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Tips from the journals of the American Society for Microbiology

American Society for Microbiology

Surprising Study Results: More Cattle Means Less Lyme Disease

The abundance of cattle is the primary influence on the prevalence of two tick-borne pathogens, according to a paper in the April Applied and Environmental Microbiology. One of these, Anaplasma phagocytophilum, causes human granulocytic anaplasmosis, and the other, Borrelia burgdorferi, causes Lyme disease. Although other studies have examined the effect of hosts on tick and tick-borne pathogen dynamics, this is the first to clarify the role of host abundance on prevalence of the two pathogens in their natural habitat, where wildlife and domestic livestock coexist.

The impetus for the research was the fact that in recent decades, gamekeepers in the study area, a wildlife preserve in the northern Iberian peninsula, had suffered Lyme disease, and had noticed an increasing abundance of ticks, says first author Francisco Ruiz-Fons, of the Instituto de Investigación en Recursos Cinegéticos, Ciudad Real, Spain. "Our working hypothesis was that wild and domestic ungulates would be primary drivers of the abundance of Ixodes ricinus ticks, and of the prevalence of pathogens transmitted by this tick species, in natural foci where they coexist and where B. burgdorferi and A.phagocytophilum are endemic."

That hypothesis held up, but somewhat differently from expected, having opposite effects on A. phagocytophilum and lyme bacterium, B. burgdorferi, in reducing prevalence of the latter. That seemingly counterintuitive finding stemmed from the fact that "cattle and wild ungulates may act as diluters of borrelias by diverting infected ticks' bites from competent reservoirs such as birds or small mammals," says Ruiz-Fons. Thus, more cattle meant reduced numbers of Lyme disease bacteria.

"The most important application of our findings is that if we want to reduce the risk of animals and humans becoming infected by [tick-borne] pathogens we should control the infestation by ticks in cattle--and perhaps in wildlife--rather than by reducing cattle or wild host abundance in areas where wild and domestic animals coexist," says Ruiz-Fons.

(F. Ruiz-Fons, I.G. Fernandez-de-Mera, P. Acevedo, C. Gortazar, and J. de la Fuente, 2012. Factors driving the abundance of Ixodes ricinus ticks and the prevalence of zoonotic I. ricinus-borne pathogens in natural foci. Appl. Environ. Microbiol. 78:2669-2676.)

Rare Emerging Disease Claims Texas Girl's Leg: PCR Plus Sequencing Identified Culprit

A 14-year-old Texas girl was finally cured of an oft-fatal emerging disease when doctors amputed her lower leg, where the infection arose, after various antimicrobials proved ineffective. The culprit was Pythium insidiosum, a fungus-like microbe which rarely causes disease in humans and then primarily in Thailand. The case "clearly highlights the need for clinicians to have the best support possible from the clinical microbiology lab," says Don Murphey of Cook Children's Medical Center, who served as attending physician during the case. The case report is published in the April Journal of Clinical Microbiology.

The girl, otherwise healthy, presented to an urgent care facility with a 2-week history of a continuously enlarging erythematous bump on her lower leg, having reported recently swimming in an algae-filled pool. "Over the course of several weeks, what started as a very small lesion grew to involve most of her leg," says first author Stephen J. Salipante, of the University of Washington, Seattle. "Initial cultures of the wound suggested that this was a bacterial infection, and it was treated as such, but without success."

"She eventually needed to be hospitalized," says Salipante. Her treatment team at Cook Children's hospital tried increasingly aggressive medical and surgical management, including different antibiotic regimens, antifungals, and surgical debridements, but the infection simply didn't respond. "Given the microscopic appearance of the organism, our working hypothesis was that this was some kind of unusual, and very aggressive fungus," says Salipante.

However, sequencing a segment of DNA that is useful for categorizing fungi, the ITS1 sequence, "revealed that this was not a fungus at all--rather, the DNA sequence very closely matched... P. insidiosum," says Salipante. This microbe has long been known to be a veterinary pathogen, primarily affecting horses and dogs, disfiguring them, often fatally. Only about 150 human cases have been reported in the literature--nearly all of them in Thailand. "Needless to say, this was an unexpected result, as this young woman had not left Texas," says Salipante.

