When antibiotics are improperly used, one consequence is resistance. Resistance is when a type of bacteria is no longer sensitive to an antibiotic. Antibiotic resistance due to improper use is quickly becoming a dangerous medical problem. Shortly after penicillin became available in 1946, resistant strains of Stapholococcus bacteria started appearing. In the 1950's as more antibiotics were discovered, strains of bacteria resistant to multiple antibiotics bagan appearing. In 1997, US doctors encountered a strain of Staphlococcus aureus bacteria that was resistant to the antibiotic vancomycin. Many strains of S. Aureus are resistant to all antibiotics except for vancomycin. Fortunately, the strain discovered in a patient to be resistant to vancomycin was treatable by other antibiotics. However, this new strain shows that it is probable for a strain of bacteria to someday be resistant to all known antibiotics. Bacteria can become resistant through several methods. They are:
Inherent (Natural) Resistance. Some Bacteria are inherently resistant to an antibiotic. A streptomycete has some gene that is responsible for resistance to its own antibiotic, and a Gram-negative bacterium has an outer membrane that establishes a permeability barrier against the antibiotic. An organism does not have a transport system for the antibiotic; or it doesn't have the target or reaction that is hit by the antibiotic.
Acquired Resistance. Bacteria can develop resistance to antibiotics, for example, bacterial populations that were sensitive previously to antibiotics become resistant. This type of resistance results from changes in the bacterial gene bank. Acquired resistance is driven by two genetic processes in bacteria: mutation and selection (sometimes referred to as vertical evolution), and the exchange of genes between strains and species (sometimes called horizontal evolution).
Vertical evolution is driven by principles of natural selection: a spontaneous mutation in the bacterial chromosome gives resistance to a member of the bacterial population. In the selective environment of the antibiotic, non mutants are killed and the resistant mutant is allowed to grow and flourish. The mutation rate for most bacterial genes is approximately 100 million. This means that if a bacterial population doubles from 100 million cells to 200 million cells, there is probably a mutant present for any given gene. Since bacteria grow to reach population densities far in excess of one billion cells, a mutant could develop from a single generation during 15 minutes of growth.
Horizontal evolution is the acquisition of genes for resistance from another living thing. For example, a streptomycete has a gene for resistance to streptomycin (its own antibiotic), but somehow that gene leaves the cell and gets into E. coli or Shigella. Or, some bacterium develops genetic resistance through the process of mutation and selection and then gives these genes to some other bacterium through one of several processes for genetic exchange that exist in bacteria.
Bacteria are able to exchange genes in nature by conjugation, transduction and transformation. Conjugation involves contact between cells as DNA crosses a sex pilus from donor to recipient. During transduction, a virus transfers the genes between bacteria that are mating. In transformation, DNA comes from the environment, having been released from another cell.
The effects of fast growth rates, high concentrations of cells, genetic processes of mutation and selection, and the ability to exchange genes combined, account for the extraordinary rates of adaptation and evolution that can be observed in the bacteria. For these reasons bacterial adaptation (resistance) to the antibiotic environment seems to take place very rapidly in evolutionary time.