Exam 3 Micro Lecture: Ch 20 Antibiotics and Resistance
The use of drugs to treat a disease
Interfere with the growth of microbes within a host
A substance produced by a microbe that, in small amounts, inhibits another microbe
A drug that kills harmful microbes without damaging the host
Main way our antibiotics work
targeting the cell wall of bacteria because we do not have cell walls
Spectrum of activity
tells us which microbes the drug most effective against
kills most gram + and - bacteria
a doc can prescribe this without having to know what is infecting you
can only be effective against a more narrow class of bacteria
5 main targets of antimicrobial drugs
1. inhibition of cell wall synthesis
2. inhibition of protein synthesis (in particular the ribosome because the bacterial ribosome , while it has the same function as our ribosomes, it is structurally different. Different size, shape, structure. It gets job done in a slightly different way and we can exploit that and develop drugs that inhibit bacterial ribosome without harming eukaryotic ribosomes)
3. inhibition of nucleic acid replication and transcription (DNA)
4. poking holes in plasma membrane (as opposed to inhibiting cell wall. Bacterial plasma membranes and our plasma membranes are very similar and so we worry about toxic effects so we have not gotten a handle on this yet)
5. inhibition of synthesis of essential metabolites and enzymes
Beta lactam antibiotics
Antibiotics that inhibit the cell wall.
Beta lactam – 3 carbons and a nitrogen all covalently bonded to form a ring. All antibiotics that have a -cillin in it, it is part of the penicillin family and have beta lactam rings.
4 groups of beta lactam antibiotics
Most effective against Gram Positives
Extended Spectrum- Amoxicillin, Ampicillin
Natural, Semisynthetic, or Synthetic varieties
Penicillin-G is original one (only effective against gram positive)
A beta lactam in which we substitute a C for a S and add a double bond
The newest group we have of beta lactam drugs
What is the bacterial cell wall composed of?
Bacterial cell wall is assembled out of long chains of NAG and NAM that form peptidoglycan cross links (long sheets may layers thick).
These layers are held together by covalently bonding them together with tetrapeptide chains that are coming off.
Tetrapeptide chains contain
1. D-isomers of amino acids
2. Diaminopimelic acid (DAP) - found no where else in nature
How do beta lactams / penicillins work?
There is an enzyme (penicillin binding protein) that forms the peptidoglycan cross links in the bacterial cell wall. Penicillin binds to the enzyme and it reacts with it in a way that it cannot let go of it. It inhibits the formation of the cross links.
What is the major limitation to the way the beta lactam drugs work?
Major limitation is that we only form the cross links when the bacteria is actively growing and dividing. If the bacteria has reached stationary phase, your antibiotic is going to be of limited use because the bacteria are going to be growing very slowly. So you want to hit bacteria when they are in exponential phase because that is when they are growing and dividing as rapidly as possible. You want to make sure you kill them all off so that is why when you have a 10-14 day supply of an antibiotic, you need to take all of it.
target cell walls
Targets NAG-NAM linkages in peptidoglycan
a glycopeptide antibiotic
B-lactamase+ Gram positive bacteria
Drug of last resort for MRSA
has to be given in a hospital through IV
Vancomycin resistant enterococci
the bacteria that are resistant to Vancomycin
organism with acid fast wall, has a thick waxy layer on top
both inhibit the waxy layer of mycobacterium from forming. Does not inhibit the bacteria.
3 antibiotics that target the cell wall
1. Beta lactam drugs
2. Glycopeptide antibiotics
3 spots on ribosome that we can target for inhibiting protein synthesis (and how they do so):
1. small subunit - the 30s section that can grab on to messenger RNA and line it up so that TRNAs are all in the right spot.... we can interfere with how the section holds on to our MRNA by changing the shape of this portion causing the code on MRNA to be read wrong (Streptomycin)
2. We can interfere with the ability of that MRNA to line up and attach with TRNA (Tetracycline)
3. large subunit - we have a third group of drugs that bind to the 50S section and inhibit peptide bonds (Chloramphenicol, Macrolides)
4 antibiotics that interfere with protein synthesis
Binds 50S subunit
inhibits peptide bond formation
Chloramphenicol is derived from _______
Interferes with tRNA attachment
4 ring antibiotic
Tetracyclines are derived from _____
Binds 50S; prevents translocation
works on gram positive bacteria only because it has giant structure which is limited to gram + because it cant get through cell wall of gram – bacteria
How do inhibitors of DNA synthesis work?
inhibits DNA unwinding proteins (DNA gyrase)
Have a giant sulfur in the center and they inhibit enzymes that are involved in synthesizing folic acid which is the step in the endomatic pathway. We get this from our food, folate, folic acid. Bacteria synthesize it themselves and it is an essential biosynthetic process.
