Genetic Cross-Talk

Bacteria can do tricks that your dog cannot do. A bacterium can acquire DNA from another completely unrelated species of bacteria just by living in the same area. For example, if you take cyanobacteria, a type of bacteria that normally live in the oceans and carry out photosynthesis, and mix them in a test tube with a population of E. coli, the bacteria that normally live in your large intestines, something odd can happen. If you let them sit together overnight, some of the E coli cells will link up with the cyanobacteria using a microscopic hose and then transfer some of their DNA into the cyanobacteria [1]. Once the E. coli DNA is inside the cyanobacterium, the cellular machinery will then splice the E. coli DNA into the chromosome. This would be like your dog somehow acquiring cat DNA and the ability to purr simply by sleeping on the same bed with the cat!This process of acquiring foreign DNA is known as lateral gene transfer (LGT) or horizontal gene transfer(HGT). A large number of scientists consider LGT to be a powerful force in bacterial evolution.

The classic example of this power can be seen in any hospital, where the spread of antibiotic-resistance genes has become so serious than many doctors and scientists are worried we will soon be unable to cure certain infectious diseases. How is it that so many bacteria have so quickly evolved resistance to our arsenal of antibiotics? Biologists Fernando de la Cruz and Julian Davies describe it as “the best known example of very rapid adaptation of bacterial populations to a strong selective pressure.” [2] They further note:

What has been learned is that this adaptation occurred (and still occurs if bacteria are challenged with a novel antibiotic) not by mutation of the menaced populations, but by acquisition and dissemination of, in most cases, simple antibiotic resistance genes by mobile genetic elements. It should be noted that studies of antibiotic resistance development are essentially retrospective, and although much has been learned, little is known of the microbial dynamics of this process. Nonetheless, the basis of antibiotic resistance development is formally simple: mobile genetic elements such as plasmids and transposons efficiently distributed the resistance determinants, singly or in clusters, among many genera and species of bacteria. [2]

The ability to spread bacterial genes from one species to another can be both massive and far-reaching. For example, a free-living bacterium known as Thermotoga maritima lives in hot springs and has the ability to metabolize all kinds of fuel sources. When its complete genome was sequenced, it was determined that almost 25% of the bacterium’s genes came from members of archaea [3]. Thermotoga also helps us to understand that a free-living, non-pathogenic bacterium can make extensive use of LGT, rendering its genome very dynamic. When two very closely related strains of Thermotoga were analyzed, about 20% of the genome was different. One strain had a vast array of genes involved in sugar transport and polysaccharide degradation that was lacking in the other strain. It also contained the V-type ATP synthases, whereas the other strain had the F-ATP synthases most commonly seen in bacteria [4]. The same basic theme is seen in E. coli, a bacterium that is radically different from Thermotoga. When the genome from E coli was analyzed, up to 18% of its genes came from foreign sources [5].

Not only does the hypothesis of life’s design allow us to predict that bacteria would engage in extensive genetic cross-talk (a relic of the strategy to maximize the chance of successful seeding), but there are many features of this cross-talk that suggest design is embedded within the process of evolution……..

1. Martin, W. 1999. Mosaic bacterial chromosomes: a challenge en route to a tree of genomes. BioEssays 21: 99-104.
2. de la Cruz F, Davies J. 2000. Horizontal gene transfer and the origin of species: lessons from bacteria. Trends Microbiol. 8:128-33.
3. Nelson KE, Clayton RA, Gill SR, Gwinn ML, Dodson RJ, Haft DH, Hickey EK, Peterson JD, Nelson WC, Ketchum KA, McDonald L, Utterback TR, Malek JA, Linher KD, Garrett MM, Stewart AM, Cotton MD, Pratt MS, Phillips CA, Richardson D, Heidelberg J, Sutton GG, Fleischmann RD, Eisen JA, White O, Salzberg SL, Smith HO, Venter JC, Fraser CM. 1999. Evidence for lateral gene transfer between Archaea and bacteria from genome sequence of Thermotoga maritima. Nature 399:323-9.
4. Nesbo CL, Nelson KE, Doolittle WF. 2002. Suppressive subtractive hybridization detects extensive genomic diversity in Thermotoga maritima. J Bacteriol. 184:4475-88.
5. Lawrence, JG. 1999. Gene transfer, speciation, and the evolution of bacterial genomes. Curr Opin Micro 2:510-523.

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