Alistair Brown Flashcards
double check recap
Define ‘bacteriostatic’.
a substance that prevents the multiplying of bacteria without destroying them.
Explain growth curve of bacteria under a bacteriostatic antibiotic.
1) introduce antibacterial during log/exponential phase of growth curve: growth curve will stay in plateau . The curve does slightly go down bc natural cell death still occurs toxins being produced.
Which drugs are bacteriostatic?
chloramphenicol erythromycin clindamycin tetracyclines sulfonamides trimethoprim
Bacteria was put under bacteriostatic antibiotic. If you transfer the bacteria back in fresh culture media, will it continue to grow?
yes.
If you give bacteriostatic antibiotic at a high enough concentration, it becomes bactericidal. However this is not given. Why?
don’t do this bc toxic effects on the host/patient cells. antibacterials/fungals/virals also cause damage to host cells too not just microorganism cells. that’s why have to be careful and not for too long, 14 days max if possible.
Define ‘bactericidal’.
a substance which kills bacteria.
Which drugs are bactericidal?
beta-lactams quinolones rifampin metronidazole vancomycin aminoglycosides
Bacteria was put under bactericidal antibiotic. If you transfer the bacteria back in fresh culture media, will it continue to grow?
only slight growth (bc 100% wont die).
Explain growth curve of bacteria under a bactericidal antibiotic.
growth curve:
usually put antibacterial in the exponential face> instead of going up during exponential phase, will plateau off then drop down bc bacteria death.
which drugs inhibit folic acid synthesis?
sulfonamides
Trimethoprim
what is tetrahydrofolate/Tetrahydrofolic acid? Why is it so important?
a folic acid derivative.
important because INVOLVED IN THE FORMATION OF NITROGONOUS BASES.
purine bases, thymine and methionine.
methionine is Amino acid> first amino acid incorporated into 70s ribosomal complex. first amino acid in translation. inhibiting dna replication, transcription translation, protein synthesis.
Explain mechanism of action of Sulfamethoxazole to clear an infection.
sulfamethoxazole inhibits (PABA) Para-aminobenzoic acid>inhibits dihydrofolic acid synthesis in bacteria, which in turn stops tetrahydrofolic acid synthess
is.
Wont kill organism by blocking this system.
what it actually does it stop the organism from growing via downstream processes.
folic acid> utilized in v important biological processes.>making nitrogenous bases and amino acids.
during infection, bacteria in unfavourable environment bc host immune syst repeatedly damage the bacteria, producing ROS-damaging its outer membrane, damages its DNA- adding free radicals onto it. Must make new DNA BC has to repair DNA and fix its membrane, produce new peptidoglycan, make new proteins. must be able to transcribe regularly. so must repair dna damage/make new DNA, Which requires nitrogenous bases. also need to make new mRNA to make the new proteins. sulfonamides target PABA and stop THF. hosts cells damages all the bacteria, and sulfonamides stop it from repairing the damage. TOGETHER the antibiotics and host immune syst. clears the infection.
Explain why folic acid synthesis pathway is a good target for antibiotics?
production of folic acid in humans is diff than bacteria. bacteria have to synthesize, but humans get from diet.
»>targeting the synth. pathway reduces amount of toxicity to host. don’t target human biosynth. of folic acid.
Explain mechanism of action of trimethoprim.
Trimethoprim inhibits dihydrofolate reductase»>inhibits formation of tetrahydrofolic acid from dihydrolic acid.»inhibits formation of nitrogenous bases and amino acids.
How do most antibiotics covered in lectures work?
WORKS TOGETHER WITH HOST CELLS TO CLEAR THE INFECTION. host cells damage bacteria DNA , antibiotics prevent repair mechanisms.
Name some DNA topoisomerase inhibitors.
- Fluoroquinolones (Cirpofloxacin, Moxifloxacin, Levofloxacin)
- Quinolones (nalidixic acid)
Explain mechanism of action of fluoroquinolones.
Fluoroquinolones bind to 2 nuclear enzymes > topoisomerase I/IV so inhibits unwinding of DNA, this inhibits dna replic.
so inhibits new DNA from being made. again, wont kill bacteria but when in host, unfavourable condition, stop bacteria repairing itself.
Name an antimicrobial which damages DNA.
Metronidazole.
Explain the 2 ways which metronidazole damages DNA.
Thioredoxin reductase (TrxR) activates metronidazole to the 2 reactive oxygen spieces (ROS). (both diagrams). the free radical version on the right binds to thiols (cysteines) of proteins and causes interactions w the disulfide bridges >damages protein OF these antioxidant defence proteins and inactivates them.
double bonded oxygen version on left interacts w oxygen spieces floating around in the organism and creates ROS or reactive nitrogen intermediates (RNI).
dual effect. activated metronidazole stops the enzymes (antioxidant defence proteins) which protect against ROS, AND makes more ROS. ROS destroy dna. so cells now more vulnerable to oxidative stress..
