Monday, August 25, 2014

New Method Shows How Bacteria Become Dangerous Pathogens

Two new papers published this month describe a new approach that combines whole genome sequencing – an ability to analyze the genetic code of bacteria in great detail – with plylogenetic reconstruction – an ability to trace the family tree of different strains of the same organism back to a common ancestor.

In the Proceedings of the National Academy of Sciences, Prof. Gordon Dougan, from the Wellcome Trust Sanger Institute in the UK, who co-authored both papers describe how they worked out that typhoid fever emerged in humans about 450 years ago. The researchers suggest the disease may have moved from animals to humans when people began to inhabit denser environments and started living close to their livestock. They suggest the bacterium followed a similar path to whooping cough and tuberculosis – it became fixed in humans and spread around the world.

Over the centuries, typhoid fever appears to have accumulated genetic mutations – not as the result of any particular event, but through a gradual process known as genetic drift.

In the second paper, published in Nature Communications, Prof. Dougan and another set of colleagues traced the genetic development of the bacterium Streptococcus agalactiae or Group B Streptococcus (GBS), a cause of serious septicemia and shock in newborns.

Over-use of tetracycline led to an evolutionary bottleneck, allowing GBS strains to acquire genes that make it resistant to the drug, transforming a harmless organism into a pathogen that found a niche in mothers and their newborns.

Tetracycline may also give GBS another advantage by eliminating the thousands of friendly species of microorganisms living in the microbiome of mothers and babies, which may stop GBS becoming too dominant.

“This is possibly the earliest case of the emergence of a new disease that can be directly associated with antibiotic use. GBS causes a distressing infection of sepsis and meningitis seen in newborn children that can result in severe illness and death, making it one of the most serious diseases in babies,” Prof. Dougan explains.

Genetic drift is the natural selection process in which mutations in the genome that are favorable for the survival of the species flourish. Because the numbers of bacterium are so high and the generations so frequent, the mutations occur relatively fast. In typhoid fever that process occurred over hundreds of years. However, the indiscriminate use of antibiotics has compressed that time frame into just a few years through “evolutionary bottleneck”.

The threat is very real, C. difficile and MRSA are both the result of indiscriminate use of antibiotic therapy. To my knowledge, there have been no studies published on the genetic drift of these organisms but the problem is widespread.

Fortunately, there are similar studies aimed at identifying the genetic structure of the healthy bacteria in the bowel as well. Over 10% of the microbiome has been identified through whole genome sequencing. In the near future, we will be able to identify each and every healthy organism in your digestive tract. That will allow to choose the right probiotic to restore balance the microbiome.

We must stop indiscriminate use of antibiotics as a society. However, it has to start with you.

Source: NIH –Tuesday, August 19, 2014

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