered by T6SS

Classes and Modes of Action

• T6SS delivers broad-spectrum antibacterial effectors.
• Effectors target recipient bacterial cells' peptidoglycan cell walls and inner membranes.
• Effector families include peptidoglycan amidases (Tae1– 4, TaeX) and peptidoglycan glycoside hydrolases (Tge1-3, VgrG-3), acting on the cell wall.
• Lipase/phospholipase (Tle1–4 and Tle5) effectors target the inner membrane.
• Specific effectors (VasX and Tse4) form pores or channels in the membrane.
• Nuclease DNases (e.g., Tde1 and RhsAB) effectors target bacterial cytoplasm.
• (Tse6/Tne1 and Tne2) Effectors can degrade essential cytoplasmic cofactors.
• T6SS can deliver ADP-ribosyltransferase toxins (Tre1) that inhibit cell division.

T6SS delivers broad-spectrum antibacterial effectors.

Effector Type	Target Site	Examples of Effectors	Mode of Action
Peptidoglycan-Targeting Effectors	Peptidoglycan Cell Wall	Tae1–4, TaeX	Peptidoglycan Amidases: Cleavage of specific peptide bonds in cell wall
Peptidoglycan-Targeting Effectors	Peptidoglycan Cell Wall	Tge1–3, VgrG-3	Peptidoglycan Glycoside Hydrolases: Cleavage of glycan chains in cell wall
Inner Membrane-Targeting Effectors	Inner Membrane	Tle1–4, Tle5	Phospholipase activity
Pores and Channels	Cell Membranes	VasX, Tse4	Formation of membrane pores/channels
Cytoplasmic-Targeting Effectors	Bacterial Cytoplasm	DNases (e.g., Tde1, RhsAB)	Degradation of DNA or RNA
Cytoplasmic-Targeting Effectors	Cytoplasmic Cofactors (e.g., NAD(P)+)	Tse6/Tne1, Tne2	Hydrolase activity affecting cofactors
ADP-Ribosyltransferase Toxins	Cytoplasm and Cellular Proteins	Tre1	Modification of essential proteins (e.g., FtsZ)

Diversity and Expansion

Direct delivery in a minority of T6SS events

• Activation
• Pre-loading
• Outer breach
• Periplasmic transit
• Inner breach
• Release
• Response

Incorporation of transmembrane domains for inner membrane traversal

• Contact between donor and recipient.
• T6SS breaches recipient's outer membrane.
• Effector proteins exploit breach.
• Anchor into inner membrane via transmembrane domains.
• Exert effects on recipient cell. 

Target cell protein-mediated import similar to Cdi toxins

• Effector Recognition
• Protein Interaction
• Import into Cytoplasm
• Effector Function

Self-Protection via Immunity Proteins: 

Role of Immunity Proteins:

• Immunity proteins are vital for bacterial cells with T6SS to:
• Protect themselves from their own T6SS-delivered effectors.
• Safeguard against harmful effectors from neighboring cells.

Dual Functionality of Immunity Proteins:

• ADP-Ribosylhydrolase Activity: Some immunity proteins can remove ADP-ribose modifications.
• Broad Resistance: This function allows them to counteract various toxins, not just their own.
• Immunity proteins serve as versatile guardians, ensuring the producing cell's safety and countering attacks from rival cells. This complexity underscores the sophistication of bacterial warfare and defense mechanisms.

Evolution and Acquisition of Effector-Immunity Pairs

Inter-Bacterial Competition:

• T6SS-mediated competition occurs within and between species.
• Diversity exists in effector– immunity pairs, even among strains of the same species.
• Effector–immunity pairs are horizontally acquired, suggesting an ongoing "arms race" between bacteria. 

Cargo Effectors and Chaperones:

• Cargo effectors are often acquired with linked chaperones or VgrG proteins for delivery in the recipient background.

Orphan Immunity Proteins:

• Orphan immunity proteins provide protection against effectors delivered by other strains.
• They can be retained from formerly active effector– immunity pairs.

Synergy and Efficacy:

• Simultaneous delivery of multiple distinct effectors enhances antibacterial activity. 
• Simultaneous targeting of multiple cellular components or enzymatic activities leads to more efficient killing.
• Multiple effectors may protect against variations in environmental conditions.

