Evolution of typhoid bacteria

Wed, 29 Nov 2006 10:47:32 PST

( from http://www.rxpgnews.com ) Typhoid fever remains a major health problem in the developing world and continues to cause disease in Europe and on the american continent. The evolutionary history and population structure of Typhi were poorly understood, partly because these bacteria show little genetic diversity. Now a team led by Mark Achtman and Philippe Roumagnac from the Max Planck Institute for Infection Biology, Berlin, has applied population genetic experience from prior work with Yersinia pestis, Escherichia coli, Helicobacter pylori and Neisseria meningitidis to provide novel insights into the evolution of this pathogen.

The team combined its resources to assemble for the first time a globally representative collection of 105 strains of Typhi and investigated the sequence diversity within 90,000 base pairs per strain. Eighty-eight informative sequence differences were detected, showing that the population structure has evolved over the last 10,000 to 43,000 years. Amazingly, the ancestral strain continues to exist today, as do many of its direct descendents, indicating a neutral population structure, whereas normally selective forces lead to extinction of intermediate genotypes. Furthermore, these bacteria are distributed globally, demonstrating that Typhi has spread inter-continentally on multiple occasions.

The authors propose that the unusual population structure of Typhi reflects long-term carriage by asymptomatic carriers, who reached public notoriety at the beginning of the 20th century with "Mr. N the milker" in England and Typhoid Mary (Mary Mallon) in the U.S.A. These individuals infected 100s of people over the decades while they worked in the food production industry. Healthy carriers may have allowed Typhi to survive in hunter-gatherer populations prior to the Neolithic expansion of city states and facilitated its intercontinental spread. Healthy carriers are also consistent with the observation that individual genotypes of Typhi persist for many decades within each country.

Increasing resistance to antibiotics in recent decades has hampered efforts of clinicians to cure typhoid fever. The indiscriminate use of fluoroquinolones, which is a cost-effective, standard treatment for typhoid fever, has been accompanied by a frightening increase in the numbers of resistant Typhi. Investigations of a large strain collection from southern Asia revealed that many different genotypes independently acquired resistance to nalidixic acid, a quinolone. One of these genotypes, H58, has become predominant throughout southern Asia and has even spread to Africa. In Vietnam, up to 95% of Typhi are now resistant to nalidixic acid and many other antibiotics. Although these cases can still be treated with newer antibiotics, those antibiotics are much more expensive than standard fluoroquinolones, which raises the cost of medical treatment. Furthermore, it is likely that Typhi will develop resistance to these antibiotics as well.

The combination of these investigations raises problems for public health measures. Indiscriminate antibiotic usage results in real-time evolution of bacteria that resist treatment. Furthermore, the healthy carrier state provides a safe reservoir for these bacteria which allows them to evade short-term antibiotic treatment and vaccination, indicating that typhoid fever will remain a major health problem for the foreseeable future.

Salmonella bacteria use RNA to assess and adjust magnesium levels

Fri, 07 Apr 2006 03:46:37 PST

( from http://www.rxpgnews.com ) Researchers at Washington University School of Medicine in St. Louis have added a gene in the bacterium Salmonella to the short list of genes regulated by a new mechanism known as the riboswitch.

The Salmonella riboswitch is the first to sense and respond to a metal ion, substantially expanding the types of molecules that riboswitches can detect to help cells assess and react to their environment.

First identified in 2002, riboswitches sense when a protein is needed and stop the creation of the protein if it isn't. That in itself isn't remarkable--scientists have been aware for decades of sensors in the cell that can cause molecules to bind to DNA to turn protein production on and off.

A riboswitch, however, doesn't rely on anything binding to DNA; instead, the switch is incorporated into messages for construction of proteins. These messages are protein-building instructions copied from DNA into strands of RNA. The riboswitch is a sensor within the RNA that can twist it into different configurations that block or facilitate the production of the protein encoded in the message.

Previously identified riboswitches respond to organic compounds such as nucleotides and sugars. The Salmonella riboswitch, reported in the April 7 issue of the journal Cell, responds to magnesium ions, key elements in the stability of cell membranes and reactants in an energy-making process that fuels most cells.

"Magnesium ions are essential to the stability of several different critical processes and structures in the cell, so there has to be a fairly intricate set of regulators to maintain consistent levels of it," says senior investigator Eduardo A. Groisman, Ph.D., professor of molecular microbiology. "To approach such a complex system, we study it in a simpler organism, the Salmonella bacterium."

Groisman and his colleagues uncovered the magnesium riboswitch while they were investigating the MgtA gene, which is controlled by the major regulator of Salmonella virulence, the phoP/phoQ system. The MgtA gene codes for a protein that can transport magnesium across the bacterium's cell membrane. Groisman's group showed 10 years ago that the phoP/phoQ system controls when Salmonella makes MgtA.

When Salmonella experiences a low-magnesium environment, phoQ chemically modifies phoP. The changed phoP binds to DNA, increasing the number of times instructions for making MgtA and over 100 other proteins are copied from DNA. But when Salmonella encounters a high-magnesium environment, phoQ deactivates phoP, and fewer copies of the instructions for making MgtA are made.

When Groisman and his colleagues created a mutant strain lacking the phoQ gene, though, they were surprised to find that production of the instructions to make the MgtA protein could still somehow respond to magnesium, producing less of its protein at high magnesium levels.

Researchers used a computer program to determine how RNA copied from the MgtA gene might be folding up. The program predicted RNA copied from the gene could have two significantly different configurations. Because of the significant differences between these configurations, Groisman, who is also a Howard Hughes Medical Institute investigator, became interested in a region at the beginning of the RNA strand that contains no protein-building instructions. He theorized that it might be a riboswitch that responded to high magnesium levels by twisting the RNA into a configuration where its protein-building instructions somehow could not be used or were invalidated.

"One of our tests to see if this was something more than a computer fantasy was to take this segment that contains no protein-building instructions off the MgtA gene and paste it into another genetic configuration," Groisman says. "We wanted to see if it conferred sensitivity to magnesium levels, which it did."

In addition, Groisman's group showed that one RNA configuration was common in low magnesium levels while another was common in high magnesium levels.

They also searched the genomes of other bacteria with MgtA genes to see if their DNA included a sequence similar to the riboswitch in Salmonella. In six other bacteria, a similar sequence precedes the MgtA gene and can twist RNA copied from it into different configurations.

"Normally you would expect to find that a DNA sequence that is conserved among different species is encoding part of a protein," Groisman says. "But here we're talking about a part of a message that does not encode a protein. So why would it be conserved? There must be some important role that the sequence is fulfilling that is leading to its conservation, such as giving the cell expanded ability to sense and respond to magnesium levels."

Follow-up inquiries are already underway to locate the riboswitch's "brain"--the section of the RNA strand that responds to magnesium; and to learn how the high-magnesium configuration of the RNA disrupts final production of the protein.

RxPG News : Salmonella

Evolution of typhoid bacteria

Salmonella bacteria use RNA to assess and adjust magnesium levels