In a recent article in Antimicrobial Agents and Chemotherapy, Steven Projan points out that there is an apparent correlation between the size of a bacterium's genome and its penchant to display multidrug resistance phenotypes.
The smaller the genome the less likely it is that antibacterial resistance will emerge and disseminate within that species. What is proposed here is that, just as there is a continuum of genome sizes among bacteria, there is a continuum in the ability or propensity of a bacterium to become "multidrug resistant" and that continuum is reflected in the size of the genome.
The author informed me that he had intended to address the correlation between genome size and the ability to carry drug-resistance plasmids, but reviewers pointed out that the "data" is anecdotal. He writes: "There has been much futility in attempting to introduce plasmids into small genome organisms (as well as move in insertion sequences)."
Steve is a very smart guy. I wonder if the correlation relates indirectly, rather than directly to size considerations. It may relate to the ability to absorb foreign DNA productively. It may also relate to the diversity of metabolic activities displayed. For example - antibiotic producers like Streptomyces have huge genomes, because they make lots of secondary metabolites with complex synthesis. They are multi-drug resistant because they make multiple drugs (and are in an antibiotic rich environment). Organisms in the same antibiotic rich environment, such as Pseudomonas, also make lots of secondary metabolites, and are good at picking up foreign DNA and incorporating it, for resistance as well as catabolic activity. Staph (which used to be Steve's favorite, and maybe still is) is a mosaic of integrated phage, so it has obviously also gotten good at tolerating and exploiting foreign DNA. This gets at the whole issue of what a microorganism's genome is...
Posted by: Paul Orwin | May 10, 2007 at 02:13 PM