The pairing of 2 sulfa drugs together because they are both inhibiting the same pathway.
Pairing gives you better than double the effect. 1+1 = 4 as opposed to 1+1 = 2. we see a greatly enhanced effect at lower doses. We can kill microbes more effectively by pairing them.
Misuse of antibiotics selects for resistance mutants
5 types of antibiotic misuse
1. Using outdated or weakened antibiotics
2. Using antibiotics for the common cold and other inappropriate conditions
3. Using antibiotics in animal feed
4. Failing to complete the prescribed regimen
5. Using someone else's leftover prescription
What are the antibiotic side effects?
When you take an antibiotic that goes after the gram bacteria that kills strep, is that antibiotic only going to kill off the strep in your throat? It is going to kill off the staph that you have in your skin or something in your gut. It is going to kill off anything that it can. So this is why people suffer side effects. Gi distress, etc. it is not your gi tract that is in distress, it is all the microbes in your gi tract that is in distress. If your microbes aren’t happy you aren't happy. So this is why they say to eat yogurt and things when you take antibiotics.
Clostridium difficile infection
A gram positive spore forming anaerobic bacteria that lives in peoples guts happily. Some strains of c diff produce a toxin and that toxin wrecks havoc with the digestive tract that causes diarrhea, bleeding, etc. it smells bad because it is an anaerobic bacteria. What happens with c diff is that patients who have chronic other problems where we give them antibiotics, the antibiotics start killing off the normal flora in the gut but not the c diff because of the spores. And the spores are not affected by the antibiotics. And so that c diff as a minor component of the gut flora, all of the sudden we have killed all the other stuff and that c diff can now colonize the areas now available and if that happens to be the one that makes the toxin, you develop c diff infection. We normally treat this with antibiotics but that is how we got into this problem to begin with. This can turn into a chronic problem.
Poop pills. We take a sample from someone who has a healthy flora and we blend it and put it through coffee filters and take that healthy flora liquid and get it into the person. you don’t drink it, they have it in pill form. These bacteria out compete the c diff and restore the normal population in your GI tract. Over 90% effective but you have to get over the ick factor.
4 mechanisms of antibiotic resistance
1. Blocking entry - Prevention of penetration of drug
2. Inactivating enzymes - break down and destruction of drug before it binds to its target
3. Alteration of drug's target site - changing what it binds to
4. Rapid ejection of the drug - let it in and pump it out
Where are resistance genes often found?
Resistance genes are often on plasmids or transposons that can be transferred between bacteria
Role of Cell Wall in Antibiotic Resistance
Some bacteria have a porin transport method. Porins allow molecules to go in and out of cell wall.
What some gram negative bacteria do is stop making the pores and stop making the proteins. If you don’t have a hole big enough, it cannot get in.
The Effect of b-Lactamases on Penicillins
The most common way of generating antibiotic resistance is to have an enzyme that will degrade the drug. Beta lactam drugs are inactivated by bacteria in this way. some variety of an enzyme that we call beta lactamase. Don’t confuse with penicillin binding protein. Chews up core structure and cannot then inhibit bacterial growth.
How does Augmentin work?
pairing b-lactam antibiotic + b-lactamase inhibitor (clavulanic acid)
a sublactam that inhibits b-lactamases
How does Vancomycin resistance work?
The bacteria had multiple genes that would resist this stuff. They had genes that would totally rearrange the synthesis pathway for the cell wall in a multiple step way. they also had developed a way to sense the drug and then turn on sensory genes when exposed to the drug.
Proteins that span the cell wall pump out the antibiotic.
As fast as it gets into cytoplasm, we burn energy and actively pump it back out.
Proteins that normally exist for other functions in the cell like enzymes that degrade food etc. they have evolved and adapted now to not just pump out whatever you want to pump out but to also pump out the drugs. We can have one set of genes but the one pump structure can pump lots of different types of drugs out where we have a multi drug efflux pump that pumps not just like tetracycline for example but also pumps out the other drugs and sulfa drugs so you cannot use those. Our choices shrink.
What is the difference in how gram + and gram - handle the efflux pump?
If gram positive they pump out one plasma membrane layer, if negative, both.
What is the #1 way of being resistant to tetracycline?
inhibit protein synythesis
Modify the ribosome so that tetracycline cannot bind to it is hard to do.
In the Alaska dig, they found that bacteria had genes that encoded for what 4 things?
2. Tetracycline resistance
3. Glycopeptide resistance
4. Vanc. resistance
We are tracking what is the something and what is the drugs it is resistant too when someone comes in for something. We swab them and track it in this antibiogram. The higher the number of susceptibility, you want to use that drug.
What 6 bacteria are called the ESKAPE bacteria and why?
Very small number of bacterial species of true concern. There is no escape from these.