by Karen Crumley

Created on: April 04, 2009

Antibiotic resistance occurs when bacteria become unaffected by an antibiotic through many possible mechanisms. Two such mechanisms include survival of the fittest and sharing of genes that allow the bacteria to exist despite the presence of the antibiotic.

Survival of the fittest rules even the smallest of organisms. In this case, the fittest bacteria will be the one that is able to circumvent the activity of an antibiotic. Every population of bacteria, as with other populations, has individuals that are different from others genetically. If the action of an antibiotic is to block a necessary biochemical pathway for the bacterium's reproduction, it may very well kill most of the bacteria in a population. There may be, however, an individual bacteria that has a different pathway for reproduction. All of the regular bacteria will die from the antibiotic, but the different bacteria will survive. It will be the only one alive to reproduce and it will hand down all of its genetic material to its offspring, including the information needed to side-step the antibiotic. Before long, there will be an entire generation of antibiotic-resistant bacteria. The presence of that particular antibiotic will no longer affect these bacteria. A new antibiotic must be used that is not based on that metabolic pathway.

One of the most common ways that bacteria become resistant is when patients do not finish their entire course of antibiotics. A patient will go to the doctor and get antibiotics for an ear infection, for example. After a couple of days of taking the medication, the patient's ear feels better and the medication is discontinued, even though the bottle clearly instructs the patient to finish all of the medication. The problem is that the only bacteria that have been killed were the ones that had absolutely no resistance to the antibiotic. There are still bacteria there, but they are more able to thrive in the presence of the medication. Those bacteria reproduce, passing down the resistance information, and the ear infection will now return. The problem is now that the bacteria will no longer respond to that antibiotic. A different antibiotic must be used. If the patient then repeats the early discontinuation of medication, the result will be a strain of bacteria that is now resistant to both of the antibiotics that have been previously used against it.

Another example of this type of resistance is seen when livestock are given feed with antibiotics in it. After only a short time, all of the bacteria in the animals will be resistant to every antibiotic it has been fed. If a human incidently becomes infected with one of these bacteria, none of these antibiotics will be effective against it.

Bacteria also have a habit of leaving little bits of genetic material around for other bacteria to pick up and use. A good example of this can be seen in hospitals where patients with resistant infections have been treated. The bacteria may die, but there may be bits of the resistance instruction left in the room. Another species of bacteria then comes along and adopts it as its own, becoming resistant in the process. In this manner, many different species of bacteria can develop resistance to common antibiotics.

Once the majority of bacteria have become resistant to an antibiotic by using different metabolic pathways, it will also be resistant to all antibiotics that use the same mechanism. A new metabolic pathway must be disrupted to kill the bacteria, thus the need for the constant development of new antibiotics. Failure to find new antibiotics will result in reversion to our pre-penicillin state of being at the mercy of overwhelming infections.

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