Anti-Infective Drugs and Antibiotics

Anti-Infective Drugs

• Antimicrobials are chemical substances that kill (biocidal) or inhibit (biostatic) the growth of microbes in diluted solutions.
• Microbes include bacteria, fungi, viruses, protozoa, and parasites.
• Specific antimicrobials exist for each category:
  • Bacteria: Antibiotics
  • Fungi: Antifungals
  • Viruses: Antivirals
  • Protozoa: Antiprotozoals
  • Parasites: Antiparasitics

Antibiotics

• Antibiotics are substances produced by microorganisms that, in dilute solutions, inhibit or destroy other microorganisms.
• They are antagonistic to life.

Mechanisms of Action of Antibiotics

• Anti-microbial action can involve:
  • Disruption of microbial cell wall development
  • Damage to the cell membrane, particularly in static/adult populations.
  • Interference with microbial protein synthesis
  • Inhibition of nucleic acid production
  • Disruption of microbial metabolic activity

Classification of Antibiotics

• Classified by their spectrum of action:

  • Narrow-spectrum antibiotics target either Gram-positive or Gram-negative bacteria.
  • Broad-spectrum antibiotics target both Gram-positive and Gram-negative bacteria, but not necessarily all.

• Classified by their effect on micro-organisms:

  • Bactericidal antibiotics kill bacteria.
  • Bacteriostatic antibiotics inhibit the growth or replication of bacteria.

Spectrum of Bacteria Types (Gram Positive/Negative)

• Cocci (spherical)
• Bacilli (rod-shaped)
• Spirochetes and spirilla (spiral-shaped)

Bactericidal vs. Bacteriostatic

• Bactericidal drugs directly kill bacteria often requiring higher concentrations or longer treatment times.
• Bacteriostatic drugs temporarily inhibit bacterial growth, often relying on the host's immune system to eliminate the bacteria.

Effectiveness of Antibiotics

• Factors influencing antibiotic effectiveness:
  • Susceptibility of the organism to the drug
  • Ability of the drug to reach the infection site at sufficient concentrations.
  • Patient's tolerance to the treatment.

Susceptibility Testing: Culture and Sensitivity

• Performed to determine the effectiveness of different antibiotics against a specific microorganism.
• Inoculation involves placing a sample on a medium with different antibiotic-containing disks
• Incubation allows microbial growth to occur or not (zones of inhibitions)
• Results allow determination of Minimum Inhibitory Concentration (MIC) and susceptibility to bacteria

Minimum Inhibitory Concentration (MIC)

• The lowest concentration of a drug that inhibits bacterial growth is the minimum inhibitory concentration.
• Further dilutions of the drug can reveal if the bacteria are susceptible or resistant to the drug
• Susceptible organisms will not have microbial growth in the lowest concentration of the drug

Time-Dependent vs. Concentration-Dependent Killing

• Time-dependent killing affects bacteria based on drug exposure time, rather than concentration needed.
• Concentration-dependent killing depends on the concentration of antibiotic needed to kill bacteria.

Patient Tolerance to Antibiotics

• Debilitated or compromised patients (e.g., neonates, geriatrics, compromised kidney or liver functions) may need therapies adjusted.

Antibiotic Resistance

• Bacteria can develop resistance to antibiotics.
• Resistance mechanisms include:
  • Genetic mutations
  • Horizontal transfer of resistance genes through plasmids.
• Resistance can lead to need for stronger antibiotics and increase in treatment time/cost.

Antibiotic Residue

• Presence of antibiotic and its byproducts in food products (animal or human).
• Residues can create antibiotic resistance, allergic reactions (humans/animals).
• Withdrawal times are used to mitigate antibiotic residues in food.

Withdrawal Times

• The time period for antibiotics to metabolize and eliminate from the animal body.
• Withdrawal times vary based on antibiotic types.