Antibiotics: what are they and how/why do they work and how long do
they live? Are some chemicals and some molds? Do they kill bacteria
because they are very toxic or for some other reason, like they feed
off of it or something? Also, how do they break down? Once we take an
antibiotic, does it die inside our bodies? Do we excrete them, and if
so, are they still alive? What causes them to die? If they are a mold
are they alive and if they are a chemical are they not alive? Thank
you.

Hello there

Perhaps we can begin by getting rid of the idea that antibiotics are
"alive."  The organism that produces the antibiotic is alive but the
drug is an extraction and not the living organism istelf.

You ask, "Are some chemicals and some molds?" - - - - Well, yes, both, and more.

Antibiotics are chemicals but the chemical is produced by - and then
extracted from a mold or a bacillum.

Here are some of the more important antibiotics and the organism which
produce them.

Penicillin - Penicillium chrysogenum
Cephalosporin - Cephalosporium acremonium
Griseofulvin - Penicillium griseofulvum
Bacitracin - Bacillus subtilis
Polymyxin B - Bacillus polymyxa
Amphotericin B - Streptomyces nodosus
Erythromycin - Streptomyces erythreus
Neomycin - Streptomyces fradiae
Streptomycin - Streptomyces griseus
Tetracycline - Streptomyces rimosus

So as you can see, not all antibiotics come from molds, some are from bacteria.

"Do they kill bacteria because they are very toxic or for some other
reason, like they feed off of it or something?"

The answer is "toxic."  However, the toxcity itself must be selected
very carefully.  - - "Several hundreds of compounds with antibiotic
activity have been isolated from microorganisms over the years, but
only a few of them are clinically useful. The reason for this is that
only compounds with selective toxicity can be used clinically - they
must be highly effective against a microorganism but have minimal
toxicity to humans. In practice, this is expressed in terms of the
therapeutic index - the ratio of the toxic dose to the therapeutic
dose. The larger the index, the better is its therapeutic value." -
quote from "Penicillin and other antibiotics" -
http://helios.bto.ed.ac.uk/bto/microbes/penicill.htm

Most of the rest of your question relates back to them being "alive"
when used and since we now know that is not the case, it is no longer
applicable.

As for excreting them, The liver and kidney are the primary organs for
metabolizing and excreting antibiotics.  People with compromised liver
and/or kidney function may have higher tissue levels of antibiotic
than a normal person, and may have violative levels of antibiotic even
after the appropriate drug withdrawal time has elapsed.

People on antibiotics may also be excreting low levels of the
compounds in their sweat, thereby contributing to the development of
antibiotic resistance in bacteria that normally live on the skin.   -
- "The excretion of benzylpenicillin in sweat may have contributed to
the development of resistant staphylococci soon after the antibiotic
was introduced more than 50 years ago, the researchers said. The
excretion of cetriaxone and especially ceftazidime in sweat may have
contributed significantly to the present worldwide spread of resistant
staphylococci, they said." - quote from The Oregonian "Science Briefs"
- http://www.oregonlive.com/news/oregonian/index.ssf?/news/oregonian/00/11/sc_31note07.frame
- Third article down the page.

So excretion of antibiotics is more than just them passing out of the
body in feces or urine.

In summary, an antibiotic is not a living thing and does not "live"
inside the human (or any other animal's body).  It is a chemical but
is of necessity produced by living molds, bacteria, etc.  It is
excreted 'as' an antibiotic through the skin (some, not all) but is
excreted as "metabolites" through the actions of the liver and
kidneys.

You may find some additional and interesting information at this website:
"MedlinePlus: Antibiotics" -
http://www.nlm.nih.gov/medlineplus/antibiotics.html - It is from the
"National Institutes of Health" and is jam-packed with articles, news
and stories about antibiotics.

Search - google
Terms - antibiotics, antibiotic production, excreting antibiotics

If I may clarify anything before rating the answer, please ask.

Cheers
Digsalot

Typically, a week after you stop taking an antibiotic 99% of the
chemical has been destroyed by body processes and/or excreted.  Neil

There are also some antibiotics that we have developed by looking at
the chemical structure of the antibiotic isolated from mold or
bacteria, and then played with various chemical entities on that
compound, thus creating new antibiotics. There are four main
mechanisms that the drugs work: inhibit the synthesis of the bacterial
cell wall (the betalactams), alter the membrane integrity(the polyenes
and azoles), inhibit protein synthesis(macrolides, aminoglycosides,
tetracyclins, etc.), and inhibit DNA synthesis (quinolones,
trimethaprim, etc.). I could give a lot more details if you wanted.
Some antibiotics just stop the growth of new bacteria, so the bacteria
won't go away until one life-time of the cell, while others actually
kill existing bacteria. Hope that helps!
--Richard

It is also worth mentioning that some antibiotics (like Salvarsan and
Sulfa-drugs) are synthetic, not produced by any microbe.

An antibiotic timeline:
http://www.life.umd.edu/classroom/bsci424/BSCI223WebSiteFiles/AntibioticsHistory.htm