Roles of Antibacterial T6SSs in Bacterial Communities

Real-life Impact

T6SS Mutants' Virulence Defects May Be Misleading

• Unraveling the Complexities: The absence of T6SSs, represented by mutants, might lead to apparent virulence defects in bacteria. However, these virulence defects can be misleading.
• Competition with Resident Microbiota: In real-life scenarios, bacteria coexist with complex microbial communities. T6SS mutants may struggle to compete effectively against the resident microbiota, impacting their overall fitness and virulence.

Antibacterial T6SSs Influence Host-Associated Communities

• Shaping Microbial Neighborhoods: It's becoming increasingly clear that antibacterial T6SSs have a profound impact on the composition of host-associated microbial communities.
• Outcomes for the Host: These systems play a role in influencing the balance of bacterial populations within a host, which, in turn, can significantly affect the health and well-being of the host organism.

Role in Gut Microbiota

• Salmonella typhimurium's T6SS in gut colonization
• Shigella sonnei's T6SS outcompetes E. coli and shown to persisted in the mouse model.
• V. cholerae's T6SS enhances colonization and activates host innate immunity genes causing inflammation response. As experimented in infant mouse model and infant rabbit model (middle small intestine) .
• Drosophila model: V. cholerae's T6SS triggers host destruction by specifically targeting and eliminating subpopulation of commensal bacteria while sparing the others which creates a niche for it to growth and outcompete.
• Zebrafish model: V. cholerae's T6SS can have a direct impact on host intestinal movements, showcasing the adaptability of T6SS-dependent interactions in different host environments. 

Bacteroidales and Colonization

• Diverse T6SS Architectures: Bacteroidales in human gut with diverse genetic diversity of T6SS.
• Intraspecies Competition: T6SS-mediated competition among B. fragilis strains.
• In the bee gut, competition driven by (T6SSs) facilitated by Rhs effector domains armed with various toxins, provide bacterial strains with a competitive edge. Through rapid exchange of these effector domains, strains can adapt and evolve, ultimately influencing the composition and dynamics of the bee gut microbiota. 
• Establishing Colonization Resistance: Antibacterial T6SS generates colonization resistance.
• Role in Protection: The symbiotic B. fragilis strain utilizes its T6SS to counteract the harmful effects of the toxigenic strain, ensuring that the host remains free from colitis.
  

Role in Plant Communities

• Diverse Presence: T6SSs in plant-associated bacteria.
• Establishing Beneficial Communities: T6SSs protect and establish plant communities.
• Rhizosphere Bacterium Combatting Pathogens: Pseudomonas putida (plant-protecting bacterium) T6SS’s reduces pathogen colonization.
• Plant Pathogen Control: A. tumefaciens T6SS effective against P. aeruginosa within plants.

Pseudomonas putida's T6SS in the Rhizosphere:

• Rhizosphere Bacterium: Pseudomonas putida defends plants in the rhizosphere.
• Counteracting Pathogens: Its T6SS reduces phytopathogen colonization, protecting plants from damage.
  

Agrobacterium tumefaciens' T6SS as a Plant Pathogen:

• Plant Pathogen: Agrobacterium tumefaciens uses its T6SS within plants.
• Eliminating Rivals: It targets bacteria like Pseudomonas aeruginosa, enhancing its pathogenicity.

Symbiosis in Squids

Symbiotic Relationships in Squid and Vibrio fischeri:

• Euprymna scolopes squid has a unique relationship with Vibrio fischeri bacteria. These bacteria colonize specific light organ crypts and produce bioluminescence, benefiting both parties.

T6SS Strain Separation in Squid:

• The Type VI Secretion System (T6SS) in Vibrio fischeri selects and separates strains within the squid's light organ, preventing incompatible strains from co-occupying crypts.
  

Biofilm Communities

1. Host-Associated Communities in Biofilm State:

• Many host-associated bacterial communities exist in a biofilm state, where bacterial cells aggregate on surfaces within an extracellular matrix.

2. T6SS Co-Regulation with Biofilm Genes:

• Type VI Secretion Systems (T6SS) are often co-regulated with genes responsible for biofilm formation, indicating their interplay in host-associated environments.

3. T6SS Aids Burkholderia's Persistence in Mixed Biofilm:

• The T6SS allows Burkholderia to persist within mixed biofilms, ensuring the stability of the bacterial community in these complex environments.

4. Extracellular Polysaccharide as a Barrier:

• In some cases, extracellular polysaccharides may act as physical barriers that reduce the effectiveness of T6SS attacks within biofilms.

