Genomic Evolution in the Blink of an Eye

If you think that genetic evolutionary processes take Milena….think again. Scientists at Allegheny General Hospital’s Center for Genomic Sciences have found that through a process known as horizontal gene transfer (HGT), bacterial pathogens can mutate multiple times during the course of a bacterial infection. In a study published by Hiller et. al. in PLoS Pathogens researchers discovered that when they compared he original strain of a pneumococcal infection with strains sequenced 7 months later, approximately 7.5 percent of the entire genome had changed.

In HGT mutations, (as opposed to single base pair mutations), DNA segments from hundreds to hundreds of thousands of bases can be incorporated from another organism’s genome without the recipient being its offspring. Previous studies have shown that Streptococcus pneumoniae (one of the most common bacteria found in childhood ear, nose and throat infections) exhibit recombination rates are 3 to 10 fold higher than DNA polymerase mutation rates and thus are highly susceptible to rampant genomic mutation.

Figure 1. Demonstration of recombination events among Streptococcus pneumoniae genomes isolated from a single patient.

Hiller’s team analyzed bacterial cultures from nasopharyngeal swabs of an 8-month old patient who had 12 clinic visits due to rhinorrhea and/or ear infections over a 7-month period. Sequencing analysis revealed that 4 out of 6 bacterial strains isolated from the patient exhibited small differences in their genomic sequence which was determined by RDP3 (a freely available statistical program) to be the result of a recombination event likely due to HGT.

As per Hiller’s conclusion, the unprecedented degree of HGT detected here within strains isolated from a single infection is strongly supportive of the distributed genome hypothesis.

In an interview with Drug Discovery News, Dr. Garth Ehrlich, scientific director of the AGH Center for Genomic Sciences and the paper’s senior author pointed out that bacterial mutations are occurring “through a dynamic, real-time process of altering their genetic code that until now has not been understood and which is counter to conventional wisdom about the typical pace of species evolution.” Furthermore, the rate at which these mutations occur coupled with HGT-based methodology enables the bacteria to constantly keep one step ahead of vaccination scientists.

Hiller NL, Ahmed A, Powell E, Martin DP, Eutsey R, Earl J, Janto B, Boissy RJ, Hogg J, Barbadora K, Sampath R, Lonergan S, Post JC, Hu FZ, & Ehrlich GD (2010). Generation of genic diversity among Streptococcus pneumoniae strains via horizontal gene transfer during a chronic polyclonal pediatric infection. PLoS pathogens, 6 (9) PMID: 20862314

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