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News From The Journals Of The American Society For Microbiology

Human Insulin Suppresses Mosquito Immune System: Increasing Cases of Type II Diabetes Could Abet Malaria’s Spread

Human insulin suppresses the mosquito immune system, according to a paper in the June Infection and Immunity. And while mosquitoes and malaria might seem to go together like baseball and hotdogs, mosquitoes’ immunological resistance to the malaria parasite actually slows its spread among H. sapiens.

“A fair portion actually fight off the infection,” says first author Nazzy Pakpour of the University of California, Davis.

But now the rate of type 2 diabetes is climbing in Africa as in most of the rest of the world, to the point where by 2030, one in five adults there are predicted to be so-afflicted. More diabetes means more hyperinsulinemia – more human insulin to inhibit mosquitoes’ immune response to Plasmodium falciparum, thus aiding and abetting transmission of this dread disease.

As horrific as the medical consequences of all this might be, the science is intriguing. “It’s crazy to think something in our blood could change how mosquitoes respond to parasites,” says Pakpour.

In earlier work, Pakpour and collaborators showed that ingested human insulin activates the insulin/IGF-1 signaling pathway in Anopheles stephensi mosquitoes, making them more vulnerable to invasion by P. falciparum. The new study showed that insulin signaling reduced expression of certain mosquito immunity genes that are under the same regulatory control, and that human insulin suppressed mosquito immunity by activating the so-called PI3K signaling pathway, and that artificially inhibiting that pathway could reverse the immunosuppressive effects of human insulin.

(N. Pakpour, V. Corby-Harris, G.P. Green, H.M. Smithers, K.W. Cheung, M.A.Riehle, and S. Luckhart, 2012. Ingested human insulin inhibits the mosquito NF-KAPPAB-dependent immune response to Plasmodium falciparum. Infect. Immun. 80:2141-2149.)

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Repeated Exposure to Triclosan Reduces Virulence in S. aureus

Repeated laboratory exposures to triclosan reduced susceptibility to antibiotics in Staphylococcus aureus, but probably not sufficiently to render commonly used antibiotics ineffective, according to a paper in the June 2012 issue of the journal Antimicrobial Agents and Chemotherapy. It also generated less virulent, less fit “small colony variants” of the pathogen.

The research is important, because concern has arisen that antiseptics and disinfectants might cause bacteria to develop reduced susceptibility to these compounds, as well as to antibiotics, to the point where for some uses, in some countries, they have been strictly regulated, says principal investigator Andrew McBain, of the University of Manchester, UK. S. aureus is an important source of hospital- and community-acquired infections. Triclosan is a broad-spectrum antibacterial and antifungal compound often used in cleaning supplies, personal care products, toys, and other consumer products, as well as in clinical settings, for example, to reduce methicillin-resistant S. aureus infections.

The research began serendipitously when, during an unrelated study, Sarah Forbes, a PhD student in McBain’s lab, created a population of small colony variants by serially exposing S. aureus ten times on concentration gradients of triclosan. “This type of selection system we used represents a worst case scenario in terms of altering bacterial susceptibility because of the repeated and continuously elevating high level exposure,” says McBain. “We then grew this strain a further ten times on triclosan-free medium to see if it could recover.” The exposed strain had reduced susceptibility to triclosan, and it was defective in its ability to form biofilms, as well as in a few other virulence-related functions. “We therefore hypothesized that if virulence had been altered at all in our strain, it had actually been reduced,” he says.

Forbes, and postdoctoral research associate Joe Latimer then grew the small colony variant, as well as the unexposed strain of S. aureus, in the wax moth larvae model, Galleria mellonella, “and found that our small colony variants were indeed less pathogenic in this test system than the unexposed strain,” says McBain. Even after it was grown another ten times in triclosan-free media, the small colony variant failed to fully regain its virulence, as well as its normal ability to form biofilms and to produce the enzyme, DNase.

“The work suggests that at least for small colony variants, long-term exposure can select for reduced susceptibility, but that the resulting organisms may also be reduced in their pathogenic capability, or fitness,” says McBain. He adds that “even though our small colony variants were less susceptible, their resistance levels remained markedly lower than commonly used concentrations so they were still probably effectively treatable.”

(J. Latimer, S. Forbes, and A.J. McBain, 2012. Attenuated virulence and biofilm formation in Staphylococcus aureus following sublethal exposure to triclosan. Antim. Agents Chemother. 56:3092-3100.)

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Changing Pigs’ Diets Alters the Gut Microbiota

Including chicory in cereal-based diets of pigs results in profound changes in gut micro-environment, morphology, and microbial population of pigs, according to a study in the June 2012 Applied and Environmental Microbiology. Some of these changes were health-promoting, says principal investigator Jan Erik Lindberg of the Swedish University of Agricultural Sciences, Uppsala.