Social Behavior and Policing

Distinguishing Self from Nonself and Formation of Boundaries:

• The T6SS is a critical bacterial tool for distinguishing self from nonself, creating physical boundaries between populations known as "Dienes lines." These boundaries separate swarms of different strains by actively targeting and eliminating nonsibling strains, establishing macroscopic separations and preserving genetic diversity.

T6SS Eliminates Nonsiblings and Policing Quorum Sensing 'Cheats':

• In Proteus mirabilis, the T6SS eliminates nonsibling strains using strain-specific effectors, ensuring that only closely related strains coexist within a specific environment.
• Within bacterial populations employing quorum sensing, spontaneous mutants known as "cheats" can exploit communal resources without contributing. Bacteria like Burkholderia thailandensis use their T6SS to police cheats by delivering toxic effectors, even to formerly immune cheats, ensuring fair resource distribution. This multifaceted role highlights the T6SS's significance in promoting fairness, cooperation, and optimal growth within microbial communities.

• A policing model where QS control of T6S contact-dependent killing (effector and immunity proteins) enforces cooperation.

A. The bacterium depicts a situation where QS co-regulates a public good with the T6SS-1 effector and immunity factors. Under these conditions, QS-proficient bacteria (cooperators, shown as white cells) limit the proliferation of a QS mutant (potential cheater, black cell) in a population. 

B. In this situation, QS no longer co-regulates public good production with the T6SS-1 killing mechanism. Without co-regulation of the killing mechanism, QS mutants (potential cheaters, black cells) outcompete the wild type cooperators (white cells).

Horizontal Gene Transfer

• Type VI Secretion Systems (T6SSs) aid in horizontal gene transfer (HGT), where bacteria acquire new genetic material.
• T6SSs facilitate DNA uptake from prey cells, transferring genes, including antibiotic resistance.
• V. cholerae's T6SS coordinates with competence machinery for enhanced DNA acquisition.
• Acinetobacter's T6SS contributes to its rapid acquisition of antibiotic resistance.

ANTI-HOST T6SS EFFECTORS

Direct Targeting of Eukaryotic Host Cells:

• Bacteria can utilize T6SSs to directly target eukaryotic cells, including those of host organisms.
• Several bacterial pathogens have shown phenotypes related to virulence, host response, and host cell interaction dependent on functional T6SSs.
• Notable examples of T6SS effectors targeting host cells include VgrG proteins with C-terminal actin crosslinking, actin ADP ribosylase, and host membrane fusion domains.
• Other effectors with tubulin-binding domains modulate microtubule-mediated bacterial internalization.
• Cargo phospholipase D effectors facilitate internalization by binding Akt and activating the PI3K pathway.
• Some T6SS effectors can act against both eukaryotic and prokaryotic cells.
• More recently, additional anti-host effectors have been identified, contributing to virulence, intracellular proliferation, and modulation of host immune responses.

Overview of the effectors, their respective bacteria, and their functions in host interactions

Effector	Bacterium	Function
PdpCD, OpiAB	Francisella tularensis	Contribute to intramacrophage growth and aid in phagosomal escape
EvpP	Edwardsiella tarda	Prevents NLRP3 inflammasome activation by inhibiting Ca2+-dependent MAPK-JNK pathway
TecA	Burkholderia cenocepacia	Activates Pyrin inflammasome via Rho GTPase deaminase activity disrupting cytoskeleton
CNF1-like toxin	Vibrio parahaemolyticus	Induces actin rearrangement in host cells
Tle4-family phospholipase	Pseudomonas aeruginosa	Disrupts endoplasmic reticulum in host cells
KatN	Enterohemorrhagic Escherichia coli (EHEC)	Acts against reactive oxygen species to enhance bacterial survival

Burkholderia cenocepacia delivers TecA from within the B. cenocepacia-containing vacuole (BcCV), causing deamidation of specific asparagine residues in Rho family GTPases RhoA and Rac1, leading to disruption of the actin cytoskeleton, activation of the pyrin inflammasome and pyroptosis and increasing bacterial clearance.

THE T6SS AS A WEAPON AGAINST MICROBIAL COMPETITORS

T6SS as a Multi-Purpose Weapon

• The Type VI Secretion System (T6SS) exhibits versatility in targeting various eukaryotic microbial competitors, expanding its influence in polymicrobial communities.
• T6SS shows adaptability against eukaryotic cells.
• The system is vital in shaping complex microbial communities.
• The T6SS acts against diverse challenges, from fungi to amoebae, reflecting its multi-purpose nature.

