5. Terminologies
Chemotherapy
• The use of drugs to treat a disease
• Selective toxicity:
– A drug that kills harmful microbes
without damaging the host
6. Terminologies
Antibacterial spectrum
Range of activity of an antibiotic
A broad spectrum
Antibiotic that can inhibit wide range of G- positive and G-
negative bacteria e.g. Carbapenems, 3-4th generation
cephalosporins, quinolones
A narrow spectrum
Antibiotic that is active only against a limited number of
bacteria e.g. penicillin G, 1-2nd generation cephalosporins,
oxazolidone
7. Bacteriostatic activity
Level of antimicrobial activity that inhibits the
growth of bacteria
Minimum inhibitory concentration (MIC)
The lowest concentration that inhibits the growth
of bacterial population
Bactericidal activity
Minimum bactericidal concentration (MBC)
The lowest concentration that kills 99.9% of
the bacterial population
8. Antibiotic combinations
Antibiotic synergism
Combination of antibiotics have enhanced activity
when tested together compared with each antibiotic
alone (e.g. 2 + 2 = 6)
e.g. ampicillin+gentamicin in entercoccal carditis
Additive effect
Combination of antibiotics has an additive effect
(e.g. 2 + 2 = 4)
e.g. combination of two ß-lactam antibiotics
Antibiotic antagonism
Combination in which the activity of one antibiotic
interferes with the activity of the other (e.g. 2 + 2 < 4)
9. Effects of Combinations of Drugs
• Synergism occurs when the effect of two drugs
together is greater than the effect of either alone.
• Antagonism occurs when the effect of two drugs
together is less than the effect of either alone.
12. Antibiotic/Antimicrobial
• Antibiotic:
– Chemical produced by a microorganism that
kills or inhibits the growth of another
microorganism
• Antimicrobial agent:
– Chemical that kills or inhibits the growth of
microorganisms
13. Antimicrobial Agents
• Disinfectant:
•antimicrobial agent used only on inanimate
objects
• Chemotherapeutic agent:
•antimicrobial agent that can be used internally
• Bactericidal:
•agent that kills bacteria
• Bacteriostatic:
•agent that inhibits the growth of bacteria
16. Mechanisms of Antimicrobial Action
• Bacteria have their own enzymes for
– Cell wall formation
– Protein synthesis
– DNA replication
– RNA synthesis
– Synthesis of essential metabolites
17. Mechanisms of Antimicrobial Action
• Viruses use host enzymes inside host cells
• Fungi and protozoa have own eukaryotic enzymes
• The more similar the pathogen and host enzymes,
the more side effects the antimicrobials will have
34. Other Inhibitors of Cell Wall Synthesis
• Cephalosporins
– 2nd, 3rd, and 4th
generations more
effective against
gram-negatives
Figure 20.9
35. Other Inhibitors of Cell Wall Synthesis
• Polypeptide antibiotics
– Bacitracin
• Topical application
• Against gram-positives
– Vancomycin
• Glycopeptide
• Important "last line" against antibiotic resistant
S. aureus
36. Other Inhibitors of Cell Wall Synthesis
• Antibiotics effective against
Mycobacteria:
– interfere with mycolic acid
synthesis or incorporation
– Isoniazid (INH)
– Ethambutol
37. Inhibitors of Protein Synthesis
• Broad spectrum, toxicity problems
• Examples
– Chloramphenicol (bone marrow)
– Aminoglycosides: Streptomycin, neomycin,
gentamycin (hearing, kidneys)
– Tetracyclines (Rickettsias & Chlamydia; GI tract)
– Macrolides: Erythromycin (gram +, used in
children)
38. Injury to the Plasma Membrane
• Polymyxin B (Gram negatives)
– Topical
– Combined with bacitracin and neomycin
(broad spectrum) in over-the-counter
preparation
39. Inhibitors of Nucleic Acid Synthesis
• Rifamycin
– Inhibits RNA synthesis
– Antituberculosis
• Quinolones and fluoroquinolones
– Ciprofloxacin
– Inhibits DNA gyrase
– Urinary tract infections
41. Antifungal Drugs
• Fungi are eukaryotes
• Have unique sterols in
their cell walls
• Pathogenic fungi are
often outside the body
42. Antiviral Drugs
• Viruses are composed of nucleic acid, protein
capsid, and host membrane containing virus proteins
• Viruses live inside host cells and use many host
enzymes
• Some viruses have unique enzymes for DNA/RNA
synthesis or protein cutting in virus assembly
Figure 20.16a
46. Antiviral Drugs Enzyme Inhibitors
• Interferons
– prevent spread of viruses to new cells (Viral
hepatitis)
• Natural products of the immune system in viral
infections
