Small is beautiful

Carsonella ruddii If you don’t want to be seen, it’s a good idea to be small. Surprisingly, that also includes not being seen by the immune system. But being small is a problem because it limits what you can do.

TIL that Carsonella ruddii has the smallest genome of any known bacterium. I feel embarrassed that I didn’t already know that, but hey, at least it shows you’r never too old to learn. The C. ruddii genome consists of a circular chromosome of 159,662 bp with a high coding density (97%) with many overlapping genes and reduced gene length. The number of predicted genes is 182, also the lowest on record. In comparison, Mycoplasma genitalium, which has the smallest genome of any free-living organism, has a genome of 521 genes. Numerous genes considered essential for life seem to be missing from C. ruddii, suggesting that the species may have achieved organelle-like status. This species is an endosymbiont that is present in all species of phloem sap-feeding insects known as psyllids.
The 160-kilobase genome of the bacterial endosymbiont Carsonella. (2006) Science 314, 267


Members of the microbial world span a great range of shapes and sizes. Differences in size are used to distinguish species and impact many aspects of microbial physiology and lifestyle. Bacterial cells, for example, range from 0.15 to 700 μm in length. In addition, for single-celled organisms, modulation of cell division or separation may significantly impact their effective size. For microbes residing in a mammalian host, size may be a determining factor in an infectious agent’s success or its clearance. Many successful pathogens have evolved strategies to modulate their effective size to accommodate these challenges. The host in turn appears to target the ability of microbes to escape its defenses with their small size. By analyzing bacteria differing only in effective size it is possible to sort out some of the independent contributions of size to pathogenesis. These studies reveal that microbial size is a battleground in the interaction between pathogen and host:

The battle with the host over microbial size. Current Opinion in Microbiology. 07 Feb 2013. pii: S1369-5274(13)00004-0. doi: 10.1016/j.mib.2013.01.001
An eponymous feature of microbes is their small size, and size affects their pathogenesis. The recognition of microbes by host factors, for example, is often dependent on the density and number of molecular interactions occurring over a limited surface area. As a consequence, certain antimicrobial substances, such as complement, appear to target particles with a larger surface area more effectively. Although microbes may inhibit these antimicrobial activities by minimizing their effective size, the host uses defenses such as agglutination by immunoglobulin to counteract this microbial evasion strategy. Some successful pathogens in turn are able to prevent immune mediated clearance by expressing virulence factors that block agglutination. Thus, microbial size is one of the battlegrounds between microbial survival and host defense.

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