Table 1.

Interactions among secretion systems in bacteria

Interacting system(s)*	Level of interaction†	Outcome and mechanism of interaction‡	Species	Ref(s)
T3SS-T5SS	Substrate level interaction (substrates: Tir and intimin)	Cooperative. Translocated Intimin Receptor (Tir) acts as a surface receptor for intimin, a T5SS substrate, owing to T3SS-mediated translocation of Tir.	Enteropathogenic Escherichia coli	[59]
T3SS-T4SS	Substrate level (BadA) and system level (T4SS) interaction	Antagonistic. BadA’s effective length creates a physical barrier between the pathogen and the host cell membrane, preventing T4SS activity.	Bartonella henselae	[65]
T2SS-T3SS	Substrate level (PCWDEs) and system level (T3SS) interaction	Cooperative. T2SS releases PCWDEs that alter plant cell wall integrity, thereby permitting T3SS-dependent effector protein translocation through.	Xanthomonas citri pv. vesicatoria	[74]
T1SS-T2SS	Substrate level interaction (substrates PrtA and PelI2)	Cooperative. T1SS substrate PrtA post-translationally modifies T2SS substrate PelI-2 to PelI-3	Dickeya dadantii	[73]
T4SS-T6SS	Substrate and system level interaction.	Cooperative. T4SS T4P promotes contact-dependent killing by T6SS by bringing adjacent prey closer to the attacker.	Pseudomonas aeruginosa; Neisseria cinerea	[86, 87]
T0SS-T6SS	Substrate level interaction (substrates: TseF and TeoL)	Cooperative. T6SS recruits T0SS (outer membrane vesicles or OMVs) either through its effector Tsef ( Cupriavidus necator ) or TeoL effector and PQS present in Pseudomonas aeruginosa OMVs.	Pseudomonas aeruginosa; Cupriavidus necator	[80, 81]
T0SS-T9SS	Substrate and system level interaction	Cooperative. T9SS effectors, typically extracellular cysteine proteases (gingipains), are anchored on outer membrane vesicles through anionic lipopolysaccharide (A-LPS) moiety of Porphyromonas gingivalis OM. This interaction results in a virulence coat contributing to P. gingivalis infections.	Porphyromonas gingivalis	[101]

*Type X secretion systems, where X stands for any number from 1 to 10, with outer membrane vesicles designated as a T0SS.

†Full names of molecules and components: BadA, Bartonella adhesin A; Tir, Translocated Intimin Receptor; TseF, T6SS effector for Fe uptake; TeoL, T6SS effector for recruitment of OMVs via lipopolysaccharide (LPS); PCWDEs, plant cell wall degrading enzymes; PQS, Pseudomonas quinolone signal; T4P, T4SS pilus. ‘Substrate’ refers to any molecule released by TXSSs (e.g. effectors, adhesins, etc.).

‡The term ‘outcome’ here refers to the way systems interact, which can be either synergistic or antagonistic.