Lecture 12

Lecture 12

1. Antibiotics

   • substance produced by an MO that is effective in killing or inhibiting growth of other MOs.
   • any synthetic drug used to treat an infectious disease
     • antibacterial
     • antifungal
     • antiprotozoal
     • antiviral
   • organic: isolated from bacteria or fungi (penicillin, erythromycin)
   • semi-synthetic: further derivatisation of organic compound for greater efficacy (amoxicillin, methicillin, oxacillin)
   • synthetic: generated in the lab (sulfonamides, quinolones)
   • qualities...
     • kill or inhibit growth of pathogens
     • cause no damage to the host
     • cause no allergic reaction to the host
     • be stable for storage as solid or liquid.
     • remain in specific tissue long enough to be effective
     • kill the pathogens before they mutate and become resistant
   • antibacterial spectrum
     • range of activity against bacteria e.g. broad or narrow
       • narrow-spectrum: only against limited variety of bacteria e.g. vancomycin (only against gram positives)
       • broad-spectrum: against a variety of gram +/- bacteria e.g. amoxicillin, tetracyline
   • bacteriostatic activity
     • level of antimicrobial activity that inhibits growth of organism
   • bacteriocidal activity
     • level of antimicrobial activity that kills organism
   • antibiotic combination
     • combining different antibiotics for: broaden the antimicrobial spectrum, treatment of polymicrobial infection, prevent emergence of resistant organism, achieve a synergistic killing effect
   • antibiotic antagonism
     • activity of one antibiotic interferes with activity of other e.g. the sum of activity is less than the most active individual drug (penicillin-chloramphenicol)
   • antibiotic synergism
     • enhanced antimicrobial activity of combined drugs compared to activity of each individual antibiotic (penicillin-streptomycin)
   • antibiogram
     • result of lab testing for sensitivity of an isolated strain to different antibiotics
     • semi-quantitative based on diffusion (Kirby-Bauer method) - antibiotic concentration highest at centre of disc and lowest at its edges the minimum inhibitory concentration can be calculated by diameter
     • quantitative based on dilution series - the last vial in which no bacteria grows contains antibiotic at minimum inhibitory concentration.
     • determining the minimum bactericidal concentration - subculturing the clear broth onto antibiotic free solid media (MBC = reduction of colony forming units (CFU) by 99.9% below control).

2. Targets

   • block cell wall synthesis
     • penicillins, cephalosporins, carbapenems
     • inhibit peptidoglycan synthesis by irreversible inhibition of peptidyl transferase = weak cell wall and osmotic lysis
     • bacteriocidal
     • PG is unique to bacteria
     • G -ve bacteria are resistant to penicillin - does not pass through outer membrane porins. therefore extended spectrum penicillins (amoxicillin, ampicillin) or semi-synthetic antibiotics needed.
     • glycopeptides (vancomycin)
       • inhibits PG synthesis by binding to the peptides of the peptidoglycan monomers = weak cell wall and osmotic lysis
       • mainly against gram +ve
       • drug of last resort e.g. for MRSA
       • bacteriocidal
   • Inhibitors of RNA transcription
     • rifamycins
     • inhibit bacterial RNA polymerase e.g. Rifampin (Rifampicin)
     • typically used against Mycobacteria (e.g. tuberculosis, leprosy), MRSA, bactericidal
   • inhibitors of nucleic acid synthesis
     • quinolone and fluroquinolones (synthetic)
     • bind DNA topoisomerases (enzymes needed for supercoiling, replication and separation of circular bacterial DNA) e.g. gyros, topoisomerase IV, helicase.
     • prevent DNA supercoiling, broad spectrum, bactericidal
     • commonly used against UTIs
   • Antimetabolites
     • sulfonamides (synthetic
       • analogues of PABA, competitively inhibits pteridine synthase
     • trimethoprim
       • inhibits bacterial dihydrofolate reductase (DHFR)
     • inhibition of tetrahydrofolic acid synthesis, cofactor needed for production of nucleic acids
     • bacteristatic, used against e,g. UTIs
   • inhibition of protein synthesis
     • binding 30S subunit
       • aminoglycosides
         • neomycin, gentamycin, kanamycin
       • freeze the 30S initiation complex (30S-mRNA-tRNA)
       • misreading of DNA due to frame-shifts
       • mostly effective against aerobic, g -ve, bacteria. bactericidal
       • synergistic w/ penicillins
       • tetracyclines
         • tetra/doxycycline
         • inhibit binding of aminoacyl-tRNA to the acceptor site on the ribosome
         • broad-spectrum, bacteriostatic, used against many bacterial infections.
     • binding the 50S subunit
       • macrolides
         • erythromycin
         • inhibit transcription of the peptidyl tRNA from the A to the P site
         • against g +ve, Mycoplasma, bacteriostatic.
       • chloramphenicol
         • broad-spectrum, bacteriostatic
         • toxic, used against bacterial meningitis

3. Mechanisms of Drug Resistance

   • Non-genetic intrinsic resistance
     • metabolic activity: most drugs only effect metabolically active cells, not dormant cells
     • lack of target structure: mycoplasmas lack cell wall (resistance to penicillins, cephalosporin, vancomycin)
     • exclusion: antibiotic can't enter cell (e.g. penicillin/g -ve bacteria)
   • genetic resistance
     • chromosomal resistance: spontaneous mutation in a gene encoding a target receptor: mutation of RNA-polymerase gene results in rifamycin resistance
     • acquired resistance due to selective pressures
   • Exclusion of antibiotic from site of action (porins)
   • Efflux pump for the antibiotic removal from site of action
   • Enzymatic modification (beta-lactamase)
     • plasmid encoded enzymes that can be transferred between bacterial strains and species
     • Methicillin, oxacillin, flucloxacillin these are unaffected by ß lactamase.
       • Clavulanic acid: ß lactamase inhibitor
       • Augmentin: amoxicillin + clavulanic acid
   • Destruction
   • Modified antibiotic insensitive target
   • New antibiotic insensitive target (alternative PBP)

4. Factor Contributing to Increased Resistance

   • Staphylococcal Cassette Chromosome mec (SCCmec)
     • integrated in bacterial genome
     • SCCmecII: mainly in hospital MRSA strains, also encodes resistance to macrolide-lincosomide-streptogramin B antibiotics and spectinmycin
     • SCCmecIV: mainly in community acquired MRSA
   • Multidrug resitant strains: resistant to more than one class of drugs
   • Pandrug-resistant strains: resistant to all clinically safe drugs
   • Methicillin resistant S. aureus (MRSA): resistant to all anti staphylococcal drugs, except vancomycin
   • Vancomycin intermediate S. aureus (VISA); resistant to usual dosage of vancomycin
   • Vancomycin resistant S. aureus (VRSA): resistant against all anti staphylococcal drugs
   • Vancomycin resistant Enterococcus (VRE)
   • Multidrug resistant Mycobacterium tuberculosis (MRTB)