Name an antimicrobial which inhibits mRNA ?
Rifampin.
Explain mechanism of action of rifampin.
RIFAMPIN (LEAD DRUG to treat TB) INHIBITS mRNA SYNTH.
inhibits mrna synth. directly. targets RpoB of RNA polymerase complex.>inhibits transcription , no mRNA, no protein, no cell wall repair , DNA repair etc. (in hosts cell, cant fix the damage caused by hosts immune syst.)
Name antibiotics targeting protein synthesis.
- chloramphenicol
- clindamycin
- linezolid
- macrolides (erythromycin, clarithromycin, azithromycin)
- aminoglycosides (gentamicin, neomycin, amikacin, tobramycin, streptomycin)
- tetracyclines (tetracycline, doxycycline, minocycline)
How does chloramphenicol work?
Chloramphenicol: inhibit formation of peptide bond between amino acids (at enzymatic activity site condesnsation reaction for formation of peptide bond). chloramphenicol blocks that active site on 50s subunit. would stop chain of amino acids formation, so no protein being formed. stop protein synth. no repair from host immune syst. repair of cell wall etc.
How do macrolides work?
macrolides e.g. erythromycin and clindamycin. binds to 50s subunit REVERSIBLY to prevent transfer of peptidyl-trna from a-site to P site , inhibiting ability of ribosome at 50s subunit moving along the mRNA, stopping translocation. jams It on mRNA. cant move forward or back (for error checks either), so falls apart, mechanisms within the organism comes and breaks the complex apart. so no protein, no damage repair…….
Which antibiotics target the 50s subunit of ribosomes in bacteria?
macrolides and chloramphenicol.
how does doxycycline and tetracycline work?
aminoacyl-tRNA (trna carring amino acid) anticodon binds to complimentary codon on mrna.
tetracycline and doxycycline v flat so bind into A position on 30s ribosome subunit in mrna translation complex, and block the aminoacyl-trna binding to the mrna-ribosome complex. no protein , no repair etc.
How do aminoglycosides work?
bind to 30s subunit and cause misreads in genetic code, interrupting the sequences, and inactivate its ability to read codons correctly. error is produced and wrong trna binds,. error reading occurs and ribosome tries to go backwards and try and start again. if it cant do this, it will stop the translation process. again no protein, no repair.
some proteins can make it all the way through and make the protein but wrong amino acid is in the sequence, bacteria spent lots of the cells nutrients and amino acids , energy atp gdp, Ribosomes. Bacteria loses that energy for proteins which are not functional.
which drugs target 30s subunit of bacteria?
aminoglycosides and tetracyclines.
bacterial Cell Wall synthesis inhibitors?
- glycopeptides (vancomycin, bacitracin,teichoplanin)
- penicillins (penicillinase sensitive+resistant types+antipseudomonals. “……cillin” e.g. amoxycillin)
- cephalosporins (I-V) (cef…../ceph……)
- carbapenems(……penem)
name peptidoglycan synthesis inhibitors?
glycopeptides - e.g. vancomycin, bacitracin, teichoplanin. .
name peptidoglycan cross linking inhibitors that target penicillin binding proteins?
penicillins, carbapenems, cephalosporins.
why is peptidoglycan a good target for antibiotics?
peptidoglycan in cell wall of wall. peptidoglycan specific to bacteria-less toxicity to host cells.
HOW DO CARBAPENEMS, PENICILLINS AND CEPHASOLPORINS WORK?
TARGET- penicillin binding proteins. if cant cross link, wont get cross linking of amino acid chains of peptidoglycan. peptidoglycan becomes v fluid and cell wall not repairable. stopping from MAKING the cross links.
penicillins cephalosporins carbapenems binds covalently (irreversible) into penicillin binding pocket stops organism from repairing its peptidoglycan. inactivates repair mechanism-killing organism by ability of organism to fix outer membrane . not targeting transcription/translation.
what are penicillin binding proteins? (PBP)
enzymes involved in creating crosslinks of amino acid chains IN PEPTIDOGLYCAN.
All β-lactam antibiotics (except for 1) bind to PBPs, which are essential for bacterial cell wall synthesis. PBPs are members of a subgroup of enzymes called transpeptidases
How does vancomycin work?
targets pbp but CAPS the actual chain. ,Vancomycin recognizes and binds to the two D-ala residues on the end of the peptide chains inhibiting them from binding to pbp and creating the cross links.
Vancomycin has a unique mode of action inhibiting the second stage of cell wall synthesis of susceptible bacteria.
