Micr 2220 Lect 17: Mech Of Antibiotic Resistance

Front 1

What is required for an antibiotic to be effective

Back 1

-need a vital target susceptible to a low concentration of the antibiotic
-antibiotic must penetrate the bacterial envelope and reach the target in sufficient quantity

Front 2

How does active efflux work

Back 2

-Pump toxic compounds from cell in proton dependent manner
-Single or multi drug
-may be non specific (e.g. transport heavy metals, solvents, detergents, antibiotics)
-"natural" proteins which may lead to significant acquired resistance due to mutation or horizontal transfer
-several families of proteins for active efflux based on sequence and substrate

Front 3

4 mechanisms of antimicrobial resistance

Back 3

-antibiotic inactivation
-permeability changes
-active efflux
-target or pathway alteration

Front 4

what is antibiotic inactivation

Back 4

reduces concentration of active antibiotic within cell

Front 5

Example of active efflux/mech of it specifically

Back 5

-MDR (multi-drug resistance) in Ps. aeruginosa.
-Active efflux transporter coded on mexA (periplasmic)-mexB (pump)-oprM (outer membrane channel) operon + RexR (regulator).
-Energy dependent pump for penicillins, tetracyclines, fluoroquinolones, chloramphenicol.
-Knockout mutations in MexA and OprM results in increased accumulation of these agents
-Related pumps identified frequently in psuedomonads and other Gram neg

Front 6

example of antibiotic inactivation, in what bacteria is it a problem, and how does it work

Back 6

Production of B-lactimases. Problem in Gram neg. Enzymes bind B-lactam rings via active serine site, cyclic amide bonds of B-lactam rings are hydrolyzed, open ring forms cannot bind to target sites

Front 7

Another example of antibiotic inactivation, in what bacteria is it a problem, and how does it work

Back 7

Chloramphenicol resistance. Widespread in gram pos and neg. Due to production of chloramphenicol acetytransferase. Converts drug to monoacetate or diacetate which cannot bind to 50S ribosomal subunit

Front 8

What is being done (e.g. in case of amoxicilin) to circumvent the B-lactamases

Back 8

Amoxiclav--the clav bit inactivates B-lactamase

Front 9

Example of permeability changes, and how does it work

Back 9

Imipenem resistant Ps. aeruginosa. Reduced OprD borin production with hyperproduction of chromosomal cephalosporinase. However porin deficient mutants are less fit

Front 10

Mechanism of efflux complex in gram neg bact

Back 10

-Efflux transporter connected to MFP (membrane fusion protein) accessory protein linked to outer membrane channel protein
-Drugs cross outer membrane and are partially inserted into bilayer of cytoplasmic membrane
-Transporter captures drug molecules and bilayer and pumps them out by pass outer membrane barrier
-for agents that cross cytoplasmic membranes rapidly the transporter should accept substrates from the cytoplasm directly or after the insertion of the drugs into the bilayer.

Front 11

How does gram positive membrane affect antimicrobial agents

Back 11

Gram pos covered by peptidoglycan which does not exclude most antimicrobial agents

Front 12

Mechanism of efflux transporter without accessory proteins

Back 12

-transporter located within the cytoplasmic membrane
-amphiphilic drugs traverse outer membrane (often via porin channels)
-become partially inserted into bilayer of cytoplasmic membrane
-transporter captures the drug in bilayer and pumps them into periplasm
-drugs then diffuse slowly through outer membrane or leave the cell via porin channels

Front 13

How does gram neg membrane affect antimicrobial agents

Back 13

Gram neg surrounded by an outer membrane which functions as an efficient permeability barrier because it contains lipopolysaccharide (LPS) and porins with narrow channels

Front 14

How does mycobacteria membrane affect antimicrobial agents

Back 14

Mycobacteria produce an exceptionally efficient mycolic acid barrier outside the peptidoglycan layer

Front 15

Example of alterations in target or pathway: Target mutation

Back 15

e.g. Penicillin resistant strep. pneumoniae
-PCPs are cell envelope proteins that are involved in cell growth and division and are the targets for B-lactams
-Each cell has several PBPs and some are non-essential
-Penicillin resistant strep. pnumoniae produce 1+ altered PBPs (esp PBP1 and 2) with reduced binding to penicillin
-resistant strains are mosaics with blocks of conserved sequence with blocks of variant sequence

Front 16

Mech of action of efflux transporter in gram pos

Back 16

-efflux transporter located within cytoplasmic membrane
-amphiphilic drugs transverse the membrane
-transporter captures the drug molecules in the bilayer and pumps them into the surrounding medium

Front 17

3 Ways alterations in target or pathway can produce resistance

Back 17

-Alterations in concentration of target due to mutations in regulating genes
-Affinity of a trug for its target reduced when target is altered
-resistance may arise by reliance on an alternate pathway that is inhibited

Front 18

Example of alterations in target or pathway: increased target concentration

Back 18

e.g D-cycloserine resistance in mycobacterium smegmatis
-resistance due to over-production of the wildtype target enzyme D-alanine racemase
-single change in the gene's promoter region resulted in elevated gene expression
-Higher antibiotic concentrations are needed to inhibit the target

Front 19

3 genetic basis of resistance

Back 19

Intrinsic, mutational, transferable

Front 20

What is intrinsic genetic basis of resistance

Back 20

-Not clinically acquired (community/wildtype)
-ususally chromosomal
-common in free living opportunists (e.g. Ps. aeruginosa)

Front 21

What is the mutational genetic basis of resistance

Back 21

-Deletion, substitution or larger scale rearrangements
-hypermutators (bact have mutation in translational machinery therefore increase rate of mutations)
-decreased fitness

Front 22

Example of alterations in target or pathway: Metabolic bypass

Back 22

e.g. glycopeptide resistance in enterococci
-Glycopeptides inhibit peptidoglycan synthesis by binding to the D-alanine-D-alanine residue in peptidoglycan
-in vancomycin resistant enterococci D-alanine-D-alanine is replaced with D-alanine-D-lactate which cannot bind bancomycin
-the vanA gene encodes and abnormal D-alanine-D-alanine ligase that synthesises the D-alanine-D-lactate dipeptide

Front 23

What is the transferrable genetic basis of resistance

Back 23

-plasmids: independent, multiple resistance genes, narrow host range
-transposons: insert into host genome, fewer resistance genes, wider host range

Front 24

Factors contributing to the emergence and spread of resistance

Back 24

-antibiotic use: humans, animals, plants
-Emergence of resistance: mutation, gene transfer
-Selection and fixation of resistance
-colonization and infection with resistant organisms
-transmission of resistant organisms: inter-human/food-animal contact, stay in healthcare institution

Front 25

Impact of antibiotic resistance for human health

Back 25

-increased mortality
-increased morbibity
-increased cost
-international focus on minimising misuse of antibiotics to control resistance

Front 26

How to control and prevent antibiotic resistance

Back 26

-Better training for prescribers
-coordination of surveillance of resistance in human and animal sectors
-better guidelines for therapy, cycling?
-restriction of antibiotic use as growth promoters in food animals
-promotion of infection control practice
-development of novel antimicrobials