Gene Transfer In Bacteria

We saw a movie clip of TB situation in Russian prisons on last thursday. The movie briefly mentioned the problems involving MDR-TB (multi-drug resistance tuberculosis) strains and the causes for their origin. The origin of these strains are mostly man-made: antibiotics used in animal feed (sub-therapeutic concentration of clinical antibiotics are often used as growth enhancers in farms), improper use of antibiotics in clinical settings, etc. A good example of evolution of a MDR strain is Staph aureus. In 1940's, one could easily eliminate S. aureus with penicillin. It's not the case anymore.

There are two major ways by which bacteria gain antibiotic resistance - lateral gene transfer via bacterial phages (viruses that infect bacteria) and plasmids (extra chromosomal DNAs found in cytoplasm that are replicate independently of chromosomal DNA, expressed only when needed).

The plasmids often carry antibiotic resistance genes that can be readily passed onto neighboring bacterial cells. They are also a very useful tool in biotechnology - cloning of any genes of interest is made possible by plasmids. Expression of human insulin in E. coli carrying genetically engineered plasmid is a good example of that. Many enzymes that are available to biologists today are cloned into plasmids.

My graduate research involved expression and purification of Listeria monocytogenes membrane proteins in E. coli. The genes that encode these membrane proteins were clones into a plasmid then expressed in E. coli. The following pictures show genes (gbuA, gbuB, and gbuC) and proteins that were expressed and isolated from E. coli:

1% agarose gel: lane 1) DNA ladder; lane 2) gbuA; lane 3) gbuB; 4) gbuC

SDS-PAGE protein gel: lane 1) marker proteins; the rest are all pure GbuA

SDS-PAGE protein gel: lane 1) marker proteins; lane 5) pure GbuB

SDS-PAGE protein gel: lane 1) marker proteins; lane 5)-8) pure GbuC

While conferring antibiotic resistance to bacteria in a lab setting can be beneficial in the context of research, uncontrolled proliferation of antibiotic resistance in deadly pathogens can be quite devastating. It is my hope that Bio440 will give you a greater understaning and appreciation of the role that we play in maintaining (or otherwise) the delicate balance of mother nature and take more responsibility for how we deal with the environment. -Min