But "this organism did not demonstrate sensitivity in vitro to any of the antifungals or antimicrobials that had some activity for other isolates," says Murphey. Efforts at treatment even included trying an experimental therapeutic vaccine for Pythium-afflicted horses," says Murphey. "When it became clear to all of us that we were not going to clear the advancing local infection, we went on to amputation."

The amputation "undoubtedly saved this young woman's life," says Salipante. Now, nearly six months later, there is no vestige of infection.

"Pythiosis is believed to be an emerging human pathogen, meaning that the number of cases are expected to go up in the future," says Salipante.

"We have demonstrated that molecular identification by PCR screening and DNA sequencing provides a strategy to allow definitive identification of a range of pathogens, even unsuspected ones," says Salipante.

"DNA sequencing is a not just a tool for discovery anymore - it provides critical data for making decisions impacting patient care," says coauthor Brad T. Cookson of the University of Washington. "I would add that medical care currently offers patients something very special - our report demonstrates the vitality and utility of collaborative, transdisciplinary approaches for solving challenging medical problems."

(S. J. Salipante, D. R. Hoogestraat, D. J. SenGupta, D. Murphey, K. Panayides, E. Hamilton, I. Castañeda-Sánchez, J. Kennedy, P. W. Monsaas, L. Mendoza, K. Stephens, J. J. Dunn, and B. T. Cookson. 2012. Molecular Diagnosis of Subcutaneous Pythium insidiosum Infection by Use of PCR Screening and DNA Sequencing. J. Clin. Microbiol. 50:1480-1483)

Intestinal Flora of Cockroaches and Termites Reflects These Insects' Family Relationships, and Divergent Diets

Researchers at the Max Planck Institute for Terrestrial Microbiology, Marburg, Germany, have compared the microbial ecosystems in the intestines termites and cockroaches, with fascinating results. The research is published in the April Applied and Environmental Microbiology.

It may be hard for people outside of certain scientific domains to muster anything but disgust for termites and cockroaches. Cockroaches, after all, infest our homes, and termites eat them. But despite their different life strategies--termites feed exclusively on wood, while cockroaches are the epitome of omnivory--these two culturally stigmatized insects are closest relatives. The microbial denizens of the termite gut have been the objects of intense study by microbiologists, with the goal of greatly boosting the conversion efficiency of cellulosic materials to biofuels, but cockroaches' intestinal inhabitants have gone ignored, despite suspicions that pathogens are among them.

"We wanted to determine to what extent, despite striking differences in diet, the gut community of cockroaches resembles that of their closest relatives, the termites," says coauthor Claire L. Thompson. "We found that termites and cockroaches contain many gut bacteria of the same families, which indicates that the evolutionary history of the host is an important factor determining the structure of the gut microbial community. However, we found also that the abundance of these different lineages differs fundamentally between termites and cockroaches, which we ascribe to their different diets." In fact, she says, the relative abundance of different bacterial groups in the cockroach gut more closely resembles that of other omnivores, such as humans and mice.

"Our research suggests that the gut microbiota of termites and cockroaches reflects both their common evolutionary origin and their different feeding habits," says Thompson. "Many bacterial lineages seem to have been associated with the cockroaches already when the termites split off more than 130 million years ago." Additionally, the researchers showed that the bacterial community of the cockroach intestine is "much more complex than it appeared from previous cultivation-based studies," and disease causing microorganisms therein "are actually quite rare."

In the paper, the researchers note that termites fall within the radiation of cockroaches, and that they "should be considered merely a family of social cockroaches." But current taxonomy has yet to catch up with these relatively recent findings.

(C. Schauer, C.L. Thompson, and A. Brune, 2012. The bacterial community in the gut of the cockroach Shelfordella lateralis reflects the close evolutionary relatedness of cockroaches and termites. Appl. Environ. Microbiol. 78:2758-2767.)

Cyanobacterium Demonstrates Promise for Biotechnology Feedstock Production

Harvard Medical School researchers have engineered a photosynthetic cyanobacterium to boost sugar production, as a first step towards potential commercial production of biofuels and other biotechnologically and industrially useful carbon compounds. As feedstock producers, cyanobacteria have advantages over plants, particularly land plants. They need little fertilizer. They don't compete with food crops, because they can grow on marginal land. At commercial scale, the engineered cyanobacteria could potentially produce five times more sugar per acre than traditional crops, including sugarcane, says first author Daniel Ducat. The research is published in the April Applied and Environmental Microbiology.

