MGY277H1 Study Guide - Final Guide: Enterobacteriaceae, Disaccharide, Restriction Fragment Length Polymorphism
8 Jun 2018
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UNIT 8: Antimicrobials and Antimicrobial resistance
• Describe the discovery of antimicrobial drugs and antibiotics.
• First produced by chemists then by organisms and now by novel sources / chemical
alteration.
• Antimicrobial drugs were first discovered by Paul Ehrlich in 1910 who discovered
Salvarsan. However, it had arsenic and was toxic although it treated syphilis really
ell if dosage as appopiate. He oied the tes agi ullet ad
heotheap.
• Gerhard Domagk discovered red dye Prontosil in 19. This as the fist sulfa dug
and had no effect outside of animals.
• In 1928 Alexander Fleming discovered that Penicillum mould was secreting Penicillin,
toxic to Staph aureus. He was unable to purify it. 10 years later (1941), Ernst Chain
and Howard Florey, and Norman Heatley purified penicillin and used it on a police
officer who was infected with Staph aureus and kept him alive until they ran out of
penicillin.
• At this tie peiilli as so peious it as eig eled fo peoples uie.
Later versions of the drug allowed the drug to be more slowly metabolized and
remain in the body for longer.
• Penicillin production increased during the war in 1944. Went up 1000x in 3 years.
• Later, Selman Waksman purified streptomycin from soil bacterium. Researchers
screened many molecules for antibiotics.
• Most modern antibiotics come from microorganisms that reside in soil. Bacteria:
Streptomyces & Bacillus & Micromonospora. Fungi: Penicillum & Cephalosporium.
• Explain how new generations of antimicrobial drugs are developed.
• Most modern antibiotics come from microorganisms that reside in soil. Bacteria:
Streptomyces & Bacillus & Micromonospora. Fungi: Penicillum & Cephalosporium.
• It as disoeed i the 96s that alteig peiilli stutue led to new drugs.
They made PenG: active against gram – bacteria, Ampicillin: a derivative of PenG,
also active against gram – bacteria, and Methicillin: less susceptible to enzymes that
destroy penicillin. There are many other derivatives of penicillin. They all have a
different chemical group attached to an amino group on the basic penicillin
backbone. These modifications can alter the stability, pharmacological availability,
and the species that are preferentially targeted.
• Most antibiotics were discovered in the United States and England.
• Companies are stopping their R & D for new antibiotics all while resistance is on the
rise.
• Describe selective toxicity; antimicrobial action; spectrum of activity;
tissue distribution/metabolism/excretion; effects of combinations;
adverse effects; and resistance to antimicrobials.
• Selective toxicity: Antimicrobial drugs must cause greater harm to the microbes
than to the patient. They do this by attacking structures only found in microbes and
not humans. Toxicity is expressed as the therapeutic index (TI). This is the lowest
does toxic to a patient / smallest effective dose used for therapy. High TI: less toxic,
Low TI: patient blood monitored for toxicity. Drugs too toxic for systemic use may be
used topically.
• Antibiotics: kill or inhibit growth of bacteria
Antifungals: kill or inhibit growth of bacteria
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Antiparasitics: kill or inhibit growth of pathogenic eukaryotes
Antivirals: inhibit the replication or entry of viruses
• Antimicrobial action: “tati ol ateiostati o fugistati dugs ihiit
ioial goth. Patiets defees ust still kill or eliminate the pathogen after its
growth is stopped.
Cidal ol ateioidal o fugiidal dugs kill ioes. “oeties ol
inhibitory depending on [ ] and stage of growth. Commonly disrupt cell wall
integrity.
• Spectrum of activity: Broad-spectrum antimicrobials → affect a wide range of
microbes. Treat acute life-threatening diseases, especially when no time to culture
for identification. Disadvantage is what it disrupts normal microbiota that aid in
excluding pathogens.
Narrow-spectrum antimicrobials → affect a limited range of microbes. Requires
identification of pathogen, testing for sensitivity but is less disruptive to the
microbiota.
Thee is o ue / lie that sepaates the to. Vancomycin only kills gram +
bacteria, it is too big for gram – bacteria.
• Effects of combinations: Drugs that interfere with each other and the outcome is
worse than either alone → antagonistic (1+1 = -1). Drugs where one enhances the
other → synergistic (1+1 = 3). Drugs where combinations are neither extra good nor
extra bad are additive (1+1 = 2).
• Tissue distribution, metabolism, an excretion: Antimicrobials behaviour is different
in the body: some are unstable at low pH, ½ life dictates frequency, short ½ life =
must be taken multiple times per day. Patients with kidney / liver dysfunction
excrete differently and so dose must be adjusted to avoid toxicity.
• Adverse effects: Toxic effects: some drugs can be toxic at high [ ]. Some people have
allergic reactions to drugs. Normal microbiota may be suppressed / altered with
antibiotics allowing some pathogens to flourish (Clostridium difficile).
• Describe how the minimum inhibitory concentration (MIC) and the
minimum bactericidal concentration (MBC) are determined.
• Determining susceptibility of bacterial strains to antibiotics can be accomplished in
multiple ways. This susceptibility is often unpredictable. MIC (Minimum Inhibitory
Concentration) → is the lowest [ ] that prevents growth in vitro. Serial dilutions of
drug are used. Inhibition does not mean treatment, this may be different in a
pesos lood. Mioes etee suseptile ad esistat ae teed
iteediate. MBC Miiu Bateioidal Coetatio→ the lowest [ ] that
kills 99.9% of cells in vitro; determined from a plate count from MIC. The following
techniques are precise, but labour intensive and expensive.