There is also evidence that vancomycin alters the permeability of the cell membrane and selectively inhibits ribonucleic acid synthesis.
Explain how bacteria can become resistant to vancomycin via change in target mechanism?
organism replaces alanine (Amino acid) with sugar lactose at the end of the side chains . vancamycin no longer caps the chain>bacteria resistance.
why target ergosterol in fungi?
ergosterol major component of fungal cell wall. ergosterol anchored into membr.
target eukaryote like fungal cell, target something mammalian cells don’t have- humans don’t have ergosterol but fungus cell wall do.
1) fungal cells require ergosterol for major structure of the cell wall, so without ergosterol, cant make cell wall, cant repair anything.
2) also ergosterol binds to outside of proteins that r anchored into membranes. - stabilizes them.
drugs target synthesis of ergosterol -by inhibiting this, it stops organism anchoring proteins into its membrane and overall integrity of fungal membrane, wont be able to replicate and repair any damage.
What do azoles (fluconazole, itraconazole, voriconazole) target in fungi against them??
TARGET and inhibit ERGOTEROL SYNTHESIS.
target and inhibit production of ergosterol from lanosterol.>target and inhibit 14-alpha-demethylase.
What do terbinafines (…..fine) target in fungi against them?
terbinafine - inhibits lanosterol synthesis - targets and inhibit squalene epoxidase. terbafine inhibits squalene epoxidase converting squalene into lanosterol (> ergosterol. )
what is problem with targeting ergosterols?
we have sterol biosynthesis in our bodies>cholesterol>sex hormones synthesis. so antifungals aren’t very good to keep taking.
why are Beta-(1,3)-d-glucans a good target against fungi?
caspofungin target complex sugars on outside cell wall. -unique to fungal cells, we don't have them - good, targets B(1,3)-d-glucans.
mechanism of caspofungin?
INHIBITS CELL WALL SYNTHESIS.
Caspofungin blocks the synthesis of β(1,3)-d-glucan of the fungal cell wall, by non-competitive inhibition of the enzyme β(1,3)-d-glucan synthase. β(1,3)-d-Glucan is an essential component of the cell wall of numerous fungal species
mechanism of amphotericin B?
FORMS MEMBRNE PORES-CELL WALL DISRUPTION
Amphotericin B, a polyene, binds irreversibly to ergosterol, resulting in disruption of membrane integrity and ultimately cell death. Increase ROS entrY.
irreversible inhibitor to ergasterol . ergosterol is packed around transporter. drug breaks interaction between pore and ergosterol> reduces rigidity of that part of the membrane and loosens it up, and loosens the protein, and so pore is more open.
these transporters have transport mechanisms involved in pumping ROS out of cell . porin stops working as well bc its no longer in its tighter conformation bc drug blocking ergosterol from packing around it.
»>creating influx of ROS into fungi>DNA damage.
HOW DOES FLUCYTOSINE WORK?
flucytosine enters fungal cell via enzyme specifically found in fungi. not found in mammalian cells.
1) Flucytosine is converted by cytosine deaminase in fungal cells to fluorouracil,»_space;which then interferes with RNA and protein synthesis.
2) Fluorouracil is metabolized to 5-fluorodeoxyuridylic acid, an inhibitor of thymidylate synthetase»DNA synthesis is then halted.
Antimicrobial resistance
3 main resistance mechanisms?
Efflux
CHANGE IN Target
Degradation
Explain efflux resistance mechanism.
MAJOR mechanism. ability of organism to efflux antimicrobial compound out of cells via ABC transporters using energy of ATP so compound does n0t get to the target.
GRAM positive only one membrane- has ABC transporters on only one membrane.
but gram negative more resistance potential bc has 2 membranes- ABC on both membranes.
drug has to first get in and at a conc high enough.
how becomes resistance? changing expression of genome- more protein more abc transp. in membr.
Explain changing target resistance mechanism .
organism will mutate its genome to introduce diff amino acids >change mrna>diff codon> diff amino acid>if the original amino acid was specific to the mode of action of the antibiotic , removing it will reduce antibiotic’s propensity or reduce activity of that antibiotic- so now the target is changed so that the antibiotic is either no longer activated / or changed so that it no longer binds to the target.
Explain degradation resistance mechanism..
organism acquires series of genes/proteins that it can express that will degrade the antibiotic
produces a number of enzymes- plasmid borne a lot of the times (plasmid is foreign DNA introduced into bacteria by congregation or transferred from environment or between bacteria). which can degrade the antibiotics - e.g. B lactamase. >that’s why some organisms resistance to penicillin.
What is the name of the class of antifungals that inhibit cell wall synthesis?
Echinocandins (caspofungin and micafungin)
