Antibiotics
Even though most bacteria are completely harmless to us, every now and then we find
some where we don’t want them to be (like, streptococcus bacteria in your sore throat).
There are many chemicals out there that will kill bacteria – unfortunately, a lot of those
chemicals kill your own cells as well. It wasn’t until the early 1900’s that we found
molecules that would harm prokaryotic cells while leaving your own cells alone. This is
possible because even though prokaryotic and eukaryotic cells use the same basic
processes to live (transcription, translation, building a cell membrane, etc.), we can
exploit subtle difference between the two types of cells.
Penicillin (and its close cousins)
This is the penicillin molecule discovered
by Alexander Fleming. Over the past few
decades, we’ve created a number of
closely related penicillin-like drugs by
adding a carbon atom here, dropping an
oxygen atom there, etc. They all work
the same way, however.
Penicillin drugs (also
known as beta-lactam
drugs) take advantage
of the fact that bacteria
have a cell wall, while
human and animal cells
do not. The cell wall is a
bit different than that of
plant cells – it’s made of
a material called
peptidoglycan. Betalactam drugs prevent the
bacterial cell from
making the building
blocks of the cell wall,
leading to holes in the
wall when individual
components wear out
and break down.
Hilarious bacterial
explosions ensue.
Aminoglycosides
Aminoglycoside antibiotics wreak havoc by
interefering with the bacterial ribosome.
Even though your cells have ribosomes
too, they are a slightly different shape than
the bacterial ones, and thus the antibiotic
does not prevent them from translating
mRNA. We don’t know exactly how
aminoglycosides such as streptomycin stop
the ribosome, but they definitely work.
Tetracyclines
Tetracyclines also interfere with the
bacterial ribosome, but in a different manner than the aminoglycosides. Tetracyclines
prevent the ribosome from picking off the amino acid from the tRNAs that come through,
so even though the ribosome is moving down the mRNA, no amino acid chain is being
made.
Sulfa Drugs
Both prokaryotic and eukaryotic cells need the molecule folic acid. The cell turns folic
acid into adenine, guanine, and thymine, and also into two of the twenty amino acids.
Interestingly, bacterial cells can make their own folic acid, while eukaryotic cells need to
pick it up from the environment. (This is why your multivitamin pill has lots of folic acid
in it.) Sulfa drugs prevent the bacterial cell from making folic acid (apparently, it can’t
pick it up from the environment.) Without it, the cell is missing three out of the four DNA
bases and is dead in the water.
There are dozens of different types of antibiotics that your doctor can give you if you
have an infection. Why do we need so many? And why are doctors and researchers
worried that we don’t have enough different types of antibiotics? Two reasons . . .
1. Not every bug can be killed by every drug. For example, the bacteria Pseudomonas
aeruginosa lives in the same environment as the Penicillium mold in the wild, and so
it has evolved defenses against natural-made penicillin. Notice the little arrow
pointing to one of the covalent bonds in the picture of the penicillin molecule? P.
aeruginosa makes a protein that breaks that bond, rendering the penicillin useless.
The bacterium has no natural defense against aminoglycoside antibiotics, however,
and so a P. aeruginosa infection can be treated by one of those drugs.
2. Bacteria can “learn” how to avoid antibiotics. The bacterium Streptococcus
pneumoniae is a common cause of earaches. When penicillin was first produced,
even very small doses of it could kill the bacteria. However, within 10 years or so, an
awful lot of S. pneumoniae bacteria could live quite happily in the presence of
penicillin. What happened?
Turns out that S. pneumoniae had
taken a lesson from P. aeruginosa.
Since there is a protein that can
break down penicillin, there must be
a gene that codes for that protein in
P. aeruginosa’s DNA. Bacteria can
pass pieces of DNA between
themselves, unlike eukaryotic cells.
Mr. Eastham can’t give some of his
DNA to Mr. Layne to use, but Bob
and Joe Bacteria can swap genes
back and forth. S. pneumoniae
(and a whole lot of other bacterial
species) started picking up the gene
for this penicillin-killing protein, and
now very few bacteria can still be
killed by penicillin. There are other
ways for bacteria to avoid
antibiotics, including just pumping
the drug back out of the cell.
The bacteria started the work of becoming resistant to being killed by antibiotics, but
human practices have made the situation much worse. Imagine you have strep throat.
You have lots of bacteria in your throat, most of which
can easily be killed by an antibiotic. There are a few
that will need a larger dose before they’re killed,
however.
The doctor gives you seven days of antibiotics. You
dutifully take the drugs, and at the end of three days,
you’ve killed off all of the easy bacteria.
If you finish the course of antibiotics, the few bacteria
left will now be exposed to much higher concentrations
of drug than they were before, and they will die.
However, you’re feeling better! You stop taking your
medicine. There’s still a few hardy bugs left, and
you’ve just cleared away all their competition. They
will multiply like crazy, and in a few days, your throat
will be filled with bacteria again. This time, the
antibiotic won’t work.
Hospitals are starting to see patients infected with bacteria that can’t be killed by ANY of
our antibiotics. Scary, no? Here’s what you can do to help stop the spread of antibiotic
resistance:
1. Don’t ask your doctor for antibiotics when you don’t have a bacterial infection.
Colds and the flu are caused by viruses, not bacteria – antibiotics will have no
effect.
2. Make sure to take all of your antibiotic medication that the doctor prescribes you.
3. Don’t take someone else’s leftover antibiotics without a doctor’s permission.
(And frankly, the person should have taken all of their antibiotics anyway and
shouldn’t have any left over!!)
4. Don’t use antibacterial soaps, lotions, or other products. Washing your hands
with plain soap and water will get rid of just as many bacteria as using the special
soap, and antibacterial kids’ toys are completely worthless.