Cyanobacteria were likely candidates for feedstock production because many freshwater species accumulate sucrose when subjected to salty environments, says Ducat, who is a postdoctoral researcher in Pamela Silver's laboratory at the Harvard Medical School. They do this to mitigate osmotic pressure, which otherwise would dehydrate them, he explains. "We hypothesized that this natural defense mechanism could be employed as a method to continuously produce sugar."

But to maintain continuous sugar production, it was necessary to provide a mechanism to continuously expel the sugar. Mechanisms for moving ions and chemical compounds in or out of cells, against osmotic gradients abound among bacteria. Ducat et al. chose a sucrose permease, which is used by other bacteria to scavenge sucrose from the environment. Since the chemical gradients between the cell and the environment are reversed in cyanobacteria, "we hypothesized that this same transporter might move sucrose out of the photosynthesizing bacteria," says Ducat.

Everything worked as expected, only better. The cyanobacteria expressing the sucrose transporter expelled sucrose at a constant rate so long as the cells were illuminated to provide energy for photosynthesis. Serendipitously, the rate of photosynthesis in the sugar-exporting cyanobacteria--which belong to the freshwater species Synechococcus elongates--was actually higher than normal. "They display more activity in the enzymes involved in harvesting sunlight--specifically the water-splitting complex, photosystem II--and are capable of fixing carbon dioxide at higher rates than those cyanobacteria not exporting sucrose," says Ducat.

Furthermore, "We found that the levels of sucrose exported in these cyanobacteria could be modulated by both the concentration of salt in the culture and the genetic background of the cyanobacteria," says Ducat.

"Our results provide good proof-of-principle that cyanobacterial cultures could be used to produce biotechnology feedstocks with great efficiency," says Ducat. The researchers also showed that the sugars could support the growth of yeast, organisms used to produce biofuels and other valuable compounds. "Therefore, the sugars produced by cyanobacteria could be used by other microbes without the need to extensively process them," says Ducat--if the process can be scaled up.

That "if" is not inconsequential, says Ducat. "One of the major problems that some earlier scale-up efforts ran into when attempting to culture open raceways of algae were competing species of microbes and algae predators," he says. An alternative is to grow cyanobacteria in a semi-enclosed reactor. Cost then becomes an issue, and "there aren't a lot of great examples of large, inexpensive, fully enclosed photobioreactors," he says.

But if scale-up can be accomplished, the much greater efficiency of production for water-borne organisms is not all that surprising, especially to Ducat's Harvard University colleague, forestry professor Michele Holbrook. In an article in the Harvard University alumni publication, Colloquy, several years ago, Holbrook explained that land-based photosynthesis seems wildly improbable when one examines the numbers. The concentration of carbon dioxide in the atmosphere, 3.8 hundredths of a percent, is far lower than in water. A plant has to hold huge quantities of air inside its leaves in order to obtain adequate CO2, but the extensive surfaces it uses for absorbing CO2 lose water fast, she told Colloquy. Thus, roughly 500 water molecules must cycle through the plant for every carbon dioxide that gets captured. "If I turned the mass of my body into sunflower leaves, I'd have to drink two liters every 30 seconds," she said.

The results of this experiment raise unanticipated scientific questions, says Ducat. One would think that removal of the sucrose in the engineered cells would render them less fit, and thus less productive--less able to produce more sugar as well as cell biomass. The fact that they can boost overall productivity suggests that wild-type cells of this species do not naturally fix carbon as rapidly as they are able. Understanding the mechanisms behind this "may pave the way towards improving photosynthetic efficiencies generally," says Ducat. "We are following up on the mechanisms that these cyanobacteria use to sense and upregulate their photosynthetic activity.

(D.C. Ducat, J.A. Avelar-Rivas, J.C. Way, and P.A. Silver, 2012. Rerouting carbon flux to enhance photosynthetic productivity. Appl. Environ. Microbiol. 8:2660-2668.)


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