Gr 11 Antibiotic Resitance

Antibiotics are chemicals produced by microbes that either kill (bactericidal) or inhibit the growth (bacteriostatic) of bacteria

Antibiotics are commonly used by man as a treatment for bacterial infections (not effective against viral infections)

Evolution of antibiotic resistance in bacteria

In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation

When treated with antibiotics, the resistant bacteria will survive and reproduce by binary fission (asexual reproduction)
The antibiotic resistant bacteria will flourish in the absence of competition from other strains of bacteria (killed by antibiotic)
Antibiotic resistant bacteria may also confer resistance to susceptible strains by transferring plasmids via bacterial conjugation
The introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)

An example of antibiotic resistance in bacteria can be seen in the evolution of Staphylococcus aureus (Golden staph)

Golden staph can cause infections to the skin (lesions and boils) as well as more serious infections (pneumonia, meningitis)
Historically, these infections were treated using the antibiotic methicillin
Bacterial strains developed that were resistant to this antibiotic (methicillin-resistant Staphylococcus aureus – or MRSA)
These strains proliferated while susceptible strains died out (methicillin-sensitive Staphylococcus aureus – or MSSA)
MRSA infections are now especially present in hospitals and nursing homes, where the use of methicillin was most common
Medical practitioners now prescribe alternate antibiotic agents to treat infections caused by Staphylococcus aureus

Antibiotic resistance in bacteria develops in several steps.

Consider the following scenario.

A woman gets a bacterial infection such as tuberculosis.
Her doctor gives her an antibiotic to kill the bacteria.
She gets better because the bacteria are largely destroyed.
By a modification of its genetic makeup, however, one bacterium is resistant to the antibiotic.
That bacterium is not killed by the antibiotic and it later multiplies in the patient’s body to make her sick again.
She goes back to the doctor and gets the same antibiotic.
This time, no result: she is still sick and asks her doctor what is wrong.
The doctor prescribes a different antibiotic that (hopefully) works.

But if the population of bacteria continues to acquire mutations, new strains could show resistance to all the antibiotics available.

Because bacteria reproduce asexually, genetically they generally do not change very often. However, there are two sources of possible change in the genetic makeup of bacteria:

mutations
plasmid transfer

Plasmid transfer involves one bacterium donating genetic information to another in a ring of nucleotides called a plasmid.
Both the donating and receiving cells open their cell walls so that the genetic material can pass from the donor to the receiver.
The development of antibiotic-resistant bacteria has happened in several cases.
New strains of syphilis, for example, have adapted to antibiotics and show multiple resistance. Some strains of tuberculosis are resistant to as many as nine different antibiotics.
There is no cure for people who get sick from such super-resistant germs, and they must rely on their immune system to save them.

Finding new antibiotics would only be a temporary solution, and pharmaceutical companies cannot nd new medications fast enough to treat these super-resistant germs. As a result, the best way to curb their expansion is to make sure that doctors minimize the use of antibiotics and that patients realize that antibiotics are not always the best solution to a health problem.

Antibiotics are used to control diseases caused by bacteria in humans. There have been increasing problems with disease-causing (pathogenic) bacteria being resistant to antibiotics. The graph (right) shows the percentage of pathogenic E. coli bacteria that were resistant to the antibiotic ciprofoxacin between 1990 and 2004. The trend with many other diseases has been similar. The theory of evolution by natural selection can explain the development of antibiotic resistance in bacteria. Genes that give resistance to an antibiotic occur in the microorganisms that naturally make that antibiotic. These antibiotic resistance genes can be transerred to a bacterium by means of a plasmid or in some other way. There is then variation in this type of bacterium as some of them are resistant to the antibiotic and some are not. If doctors or vets use the antibiotic to control bacteria it will kill bacteria that are susceptible to the antibiotic, but not those that are resistant. This is an example of natural selection, even though it is caused by humans using antibiotics. The antibiotic-resistant bacteria reproduce and pass on the resistance gene to their offspring. These bacteria spread from person to person by cross-infection. The more an antibiotic is used, the more bacteria resistant to it there will be and the fewer that are non-resistant. As a result of excessive use of an antibiotic, most of the bacteria may eventually be resistant.

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