1. Antimicrobial treatments and their mechanisms of action
Regis P. Kowalski, MS,[M]ASCP
Research Professor of Ophthalmology
Executive Director of The Charles T Campbell Laboratory
University of Pittsburgh, Pittsburgh, PA
The Charles T. Campbell Ophthalmic Microbiology Laboratory

2. Areas of Infection

3. Normal Flora

5. Treatment Administration
Topical – Most common (conjunctivitis, keratitis, blepharitis, prophylaxis)
Intra-vitreal injection – endophthalmitis
Systemic – not commonly used
Subconjunctival – used to provide a constant flow of anti-infective to ocular surface.

6. Antibacterials
Antibiotics – No Topical Standards for Interpreting Susceptibility
Use Serum Systemic Susceptibility Standards - But we must assume that “The antibiotic concentrations reached in the ocular tissue by topical therapy is equal to or greater than the concentration of antibiotic in the blood serum”.
Ocular antibiotics are developed from systemic antibiotics for conjunctivitis, keratitis

7. Antibiotics Parameters
Concentration–dependent
fluoroquinolones, aminoglycosides
Time-dependent
vancomycin, cefazolin
Bactericidal – kill
FQs, vancomycin,
Bacteristatic – inhibit
erythromycin, azithromycin, sulfa
Antiseptics – PI – kill immediately

8. Susceptibility Parameters
MIC
MBC
MPC
Growth
No growth but viable
Death but possible mutations
Death and NO mutations

9. True Resistance Organism acquires resistance to an antibiotic.
Resistance spreads to other patients
No antibiotic to cover resistance

10. Class Mode of Action Primary Indication
besides conjunctivitis
Fluoroquinolones DNA synthesis keratitis, surgical prophylaxis
(ciprofloxacin, ofloxacin, bactericidal Broad-spectrum coverage
levofloxacin, gatifloxacin,
moxifloxacin) besifloxacin
Aminoglycosides protein synthesis keratitis, endophthalmitis
(gentamicin, tobramycin, cell wall surgical prophylaxis
amikacin) bactericidal Broad-spectrum coverage
Cephalosporins cell wall keratitis, endophthalmitis
(cefazolin, ceftazidime) bactericidal Gram-positive coverage
Glycopeptides cell wall keratitis, endophthalmitis
(vancomycin) bactericidal MRSA prophylaxis
Gram-positive coverage
Macrolides protein synthesis blepharitis
(erythromycin, azithro) bacteristatic Gram-positive coverage
Peptides cell wall blepharitis, keratitis
(bacitracin, polymycin B) bactericidal Bac – GMpos PB – GMneg
Sulfa drugs enzyme inhibitor keratitis, 2nd – line MRSA
(sulfacetamide) bacteristatic Broad-spectrum coverage

11. Anti-Infective Test Batteries
Keratitis Endophthalmitis Conjunctivitis bacitracin vancomycin bacitracin vancomycin gentamicin erythromycin ciprofloxacin ciprofloxacin gentamicin ofloxacin ofloxacin ciprofloxacin polymyxin B cefazolin ofloxacin cefazolin amikacin trimethoprim tobramycin ceftazidime polymyxin B sulfisoxazole cefoxitin tobramycin cefoxitin ampicillin sulfisoxazole gentamicin clindamycin azithromycin gatifloxacin gatifloxacin gatifloxacin moxifloxacin moxifloxacin moxifloxacin

12. Interactive Testing
“Synergism” is a positive effect where the combination of two antibiotics are more active than when either antibiotic is used independently.
An “additive effect” : The result with two drugs is equal to the sum of the results for each drug used separately.

13. Interactive Testing
“Antagonism” is a negative effect whereas the combination of both drugs is effectively less than the separate independent effects of both antibiotics.
An “indifferent interaction” indicates that both antibiotics react independent from each other.

14. Checkerboard Testing
Fractional Inhibitory Concentrations A/MICa + B/MICb = FIC index
- “Synergism” = FIC index of 0.5 or less
- “Additive” = >0.5 to 1.0
- “Indifferent” = 1.0 to 2.0
- “Antagonism” = an FIC index greater than

15. Indifference
GAT
0.06 B A K
1.0
0.06/ /1 = 2.0

16. Antagonism
1/ /256 = 5

17. Time-Kill Testing
A “Synergistic” interaction between two antibiotics is indicated with an additional 2 log decrease in colony counts with the antibiotic combination compared with the most effective single antibiotic.
An “antagonistic” interaction between two antibiotics is indicated with an additional 2 log increase in colony counts with the antibiotic combination compared to the most effective single antibiotic.
An “indifferent” effect between two antibiotics is indicated with an equivalent colony count between the antibiotic combination and the most effective single antibiotic.

18. A
Synergism B
A + B A
Antagonism
A + B B A
Indifference
A + B B

19. Keratitis Models 24-48 hour results
New Zealand White Rabbits
One- or Two-eye design
Corneas infected intrastromally with bacteria
After 4 hours, treat with Anti-Infectives (5-8 hours)
Clinically evaluate rabbits
Determine viable bacteria
Statistical Analysis
Bactericidal Effect (3 log decrease)

20. In Vivo Interaction Testing
Staphylococcus aureus
Onset
≈ 99.9% kill
99.9 % Kill

21. Thank You!!