Trading resistance via nanotubes?

Evidence for the first ever nanotubes that allow inter-species transfer of macromolecules as big as green fluorescent protein (GFP) was reported today (February 17) in __Cell,__ suggesting a new mechanism of bacterial communication in biofilms with implications on how antibiotic resistance spreads. However, many in the community are holding out for more evidence. "I think it's going to create a sensation," said microbiologist linkurl:Richard Losick;http://www.mcb.harvard.edu/Losick/Research/ from Harvard University, who was not involved in the research. "It's really so surprising." There are other examples of tube-like structures between cells, such as the well-studied plasmodesmata that allow transport between cells in some plant species, as well as recently discovered nanotubes in linkurl:mammalian cells;http://www.ncbi.nlm.nih.gov/pubmed/2818746 that transfer vesicles and organelles. But this is the first evidence that bacteria may also have such nanotubule structures, and the first time such tubes have been suggested to transfer molecules between different species.

Scanning electron micrographs of __B. subtilis
Image: Courtesy of the Ben-Yehuda laboratory

"Nothing like this has been seen in bacteria," said Losick. "Bacteria communicate with each other by sending molecules out and delivering small molecules to each other." While bacteria do make conjugative bridges that pass plasmids during a sort of sexual reproduction, "there has been no evidence that large proteins could pass through," he added.linkurl:Sigal Ben-Yehuda;http://research.ekmd.huji.ac.il/researchers.asp?id=180 and Gyanendra Dubey from The Hebrew University of Jerusalem found the nanotubes after observing what looked like GFP-leakage in their culture plates: Bacteria genetically modified to express GFP appeared to transmit the green glowing protein to nearby bacteria of the same species that lacked the GFP gene. Later, they observed the same transmission between different species, even as distantly related as gram positive and gram negative bacteria.Interestingly, they observed that the fluorescence in the donor cell decreased over time, as the fluorescence increased in the recipient cell. Searching for an explanation for this observation, the team took high resolution scanning electron microscopy photos and noticed large and small filaments connecting the bacteria. To test whether these extensions might have transmitted the GFP, the researchers fixed and stained the bacteria with gold-labeled antibodies against GFP. Indeed, the authors observed the gold particles in the tubes connecting two bacteria. The authors then tested whether bacteria might be able to transmit antibiotic resistance proteins. They constructed two Bacillus subtilis strains -- one resistant to chloramphenicol and the other resistant to lincomycin. Each strain could survive being cultured with its respective antibiotic, but ceased to grow in the presence of the other drug. However, when the two strains were mixed and cultured on plates containing both antibiotics, both strains continued to grow, suggesting that bacteria had shared antibiotic proteins. The transfer of resistance was only temporary, however; when each strain was again plated in isolation, it no longer displayed double resistance. When Ben-Yehuda presented her team's unpublished results at a conference last September, many of the attending researchers expressed skepticism about the work. "I guess it was surprising. They saw it for the first time," said Ben-Yehuda about the evidence she presented. "I've seen it for two years." But the skepticism remains. Researchers question, for example, whether the filaments are really tubular, allowing molecules to be transmitted from one bacterium to another. "Alternative explanations for many observations seem not to have been ruled out," linkurl:S. Dusko Ehrlich;http://www.international.inra.fr/join_us/working_for_inra/portraits/stanislav_dusko_ehrlich a microbiologist from INRA said in an email. For example, the recipient GFP cells show a low level of fluorescence, which could be due to "optical or electronic artifacts," he said. And the EM photographs showing connections between bacteria "could be exopolysaccharides and not nanotubes." Futhermore, the observation that two antibiotic resistant strains both grew on double-antibiotic laden culture could be explained by the fact that antibiotic resistant bacteria will often degrade or sequester antibiotic in its vicinity, he added. "A critical test" of the nanotubes theory would have been to engineer bacteria which expressed one piece of a two-part protein, such as a two-part GFP, Ehrlich said. Only if the proteins were exchanged would the bacteria be able to construct a complete GFP molecule and fluoresce. Other researchers wanted more information on both the genetic basis of the tubes and how they might function. "They have not addressed the structure of the nanotubes," said linkurl:Hans Hermann Gerdes;http://www.uib.no/persons/Hans-Hermann.Gerdes from the University of Bergen, who observed the first linkurl:nanotubes in mammalian cells.;http://f1000.com/1017368Still, "this is a really nice first step," said linkurl:Derek Lovley;http://www.bio.umass.edu/micro/faculty/lovley.html of the Univeristy of Massachusetts. "It's one of those ideas -- people are skeptical until the overwhelming burden of evidence proves that it's true," added Lovley, whose work on conductive nanowires in bacteria garnered its own share of skepticism when first published. "I don't know if Sigal Ben-Yehuda is right or not," said microbiologist linkurl:Antoine Danchin,;http://www.normalesup.org/%7Eadanchin/AD/CVenglish_AD.html CEO of AMAbiotics SAS, a bioremediation company, "but I think that the evidence she shows suggests that it is possible."G. P. Dubey and S. Ben-Yehuda, "Intercellular Nanotubes Mediate Bacterial Communication," Cell, 144:590-600, 2011. DOI: 10.1016/j.cell.2011.01.015
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[17th February 2011]