Lecture slides on Antibiotics

1. ANTIBIOTICS

2. OUTLINE
a. Bacterial cell structure
b. Antibiotic groups and sites of action
c. Mechanisms of Resistance in Bacteria
d. Sensitivity testing

3. Principles and Definitions
• Selectivity
– Selectivetoxicity
• Therapeutic index
– Toxic dose/ Effective dose
• Categories of antibiotics
– Bactericidal
• Usually antibiotic of choice
– Bacteriostatic
• Duration of treatment sufficient for host
defenses

4. Principles and Definitions
• Antibiotic susceptibility testing (in vitro)
– Minimum inhibitory concentration (MIC)
• Lowest concentration that results in inhibition of
visible growth
– Minimum bactericidal concentration (MBC)
• Lowest concentration that kills 99.9% of the
original inoculum

7. Inhibitors of cell wall synthesis
A Beta lactam antibiotics
Penicillins
Cephalosporins
B Glycopeptides
C Bacitracin
spheroplasts and protoplasts

8. CytoplasmCytoplasm
Lipoteichoic acid Peptidoglycan-teichoic acid
Cytoplasmic membrane
GRAM POSITIVE CELLGRAM POSITIVE CELL
ENVELOPEENVELOPE
Degradative enzyme

9. GRAM NEGATIVEGRAM NEGATIVE
CELL ENVELOPECELL ENVELOPE
CytoplasmCytoplasm
Inner (cytoplasmic) membrane
Outer Membrane
(Major permeability barrier) LipopolysaccharidePorin
Braun lipoprotein
Periplasmic space Degradative enzyme
Periplasmic binding protein
Permease

10. Inhibitors of Cell Membrane function
1. Detergents: Colistin and Polymyxins
2. Inhibition of Biosynthetic Functions:
Nalidixic acid and Novobiocin Polyenes
3. Ionophores: eg K+ loss
Antifungals

11. Inhibitors of protein synthesis
70S versus 80S ribosomes
30S
AMINOGLYCOSIDES
TETRACYCLINES
OXAZOLIDINONES
50S
CHLORAMPHENICOL
MACROLIDES
LINCOMYCIN

12. Protein Synthesis Inhibitors
• Mostly bacteriostatic
• Selectivity due to differences in
prokaryotic and eukaryotic ribosomes
• Some toxicity -

13. • Aminoglycosides
1 receptor attachment
2 prevents formation of initiation
complex
3 misreading
4 polysomes break into monosomes

15. Inhibition of Nucleic Acid
Synthesis
DNA
• Quinolones DNA gyrase
• Sulfonamides
• Trimethoprim
RNA
• Rifampicin RNA polymerase

16. Inhibitors of RNA Synthesis
Selectivity due to differences between
prokaryotic and eukaryotic RNA polymerase

17. Inhibitors of DNA Synthesis
Selectivity due to differences between
prokaryotic and eukaryotic enzymes

18. Quinolones (bactericidal)
nalidixic acid, ciprofloxacin, ofloxacin, norfloxacin,
levofloxacin, lomefloxacin, sparfloxacin
• Mode of action - These antimicrobials bind to the A subunit of
DNA gyrase (topoisomerase) and prevent supercoiling of DNA,
thereby inhibiting DNA synthesis.
• Spectrum of activity - Gram-positive cocci and urinary tract
infections
• Resistance - Common for nalidixic acid; developing for
ciprofloxacin

19. Rifampin, Rifamycin,
Rifampicin, Rifabutin (bactericidal)
• Mode of action - These antimicrobials bind to DNA-dependent
RNA polymerase and inhibit initiation of mRNA synthesis.
• Spectrum of activity - Broad spectrum but is used most
commonly in the treatment of tuberculosis
• Resistance - Common
• Combination therapy - Since resistance is common, rifampin
is usually used in combination therapy.

20. Inhibitors of Folic Acid Synthesis
• Basis of
Selectivity
• Review of
Folic Acid
Metabolism
PABA+ Pteridine
Dihydropteroic
acid
Dihydrofolic
acid
Tetrahydrofolic
acid
Pteridine
synthetase
Dihydrofolate
synthetase
Dihydrofolate
reductase
Purine
Trimethoprim
Sulfonamide

21. • Most antimicrobials have more than one
mechanism of action.

23. Mechanisms of Antimicrobial
Resistance

24. Beta-lactam Antibiotics

25. Mechanism of Action of
Beta-lactams

26. Penicilllin,
action of β-lactamase
Site of amidase action
Site of penicillinase action
(break in β-lactam ring)

27. Beta-lactamase resistant
Beta-lactams

28. Mechanism of Methicillin
Resistance

29. SensitivityTesting

30. Antibiotic susceptibility testing
(in vitro)
Minimum inhibitory concentration (MIC)
Lowest concentration that results in inhibition of
visible growth
Minimum bactericidal concentration (MBC)
Lowest concentration that kills 99.9% of the
original inoculum

31. Antibiotic Susceptibility
Testing
8 4 02 1
Tetracycline (g/ml)
MIC = 2 g/ml
Determination of MIC
Chl Amp
Ery
Str
Tet
Disk Diffusion Test

32. Media used for Sensitivity
Testing
• Mueller Hinton Agar
• DST
• Special media e.g. Chocolate agar

33. Disc Diffusion Testing
• A. Kirby Bauer method……
CLSI recommended
• B. Stokes Comparitive method

34. Kirby Bauer Test

36. MIC evaluation by E test

37. • Inoculum Standardization
0.5 McFarland Standard

38. Review
• Sites of action
• Mechanisms of action
• Major groups
• Sensitivity testing methodology