Infectious Diseases News Brief - November 27, 2709

[Current Issue -Table of contents]

A Novel Study Discovers A New Communication System Between Streptococci

Group A streptococcus (GAS) is a human pathogenic bacteria. These bacteria can cause a variety of human diseases ranging from superficial skin and throat infections to highly invasive life-threatening diseases such as toxic shock and necrotizing fasciitis, commonly known as the flesh-eating bacteria. Along with the consequences of autoimmune complications of rheumatic fever and rheumatic heart diseases, a conservative estimate of 500,000 deaths per year globally due to GAS infections has been calculated, placing this bacterium as one of the top 10 infectious causes of mortality. Little is known about what controls the conversion of the bacteria from a non-harming form to the pathogenic state in GAS infections. Since these bacteria generally exist as communities and not as solitary microorganisms, bacterial communication systems are key elements in determining host-bacterial interactions. Most communication between bacterial cells is done by signaling molecules secreted and sensed by the bacteria. When the level of the signaling molecules is high enough, they can activate the expression of genes that coordinate their behavior. This activation only takes place in the presence of a sufficient number (a quorum) of bacteria, giving this mechanism the name quorum-sensing. A novel research, led by Professor Emanuel Hanski from the Department of Microbiology and Molecular Genetics at the Medical School of the Hebrew University of Jerusalem, identified a new array of genes in GAS and in a close relative, Group G Streptococcus (GGS), usually considered a commensal that do not harm the host. These genes are activated by a quorum-sensing peptide termed SilCR. SilCR is not functional in highly invasive GAS strains, suggesting that this array of genes may be involved in colonization and establishment of commensal host-bacterial relationships. The researchers further show that GAS and GGS strains can sense their respective SilCR molecules, thus coordinating their pathogenicity, and comprising a novel communication system between these bacteria.

Source: Medical News Today, November 10, 2009 http://www.medicalnewstoday.com/articles/170441.php

How Ticks Transmit Lyme Disease to Humans: Imaging Technique Leads to Better Understanding

Using a powerful microscopic live imaging technique, a research team led by Dr. Justin Radolf, professor in the Departments of Medicine and Genetics and Developmental Biology at the University of Connecticut Health Center, has discovered the way ticks transmit Lyme disease to humans is different than previously thought. Lyme disease is caused by transmission of the spirochete bacterium Borrelia burgdorferi from ticks to humans but for a number of technical reasons, the transmission process has been difficult to study. Radolf and researchers Star Dunham-Ems and Melissa Caimano tried a novel approach. They genetically modified a virulent strain of B. burgdorferi to express green fluorescent protein (GFP). Spirochetes in culture are highly motile, and it is widely believed that during feeding, the spirochetes in the midgut rapidly move through the wall of the midgut. But Radolf and his team found that during much of the feeding period, the spirochetes do not move. They actually divide and surround the cells of the midgut lining or epithelium, forming tight networks. Eventually, spirochetes in the networks reach the base of the epithelium by completely surrounding the epithelial cells. At this point, they become motile, detach, and completely penetrate the midgut, although in very small numbers. These few bacteria then swim to the salivary glands, which they penetrate en route to the mouse. Lyme disease is the most prevalent vector-borne infection in the United States with more than 25,000 new cases reported annually. A substantial percentage of these cases occur in Connecticut.

Source:Science Daily November 17, 2009
http://www.sciencedaily.com/releases/2009/11/091116180134.htm

Population Movement Can Be Critical Factor in Dengue's Spread

Human movement is a key factor of dengue virus inflow in Rio de Janeiro, according to results from researchers based at the Oswaldo Cruz Foundation (Fiocruz) in Brazil. The results, based on data from a severe epidemic in 2007-2008, contribute to new understanding on the dynamics of dengue fever in the second largest city in Brazil. Dengue fever is a major public health problem in many tropical regions of the world. It is a vector-borne disease, transmitted most often by the mosquito Aedes aegypti. According to the WHO, the prevalence of dengue is highest in tropical areas of Asia and the Americas, with 50-100 million estimated cases of dengue fever and 250,000-500,000 cases of dengue hemorrhagic fever occurring annually worldwide as explosive outbreaks in urban areas. In Brazil, three dengue virus serotypes (DENV) have been introduced in the past three decades. In 2007-2008, a dengue fever epidemic in Rio de Janeiro led to 240 deaths registered (100 deaths due to dengue hemorrhagic fever and 140 due to other dengue-related complications). This populous city presents highly favorable conditions for transmission of dengue. Dengue surveillance and control in large urban areas with high levels of dengue transmission pose important challenges. Therefore, consistent knowledge of the dynamics of this disease that integrates epidemiological and entomological data is essential. The present research combines data on dengue fever seroprevalence, recent dengue infection, and vector density in three neighborhoods of Rio de Janeiro: an urban, a suburban, and a slum area. Serological surveys were conducted before and during the epidemic period. Entomological surveys consisted of weekly collections of A. aegypti eggs and adults from traps.

This integrated entomological-serological survey showed evidence of silent transmission even during a severe epidemic. No association was observed between household infestation index and risk of dengue infection in these areas, raising new questions about where transmission occurs -- in the household, at work or elsewhere. When combined, the neighborhood-specific seroprevalence maps correlated significantly higher risk with areas of intense people traffic.

Source: Science Daily November 12, 2009
http://www.sciencedaily.com/releases/2009/11/091110065920.htm