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Attention cheaters: bacterial police are coming

May 29, 2011
Courtesy of Indiana University
and World Science staff

At least some bac­te­ria can “po­lice” cheat­ers in their midst, a study has found, al­though how they do so is un­clear.

“Even sim­ple or­gan­isms such as bac­te­ria can evolve to sup­press so­cial cheat­ers,” said In­di­ana Un­ivers­ity Bloom­ing­ton bi­ol­o­gist Greg­o­ry Ve­li­cer, co-author of a re­port on the find­ings in the May 17 is­sue of the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences.

When environmental conditions are hospitable, Myxococcus xanthus takes a rod-shaped form (yellow), swarming, dividing, and competing with other cells for nutrients. When stressed, the bacterium becomes more social, collaborating with other cells to produce round spores (green) that can withstand stress. Image courtesy of Juergen Berger and Supriya Kadam


Ve­li­cer, with Ph.D. stu­dent Paul­ine Man­hes, stud­ied Myx­o­coc­cus xan­thus, a pred­a­to­ry bac­te­ri­um that swarms through soil, kill­ing and eat­ing oth­er mi­crobes by re­leas­ing tox­ic and di­ges­tive com­pounds. 

Like many co­op­er­a­tive crea­tures, M. xan­thus al­so comes to­geth­er in hard times. When fam­ine hits, a col­o­ny will gath­er it­self to­geth­er in an orch­est­rated pro­cess aimed at hun­ker­ing down in a state that can wait out the hard times, or mov­ing its mem­bers to green­er pas­tures. The catch: many mem­bers will have to die help­ing the oth­ers through this tran­si­tion. The mi­cro­bi­al equiv­a­lent of a lot­tery de­ter­mines who dies, and who gets a new lease on life.

And there are cheat­er­s—w­hole strains of bac­te­ria that game the sys­tem of chem­i­cal sig­nals gov­ern­ing the live-or-die pro­cess to boost their chances at a win­ning tick­et, so to speak. Their gain, of course, comes at the ex­pense of the “good cit­i­zens.”

Ve­li­cer and Man­hes ex­posed an “hon­est” strain of M. xan­thus to a “cheat­ing” strain over many cy­cles of this pro­cess, called fruit­ing body de­vel­op­ment. They found that over time, the freeload­ers be­came less and less suc­cess­ful—as long as they were pre­vented from im­prov­ing their ar­se­nal in an arms race that seems to go on con­tin­u­ally be­tween com­pet­ing strains. The cheat­ers grad­u­ally found them­selves with few­er and few­er of their mem­bers in­ducted in­to the sur­viv­ing group, while more up­stand­ing strains in­creas­ingly dom­i­nat­ed that priv­i­leged or­der.

The sci­en­tists pre­vented the cheat­ers from adapt­ing, or evolv­ing, by kill­ing them all at the end of each cy­cle us­ing a tar­geted an­ti­bi­ot­ic. They would then re-intro-duce mem­bers from their orig­i­nal strain in the next cy­cle.

The ex­pe­ri­ment re­vealed that “hon­est” strains con­tin­u­ally act and adapt to keep their un­sav­ory kin in check, though their ex­act strate­gies for do­ing so re­main a mys­tery, Velicer said. The hon­est in­di­vid­u­als might pro­duce some chem­i­cal that crip­ples the cheat­ers, he spec­u­lat­ed.

The re­search­ers al­so tried pit­ting two “hon­est” strains against each oth­er, the only known dif­fer­ence be­tween them be­ing that one had evolved to han­dle the cheat­ers, and the oth­er had­n’t. They found that the first group out­com­peted the sec­ond, but only when cheat­ers were around—a con­firma­t­ion that their new adapta­t­ions were spe­cif­ic­ally cheat­er-oriented, Velicer con­tends.

“Mech­a­nisms that pre­vent, mit­i­gate or elim­i­nate so­cial con­flict among in­ter­act­ing in­di­vid­u­als are re­quired for coop­era­t­ion or mul­ti­cel­lu­lar­ity to suc­ceed,” Ve­li­cer said. “Polic­ing is one such mech­an­ism. This study shows that bac­te­ria have the po­ten­tial to evolve be­hav­iors that elim­i­nate fit­ness ad­van­tages de­rived from cheat­ing with­in so­cial groups.”

