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Contact: Jim Sliwa
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202-942-9297
American Society for Microbiology

Tips from the journals of the American Society for Microbiology

November 2012

Cigarette Smoke Boosts Virulence in Staphylococcus aureus

Exposure to cigarette smoke has long been associated with increased frequency of respiratory infections—which are harder to treat in smoke-exposed people than in those who lack such exposures. Now Ritwij Kulkarni of Columbia University, New York, NY, and colleagues show that cigarette smoke actually boosts virulence of Staphylococcus aureus bacteria. Their study appears in the November 2012 issue of the journal Infection and Immunity.

S. aureus is a normally harmless inhabitant of the upper respiratory tract, but one which can morph into a dangerous pathogen capable of causing severe, and even fatal infections, says Kulkarni. The new research shows that cigarette smoke can aid and abet that transformation.

Cigarette smoke does so by enhancing S. aureus' ability to form biofilms, which are an important virulence factor, according to the study. The research showed further that reactive oxygen species, such as H2O2, which are concentrated in cigarette smoke, drive biofilm formation, says Kulkarni.

Kulkarni notes that a recent paper, from another group, showed that reactive oxygen species suppress the gene regulator, "Accessory Gene Regulator," or agr for short. "That fits nicely with our story," he says. "We think control of biofilm formation [and of numerous other virulence factors in S. aureus] proceeds via agr."

(R. Kulkarni, S. Antala, A. Wang, F.E. Amaral, R. Rampersaud, S.J. LaRussa, P.J. Planet, and A.J. Ratner, 2012. Cigarette smoke increases Staphylococcus aureus biofilm formation via oxidative stress. Infect. Immun. 80:3804-3811.)

Download a copy of the journal article at http://bit.ly/asmtip1112a.


Norovirus Disinfection: How Much is Enough?

A variety of institutions and governments have developed "commonsense-based" disinfection guidelines to control norovirus contamination, but now, for the first time, a Dutch team has come up with science-based guidelines. The research is published in the November 2012 issue of the journal Applied and Environmental Microbiology.

Norovirus is the most common cause of gastroenteritis, according to the Centers for Disease Control and Prevention (CDC). This fecal-orally transmitted virus is notorious for spreading like wildfire in schools, on ocean cruises, and where-ever one infected person may be involved in the feeding of hundreds more, leaving victims tossing their cookies for as long as several days. Norovirus is especially problematic when it strikes hospitals, as both staff and patients are laid low.

In the study the researchers first determined how low the concentration of virions needed to go for transmission to become unlikely. They then tested different methods of cleaning hard surfaces, by using water, soap, or chlorine bleach solutions to determine the best method, or combination of methods for achieving a sufficiently low concentration to prevent virus transmission from hard surfaces, such as your kitchen counters.

The researchers prescribe a two-step process: wipe with a wet cloth, and then disinfect with chlorine. Their prescription is directed at hospitals, schools, restaurants, and other food-handling establishments. In most cases, a 250 ppm solution of chlorine is sufficient, but for high levels of contamination, they recommend 1,000 ppm, says principal investigator Erwin Duizer, of the National Institute for Public Health and the Environment, Bilthoven, the Netherlands.

To achieve 1,000 ppm, one Suma Tab D4 tablet must be dissolved in 1.5 liters of water (about a quart and a half). Household bleach, says Duizer, is an average about 5 percent chlorine when new (concentration declines with age), or 50,000 PPM, and thus, can be diluted 50-fold to achieve the 1,000 ppm.

Norovirus is apparently no more resistant to cleaning and disinfection than other pathogens, says Duizer. The virus' efficiency in causing outbreaks "is more likely due to their extremely low infectious dose," resulting in the requirement of a very low level of residual contamination in order to prevent further transmission. "Fortunately, reducing the level of residual contamination to that low level is not that difficult and can be achieved without extreme measures," says Duizer.

"The current guideline for norovirus outbreaks in the Netherlands is quite stringent in some aspects," says Duizer. "During recent years I have heard many complaints from people in the field that 'it just can't be done.'" The new guidelines will be more practical, and thus more effective, he says.

(E. Tuladhar, W.C. Hazeleger, M. Koopmans, M.H. Zwietering, R.R. Beumer, and E. Deuzer, 2012. Residual viral and bacterial contamination of surfaces after cleaning and disinfection. Appl. Environ. Microbiol. 78:7769-7775.)

Download a copy of the journal article at http://bit.ly/asmtip1112b.


