Cell Host & Microbe

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Type VI Secretion System Helps Find a Niche

Nicole Kapitein1,2 and Axel Mogk1,2,*1Zentrum fur Molekulare Biologie der Universitat Heidelberg (ZMBH), DKFZ-ZMBH Alliance, Im Neuenheimer Feld 282, 69120 Heidelberg,Germany2Deutsches Krebsforschungszentrum (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany*Correspondence: a.mogk@zmbh.uni-heidelberg.dehttp://dx.doi.org/10.1016/j.chom.2014.06.012

Type VI secretion systems (T6SSs) deliver toxins into target cells and thus play a role in bacterial warfare. Inthis issue of Cell Host & Microbe, Ma et al. (2014) demonstrate that T6SS-dependent attack during interbac-terial competition in the host context enables niche colonization by Agrobacterium tumefaciens.

Competitive pressure rules the basic in-

teractions between all organisms. The

competition for nutrients and space

essentially influences how microorg-

anisms emerge and vanish in specific

habitats. Bacteria extensively use two

strategies to compete with each other.

In a passive scramble competition,

limiting resources are used up rapidly.

The secretion of siderophores that effi-

ciently sequester extracellular iron, pre-

venting iron uptake by competing cells,

exemplifies this deprivation of supplies.

A more active contest competition in-

volves direct interaction with competitors.

The latter strategy encompasses the

secretion into the extracellular space of

small antimicrobial compounds including

antibiotics or bacteriocins to poison

opposing strains.

A recently described specialized secre-

tion machinery, the type VI secretion sys-

tem (T6SS), is used by a wide variety of

Gram-negative bacteria to directly deliver

toxins upon cell-to-cell contact into target

cells in order to kill. This cytotoxic activity

of the T6SS was demonstrated to be

targeted against both prokaryotic and

eukaryotic competitors (Hood et al.,

2010; Pukatzki et al., 2006). T6SS activity

is not restricted to competitor cells but

can also play important roles in patho-

genicity in mammalian hosts.

T6SSs have been extensively studied

in the past years. They are typically en-

coded within a single gene cluster

consisting of 13 conserved core com-

ponents and a number of accessory

ones. Together, those proteins form a

membrane-embedded, syringe-like sys-

tem strikingly similar to the injection

machinery of bacteriophages. Analogous

to the phage tail sheath proteins, the

T6SS components VipA/VipB (TssB/

TssC) form a contractile sheath around

a hollow, inner tube composed of

Hcp (hemolysin-coregulated protein).

Attached on top of the Hcp tube is a

trimeric, spike-like cap consisting of

VgrG (valine-glycine repeat protein G),

similar to the tail spike complex of bacte-

riophages. Upon contraction of the VipA/

VipB (TssB/TssC) sheath, the Hcp tube

together with VgrG is pushed outward

and penetrates the target cell (for review

see Kapitein and Mogk, 2013). The pene-

tration of target cells is accompanied by

the delivery of specific toxins. Effectors

can be attached to the VgrG spike via

specific PAAR domain-containing adap-

tors or are covalently fused to VgrG

(Shneider et al., 2013). Alternatively, at

least small toxins might be delivered

directly through the hollow channel of

the Hcp tube, ending up in the target cell

after the VgrG cap has detached (Silver-

man et al., 2013).

The number of T6SS-dependent effec-

tors is manifold and mostly directed

against highly conserved, indispensable

cellular components: the peptidoglycan

layer, membranes, and DNA. T6SS

effectors co-occur in tandem with corre-

sponding immunity proteins that inhibit

the activity of the cognate toxin, prevent-

ing self-intoxication. The composition of

the effector-immunity arsenal differs sub-

stantially even between closely related

species, and the combination selected

out of this big armory will determine the

winners of interbacterial duels.

T6SS function in interbacterial competi-

tion is also reflected in the control of its

activity. In Pseudomonas aeruginosa,

T6SS activity is controlled by the kinase

PpkA, which is in turn regulated by the

membrane-localized TagQRST protein

complex. This membrane system senses

Cell Host & Mic

perturbations of the bacterial mem-

branes, caused by an opponent cell, and

transduces a yet-to-be-determined signal

to ultimately trigger a T6SS counterstrike

immediately after the initial attack (Basler

et al., 2013).

By now, multiple cases have been

described where T6SS-positive organ-

isms are able to efficiently outcompete

competitor cells. This drives the hypo-

thesis that T6SSs are important evolu-

tionary factors helping bacteria to

conquer ecological niches. Such a role

could be also important for pathogens,

enabling T6SS-positive populations

to outcompete commensal bacteria,

thereby indirectly supporting pathogen-

esis. While prior studies could demon-

strate that T6SSs in Vibrio cholera are

active upon colonization of the host in-

testinal tract, evidence for V. cholerae

T6SS function in the replacement of the

commensal microbiota is outstanding

(Fu et al., 2013). Also, bacterial duels

were so far not tested in physiological

relevant environments, leaving the hy-

pothesis, while attractive and reason-

able, unproven.

This open issue of the role of T6SS

in interbacterial competition in situ has

now been addressed by Ma et al. (2014).