In an in­tri­guing twist, he added, some popula­t­ions de­scended from the “hon­est” an­ces­tors be­came cheat­ers them­selves, but of a new kind that could some­times ex­ploit both the co­op­er­a­tive an­ces­tor and the non-e­volv­ing cheat­er.

The study may cast a shad­ow on re­cent pro­pos­als that cheat­ers might be used to thwart in­fec­tions of bac­te­ria that coop­erate with each oth­er to cause dis­ease in hu­mans, Ve­li­cer said. The bas­ic idea of such pro­pos­als is to in­tro­duce cheat­ers that will dis­rupt the so­cial co­he­sion of in­fect­ing bac­te­ri­al popula­t­ions. But just as bac­te­ria readily evolve re­sist­ance to an­ti­bi­ot­ics, co­op­er­a­tive bac­te­ria that in­fect hu­mans or an­i­mals may evolve to beat the cheats, he warned.


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At least some bacteria can “police” cheaters in their midst, a study has found, although how they do so is unclear. “Even simple organisms such as bacteria can evolve to suppress social cheaters,” said Indiana University Bloomington biologist Gregory Velicer, co-author of a report on the findings in the May 17 issue of the journal Proceedings of the National Academy of Sciences. Velicer, with Ph.D. student Pauline Manhes, studied Myxococcus xanthus, a predatory bacterium that swarms through soil, killing and eating other microbes by secreting toxic and digestive compounds. Like many cooperative creatures, M. xanthus also comes together in hard times. When food is short, a colony gathers itself together in an organized attempt to move to more promising areas. The catch: many members will have to die helping the others to get away. The microbial equivalent of a lottery determines who dies, and who gets a new lease on life. And there are cheaters—whole strains of bacteria that game the system of chemical signals governing the live-or-die process to boost their chances at a winning ticket, so to speak. Their gain, of course, comes at the expense of the “good citizens.” Velicer and Manhes exposed an “honest” strain of M. xanthus to a “cheating” strain over many cycles of this process, called fruiting body development. They found that over time, the freeloaders became less and less successful—as long as they were prevented from improving their arsenal in an arms race that seems to go on continually between competing strains. The cheaters gradually found themselves with fewer and fewer of their members inducted into the surviving group, while more upstanding strains increasingly dominated that privileged order. The scientists prevented the cheaters from adapting, or evolving, by killing them all at the end of each cycle using a targeted antibiotic, then re-introducing members from an original strain in the next cycle. The experiment revealed, in other words, that “honest” strains continually act and adapt to keep cheaters in check, though their exact strategies for doing so remain a mystery, Velicer said. The honest individuals might produce some chemical that cripples the cheaters, he speculated. The researchers also tried pitting two “honest” strains against each other, the only known difference between them being that one had evolved to handle the cheaters, and the other hadn’t. They found that the first group outcompeted the second, but only when cheaters were around—a confirmation that their new adaptations were specifically cheater-oriented, Velicer contends. “Mechanisms that prevent, mitigate or eliminate social conflict among interacting individuals are required for cooperation or multicellularity to succeed,” Velicer said. “Policing is one such mechanism. This study shows that bacteria have the potential to evolve behaviors that eliminate fitness advantages derived from cheating within social groups.” In an intriguing twist, he added, some populations descended from the “honest” ancestors became cheaters themselves, but of a new kind that could sometimes exploit both the cooperative ancestor and the non-evolving cheater. The study may cast a shadow on recent proposals that cheaters might be used to thwart infections of bacteria that cooperate with each other to cause disease in humans, Velicer said. The basic idea of such proposals is to introduce cheaters that will disrupt the social cohesion of infecting bacterial populations. But just as bacteria readily evolve resistance to antibiotics, cooperative bacteria that infect humans or animals may evolve to beat the cheats, he warned.