Activities of Certain Antibiotics

Activities of Certain Antibiotics

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There are thousands of known antibiotics of which only a few are approved for human use because many also affect mammalian systems. So let's talk about a few that generally don't affect human systems.

Generally they don't affect humans for one of two reasons - the human doesn't have the targetted structure, but the bacterium does (think "cell wall" for example), or the antibiotic cannot get into the eukaryotic cell - cannot get through our membranes.

Penicillin and its relatives inhibits the linking of d-amino acids together to make one of the structures in bacterial cell walls.
Streptomycin and its relatives bind to a portion of the small ribosomal subunit and prevent the ribosome from ratchetting along the mRNA - sort of like sand in the gears. Str-resistant bacteria have a mutated r-protein that does not bind streptomycin. Hence, no amount of str will affect resistant mutants. Str cannot get into most mammalian cells. However, it does leak into the cells of the 5th cranial nerve and causes loss of smell. Better to lose one's smeller than one's life to typhoid fever. But other antibiotics are now used, and str is very rarely prescribed any longer.
Tetracycline probably blocks bacterial pores. Since their transmembrane proteins are different from ours, tet has little damaging affect on our cells.
Sulfa drugs (sulfanilamides) are analogs of a p-amino-benzoic acid (PABA), which is a building block for making the coenzyme folic acid (vitamin Bc), which is used in moving methyl groups around such as in making methionine. Because most mammals require fully pre-made folic acid in the diet, the addition of confusing analogs of PABA has no effect on us and is merely excreted. But to bacteria and some fungi, which can make folic acid de novo (from scratch), if you add some PABA in their medium they will turn off the pathways used to make PABA (why waste time). If sulfa is added, it also turns off the PABA synthesis and so the bug ends up with no folic acid and starves to death.

Of course, there are other antibiotics that block mRNA synthesis, others block translation (as does str), some block DNA synthesis, and others - such as what Milton Saier's groups plays with (methylated glucoses) get pumped into bacteria and then cannot enter glycolysis and neither can they be pumped out. So the cells clog with this stuff. Our cells can discriminate against methylated sugars and exclude them. Interestingly, the hotness of pepper is mostly methylated glycosides - and it has been shown that people in the tropics who eat a lot of spicey foods have fewer intestinal diseases.

Some of the goals for fighting HIV and other retroviruses is to find agents that inactivate reverse transcriptase, which normal cells don't have.

Other antiviral agents take one of two strategies - either target the attachment devices on the virus particles, or bind to the places on cells that the viruses would attach to. So if the virus's claws are clipped it cannot attach to cells, or if the landing sites are already occupied, the viruses cannot land. (A corrolary to blocking the landing sites, is to destroy them with enzymes.)

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