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Lecture 4

MICB 201 Lecture 4: Chapter 4 Notes

14 Pages

Course Code
MICB 201
Dave Oliver

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Origin of ABs Archaea dont cause illnesses in humans information we have regarding antibiotics only applies to bacteria Deaths from infectious diseases has dramatically declined since 1900 b/c improved sanitation, sewage treatment, medical care, decrease in poverty + introduction of antibiotics However, antibiotic-resistant bacteria spring up v fast after new AB development 75% ABs today = naturally synthesized by soil microbes (esp. Members of mycelial Actinobacteria) Why do some bacteria synthesize ABs? reason unclear, assumed to be for biological warfare but ABs only effective at high concentrations unlikely to be reached in natural environments Semi-synthetic = natural ABs modified chemically, eg. -lactam ABs derived from penicillin At clinically-relevant (high) concentrations, antibiotics work by: disrupting cellular process/structure unique to target organism, or Disrupting cellular process/structure in targets without affecting same process in self Eg. interfering w/ PG synthesis, translation, transcription, replication + membrane damage New PG synthesis and Effects of some ABs, eg. -lactam Bacterial cell must double all components in order to divide, including PG Diff modes of cell division (specifically creation of new PG) for diff bacteria eg. for unicellular Gm +ve bacteria: Cocci Division synthesis New PG synthesized on either side of cell division site junction/ridge btwn new + old PG = wall band Non- Division + elongation synthesis spherical new PG added on either side of cell division site new PG additionally synthesized at sites around cell body (but not at ends), or only at ends New PG added by breaking bonds in preexisting PG (by autolysins) then adding disaccharide-peptide units to existing PG Autolysins (periplasmic enzymes) catalyze breakage of O-glycosidic bonds btwn NAM + NAG sugars, producing free end to add new PG Disaccharide-peptide unit = NAM-NAG disaccharide w/ pentapeptide Made in cytoplasm/cytoplasmic membrane + transported to periplasm In periplasm, enzymes catalyze transglycosylation rxn (O-glycosidic bond formation btwn NAM + NAG) and transpeptidation rxn (peptide bond formation which leads to release of 1 aa tetrapeptide) Most clinically important ABs = -lactams inhibit PG synthesis by binding to enzyme that catalyzes transpeptidation, preventing transpeptidation rxn from occurring Many diff classes, eg. penicillins (penams), cephalosporins + carbapenems, but all have same fxn and possess -lactam ring Vancomycin = inhibits PG synthesis by binding to AA 4d AA o5pentapeptide (if both are D-Ala), preventing both transpeptidation + transglycosylation reactions Cant be used for Gm -ve b/c OM; diffuses v slowly across OM + excluded by all porin channels Polymyxin = one of few ABs which directly disrupts cellular structure (OM) instead of inhibiting cellular process (eg. transpeptidation rxn) Has polycationic ring (Full of NH s) interacts w/ negatively-charged 2+ 32+ LPS core, displacing the Ca /Mg ions and breaking down OM After OM breakdown, polymyxin disrupts inner membrane by inserting its hydrocarbon tail into membrane (detergent-like mechanism) Only effective against Gm -ve for some reason Chemical characteristics of ABs + Barriers to AB penetration ABs = medium-sized chemicals (~100-1000 Daltons) in same category as aas, nucleotides, sugars Diff ABs = diff solubilities hydrophilic = chemicals v soluble in water, hydrophobic = v not soluble in water Correlated w/ relative amounts of polar/nonpolar content + presence of charged groups However, all ABs = high polar content + most are charged Only the CM + OM are significant barriers to AB penetration capsular matrix, S-layer pores + PG meshwork gaps = easy for ABs to pass thru Although most ABs = v polar + charged, many are somehow able to diffuse across CM freely Some are passively/actively transported by CM permeases if structurally similar to normal substrate, eg. Streptomycin OM = less fluid + better barrier than CM, but allows ABs w/ high nonpolar content to diffuse (eg. erythromycin + rifampin) Some are transported into periplasm by porins -lactams b/c structurally similar to aa Zwitterionic penams (eg. ampicillin + amoxicillin) penetrate porins faster than anionic penams more effective ABs Self-promoted uptake: some ABs cross OM by disrupting structure, eg. polymyxin + streptomycin An AB doesnt either diffuse thru bilayer or travel thru porins; some can take both routes depending on pH (affects ionization) Factors contributing to a bacterias resistance to ABs: Factor How it helps with resistance How to acquire Can Gm -ve generally more resistant antibiotic b/c OM; some ABs diffuse so slowly pass thru basically useless barriers? in Gm -ve, also consider ratio Gm -ve or of LPS vs. PL in OM, amount of Gm +ve porin channels + size/selectivity OM issues of porin channels Can Bacteria will be resistant to ABs if Alteration to antibiotic they lack the target sites of the ABs target so AB cant bind? eg. Mycoplasma sp. = no PG, so bind Presence resistant against -lactam ABs or replacement + suitability of Vancomycin needs D-Ala @ with a different, but target sites AA4 + AA5 functionally -lactam ABs target equivalent, transpeptidase enzymes, but since molecule that AB diff bacteria synthesize cant bind to transpeptidase enzymes w/ slightly diff structures, variations exist btwn bacterial resistance Antibiotic- Some bacteria can chemically Acquisition of modifying modify + inactivate ABs w/ enzyme which enzymes enzymes chemically modifies present? -lactamases/Bla proteins + deactivates AB catalyze hydrolysis of -lactam rings Chloroamphenicol transcetylase (Cat) catalyzes acetylation of chloramphenicol Antibiotic Some bacteria have efflux protein Change in cell efflux and/or which transports AB out of cell envelope decreased when it enters (specificity varies composition that influx btwn organisms) decreases AB influx Acquire new efflux protein Basically, acquired AB resistance (phenotype changes) comes from mutation of existing genes or addition of new genes (genotype changes).
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