“Certain types of substrates, such as dietary fiber, escape digestion in the foregut and reach the hindgut of humans and mono-gastric animals, and are becoming important in applied nutrition,” says Lindberg. These substrates, called “prebiotics,” can alter gut bacterial composition, modify intestinal fermentation processes, promote gut development, and possibly improve health.

The changes were similar in the small and large intestines, but they differed for chicory forage and chicory root, says Lindberg. They also differed for both chicory forage and root as compared to the control diet, in potentially important ways, according to the report. For example, a lactic acid bacterium, Lactobacillus johnsonii, is involved in regulating production of the immune compound interleukin-12, while Lactobacillus mucosae is reported to possess probiotic mucus-binding ability. Both lactobacilli were dominant when chicory was included in the diet.

Additionally, the presence of chicory forage in the feed boosted the numbers of bacteria that produce butyrate, a key substrate for the epithelial cells that line the colon, as well as a signaling molecule for the gut immune system, says Lindberg. The major butyrate-producing bacteria were Faecalibacterium prausnitzii, Eubacterium rectale, and Roseburia sp. As their numbers rose, those of Prevotella spp.declined.

Prevotella are strictly anaerobic bacteria that have been identified as a dominant species in the large intestine of pigs, and are also abundant in the ileum. Additionally, they were found (by other researchers) to be dominant in the gut microbiota of rural African children living largely on millet grain, sorghum, legumes, and vegetables.

Pigs fed chicory roots contained copious Megashaera elsdenii, a bacterium which is abundant in the colon of pigs fed a diet designed to prevent swine dysentery.

The research originated in the search for prebiotic fiber sources, says Lindberg. “In this context, chicory was a good candidate as both the root and the above-ground biomass, the forage, can be eaten by animals and humans. We knew that there are many members of the chicory family that we regularly eat in salad.”

“Prebiotic dietary effects can be used to minimize the occurrence of enteric disease, thereby reducing the need to use antimicrobials,” says Lindberg. “This will improve animal productivity and welfare, and will also minimize the occurrence of contaminated products that can pose threats in the human food chain.” Lindberg also says that he would expect a similar response to these dietary changes in humans.

(H. Liu, E. Ivarsson, J. Dicksved, T. Lundh, and J.E. Lindberg, 2012. Inclusion of chicory (Cichorium intybus L.) in pigs’ diets affects the intestinal microenvironment and the gut microbiota. Appl. Environ. Microbiol. 78:4102-4109.)

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Researchers Determine 3D Structure of Adeno-Associated Virus 9: Aim to Boost Gene Therapy

A team of researchers led by the University of Florida, Gainesville, has determined the precise structure of a virus that has promise as a delivery vehicle for gene therapy. The research appears in the June 2012 issue of the Journal of Virology.

Adeno-associated viruses are benign in humans, and are highly promising in gene therapy as delivery devices to place healthy genes into the genome, in order to compensate for malfunctioning genes. These viruses come in 12 different serotypes (sets of antigens). Adeno-associated virus 9 (AAV9) is currently under development as a delivery vehicle for treating neurodegenerative diseases, such as spinal muscle atrophy, amyotrophic lateral sclerosis, and Parkinson’s disease.

Mavis Agbandje-McKenna of the University of Florida, Gainesville and her colleagues have determined the precise structure of AAV9, work she says “will help us to understand which parts of the capsid we can alter or modify to make safer, more efficient vectors, and which regions should not be modified as we try to engineer capsids to treat specific diseases.” The capsid is the shell that protects the viral nucleic acid.

The researchers applied X-ray crystallography, the technique that was used to determine DNA’s structure nearly 60 years ago, as well as a complementary, and much newer technique called cryo-electron microscopy and image reconstruction, to determining AAV9′s structure. That work revealed the precise position in space of every atom of the AAV9 capsid. They then compared that structure to other adeno-associated viruses for which structures have been determined, and identified which regions are conserved, and which vary, in comparison to AAV9. They then annotated these regions with respect to function: different parts of the capsid are involved in different functions such as receptor attachment, determining the efficiency of transduction in specific tissues, and antibody recognition.

“Our goal is to use 3D information to inform the design of gene delivery vectors that will have improved efficacy with respect to tissue targeted delivery of therapies, and reduced host immune antibody response recognition,” says Agbandje-McKenna. “The information that we have obtained is guiding further research in our group as well as groups elsewhere who are trying to understand the functions of the capsid, in an effort to improve gene delivery via AAV.” She notes that AAV9 is especially important in these areas because of its ability to cross the blood brain barrier, adding that this makes it particularly useful for treating brain diseases, “for which current therapies are quite limited.”

(M.A. DeMattia, H.-J. Nam, K. Van Vliet, M. Agbandje-McKenna, et al. Structural insight into the unique properties of adeno-associated virus serotype 9. J. Virol. 86:6947-6958.)

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American Society for Microbiology