Escherichia coli and antibiotic resistance

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Escherichia coli, short E. coli is an important bacteria that are found in the lower part of intestines of warm blooded animals, including human. People’s feces for a day usually contain about 100 million or 10 trillion individuals of E. coli. E. coli is a major component of a bacterial system that helps to digest food particles. E. coli also has commercial uses. For instance, is used as an organic indicator for the sewage treatment to indicate the level of the concentration of poop pollutions in the water. However, uncontrolled growth of E. coli bacteria or the allocation of E. coli to other parts of the human body can be dangerous or even fatal. E. coli is also used in many laboratory experiments because they are cheap to acquire and easy to culture.

When people are referring to bacteria that are resistant against penicillin, for instance penicillin resistant E. coli, these people are referring specifically to the ability of this bacteria population to produce an enzyme called penincillinase. Penincillinase is a general term describing a wide variety of enzymes that are capable of deterring penicillin invasion. A common type of penincillinase is plasmid-mediated extended-spectrum b-lactamases (ESbl) (bacteria can transfer genetic material through replication or plasmid). Plasmid-mediated means that the enzyme is found or transferred by plasmid; extended-spectrum means that this enzyme (b-lactamases) has resistance against multiple antibiotic drugs. Beta lactamases is the real charm here. Beta lactamases breaks the lactamine rings in the penicillin structure and destroying penicillin molecules; thus successfully stops a penicillin attack.

Penicillin is a general term describing a broad-spectrum of antibiotics obtained from penicillium molds or produced synthetically to fight gram-positive bacteria (only prokaryotic cells). Penicillin destroys gram-positive bacteria by destroying the peptidoglycan wall. In a typical bacteria environment, rapture in cell wall will almost always cause an explosion or disfigurement of the bacteria, thus destroying the bacteria. Penicillin does not work well against gram-negative bacteria because peptidoglycan wall is located deeper within the cell wall, thus making it difficult for penicillin to destroy the bacteria.

Identifying Selection Pressures:

At the peak of antibiotic drugs, there were considered “magic bullets.” But as more and more antibiotics are used, whether as a medicine for people or crops, many antibiotic resistant cases have been discovered and antibiotic resistant organic beings now pose a serious problem to man kind. The principles behind the appearance of antibiotics are rather simple. Microorganisms such as bacteria have gone through a miniature evolution. Bacteria have shorter life span than human and thus undergo many generations within a single human generation. As Charles Darwin pointed out in his books, time is the essential gradient as any kind of selection that leads to evolution require a long time (in respective to the bacteria).

There are two fundamental ways in which an organic being can develop resistance to antibiotic drugs; first is through prolonged exposure to the antibiotic drugs; secondly the population has resistant genes to begin with. A native organic population, whether be E. coli bacteria or insects is most susceptible to the new a type of foreign invasion because the organic population has previous experience with the new force and thus has no defense against it. However, each individual within species has exchangeable genetic information but with subtle differences in the genetic information within each individual to allow slightly different interpretations of the genetic information (how wonderful). A result of this difference is the variations between individuals within single specie (as Darwin pointed out also). If a native bacteria population has been exposed to penicillin, some individuals within this population will have slight variations in genes that allow them to counter the penicillin and survive. Those that survived will have different genes that offered protection against penicillin than those that did not survive.

These genes are called polymorphic genes and are slowly being identified by scientists/biologists. Those that survive are mostly able to reproduce and these genes that are resistant to penicillin are preserved and accumulated in a directional selection towards new pure strained specie (maybe not quite as new specie) that is immune towards penicillin. And this my friend, is the principle of The Fittest Survives, this is Natural Selection at work. In the second scenario, the population injected with antibiotic drugs already has immunity within the population due to previous experiences with these particular antibiotic drugs. The third possible explanation of antibiotic bacteria is mutations. However, this process is least likely to happen. In most cases, mutation tends to destroy organic beings but on rare conditions, mutation can destroy a particular gene that controls the production of penincillinase, which maybe produced in large quantities to destroy penicillin.


What effect does penicillin, an antibiotic drug has on different groups of E. coli bacteria?


Native E. coli, such as those that are found on feral animal excretions are extremely sensitive to any antibiotic drugs and therefore most susceptible to any antibiotic drugs. However, those populations that already contain resistance to antibiotic drugs, or those that have survived prolonged exposure to antibiotic drugs will have a stronger resistance against penicillin.


According to past experiments, those E. coli bacteria populations that have weak or no resistance against penicillin will have the lowest diameter of the zone of inhibition because the drug will prohibit the bacteria to grow and thus limits the size in the petri dish that the bacteria will occupy. Any E. coli population with the strongest antibiotic resistance against penicillin will have the largest diameter of the zone of inhibition because the antibiotic resistance will allow this population to overcome the antibiotics and therefore will occupy a larger area in the petri dish. Any population that has intermediate resistance to antibiotics will have a diameter of zone of inhibition between that of the strongest resistance population and the weakest resistance population.

Experimental Design:

There is a huge problem with this experiment. Last time I checked E. coli is gram-negative bacteria and penicillin does not work well against it! This means all E. coli population has already in possession a form of resistance against penicillin and as result, the zone of inhibition produced will not be as clear cut if the bacteria tested were gram-positive bacteria. Nevertheless, predictions, hypothesis and all the variables still hold true within the circumstance within this experiment. The major manipulated components of this experiment are the populations of E. coli bacteria that have different susceptibilities to antibiotic drugs. As described before, native E. coli bacteria is most susceptible to any antibiotic drugs and therefore be used as the least strongest of E. coli population.

As described in this background information, E. coli bacteria populations that have strongest resistance to penicillin are capable of producing sufficient amount of penincillinase to counter with penicillin. Finally, three different intermediate types of E. coli will be used to compare with the strongest and weakest E. coli population. The controlled variables in this experiment would be factors/conditions that may effect E. coli growth. First, all E. coli bacteria used must be pure strains, which means a E. coli population only has E. coli and nothing else. To achieve this effect, the sterile method is used in preparing the population. Bacteria need food to grow on. Each testing E. coli population will be separated into separate petri dishes containing the identical amount and type of mediums. To quicken the bacteria growth, the bacteria petri dishes are incubated to speed up bacteria growth. Finally, the responding variable will be how much space does each population of E. coli occupies.

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