Antibiotics such as tetracycline, streptomycin, and bacitracin are small organic molecules that are synthesized by particular species of bacteria. Microbiologists have hypothesized that the reason why certain bacteria make antibiotics is to kill other species that occupy the same environment. Bacteria that produce an antibiotic may be able to kill competing species. This provides more resources for the antibiotic-producing bacteria. In addition, bacteria that have the genes necessary for antibiotic biosynthesis contain genes that confer resistance to the same antibiotic. For example, tetracycline is made by the soil bacterium Streptomyces aureofaciens. Besides the genes that are needed to make tetracycline, S. aureofaciens also contains genes that confer tetracycline resistance; otherwise, it would kill itself when it makes tetracycline. In recent years, however, many other species of bacteria that do not synthesize tetracycline have acquired the genes that confer tetracycline resistance. For example, certain strains of E. coli carry tetracycline-resistance genes, even though E. coli does not synthesize tetracycline. When these genes were analyzed at the molecular level, it was found that they are evolutionarily related to the genes in S. aureofaciens. This observation indicates that the genes from S. aureofaciens have been transferred to E. coli.
A. What form of genetic transfer (i.e., conjugation, transduction, or transformation) would be the most likely mechanism of inter¬species gene transfer?
B. Because S. aureofaciens is a nonpathogenic soil bacterium and E. coli is an enteric bacterium, do you think it was direct gene transfer, or do you think it may have occurred in multiple steps (i.e., from S. aureofaciens to other bacterial species and then to E. coli)?
C. How could the widespread use of antibiotics to treat diseases have contributed to the proliferation of many bacterial species that are resistant to antibiotics?