antibacterials

• What is an antibiotic?

  An antimicrobial compound produced by a microorganism that selectively inhibits or kills bacteria.
• How does an antibacterial differ from an antimicrobial?

  Antibacterials are antimicrobials that specifically target bacteria.
• What are the main categories of antimicrobial compounds based on their origin?

  Natural, semisynthetic, and synthetic.
• How are antimicrobial compounds classified based on their target microorganisms?

  • Antiviral: target viruses
  • Antibacterial: target bacteria
  • Antifungal: target fungi
  • Antiprotozoal: target protozoa
• What are the two primary modes of antimicrobial activity?

  Bacteriostatic and bactericidal.
• What does selective toxicity refer to in the context of antimicrobials?

  Exploiting the biochemical differences between the invading organism and the host.
• What are some biochemical targets exploited by antimicrobials?

  • Ribosomes
  • Plasma membrane
  • Cell wall
  • DNA
  • Soluble proteins
  • Outer membrane
• Which class of drugs are inhibitors of cell wall synthesis?

  β-lactams.
• What is the target of β-lactams in bacterial cell walls?

  Peptidoglycan layer synthesis.
• What is the R-site of substitution in β-lactams?

  The R-site determines the specific substituent attached to the β-lactam ring.
• What are the different generations of cephalosporins?

  • 1st generation
  • 2nd generation
  • 3rd generation
  • 4th generation
  • 5th generation
• What is the mode of action of cephalosporins?

  They bind to penicillin-binding proteins (PBPs), leading to bactericidal activity.
• How do polymyxins work against bacteria?

  • Interact with lipopolysaccharides in the outer membrane
  • Disrupt both outer membrane (OM) and cytoplasmatic membrane
  • Result in bactericidal action
• What are the main classes of antimicrobial drugs that disrupt metabolic pathways?

  • Sulfonamides (structural analogues of PABA)
  • Diaminopyrimidines (trimethoprim)
• Which antimicrobial classes disrupt protein synthesis?

  • Tetracyclines
  • Aminoglycosides
  • Chloramphenicol
  • Macrolides and Lincosamides
• What is the mode of action of aminoglycosides?

  • Bind to the A-site on the 16S rRNA of the 30S subunit
  • Alter ribosome conformation
  • Induce codon misreading
  • Cause error-prone protein synthesis
  • Impair genetic code proofreading
  • Result in bactericidal activity
• What are the resistance mechanisms for aminoglycosides?

  • Altered ribosomal binding sites
  • Aminoglycoside modifying enzymes (plasmid mediated)
  • Reduced antibiotic uptake
  • Decreased cell permeability
  • Efflux pumps
• What is the main severe side effect associated with chloramphenicol in humans?

  Idiosyncratic fatal aplastic anaemia.
• How does chloramphenicol inhibit bacterial protein synthesis?

  It irreversibly binds to peptidyl transferase on the 50S ribosomal subunit.
• What are the different macrolide ring sizes and corresponding classifications?

  • 14-membered ring: Erm group
  • 15-membered ring: azalides (e.g., azithromycin)
  • 16-membered ring: ketolides (e.g., spiramycin, tylosin, tilmicosin)
• What are the resistance mechanisms for fluoroquinolones?

  • Decreased porin permeability
  • Point mutations
• How do fluoroquinolones inhibit bacterial DNA replication?

  • Inhibition of DNA-Gyrase by forming a FQ-DNA Gyrase ternary complex
  • Formation of FQ-topoisomerase-DNA ternary complex
  • Leads to fragmentation of daughter DNA strands
• How does rifampicin inhibit RNA synthesis?

  It inhibits DNA-dependent RNA polymerase activity by forming a stable complex with the enzyme.
• What happens when microorganisms develop antimicrobial resistance?

  • Antimicrobial drugs become ineffective
  • Infections persist in the body
  • Increased risk of spread to others
• By which year are AMR-attributable deaths estimated to reach 10 million?

  2050.
• What are some global and national strategies to combat antimicrobial resistance?

  • WHO Global Action Plan on Antimicrobial Resistance
  • World Antibiotic Awareness Week
  • European Antibiotic Awareness Day
  • National Action Plans on AMR
• What are the genetic and structural mechanisms used by bacteria to avoid antimicrobial action?

  • Genetic Mechanisms:
  • Mutations
  • Horizontal gene transfer

  
  

  • Structural Adaptations:
  • Biofilm formation
  • Efflux pumps
  • Altered target sites

  
  

  • Enzymatic Strategies:
  • Drug inactivation
  • Modification of drug
  • Modification of target site
• What is intrinsic antimicrobial resistance?