In their paper published in this issue of

Cell Host & Microbe, Ma et al., (2014)

used Agrobacterium tumefaciens, a

Gram-negative soil bacterium that lives

in the phyllosphere and causes the crown

gall disease in infected plants, as the

model T6SS expressing bacterium. Ma

et al. (2014) show that A. tumefaciens

T6SS is important for host colonization

and involved both in intra- and

interspecies competition with the

T6SS elaborating soil bacterium Pseudo-

monas aeruginosa. When bacterial

robe 16, July 9, 2014 ª2014 Elsevier Inc. 5

A B

Figure 1. The Natural Habitat Enables A. tumefaciens to Outcompete Opponent Bacteria via T6SS Activity(A and B) Bacterial duels between Agrobacterium tumefaciens and Pseudomonas aeruginosa. Pseudomonas aeruginosa efficiently outcompetes Agrobacteriumtumefaciens in vitro (A), while the outcome of interbacterial competition is reversed in planta (B). Here, an unknown environmental signal presumably increasesA. tumefaciens T6SS activity, leading to efficient delivery of Tde effectors that degrade chromosomal DNA of competitor cells. The immunity protein Tdi inacti-vates Tde and protects A. tumefaciens from self-toxication.

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duels between A. tumefaciens and

P. aeruginosa were performed in vitro on

agar plates, A. tumefacienswas efficiently

outcompeted (Figure 1). The competitive

advantage of P. aeruginosa included a

T6SS-mediated counterstrike. Strikingly,

the outcome of the duel was reverted in

planta, when coinfection experiments in

tobacco plants were performed. Here,

A. tumefaciens exhibited a T6SS-

mediated growth advantage toward

P. aeruginosa, providing first evidence

that T6SSs enable bacteria to settle in

natural habitats and underlining the

impact of the plant environment in

this process. Competitive fitness of

A. tumefaciens in planta is mediated by

a novel class of T6SS-dependent DNase

effectors, termed Tde (Figure 1). The Tde

superfamily is likely VgrG linked and

thereby delivered through association

with the needle itself. Tde forms a clas-

sical toxin-antitoxin pair with the accom-

panying immunity protein Tdi. The authors

confirm the widespread conservation of

the Tde superfamily within Gram-negative

plant pathogens and symbionts, thereby

supporting a global role within plant

colonization.

The basis for the observed advantage

enjoyed byA. tumefacienswithin the plant

habitat compared to an in vitro setting

remains unclear. One possibility would

6 Cell Host & Microbe 16, July 9, 2014 ª2014

be that the A. tumefaciens T6SS activity

is simply higher in planta, thereby leap-

frogging Pseudomonas aeruginosa in its

counterstrike activity. A. tumefaciens

T6SSs can be controlled at transcriptional

and posttranslational levels. Low pH

increases expression of A. tumefaciens

T6SS (Wu et al., 2012), but whether differ-

ences in acidity between in vitro and in

planta experiments are causative of the

opposing competition outcomes remains

to be investigated. A. tumefaciens T6SS

activity is also regulated by PpkA-

mediated phosphorylation, but TagQRST

homologs are not present, leaving PpkA

activity control unresolved (Lin et al.,

2014). Therefore, further experimental

advances aimed at identifying the habitat

and host-specific signals that regulate the

activation cascade of the A. tumefaciens

T6SS would be required.

The presented work sets the stage for

deeper analysis of T6SSs in the natural

environment and how their activities

might be modulated by host factors. It

will be important to broaden the now

proven concept of T6SS function in niche

occupation for other bacterial species.

This understanding should also prove

valuable for analyzing and understanding

the contribution of T6SSs in acute and

chronic diseases involving displacement

of commensal bacteria.

Elsevier Inc.

ACKNOWLEDGMENTS

This work was supported by grants of the Germanresearch council (DFG) to A.M. (MO970/3).

REFERENCES

Basler, M., Ho, B.T., and Mekalanos, J.J. (2013).Cell 152, 884–894.

Fu, Y., Waldor, M.K., and Mekalanos, J.J. (2013).Cell Host Microbe 14, 652–663.

Hood, R.D., Singh, P., Hsu, F., Guvener, T., Carl,M.A., Trinidad, R.R., Silverman, J.M., Ohlson,B.B., Hicks, K.G., Plemel, R.L., et al. (2010). CellHost Microbe 7, 25–37.

Kapitein, N., and Mogk, A. (2013). Curr. Opin.Microbiol. 16, 52–58.

Lin, J.S., Wu, H.H., Hsu, P.H., Ma, L.S., Pang, Y.Y.,Tsai, M.D., and Lai, E.M. (2014). PLoS Pathog. 10,e1003991.

Ma, L.-S., Hachani, A., Lin, J.-S., Filloux, A., andLai, E.-M. (2014). Cell Host Microbe 16, this issue,94–104.

Pukatzki, S., Ma, A.T., Sturtevant, D., Krastins, B.,Sarracino, D., Nelson, W.C., Heidelberg, J.F., andMekalanos, J.J. (2006). Proc. Natl. Acad. Sci.USA 103, 1528–1533.

Shneider, M.M., Buth, S.A., Ho, B.T., Basler, M.,Mekalanos, J.J., and Leiman, P.G. (2013). Nature500, 350–353.

Silverman, J.M., Agnello, D.M., Zheng, H.,Andrews, B.T., Li, M., Catalano, C.E., Gonen, T.,and Mougous, J.D. (2013). Mol. Cell 51, 584–593.

Wu, C.F., Lin, J.S., Shaw, G.C., and Lai, E.M.(2012). PLoS Pathog. 8, e1002938.