Antimicrobial Resistance – Mechanisms and controlGenetic basis of antimicrobial resistance1. Intrinsic resistance2. Mutations3. Acquisition of foreign DNAIntrinsic resistance• Predict potential problems that may emerge under selective pressures• Stenotrophomonas maltophilia, environmental bacteria cause infection to IC patients: intrinsically

resistant imipenem• Enterococci: intrinsically resistant cephalosporins and amyloglycosidesMutationsSignificant clinical impact: emergence of resistance during therapy, resistance develops on treatmentAcquisition

Transduction Acquire with help by flagellaTransformation Organisms acquire DNA which resists antibiotic from dead bodyConjugation Acquire by direct contact

Mechanisms of antimicrobial resistance1. Beta-lactam2. Aminoglycosides3. Glycopeptides4. Quinolones5. Marcolides

6. Sulphonamides7. Trimethoprim8. Tetracylines9. Chloramphenicol

Beta-lactam1. Enzymatic modification - Beta-lactamase (plasmid-mediated)

• Plasmid: for containing genes, can cross species even genus• There are chromosome-mediated but slower and within species transfer only• Gram-negative bacteria (E. coli, Salmonella, etc)• (Beta-lactamase genes: TEM-1, OXA-1, etc)

2. Modification of penicillin binding proteins (mutation), which for cell wall synthesis• Penicillin-resistant Streptococcus pneumonia• Penicillin-resistant Neisseria gonorrhoeae, almost 100% resistant

3. Bypass resistance: alternative routes of peptidoglycan synthesis• Produce other PBPs which do not bind to penicillin to synthesis• Methicillin-resistant Staphylococcus aureus, mec gene

4. Impermeability: porin loss (mutation)• Porin loss to prevent antibiotics get in• Imipenem-resistant Pseudomonas aeruginosa

Aminoglycosides1. Enzymatic modification:

• aminoglycoside-modifying enzymes (AME) (plasmid-encoded)• Gram-negative bacteria• Gram-positive bacteria, both produce it

2. Ribosomal resistance• Rare• Mutations in ribosomal genes• High-level streptomycin resistance in enterococci• Streptomycin resistance in M. tuberculosis (low level: intrinsic, aminoglycosides + penicillin →

synergistic effect3. Ineffective transport

• O2 transport system, no O2 transport system in anaerobes, so all anaerobes are resistant to aminoglycosides

Glycopeptides

1. Permeability• Gram negative bacteria naturally resistant because glycopeptides are large molecules, cannot

pass through outer membrane porins• Vancomycin is effective on Gram positive bacteria only

2. Acquired vancomycin resistance• Genes encode ligases (for protein synthesis) which incorporate D-lactate in place of D-alanine as

terminal amino acid of the peptide chain → ↓ affinity for glycopeptides antibioticsQuinolones1. Mutations (most common)

• DNA gyrase twist DNA chromosome into small packing & type 4 topoisomerase for DNA replication, mutation on gene coding for DNA gyrase & type 4 DNA topoisomerase, both are target of quinolones

2. Altered permeability• ↓ quantities of OmpF outer membrane protein, OmpF is porin for quinolones to get inside, Omp:

outermembrane protein3. Efflux

• Efflux pumps4. qnr

• Qnr proteins protect DNA gyrase from binding of quinolones5. Ciprofloxacin-modifying enzymeMacrolides1. Target site alternation:

• Modification of 23S rRNA by methylase, encoded by a class of genes erm (erythromycin ribosome methylation)

2. Drug modification: • macrolide-modifying enzymes

3. Altered transport (efflux)4. ↓permeabilitySulphonamides1. Overproduction of para-aminobenzoic acid (PABA)

• Compete with sulphonamides2. Change in dihydropteroic acid synthetase enzyme3. Mutations in chromosomal genes/ plasmid-mediated4. ↓ cell permeabilityTrimethoprim1. Mutation in gene coding for production of dihydrofolate reductase (DHFR)

• ↑ production of/ change in DHFR2. Thymine auxotrophyBacteria cannot utilize exogenous thymine; gene mutation allows them to use it3. ↓ cell permeability (loss of OMP)4. Resistance genes integrated into transposons, on plasmid or chromosome

Pteridine + PABA –(dihydropteroic acid synthetase, target of sulphonamides)--> dihydropteroic acid --> dihydrofolic acid –(dihydrofolate reductase, DHFR, target of trimethoprim)--> tetrahydrofolic acid --> thymidineTetracycline1. Energy-dependent efflux2. Enzymatic modification (rare)3. Ribosomal protection proteins, target of tetracycline is ribosomeChloramphenicol1. Enzymatic modification

• Chloramphenicol acetyltransferase (CAT): acetylated chloramphenicol cannot bind to ribosome

2. Active efflux: Pseudomonas aeruginosaControl

• Resistance in natural, intestine of human & man• Resistance will develop whenever antibiotics are used• Avoid indiscriminate use in human, animals, think before use• Surveillance• Avoid sub-therapeutic doses which cannot kill bacteria and cause resistance, selective pressure• Patient compliance, complete the course• New antibiotics with other modes of action