  A result of inherent structural or functional characteristics that prevent antimicrobial action.
• What factors influence the mutation rate in bacteria?

  Rate of bacterial replication, periods of latency/dormancy, and mutation repair mechanisms.
• What types of mutations contribute to acquired antimicrobial resistance?

  • Substitution (synonymous/nonsynonymous, single nucleotide polymorphism)
  • Insertions
  • Deletions
  • Inversions
  • Duplications
• What are the genetic mechanisms bacteria use to avoid antimicrobial action?

  Mutations, horizontal gene transfer, structural adaptations, biofilm formation, efflux pumps, altered target sites, enzymatic strategies, drug inactivation, modification of drug, and modification of target site.
• What is intrinsic antimicrobial resistance?

  • A result of inherent structural or functional characteristics
  • May involve the lack of a target site
  • Can include increased efflux pumps
  • Found naturally in some bacteria
• How can acquired antimicrobial resistance be generated through mutations?

  • Mutations in target genes
  • Substitutions: synonymous/nonsynonymous
  • Single nucleotide polymorphisms (SNPs)
  • Insertions/deletions
  • Inversions
  • Duplications
  • Differences in mutation rate
  • Influenced by:
  • Rate of bacterial replication
  • Periods of latency/dormancy
  • Mutation repair mechanisms
  • Mutations as a result of:
  • Errors in DNA replication
  • Adaptation to selective pressure
  • Vertical descent with modification/mutation
• What are the main mechanisms of horizontal DNA transfer that contribute to acquired antimicrobial resistance?

  • Transduction:
  • A temperate phage inserts its DNA as a prophage into the chromosome.
  • Replicates occasionally packaging host DNA (generalized) or its own DNA (specialized).
  • Lysing the cell infects a recipient where the novel DNA recombines.
  • Conjugation:
  • Uses a protein structure (pilus) to connect with recipient cells.
  • Transfers conjugative plasmids or integrated conjugative elements (ICEs).
  • Alternatively, small multicopy plasmids, defective genomic islands, or the entire bacterial chromosome can be transferred.
  • Transferred elements can insert into the chromosome or replicate independently.
  • Transposition:
  • Transposons integrate into new sites on chromosomes or plasmids via non-homologous recombination.
  • Integrons use similar mechanisms to exchange single gene cassettes.
  • Transformation:
  • Uptake of extracellular DNA
• How do structural adaptations contribute to antimicrobial resistance?

  • Biofilm formation:
  • Creates organized microbial communities encased in a matrix of extracellular polymeric substances.
  • Prevents intracellular accumulation of the drug.
  • Change in membrane:
  • Prevents antibiotic binding.
  • Prevents transport across the cytoplasmic membrane.
  • Efflux pumps:
  • Transport proteins that actively pump antibiotics out of the bacterial cell.
  • Altered target sites:
  • Structures such as bacterial ribosomes, enzymes involved in cell wall synthesis, DNA replication, or metabolic processes show reduced binding affinity for antibiotics.
• What are the enzymatic strategies bacteria use to inactivate antibiotics?

  • Enzymatic inactivation:
  • Enzymes target certain structures/bonds of the antibiotic, rendering it inactive.
  • Examples: beta-lactamases, macrolide esterases.
  • Enzymatic modification:
  • Enzymes chemically modify the structure of antibiotics, reducing their ability to bind to their target.
  • Examples: phosphorylating or adenylating enzymes altering aminoglycosides.
  • Enzymatic modification of target sites:
  • Reduces binding affinity of antibiotics, e.g., through methylation.
• How does inactivation of antibiotics through hydrolysis contribute to antimicrobial resistance?

  • Enzymatic hydrolysis via acquired genes:
  • Beta-lactamases: cleave the beta-lactam ring.
  • Macrolide esterases: hydrolyze the ester bond in macrolides, preventing binding to the bacterial ribosome.
  • Steric hindrance via acquired genes:
  • Aminoglycoside-binding enzymes (acetyltransferases, adenyltransferases, phosphotransferases) reduce binding to the bacterial ribosome.
• How are antibiotic-binding targets modified to create resistance?

  • Mutations in the target site:
  • Reduce the affinity of antibiotic binding.
  • Examples:
  • Fluoroquinolones (topoisomerase)
  • Recombination resulting in a mosaic allele (Pbp in streptococci and gonococci)
  • Protection of the target site by added chemical groups:
  • E.g., methylation (no mutation required).
• What enzyme is responsible for resistance to beta-lactams in Gram-positive bacteria such as Staphylococcus aureus?

  Beta-lactamases
• What is the primary function of beta-lactamase inhibitors?

  They bind irreversibly to beta-lactamases.