Antibiotics Disrupt Gut Flora in Infants: Recovery Still Incomplete After 8 Weeks

Eight weeks after antibiotic treatment of infants, the diversity of gastrointestinal flora remained diminished, although the number of individual bacteria was back to normal, according to a paper in the November 2012 issue of the journal Antimicrobial Agents and Chemotherapy. Additionally, the potentially disease-causing Proteobacteria were now the dominant population in the treated infants.

"This is the first sequencing-based study to demonstrate the negative effects of short-term antibiotic treatment on the beneficial gut bacteria populations in infants," says coauthor Catherine Stanton of the Teagasc Food Research Centre, Fermoy, Cork, Ireland.

In the study, nine infants were treated with intravenous ampicillin/gentamicin within 48 hours of birth, and over the two month study period, their gastrointestinal flora were compared to that in nine control infants. At four weeks, bacteria from the beneficial genera, the Bifidobacteria and the Lactobacilli, were significantly reduced, and although the numbers bounced back by the study's end, the species diversity did not. The researchers used advanced DNA sequencing to identify the species of gut flora, and to quantify their numbers, says Stanton.

By altering the gut microbiota, and thus the immune system very early in life, the antibiotics could negatively influence long-term health, particularly by boosting the risk of developing asthma, allergy, and obesity, according to the report. The risk is heightened by the fact that the antibiotic-driven disruption of the microbiota comes at a time "when this population is in rapid flux and can easily be unbalanced," according to the report.

These results notwithstanding, because the sequencing data reveal only proportions of species present, rather than absolute numbers, it remains unclear whether the potentially harmful Proteobacteria predominate because their population has grown or because the other populations have shrunk, the researchers write. However, the data suggest the former, which jibes with previous research.

"This research suggests that the merits of administering broad spectrum antibiotics—those that kill many bacterial species—in infants should be reassessed, to examine the potential to use more targeted, narrow-spectrum antibiotics, for the shortest period possible," says Stanton.

(F. Fouhy, C.M. Guinane, S. Hussey, R. Wall, C.A. Ryan, E.M. Dempsey, B. Murphy, R.P. Ross, G.F. Fitzgerald, C. Stanton, and P.D. Cotter, 2012. High-throughput sequencing reveals the incomplete, short-term recovery of infant gut microbiota following parenteral antibiotic treatment with ampicillin and gentamicin. Antim. Agents Chemother. 56:5811-5820.)

Download a copy of the journal article at http://bit.ly/asmtip1112c.


Grape Seed Extract Bollixes Norovirus

Norovirus causes more than half of all food-born illnesses in the United States, and is the second greatest source of reported food borne illness outbreaks in the European Union. A recent study found that grape seed extract could reduce the infectivity of Norovirus surrogates (Norovirus surrogates are viruses that share pathological and/or biological features with human norovirus). Now, Dan Li of Ghent University, Ghent, Belgium and collaborators have shown that grape seed extract does so by denaturing the capsid protein, which is the coat of the virus, thereby disabling the virus. The research is published in the November 2012 issue of Applied and Environmental Microbiology.

In the study, the researchers observed that under treatment with grape seed extract, at low doses, the spherically-shaped murine (mouse) norovirus-1 coat proteins clumped, and showed "obvious deformation and inflation," according to the report. At higher doses, the researchers saw no coat proteins, only protein debris. "This provides evidence that [grape seed extract] could effectively damage the [norovirus] capsid protein, which could reduce viral binding ability and infectivity accordingly," according to the report.

The researchers used surrogate viruses because there are no suitable animal models of norovirus, and human norovirus has been impossible to propagate in cell cultures. The surrogate virus, murine norovirus-1, can be grown in cell culture, and belongs to the same genus as human norovirus, and has a very similar genome structure, and morphology. Nonetheless, the researchers were able to measure the specific binding strength of human norovirus by two different methods, finding that it declined precipitously under the influence of grape seed extract, providing further support to their results.

Norovirus is transmitted mainly fecal-orally, and infected food handlers, contaminated water, and surfaces can be identified as important sources of transmission, "which could further contaminate ready-to-eat foods, drinking water, shellfish, and fresh produce," says Li. A mere 10-100 virus particles are sufficient to transmit the disease.

(D. Li, L. Baert, D. Zhang, M. Xia, W. Zhong, E. Van Coillie, X. Jiang, and M. Uyttendaele, 2012. Effect of grape seed extract on human norovirus GII.4 and murine norovirus 1 in viral suspensions, on stainless steel discs, and in lettuce wash water. Appl. Environ. Microbiol. 78:7572-7578.)

Download a copy of the journal article at http://bit.ly/asmtip1112d.

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