May 18, 2010 at 3:08 am #13328
Why do bacteria die off in a broad band around an introduced antibiotic, but die off only in the immediate vacinity of discintfectant?
May 18, 2010 at 3:24 am #99770
My daughter’s biology teacher has failed to connect anything. The students are flailing around totally confused. My reading tells me that the action of antibiotics is transformation – passing on damaged genes to other cells which all eventually die (ideally). So the mechanism of anti-biotics is basically genetic.
Disinfectant is poison that kills on contact but is not passed to other bacteria.
But then I read more and find -cidal vs static action neither one of which described genetics.
Am I way off?
Shouldn’t a biology teacher be able to explain things in simple
general terms (with a myriad of caveats)?
May 18, 2010 at 6:52 am #99773biohazardParticipantquote highschooldropout:
Antibiotics work in many ways, but this "passing of damaged genes" mechanism is not one of them. Antibiotics usually target such structures in a bacterial cell that differ from eukaryotic cells (e.g. human cells). Very efficient targets are for example the bacterial cell wall (or its synthesis) or bacterial ribosomes. Because these structures are absent or different in humans, they remain unaffected.
The reason why bacteria die around of introduced antibiotics is diffusion: the antibiotic spreads along the medium up to a certain distance and kills bacteria within this area. Gradually the concentration of the antibiotic becomes lower and the bacteria manage to grow when they are further from the source of the antibiotic. The exact pattern of inhibition/growth depends on the medium (e.g. agar) used, and the type and concentration of the antibiotic.
If disinfectants are used in a similar manner, they also produce an area of growth inhibition around the site of administration when the disinfectant spreads. However, most disinfectants evaporate quickly or lose their potency when diluted or mixed with another substances. Thus, disinfectants usually kill bacteria only from the area where they are administrated and then evaporate before they have time to cause this pattern of growth inhibition typical for antibiotics.
Bacteriocidal and -static properties of antibiotics mean that some antibiotics (like tetracyclins) simply stop the bacteria from dividing. This is usually gives the body enough time to kill out the remaining bacteria because they cannot spread. Bacteriocidal antibiotics simply kill bacteria.
Even though the effect of antibiotics does not get passed on to other bacteria by the transfer of damaged genes as you suggested, an exact opposite phenomenom can often be seen: bacteria that survive from the use of antibiotics do spread the antibiotic resistance to other bacteria. Antibiotic resistance is often made possible by small genetic elements called plasmids, and suriving bacteria transfer these plasmids also to other bacteria in their environment, turning them as well into antibiotic resistant forms. This is one important mechanism in the formation of multiresistant bacterial strains often found in hospitals and elderly houses etc. Spontaneous mutations can also create resistant strains, especially under prolonged pressure from antibiotics.
May 18, 2010 at 11:03 am #99778
Not what I was expecting. So it is the environment!
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