BACTERIA can find themselves in the rather undesirable position of being addicted to parasites. The parasites in question are not of the blood-sucking sort however, but rather of the gene-sucking sort.
In nature there are numerous genetic entities, various forms of DNA, that parasitise bacteria:
- bacteriophages (viruses that infect only bacteria),
- plasmids (usually a circular strand of DNA that exists separately to the bacteria’s chromosome),
- transposons (a unit that consists of a collection of genes that inserts itself into the host’s chromosome, but can cut itself free and reinsert itself elsewhere on the chromosome) and conjugative transposons (also capable of transferring themselves from cell to cell between bacteria),
- genomic islands (again, a collection of genes that usually encode particular functions – disease-causing factors or antibiotic resistance – that have arrived from another organism and have become fixed in the chromosome).
We also have integrative conjugative elements (ICEs) that, like conjugative transposons, insert themselves into the host’s chromosome where they are replicated along with the host’s DNA, but then periodically (often under stress) cut themselves free and mail a copy off to another host cell.
Transfer of any of the above genetic entities can result in a bacterial cell acquiring new and desirable traits as such as the ability to consume new food sources, or resist antibiotics, or be more invasive. These traits have been picked up via the many occasions that these elements have jumped into and out of bacterial chromosomes, taking bits of those chromosomes with them.
The transfer of new traits by these genetic entities is referred to as Horizontal Gene Transfer (HGT), which is a term that is perhaps easier to understand if we consider that sexual reproduction, the process by which your parents produced you, is a form of vertical gene transfer; so too is the division of a single bacterial cell to produce a copy of itself and a ‘daughter’ cell. By comparison, horizontal gene transfer might be likened to you reaching out to touch your cousin and acquiring his or her ginger hair and freckles.
The thing that unites these genetic elements is that, being parasites, they need the host cell in order to produce more of themselves. Sometimes these elements don’t provide anything useful to the cell, sometimes they’re more of a burden, but some of these genetic parasites have evolved ways to ensure that the cell doesn’t toss them aside.