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January 18, 2011

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In war on germs, “backstabbers” could be our friends on the inside

Sept. 6, 2010
Courtesy of the University of Nottingham
and World Science staff

Days af­ter sci­en­tists re­ported dis­cov­er­ing “char­i­ta­ble” bac­te­ria whose ac­ti­vi­ties might help re­veal new in­fec­tion-fighting strate­gies, oth­er re­search­ers have an­nounced an op­po­site sort of find.

Un­like the “char­i­ta­ble” bugs—which use their rare an­ti­bi­ot­ic-re­sis­tant abil­i­ties to help shield their whole group from an­ti­bi­ot­ic at­tack—the newly re­ported bac­te­ria are said to be back­stab­bers. They free-ride on their own popula­t­ion of in­fec­tious bac­te­ria, sci­en­tists say, bur­den­ing their group by not help­ing to pro­duce a tox­in that keeps the in­fec­tion go­ing.

This is of course nice for the af­flicted pa­tients, researchers add. By sab­o­tag­ing their own com­pan­ions, free­loading bac­te­ria could make treat­ment eas­i­er for us, ac­cord­ing to the stu­dy, pre­sented at the So­ci­e­ty for Gen­er­al Mi­cro­bi­ol­o­gy’s au­tumn meet­ing on Sept. 6 at the Uni­vers­ity of Not­ting­ham, U.K.

Bac­te­ria are be­lieved to co­op­er­ate by us­ing a com­mu­nica­t­ion sys­tem called Quor­um Sens­ing. Dur­ing in­fec­tion, bac­te­ria use quor­um sens­ing to co­or­di­nate the re­lease of tox­ins.

The re­search­ers, al­so from the Uni­vers­ity of Not­ting­ham, found that in Staph­y­lo­coc­cus au­re­us in­fec­tions, bac­te­ria de­fec­tive in quor­um sens­ing can “opt out” of tox­in pro­duc­tion. By do­ing so, they can in­vest more en­er­gy in re­pro­duc­ing—while tak­ing ad­van­tage of the nutrient-rich in­fec­tion main­tained by their neigh­bours.

The freeload­ers thus soon out­num­ber bac­te­ria that are busy pro­duc­ing tox­ins, and the over­all in­fec­tion sev­er­ity drops. “This opens up the in­ter­est­ing pos­si­bil­ity of us­ing these un­co­op­er­a­tive bac­te­ria to treat in­fec­tion,” said Er­ic Pol­litt of the uni­vers­ity, who is pre­sent­ing the stu­dy. (Click here for a full-length inter­view with study col­lab­orator Ste­phen P. Dig­gle).

The “back­stab­bers” are known as quor­um sens­ing-deficient, or QS-deficient, bac­te­ria.

The group tested the effects of these microbes by in­fect­ing wax­worms with them. “We found that the QS-deficient bac­te­ria could not only out­grow nor­mal bac­te­ria in the same popula­t­ion, but that they could al­so in­vade oth­er, co­op­er­at­ing popula­t­ions,” ex­plained Pol­litt. “This means that we could po­ten­tially iso­late QS-deficient bac­te­ria and use them to treat clin­i­cal S. au­re­us in­fec­tions.”

* * *

Days after scientists reported discovering “charitable” bacteria whose activities might help reveal new infection-fighting strategies, other researchers have announced an opposite sort of find. Unlike the “charitable” bugs—which use their rare antibiotic-resistant abilities to help shield their whole group from antibiotic attack—the newly reported bacteria are said to be backstabbers. They free-ride on their own population of infectious bacteria, scientists say, burdening their group by not helping to produce a toxin that keeps the infection going. This is of course nice for the afflicted patients, scientists say. By sabotaging their own companions, “backstabbing” bacteria make treatment easier for us, according to the study, presented at the Society for General Microbiology’s autumn meeting on Sept. 5 at the University of Nottingham, U.K.. Bacteria are believed to cooperate by using a communication system called Quorum Sensing. During infection, bacteria use quorum sensing to coordinate the release of toxins. The researchers, also from the University of Nottingham, found that in Staphylococcus aureus infections, bacteria defective in quorum sensing can benefit from “opting out” of toxin production. By doing so, they can invest more energy in reproducing—whilst taking advantage of the nutrient-rich infection maintained by their neighbours. The freeloaders thus soon outnumber bacteria that are busy producing toxins, and the overall infection severity drops. “This opens up the interesting possibility of using these uncooperative bacteria to treat infection,” said Eric Pollitt of the university, who is presenting the study. The “backstabbers” are known as quorum sensing-deficient, or QS-deficient, bacteria. The group tested the theory by infecting waxworms. “We found that the QS-deficient bacteria could not only outgrow normal bacteria in the same population, but that they could also invade other cooperating populations to reduce the severity of infection,” explained Pollitt. “This means that we could potentially isolate QS-deficient bacteria and use them to treat clinical S. aureus infections.”