Antibiotics I Flashcards
Why do organisms make antibiotics?
Bacteria grow in competition with other organisms. In order to effectively compete, they produce things that will inhibit the growth of other organisms. Among these
are antibiotics. We’ve taken advantage of these naturally produced compounds, and learned to purify, amplify, or even modify them to increase their bioavailability by increasing their solubility.
What are the major classes of antibiotics?
Penicillin and other beta-lactam antibiotics, Streptomycin and Aminoglycosides, Tetracyclines.
What did Flemming discover?
Fleming realized that the fungi were secreting some substance that inhibited the growth of bacteria. Seemed to work well against bacteria that were gram positives but not gram negative bacteria, aside from neisseria. Neisseria cause ghonneria, an STD. Penicillin is a primary treatment for ghonneria.Also found out about lysozyme.
What’s an important thing to take into account when making antibiotics?
When making a new antibiotic, one thing you have to take into account is specificity. Is it going to be selective for the target organisms or is it going to harm the host given that we often share the same or similar
metabolic pathways. In order to choose a potential antibiotic target, ideally you want to find a unique structural feature of bacteria or fungi, that differentiate them from our cells. If thats not available then the
other way is to look at binding affinities. We’ve seen the latter with some sulfanilamides and some other drugs that actually do target enzymes found in both prokaryotes and eukaryotes, but they have a sufficiently
different binding affinity so that the therapeutic index is fairly high. Its often difficult to find distinct structural features when dealing with fungi, so this strategy is often used.
What distinguishes an antibiotic from other antibacterial agents?
Antibiotics are produced by other living organisms. Other general antibacterial agents, such as sulfanilamides, are synthetically
produced. Sulfanilamides are derived from the red dye prontosil, never found naturally in other living organisms. Antibiotics are primarily made by the bacteria genus streptomyeces.
How is a gram stain done?
Gram stain: Done by adding two different stains. The idea is that the first stain added is retained in the bacteria (blue or purple color), however it can be removed with a subsequent wash with an
alcohol solution in some bacteria, whereas other bacteria with the stain will retain it even after the wash. The ones that retain the stain are called gram positive, the ones that are cleared
with the wash are gram negative. This separates almost all bacteria into two basic groups. The ones that have the stain washed out can then be stained with a red stain so that you can
quantify them. These differences in staining have to do with the cell wall structure of the bacteria.
Why was Flemming not able to put penicillin to full use?
Although Fleming discovered penicillin, he wasnt able to figure out how to get large quantities of it, so he gave up. Later, florey and chain discovered how to grow and purify enough of it to make it useful in WW2. Before they could make it in such large quantities, the alternative was to collect urine from individuals treated with penicillin and purify it from that to reuse it.
How was the MOA of penicillin discovered?
Penicillin was found to be causing lysis of the bacteria. The researchers looked at 3 different parameters: the viable count, going down with the addition of penicillin. Then decreased turbidity (even though the
line is shown to be going up, it actually represents a decrease in the turbidity of the bacterial culture. When you grown bacteria in solution the growth media becomes more and more turbid as the bacteria grow more
and more and you can use a spectrophotometer to quantify this by looking at the increased absorbance of light. The key discovery was that when they centrifuged the culture grown in the presence of penicillin, they
found that normally you could isolate bacterial DNA from the pellet, which makes sense, but when they did the same thing after treatment with penicillin, there was decreased DNA found in the pellet and increased
DNA found in the supernatant. This hints that the DNA is being released as a result of cell lyses.
What’s isotonic mean?
Isotonic: ions are in equal concentrations inside and outside the
cells. This means that the flow of water is not going to go to one side or the other, so no potential for lysis.
What does hypotonic mean?
Hypotonic: less dissolved solutles on the outside, so water will flow into
the cell and the cell will burst. Hypertonic is the opposite and youll
get cell shrinkage.
What hinted towards penicillin disrupting the formation of the cell wall? (Specific media setup)
If you grow the cells in isotonic media, you still see changes in morphology.
This was the first clue. The cell wall controls the morphology, so this hinted
that penicillin was affecting the cell wall structure. Unlike mammalian cells, bacterial cells can normally survive a hypontonic solution without rupture, however when treated with penicillin, the bacterial cells were observed to undergo lysis when in a hypotonic solution.
What does penicillin do to nongrowing bacteria? What does this show?
Non-growing bacteria were not significantly affected by penicillin. So this was an indication that unlike the action of lysozyme, which basically breaks down a pre-existing cell wall, what it looks like is happening is that the formation of the cell wall is being inhibited. This was supported by finding that further examination of the supernatant, you could detect some precursors of cell wall syntheiss.
What’s different about sulfanilamides and penicillin?
Sulfanilamides are bacteriostatic: they stop bacteria from growing after a certain lag point. Penicillin is bacteriocidal in that you get a decrease in the number of viable bacteria after adding the agent. So penicillin actually kills the bacteria, so even in someone who is immunocomprimised, penicillin will still work effectively because you dont need the immune system to clear out the bacteria. Gram positve bacteria have a very extensive peptidyl-glycan structured cell wall outside of the plasma membrane. They additionally have teichoic acid which is not made by gram negative bacteria. Gram negative bacteria have a more complex structure of cell wall. They have a plasma membrane, then a peptidyl glycan layer that is significantly thinner than what you see in gram positives. The thickness of the peptidyl-glycan layer explains the staining difference between the bacteria in that the thicker one more effectively traps the dye in the cells and gives gram positives their positive appearance. Additionally, gram negatives have an outer layer of polysaccharides which is essentially an additional membrane. Gram negatives also have bacterial pores that are formed by porins which are important for regulated entry of molecules in gram negative bacteria, including penicillin. Standard penicillin cannot penetrate gram negative bacteria. because it cant fit through the porins.
What part of the cell wall is the target for penicillin?
Peptidyl-glycan layer is the target for
penicillin.
If Neisseria is gram negative, how is penicillin able to affect it?
Neisseria have a different type of porin present, which penicillin is able to penetrate, which explains why penicillin can be used to treat infections of neisseria (ghonneria).
What allows penicillin to affect gram positive but not gram negative bacteria?
The process of cell wall formation is similar in gram negative and gram positive cells, so it was clear that the
selectivity of penicillin form gram positive cells was not due to some inherritant differnce in the cell wall formation,
but rather the selective pores in gram negative. Both have alternating subunits of N-acetylglucosamine and N-acetylmuranic
acid. A series of side chains are formed that are built by non-ribosomal amino acid attachment (carried out by transpeptidases instead).
What step in cell wall formation is affected by penicillin? Specifically what type of proteins are targeted?
Transpeptidases were first identified as penicillin-binding proteins. It appears that the last
cross-linking step described to the right is the key step that is interfered with by penicillin
and leads to a weakened cell wall, which leaves the bacteria being susceptible to lysis. If the
bacteria are not actively
growing then this wont
really affect them.
