In inflammation of the external eye, where bacterial infection may be implicated, isolation of bacteria may aid the ophthalmologist in making the correct diagnosis and determining the choice of antibacterial agent. If bacterial infection is suspected the isolation must be undertaken as one of the first steps in the ophthalmological examination. It is important to recognize that almost all bacterial infections of the external eye are secondary to either ocular disease (eg. KCS, entropion, lagophthalmos, etc.) or trauma, which all imply an epithelial defect, making bacterial invasion possible (Evans 2001). A resent publication shows that there is no significant difference in the prevalence of the five predominant bacterial species (Staphylococci, Streptococci, Pseudomonas, Diphtheroids and E. coli) in eyes with evidence of infection of the external eye and clinically normal eyes (Evans 2001). It is thus not possible to diagnose clinical infection alone on the identified bacteria, as many bacteria may be potentially pathogenic.

So far most of the published investigations have only divided bacteria into crude groups (eg. Coagulase-positive vs Coagulase-negative Staphylococci) and PCR and other accurate tests may reveal that certain specific bacterial species (as well as subspecies) are more pathogenic in canine ocular infection than others. A method of quantification of bacteria has been described (Petersen-Jones 1997), but has presumably not gained widespread acceptance and application. There are many ways to isolate bacteria, but using a moistened (physiological saline or broth) Calcium Arginate Swab had been identified as a promising method (Benson and Lanier 1992, Byrne et al 1995). Using a moistened Dacron/rayon swab has also been reported to result in a high yield of bacteria (Varga et al 1992). The number of bacterial species isolated (conjunctival culture in dogs) per animal was significantly greater using a wet swab than a dry swab (Hacker et al 1979).

While making a swab it is of importance not to touch the palpebral margin with the swab, as this will result in isolation of the palpebral flora (Byrne et al 1995). If isolating bacteria from a corneal ulcer, it is advisable to swab and scrape (eg. a scalpel blade) from the margin between normal cornea and the ulcer. After the swab has been done, direct incubation is advised as bacteria may be sparse or die in the transport media underway to the diagnostic laboratory. After isolation the swab is usually spread on a Blood agar plate. If fungal infection is suspected a Sabourand's dextrose agar may be included. After isolation of single colonies and incubation, the bacteria may be identified by numerous biochemical tests (Holt et al 1994) or commercial test-kits (eg. Analytical Profile Index (API) Biochemical Identification) may be utilized (Byrne et al 1995). If time is of great importance, the bacteria is spread over the entire surface of the Blood Agar Plate, followed by application of antimicrobial discs, and it is possible to have a sensitivity result in 24hrs and not 48hrs as usual. Bacterial susceptibility towards various antimicrobial agents may at present be best correlated with MIC-values obtained by commercial methods such as disc diffusion (eg. Neo-Sensitabs®, Rosco A/S, Taastrup, Denmark) or Dilution Susceptibility Tests (eg. Sensititre, AcuMed) according to NCCLS standards (NCCLS 1997). It is important to know the obtainable concentration of antibiotics in ocular tissues, as this concentration often is higher than the plasma level in systemic antibacterial therapy. Comparison of the tear concentration of an antibiotic to the MIC-value of the isolated bacteria may prove to be valuable (Evans 2001).

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This page was authored by T. F. Evans July 2001.