A family tree can be very illuminating, but there can be some surprises in those branches, too. A recent mBio paper delves into the ancestry of Streptococcus agalactiae, a.k.a. Group B Strep (GBS), one of the more recent additions to the lineup of human pathogens, and uncovers some unexpected facts about the bacterium’s family history.
GBS is the most prevalent cause of invasive and often fatal infections in newborn babies, but it’s only become a significant problem in the last several decades. Sørensen at al. examined 15 housekeeping genes to plot the phylogeny of a collection of 238 bovine and human GBS isolates from nine countries to determine what evolutionary processes might be behind the emergence of the disease.
“The big question seemed to have been, ‘Did this emerge from bovine strains?’” says Michael Russell of the University at Buffalo and a member of mBio’s Board of Editors. Russell notes that prior work suggested the GBS humans suffer from may have evolved from a strain of GBS found in cattle (Bisharat et al., 2004; Hery-Arnaud et al;., 2005), but those studies anayzed fewer strains and used older technology.
Surprisingly, the new data revealed another story: bovine and human pathogens seem to have emerged independently from a genetically heterogeneous core population, so a simple monophyletic model does not exist for their evolution. The data also show that GBS genomes are like shuffled mosaics of long genome segments, indicating that the virulent lineages (like the one that infects human infants) emerge occasionally as a result of recombination in major segments of the genome.
“I think this puts the population genetics history of these infections in a rather new light,” says Russell. “The authors were able to point out why the older analysis of GBS strains, based on a smaller number of strains and using different technology, led to different conclusions. This strengthens the new conclusions,” he says.