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Antibiotic Resistance

  • Timeline
  • Gene transfer – chromosomal & extra-chromosomal
  • Mechanisms
    • target modification
    • antibiotic breakdown
    • increased entry/efflux
    • alternative pathways
    • Examples
      • b-lactam antibiotics
      • MRSA
      • Vancomycin
  • Contributing factors to resistance
  • Circumventing resistance

Resistance to Antibiotics

  • The effectiveness of nearly all chemotherapeutic treatment is limited by the development of resistance
  • When antibiotics were first introduced development of resistance was thought unlikely because the frequency of mutation in bacteria was too low, but resistance nevertheless appeared
    • 1935 Sulphonamides introduced
    • late 1940s resistant strains Nesseria gonorrhoeae and Streptococcus pyogenes appear
    • 1940 Penicillin introduced
    • late 1940s resistant strains of Staphylococcus aureus appear
    • mid 1970s resistant strains of Nesseria gonorrhoeae appear
    • late 1950s appearance of strains of multi drug resistant Shigella dysenteria resistant to
      • sulphonamides
      • tetracyclines
      • streptomycin
      • chloramphenicol
  • genes determining resistance
    • pass between generations (Vertical transfer)
    • move easily from chromosome to extrachromosomal plasmids
    • can be transferred readily between bacteria
      • same species
      • different species (Horizontal transfer)
  • Genes providing resistance to one specific drug are often part of a larger package of resistance-associated genes located on plasmids
  • collectively these genes can confer Multidrug Resistance
  • use of one type of antibiotic will select also for resistance to a number of different antibacterial agents
  • After half a century of control over microbial disease in the developed countries, there is now worldwide resurgence of bacterial disease brought about in large measure by acquisition of resistance genes by virtually all major pathogenic bacteria

Mechanisms underlying resistance to antibacterial agents

Inactivation of antibiotics

  • b-lactams by b-lactamases
    • All bacteria possess some form of b-lactamase
    • In gram –ve bacteria, enzymes are secreted into periplasmic space; so resistance is a single cell phenomenon
    • In gram +ve bacteria enzymes are secreted into surroundings, so resistance is a population phenomenon
    • Some b-lactamases are inducible
    • b-lactamases may be plasmid or chromosomal encoded
  • There are many b-lactamases with different substrate preferences; eg aminoglycosides via Acetyl transferases
  • Phosphotransferases
  • Adenyltransferases

Modifications to target sites so rendering them insensitive to drug action

  • Acquisition of new genes encoding a new target with low affinity for antibiotic
    • eg resistance to sulfonamides and trimethoprim
  • Production of increased amounts of normal target
    • eg resistance to trimethoprim
  • Acquisition of new genes that bring about enzymic modification of targets
    • eg resistance to macrolides by methylation of 23S rRNA
  • Mutational events resulting in lower affinity of existing targets
    • eg resistance to b-lactams by mutations in penicillin binding proteins(Pbp’s)
    • Eg. Resistance to quinolones by point mutations in DNA gyrases
    • Eg. resistance to aminoglycosides by changes in rRNA structure
    • Eg. resistance to rifampicin by changes in RNA polymerases subunit
  • Examples of clinically important resistance based on changes in targets
    • Methicillin resistant staphylococcus aureus (MRSA)
      • Resistant cells contain the mecA gene of non-staphylococcal origin
      • mecA encodes Penicillin Binding Protein with a low affinity for b-lactams
      • this confers low level resistance
      • Auxiliary genes femA,B,D (factor essential for the expression of methicillin resistance) generate products that control biosynthesis of precursor muropeptides
      • Normal precursor muropeptides are needed to compete with methicillin for the active site of the enzyme and so produce high level resistance
    • Vancomycin resistance
      • Sensitive bacteria synthesise peptidoglycan strands that terminate in D-Ala-D-Ala
      • Vancomycin resistant cells contain a transposable element encoding 9 resistance-associated genes, the combined activities of their products leading to synthesis of D-Ala-D-Lact
      • Vancomyicin binds with 3 orders of magnitude lower affinity to D-Ala-D-Lact than to D-Ala-D-Ala

Decreased antibiotic accumulation at the target site

impaired uptake or enhanced efflux

Entry

  • There may be changes in permeability barriers and active transport
  • Decreases in outer membrane permeability via mutational loss of Outer Membrane Porins (Omps): eg low level resistance to
    • Chloramphenicol
    • Tetracyclines
    • b-lactams (in synergy with b-lactamases)
  • Changes in existing uptake systems in the cytoplasmic membrane
    • eg D-alanine transport producing resistance to Cycloserine
    • eg hexose-6-phosphate uptake producing resistance to fosfomycin
  • Changes in specialised transport systems eg defects in electron-transport and proton motive force producing resistance to
    • aminoglycoside
    • tetracycline

Efflux

  • There may be increased active efflux which often occurs together with repression of porin synthesis
  • Bacteria have many efflux systems including drug/H antiporters and translocases
  • Some can handle a wide variety of drugs hence causing multidrug resistance
  • In gram-ve bacteria, accessory proteins span the periplasmic space and link the transporter in the cytoplasmic membrane with a channel in the outer membrance
  • Drugs can thus be extruded directly into the surrounding medium
  • Active drug efflux is the basis of resistance to many antibiotics; eg
    • tetracyclines,
    • macrolides,
    • chloramphenicol,
    • quinolones

Contribution factors for selection of resistant bacteria and the spread of resistance

  • Extensive use of antibiotics in medicine
  • Increased frequency of invasive medical intervention
  • Frequent over prescribing of antibacterial agents in clinical practice
  • Increased numbers of immunologically compromised individuals
  • Prolonged survival of patients with chronic debilitating disease requiring extensive antibiotic treatment
  • Increased mobility of human populations encouraging rapid dissemination of resistance
  • Extensive use of antibiotic agents in agriculture, fisheries and animal husbandry

Strategies for circumventing or limiting resistance

  • Designing derivatives of existing drugs that are not metabolised or effluxed by the resistant bacteria eg
    • b-lactams,
    • chloramphenicol,
    • tetracyclines
  • Using adjuvant drugs that inhibit drug inactivation eg
    • clavulanic acid
  • Reducing usage of antibacterial agents so decreasing selection pressures