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2002 Ocular Microbiology and Immunology Group, Abstract 10

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The In Vitro Susceptibility of Bacterial Endophthalmitis Isolates to Azithromycin
R. Mather, R.P. Kowalski, J.P.Whitcher. The Charles T. Campbell Laboratory, University of Pittsburgh, PA and The Francis I Proctor Foundation, University of California, San Francisco, CA

Purpose: Oral azithromycin has demonstrated an extended elimination half-life and is reported to have good ocular tissue penetration. Oral azithromycin may be efficacious in the initial management of 'high risk' patients presenting with penetrating trauma associated with organic matter contamination. This in vitro study tests the susceptibilities of 93 bacterial endophthalmitis isolates against azithromycin.

Methods: The MICs (mcg/ml) of 93 bacterial endophthalmitis isolates were determined to azithromycin using E-tests. The median MIC values were calculated for each bacterial species. Susceptibility to azithromycin was defined as an MIC value #2 mcg/ml, based on the National Committee of Clinical Lab Standards (NCCLS).

Results: Median MICs (mcg/ml) and susceptibility of 93 bacterial endophthalmitis isolates to azithromycin.

    Median %
  N MIC Susceptible
Fluoroquinolone-susceptible/ methicillin susceptible S. aureus 6 2 83.3
Fluoroquinolone -resistant /methicillin susceptible S.aureus 4 >256 0
Fluoroquinolone -resistant /methicillin resistant S. aureus 4 >256 0
CoagNeg Staph-fluoroquinolone resistant/methicillin susceptible 5 >256 0
CoagNeg Staph-fluoroquinolone resistant/methicillin resistant 5 >256 0
CoagNeg Staph-fluoroquinolone susceptible/methicillin resistant 5 >256 0
CoagNeg Staph-fluoroquinolone susceptible/methicillin suscept 5 1.5 80
Streptococcus pneumoniae 10 0.125 90
Streptococcus viridans group 10 4.25 50
Beta-hem Strep 5 1.5 100
Enterococcus species 9 8 11.1
Bacillus species 9 1 88.9
Haemophilus species 4 2.25 50.0
Other Gram-negative bacteria 12 8 41.7

Conclusion: Our In vitro study suggests that azithromycin is effective against Bacillus sp. (88.9% susceptible), Streptococcus pneumoniae (90%), and beta-hemolytic strep (100%). However, azithromycin does not show acceptable activity against S. aureus (except for FQS-MS isolates); Coagulase-negative Staph (except FQS-MS isolates), Strep viridans group, Gram negative rods, Haemophilus sp.and Enterococcus. In vivo studies are required to evaluate the effectiveness of oral azithromycin against infectious endophthalmitis.

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