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Arthur T Knackerbracket has found the following story:

We receive half of our genes from each biological parent, so there's no avoiding inheriting a blend of characteristics from both. Yet, for single-celled organisms like bacteria that reproduce by splitting into two identical cells, injecting variety into the gene pool isn't so easy. Random mutations add some diversity, but there's a much faster way for bacteria to reshuffle their genes and confer evolutionary advantages like antibiotic resistance or pathogenicity.

Known as horizontal gene transfer, this process permits bacteria to pass pieces of DNA to their peers, in some cases allowing those genes to be integrated into the recipient's genome and passed down to the next generation.

The Grossman lab in the MIT Department of Biology studies one class of mobile DNA, known as integrative and conjugative elements (ICEs). While ICEs contain genes that can be beneficial to the recipient bacterium, there's also a catch—receiving a duplicate copy of an ICE is wasteful, and possibly lethal. The biologists recently uncovered a new system by which one particular ICE, ICEBs1, blocks a donor bacterium from delivering a second, potentially deadly copy.

"Understanding how these elements function and how they're regulated will allow us to determine what drives microbial evolution," says Alan Grossman, department head and senior author on the study. "These findings not only provide insight into how bacteria block unwanted genetic transfer, but also how we might eventually engineer this system to our own advantage."

[...] Although plasmids are perhaps the best-known mediators of horizontal transfer, ICEs not only outnumber plasmids in most bacterial species, they also come with their own tools to exit the donor, enter the recipient, and integrate themselves into the recipient's chromosome. Once the donor bacterium makes contact with the recipient, the machinery encoded by the ICE can pump the ICE DNA from one cell to the other through a tiny channel.

For horizontal transfer to proceed, there are physical barriers to overcome, especially in so-called Gram-positive bacteria, which boast thicker cell walls than their Gram-negative counterparts, despite being less widely studied. According to Davis, the transfer machinery essentially has to "punch a hole" through the recipient cell. "It's a rough ride and a waste of energy for the recipient if that cell already contains an ICE with a specific set of genes," she says.

Sure, ICEs are "selfish bits of DNA" that persist by spreading themselves as widely as possible, but in order to do so they must not interfere with their host cell's ability to survive. As Avello explains, ICEs can't just disseminate their DNA "without certain checks and balances."

"There comes a point where this transfer comes at a cost to the bacteria or doesn't make sense for the element," she says. "This study is beginning to get at the question of when, why, and how ICEs might want to block transfer."

[...] "We had suspected that Gram-positive ICEs might be capable of exclusion, but we didn't have proof before this," Avello says. Now, researchers can start to speculate about how pathogenic Gram-positive species might control the movement of ICEs throughout a bacterial population, with possible ramifications for disease research.

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