• MIC → just put bacteria in solutions of differently diluted antimicrobials. If the
bacteria grows then the flask goes from yellow → pink. The MIC is the serial dilution
just above the one that changes colour.
• Compare and contrast the Kirby-Bauer disc diffusion test with
commercial modifications of antimicrobial susceptibility testing.
• Conventional Disc Method → Also known and Kirby-Bauer disc diffusion test is
commonly used to determine susceptibility. A standard [ ] of the bacteria is spread
o a aga plate ad the diss of diffeetl [ ]d atiioials ae plaed o the
jelly. The diameter of the circle around them they create indicates the ateias
suseptiilit. This ile is alled a zoe of ihiitio ad is opaed ith
premade charts to establish susceptibility. The size of the zone depends on the [ ] of
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microbes, the amount of drug in the disc, as well as the molecular weight, stability,
and solubility of the drug.
• Less labour-intensive, often faster results can be obtained by using commercial
modification of susceptibility testing. Take 6-15 hours. E-Test is a modification of
disc diffusion test, using a strip with gradient of the drug. Using this test, the
intersection of the zone of inhibition indicates the MIC.
• We measure the [ ] of antimicrobial drug in blood and other bodily fluids to
determine blood toxicity and achievable levels for new drugs. A diffusion bioassay
copaes ko [ ]s ith patiet saples poduig a stadad ue hih a
opae ield [ ]s.
• Describe the ß-lactam drugs and other antimicrobials that inhibit cell
wall synthesis.
• Drugs target the cell wall (peptidoglycan synthesis), nucleic acid synthesis, protein
synthesis, metabolic pathways, and cell membrane integrity. Off-target effects can
ou he the dug tagets ezes it didt ea to.
• Bacterial cell walls are unique in that they contain peptidoglycan which is a great
target for drugs which often have a high therapeutic index. Wall peptides end in 2 D-
alaies. Oe gets eoed he atiioials id.
B-Lactam drugs competitively inhibit enzymes that help form peptides bridges
between adjacent glycan chains.
Vancomycin binds to the AA side chain of NAM molecules, blocking peptidoglycan
synthesis.
Bacitracin interferes with the transport of peptidoglycan precursors across the
cytoplasmic membrane.
• Peiillis, Cephalospois, ad othe B-Lactam Drugs competitively inhibit
enzymes that catalyze formation of peptide bridges between adjacent glycan
strands. Disrupt peptidoglycan synthesis, weakens cell walls, leads to cell lysis. All of
these have a B-Lactam ring which irreversibly binds to the enzyme that links wall
peptides together (transpeptidase?). These enzymes are called penicillin-binding
poteis PBPs sie the id peiilli. These ae ol effetie agaist atiel
growing cells. Penicillin and cephalosporin mimic the D-alanine – D-alanine link.
• PBPs diffe i atiit due to ell all stutue. The peptidogla of ga +
bacteria are exposed whilst the outer membrane of gram – bacteria blocks access to
the target. They also differ in aerobes vs anaerobes, and in different species. Some
bacteria synthesize B-lactamase, which inactivates antibiotic by destroying the B-
lactam ring. Penicillinase inactivates members of the penicillin family, Extended-
spectrum B-lataases E“BLs iatiate alost all B-lactam ring drugs, gram –s
podue a oe etesie aa of peiilliases ad E“BLs tha ga +s.
• There are 5 general groups of penicillins (work to kill bacteria): Natural, Penicillinase-
resistant, Broad-spectrum, Extended-spectrum, and Penicillins + B-lactamase
inhibitors.
Natural are from Penicillium chrysogenum, are narrow-spectrum, acting against
gram + and few gram – bacteria.
Penicillinase-resistant developed in response to S.aureus strains. Examples include
methicillin and dicloxacillin. Some microbes are now able to produce altered cell
wall synthesis enzymes to which B-Lactam drugs do not bind such as MRSA.
Broad-spectrum act against gram + / - due to modified side chain. They are
inactivated by many B-lactamase. Examples include ampicillin and amoxicillin
Extended-spectrum have greater activity against Pseudomonas species. They have
reduced activity against gram + and are also destroyed by many B-lactamases.
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Document Summary
Unit 8: antimicrobials and antimicrobial resistance: describe the discovery of antimicrobial drugs and antibiotics. First produced by chemists then by organisms and now by novel sources / chemical alteration: antimicrobial drugs were first discovered by paul ehrlich in 1910 who discovered. However, it had arsenic and was toxic although it treated syphilis really (cid:449)ell if dosage (cid:449)as app(cid:396)op(cid:396)iate. He (cid:272)oi(cid:374)ed the te(cid:396)(cid:373)s (cid:862)(cid:373)agi(cid:272) (cid:271)ullet(cid:863) a(cid:374)d (cid:862)(cid:272)he(cid:373)othe(cid:396)ap(cid:455)(cid:863): gerhard domagk discovered red dye prontosil in 19(cid:1007)(cid:1006). This (cid:449)as the fi(cid:396)st (cid:858)sulfa(cid:859) d(cid:396)ug and had no effect outside of animals. In 1928 alexander fleming discovered that penicillum mould was secreting penicillin, toxic to staph aureus. Later versions of the drug allowed the drug to be more slowly metabolized and remain in the body for longer: penicillin production increased during the war in 1944. Later, selman waksman purified streptomycin from soil bacterium. Researchers screened many molecules for antibiotics: most modern antibiotics come from microorganisms that reside in soil.
