Chemotherapeutic Antibiotics: Mechanisms of Bacterial Resistance (DONE)

Question 1

Bacterial resistance to B-lactams general methods

Answer 1

Decreasing drug amount that penetrates into the bacterial cell
Enzymatic inactivation
Modification/duplication of target sites- new target site becomes insensitive

Question 2

Decreasing drug amount that penetrates into the cell- B lactams

Answer 2

Impermeability- antibiotic unable to penetrate the outer membrane of gram-negative bacteria e.g. benzyl penicillin
Changes in permeability- membrane composition e.g. carbenicillin

Question 3

Enzymatic inactivation to B lactams

Answer 3

Induction of B lactamases
These enzymes are liberated extracellularly and hydrolyse the B lactam ring
Gram negative produce B lactamases within their cell membrane (periplasmic space), usually chromosomally encoded

Question 4

Examples of enzymatic inactivation- B lactams resistance

Answer 4

Amoxicillin: complete cross resistance with ampicillin
Carbenicillin: cross resistance possible with other antipseudomonal penicillins
Meticillin: cross resistance to other penicillins including penicillinase resistant ones, cloxacillin, flucloxacillin and cephalosporins

Question 5

Cephalosporins and B-lactamases

Answer 5

Second generation (cefuroxime, cefaclor) less susceptible to B lactamases
Fourth generation (cefepime) less susceptible chromosomally encoded B-lactamases

Question 6

Modification/duplication of target sites

Answer 6

New target site becomes insensitive
Most antibiotics have unique mechanisms of action against very specific target site, if target site is modified leads to resistance

Question 7

Bacterial resistance to quinolones general methods

Answer 7

Decrease in the number of porins
Efflux pump
Alterations of target sites
Protection of targets

Question 8

Decrease in the number of porins- quinolones

Answer 8

Porins: beta barrel proteins that cross a cellular membrane and act as a pore through which molecules can diffuse
By decreasing the number of porins, makes it harder for quinolone to access cell

Question 9

Mutations in active sites- quinolones

Answer 9

Point mutations located within a part of GyrA termed the quinolone-resistance-determining region (QRDR)
Mutations result in conformational changes in DNA gyrase reducing the binding of quinolones
Level of resistance to quinolones is related to the number and type of mutations accumulated in the QRDR

Question 10

Efflux pumps- quinolones

Answer 10

Anything the bacteria doesn’t like will be ejected from the cell
Most efflux pumps are proton pumps, ABC uses ATP
ABC super family works on multiple drugs
Primary pumps: efflux ATPases
Secondary cation/proton pumps: use electrochemical proton gradient (proton motive force) as energy source

Question 11

Gene regulation

Answer 11

Operon: a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter
Promoter: a nucleotide sequence that enables a gene to be transcribed, recognised by RNA polymerase
Regulator: codes for and produces a repressor, works with inducers and corepressors
Operator: a segment of DNA that a repressor binds to
Structural genes

Question 12

Efflux- decreased accumulation: quinolones

Answer 12

Increase number of efflux pump (expression)
Regulatory genes-efflux pumps
Mutation in regulator genes

Question 13

Protection of target- quinolones

Answer 13

Ciprofloxacin: qnr produces a pentapeptide involved in protein-protein interactions

Question 14

Bacterial resistance to macrolides general methods

Answer 14

Increase number of efflux pump (expression)
Enzymatic inactivation- esterases
Alterations of target sites- ribosomes

Question 15

Decreased accumulation- efflux: macrolides

Answer 15

Increase number of efflux pump (expression)
Macrolides- MRSA in S. aureus; MEFA in streptococcus pneumonia confers resistance only to 14- and 15- membered ring molecules

Question 16

Enzymatic inactivation of antibiotics- esterases: macrolides

Answer 16

Hydrolysis of the lactone ring by plasmid encoded esterases

Inactivation of erythromycin and oleandomycin in enterobacteria

Question 17

Alterations of target sites- ribosomes: macrolides

Answer 17

Loss of macrolides binding to the ribosome (plasmid/transposon mediated)
S. aureus: mono- or di-methylation of an adenine residue
E. coli: ribosomal protein alteration, altering maturation of ribosomal RNA and a 30S ribosomal subunit protein

Question 18

Bacterial resistance to glycopeptides general methods

Answer 18

Alterations of target sites- target site becomes insensitive

Increase in target sites

Question 19

Alterations of target sites- glycopeptides

Answer 19

Glycopeptides: biosynthesis of peptidoglycan with altered glycopeptide recognition sites- binding of glycopeptides is reduced

Question 20

Increase in target sites- glycopeptides

Answer 20

Thickening of the cell wall (target)- vancomycin and teicoplanin

Question 21

Bacterial resistance to AGACs general methods

Answer 21

Alterations of target sites- target site becomes insensitive
Enzymatic inactivation
Impaired uptake

Question 22

Alterations of target sites- AGACs

Answer 22

Single step mutations in ribosomal proteins

Modification interferes with binding to the target 16S rRNA in the decoding region of the A-site of the ribosome

Question 23

Enzymatic inactivation- AGACs

Answer 23

AAC (acetyltransferases)
ANT (nucleotidyltransferases or adenyltransferases)
APH (phosphotransferases)

Question 24

Impaired uptake- AGACs

Answer 24

Alteration of transport system:
Absence of or alteration in the aminoglycoside transport system
Inadequate membrane potential
Modification in LPS structure

Question 25

Chloramphenicol

Answer 25

Enzymatic inactivation of antibiotic- acetyltransferases
Porin mutations affecting chloramphenicol transport in gram negative bacteria
Increase number of efflux pump (plasmid/transposon-mediated)

Question 26

Sulphonamides/Trimethoprim

Answer 26

Alterations of target sites
Changes in permeability
Increase in the number of target sites

Question 27

Mupirocin

Answer 27

Duplication of target site, plasmid encoded high level resistance

Question 28

Fusidic acid

Answer 28

Impermeability; unable to penetrate the outer membrane of gram negative bacteria
Protection of target; modified translocation factor protein with lowered affinity for antibiotic

Question 29

Tetracyclines

Answer 29

Ribosomal protection factors; dislodging tetracycline from the ribosome
Increase number of efflux pumps- plasmid/transposon mediated