Commonly, gene transfer is thought of as a vertical line from parent to offspring, along which all evolutionary traits are passed. However, as we began delving into genomic sequences, we found that this may not be true and that the lines between "species," especially on the microbial level, are quite fuzzy.

Horizontal gene transfer is the transfer of genetic elements between species. The microbial world is filled with examples of this phenomenon. This article is the first in a 3 part series that will explore the ever fluctuating genetic world of our microbial majority.

Escherichia coli is perhaps the most well studied and utilized bacterial organism on the planet. This bug also has the capability for a wide range of illness in humans including: non-pathogenic, enterohemorrhagic (such as popular strain 0157:H7), enterotoxigenic, and uropathogenic. The question is obviously raised--how can the same organism have such pathotypes, and what allows it to be versatile enough to thrive in such different environments as the urinary tract vs. the intestinal tract.

An article coming out of the University of Wisconsin-Madison shows that E. coli consists of a general common backbone that is nearly 100% identical across strains, yet only takes up 75%of the genome. However, they show that within this backbone are regions (islands) that are highly divergent between strains (See figure at left). It is no surprise that within these divergent regions contain the genes which allow for survival in different environments (pathogenicity islands)

Read more about E. coli mosaicism at Blogging for Bacteriophage.