Fine Reading: Killing by Type VI Secretion...

by Elio
 

A recent paper by McNally et al. entitled: 'Killing by Type VI secretion drives genetic phase se­pa­ra­tion and correlates with increased cooperation' begins with the felicitous paragraph below. I think that it encapsulates the main features of the paradigm shift that has occurred recently, from bac­te­ria as lone (planktonic) individuals to organisms with a communal (often sessile) lifestyle.
 

"Microbes are fundamentally social organisms^1–5. They often live in dense, surf ace attached com­munities, and participate in a range of social behaviours mediated through the production and con­sumption of extra-cellular proteins and metabolites. Paradigmatic examples include the co­op­e­ra­tive production of digestive enzymes⁶, metal chelators⁷, signalling molecules⁶ and the struc­tu­ral components of biofilms⁸. Many of these extracellular compounds are susceptible to social ex­ploi­tation, in which non-producing 'cheats' gain an evolutionary advantage. If unchecked, this so­cial ex­ploitation can lead to the extinction of cooperative genotypes^9,10."
 

Figure 1. The Type VI secretion system depicted schematically. CC BY-SA 4.0 Maite.plummer (mo­­di­fied)

The paper goes on to consider the effect of Vibrio cho­le­rae killing adjacent bacteria, a social interaction if there ever was one. The authors use what to me are advanced mathematical computations leading to models that pre­dict the behavior in two-dimensional space of killers and adjacent sensitive cells. The general conclusion is that a kind of self-organization takes place. But there is more to it and cooperation between like cells is involved as well. They say: "We mathematically demonstrate that T6SS-me­di­a­ted killing should favour the evolution of public goods cooperation, and empirically support this prediction using a phylogenetic comparative analysis. This work il­lus­trates the twin role played by the T6SS, dealing death to local competitors while simultaneously creating con­di­tions potentially favouring the evolution of cooperation with kin."
 

Anyhow, the paper enlarges the role of Type VI secretion, that fascinating mechanism for de­li­ver­ing proteins, mainly toxic ones, to adjacent cells, both prokaryotic and eukaryotic. The cells that use this system prevent self-killing by producing toxin-specific immunity determinants. Fun is that the system uses what looks like a phage tail, a theme that recurs again and again. See here and here.
 
 

Indeed, microbes socialize with great intensity, not necessarily in ways benign.
 

 

References

1. Nadell CD, Drescher K, Foster KR. 2016. Spatial structure, cooperation and competition in biofilms. Nat Rev Microbiol, 14 (9), 589 – 600  PMID 27452230
2. Kümmerli R, Griffin AS, West SA, Buckling A, Harrison F. 2009. Viscous medium promotes cooperation in the pathogenic bacterium Pseudomonas aeruginosa. Proc Biol Sci, 276 (1672), 3531 – 3538 PMID 19605393 

3. West SA, Griffin AS, Gardner A, Diggle SP. 2006. Social evolution theory for microorganisms. Nat Rev Microbiol, 4 (8), 597 – 607 (2006) PMID 16845430 

4. Oliveira NM, Niehus R, Foster KR. 2014. Evolutionary limits to cooperation in microbial communities. Proc Natl Acad Sci U S A, 111 (50), 17941 – 17946 PMID 25453102 

5. Hibbing ME, Fuqua C, Parsek MR, Peterson SB. 2010. Bacterial competition: surviving and thriving in the microbial jungle. Nat Rev Microbiol, 8 (1), 8, 15 – 25 PMID 19946288 

6. Diggle SP, Griffin AS, Campbell GS, West SA. 2007. Cooperation and conflict in quorum-sensing bacterial populations. Nature, 450 (7168), 411 – 414 PMID 18004383 

7. Buckling A, Harrison F, Vos M, Brockhurst MA, Gardner A, West SA, Griffin A. 2007. Siderophore-mediated cooperation and virulence in Pseudomonas aeruginosa. FEMS Microbiol Ecol, 62 (2), 135 – 141 PMID 17919300 

8. Hammerschmidt K, Rose CJ, Kerr B, Rainey PB. 2014. Life cycles, fitness decoupling and the evolution of multicellularity. Nature, 515 (7525), 75 – 79 PMID 25373677 

9. Sanchez A, Gore J. 2013. feedback between population and evolutionary dynamics determines the fate of social microbial populations. PLoS Biol, 11, e1001547 PMID 23637571 

10. Travisano M, Velicer GJ. 2004. Strategies of microbial cheater control. Trends Microbiol, 12 (2), 72 – 78 PMID 15036323

(Open Access: bold)