How does an antibiotic work?
Various antibiotics work in different ways. If you think about it, there are several ways that a bacterium could be prevented from growing or living.
How is antibiotic resistance acquired?
In the absence of human involvement, bacteria in the wild do not usually have resistance to antibiotics. In order for a bacterium to gain resistance to a given antibiotic, there must be either a natural mutation in a gene within the bacterial chromosome (less common), or, the system that leads to resistance must be acquired. Acquisition of antibiotic resistance occurs when genetic material is taken up by the bacterial cell and either incorporated into the chromosome, or, that is able to exist in a stable form independent of the chromosome. Such stable genetic elements that can not only exist but that can replicate independently of the chromosome, are known as plasmids. If the genetic information encoded by a plasmid leads to resistance against a particular antibiotic, this plasmid is known as a resistance plasmid. The bacterium that acquired the new genetic information that leads to resistance, is now known as an antibiotic-resistant strain of that bacterial species. Examples of such bacterial strains include Vancomycin Resistant enterococci (VRE) and Methicillin Resistant Staphylococcus aureus (MRSE).
If one cell of a bacterial species is resistant to a given antibiotic,
are all bacterial cells of that species resistant to the same
No; only that strain and subsequent offspring of that strain will be resistant to that antibiotic. For example, if a bacterial cell acquires a system that destroys penicillin or methicillin, all other cells of that species may still be sensitive to the killing action of the antibiotic. However, the cell that acquired resistance to methicillin will usually be able to pass on this resistance in the form of stable genetic information to all of the progeny of that one cell. Since bacterial cells can divide every 20 minutes, in a very short time there can be many, many cells of the new, resistant strain that will no longer be killed by methicillin.
What difference does it make if a bacterial cell or two acquires
The overwhelming majority of bacteria are not only helpful to us but necessary for our good health. This preponderance of good bacteria means that there are fewer spaces available for harmful (pathogenic - disease causing) bacteria to occupy and enormous competition for the limited nutrients available. Consequently, it's tough for bad bacteria to gain a foothold and to exist in large numbers - special conditions must usually occur in order for such a thing to happen. One such special condition is the loss of competition for space and food. Since antibiotics kill both good and bad bacteria alike, if there is a disease-causing bacterial cell that happens to be resistant to an antibiotic, and most of the good bacteria are killed - this antibiotic resistant bad bacterium now has a significantly better competitive advantage. This strain may now increase in number, gain a foothold and cause disease - and all members of this strain will be resistant to at least one antibiotic that could kill them, to boot.
Under what conditions do bacteria most easily acquire resistance to a
When bacteria are in a limited geographic environment with routine, consistent exposure to antibiotics - such as a hospital - or within a single individual on long-term antibiotic therapy - or within farm animals treated with low, non-therapeutic doses of antibiotics for weight gain. If antibiotics are used extensively, there is an increased likelihood of selection of a bacterial cell that is resistant to the effects of a given antibiotic. Look at it this way - if there were 100 people in a room and only one individual had an umbrella, spraying water from the ceiling would select that one person for dryness - resistance to getting wet. If this person and their umbrella divided every 20 minutes, soon, the room would be filled only with individuals who had umbrellas and all could remain dry. That individual would represent a strain of bacterium, resistant to an antibiotic, and allowed to increase in number and to perhaps become the predominant occupant of the room.
Another situation that encourages selection for antibiotic resistance is indiscriminate usage of antibiotics. If an antibiotic isn't needed to treat a disease, such as a disease caused by a virus (antibiotics work only on bacteria), the use of antibiotics in this situation increases the opportunity to select a bacterial strain that is resistant to that antibiotic. One particularly troublesome situation in this area is the routine use by the animal food industry of low, non-therapeutic dosage levels of antibiotics. These antibiotics are either the same or very similar to the antibiotics used for human health.
Non-therapeutic use of antibiotics means that the animals are not treated simply to prevent spread of an on-going disease, but healthy animals are treated to prevent spread of disease and to also increase weight gain. It is not yet understood how low levels of antibiotics cause the animals to gain weight. However, the widespread use in this manner of several different antibiotics is thought to increase the risk of development of antibiotic resistance among bacteria dangerous to human health. Antibiotic-resistant bacteria would have a significantly higher chance under such conditions of being selected as survivors.
Is anyone doing anything about this issue?
There is increasing awareness of what we are doing to increase the presence of antibiotic-resistant strains of disease-causing bacteria - both in the medical community and in the animal food industry. There are many more discussions and recommendations being made that are resulting in less indiscriminate use of antibiotics. However, this issue is a world-wide problem and continuous efforts are being made to reduce this threat to human health. One recent success appears to have occurred in Europe. A recent article in the journal Science attests to the effect of a ban on use of the antibiotic, Avoparcin, in animal feed on the prevalence of antibiotic resistance in human hospital patients (Ferber, Dan (2002) Livestock Feed Ban Preserves Drug's Power. Science, 295:, 27-28. The data for the Science article were presented by Belgian scientists at The Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago, 16-19, December 2001, that was sponsored by the American Society for Microbiology.
Studies had shown that bacteria isolated from the intestines of animals that were resistant to Avoparcin were also resistant to Vancomycin. Both of these antibiotics work by preventing the cell wall of bacteria to form properly - without cell wall integrity, bacterial cells will rupture and die. As mentioned above, Vancomycin-resistant enterococci exist and are a significant concern for surgery patients who may become infected with these bacteria. Scientists felt that reduction of the use of Avoparcin among animals might eventually reduce the prevalence of Vancomycin resistant bacteria among humans.
The concern that antibiotics in animal feed could eventually select for antibiotic resistant bacterial strains, and that these strains could enter the human intestines through the food chain, caused the European Union to reach an important decision - that is, to ban in 1997 any further use of Avoparcin in livestock feed. Researchers in Europe have been monitoring the effects of this ban. Since the ban was instituted a significant decrease in Vancomycin-resistant enterococci has been observed in bacteria isolated from chickens, pigs and in chicken meat in stores. Perhaps most importantly, in an Antwerp, Belgium hospital patient study in June, 2001, an approximate ten-fold decrease in Vancomycin resistant enterococci were observed relative to when the ban was begun. Since the use of Vancomycin in Belgian hospitals had not changed since 1997, this decrease in Vancomycin resistance appears to be clearly associated with the decrease in use of the similar antibiotic, Avoparcin, in farm animals. In the United States, the U.S. Food and Drug Administration (FDA) has recommended that two different antibiotics be banned from use in livestock feed. The final decision on such a ban has not yet been made.
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