Bacterium	Target Microbial Competitors	T6SS Effector	Outcome of Interaction
Pseudomonas syringae	Cryptococcus carnescens	Tfe1 and Tfe2	Fungal cell death
Serratia marcescens	Saccharomyces cerevisiae, Candida spp.	Tfe1 and Tfe2	Fungal cell death
Vibrio cholerae	Dictyostelium discoideum	VasX and VgrG-1	Resistance and virulence

• The Type VI Secretion System (T6SS) delivers antifungal effector proteins from Serratia marcescens to fungal cells. 
• In the secreting bacterial cell, the TssBC sheath contracts, propelling a cell-puncturing structure with effector proteins into a neighboring target cell. This structure includes a tube (Hcp proteins) and a spike (VgrG and PAAR proteins), carrying effector proteins like Tfe1 and Tfe2. 
• Once inside the target cell, these antifungal effectors disrupt cell function. 
• Tfe1 depolarizes the plasma membrane, while Tfe2 disrupts nutrient uptake and amino acid metabolism, inducing autophagy. 
• Serratia marcescens T6SS also delivers antibacterial effectors for bacterial competitors.

Bacterium	Target Microbial Competitors
S. marcescens	S. cerevisiae, Candida spp.

A CONTACT-INDEPENDENT ROLE FOR T6SS IN METAL UPTAKE T6SS EFFECTORS

T6SS's Contact-Independent Role in Metal Uptake

In addition to its traditional functions, the Type VI Secretion System (T6SS) plays a vital role in metal acquisition, often in a contact-independent manner. T6SS-mediated secretion of metal-binding proteins allows bacteria to scavenge scarce metal ions from their surroundings, promoting survival in challenging conditions, including oxidative stress.

• Yersinia pseudotuberculosis: T6SS4 secretes the Zn2+-binding protein YezP under oxidative stress conditions, enabling the scavenging of Zn2+ ions, contributing to host survival.
• Burkholderia thailandensis: T6SS-4 secretes metallophore effectors TseZ and TseM, facilitating Zn2+ and Mn2+ uptake under oxidative stress. These effectors interact with outer membrane receptors, enhancing metal transport.
• Competitive Advantage: The possession of TseZ/HmuR or TseM/MnoT, along with T6SS-4, provides a competitive edge during co-culture under metal-limiting conditions without direct harm to other bacteria.
• Virulence in Hosts: Both metal uptake systems are crucial for full virulence in host environments, exemplified in a Galleria model.
• T6SS Function Variety: The contact-independent role of T6SS in metal acquisition underscores its versatility and the potential for extracellular proteins to utilize the system for secretion.

Bacterium	T6SS	Metallophore Effector	Metal Ion	Role
Yersinia pseudotuberculosis	T6SS4	YezP	Zn²⁺	Scavenging Zn²⁺ under oxidative stress
Burkholderia thailandensis	T6SS-4	TseZ	Zn²⁺	Enhancing Zn²⁺ uptake under oxidative stress
Burkholderia thailandensis	T6SS-4	TseM	Mn²⁺	Facilitating Mn²⁺ uptake under oxidative stress
Pseudomonas aeruginosa	H3-T6SS	TseF	Fe²⁺	Mediating Fe²⁺ uptake from PQS-Fe²⁺ complexes

B. thailandensis uses its T6SS-4 to translocate TseM to the extracellular milieu, where it binds Mn2+. TseM loaded with Mn2+ interacts with the outer membrane TonB dependent receptor MnoT, which is associated with a TonBExbD-ExbB complex, transferring Mn2+ from TseM to MnoT and allowing its active import across the outer membrane. Either the SitABCD or MntH transporters may then be utilized to import Mn2+ across the inner membrane

Conclusion

The Type VI Secretion System (T6SS) is a versatile bacterial tool that enhances competitiveness in various ways:

• Versatile Competition: Antibacterial, anti-fungal, and anti-host T6SSs enable bacteria to compete effectively, shaping microbial communities.
• Diverse Effectors: A wide range of effector proteins diversify T6SS functions, including non-toxic extracellular roles.
• Broad T6SS Systems: Beyond T6SSi, multiple T6SS systems offer distinct roles in complex communities.
• Real-Life Impact: T6SS plays a pivotal role in shaping real-life microbial communities.
• Antimicrobial Potential: T6SS opens doors for innovative antimicrobial strategies.

Reference

Coulthurst S. (2019). The Type VI secretion system: a versatile bacterial weapon. Microbiology (Reading, England), 165(5), 503–515. https://doi.org/10.1099/mic.0.000789