Discussion
Common etiologic agents of post-surgical inner eye infections include Staphylococcus epidermidis, Staphylococcus aureus, Streptococcus pneumoniae, enteric gram negative bacilli such as Proteus species, Pseudomonas species, and fungi such as Candida albicans and Aspergillus species. Infections may not be apparent for several weeks after surgery and delayed infections may be caused by the more indolent pathogens such as Corynebacterium species, Propionibacterium species, mycobacteria, actinomycetes, and some fungi. As in this case, very unusual organisms may be isolated from post-surgical inner eye infections.
Due to the potential for loss of vision and/or complete loss of the eye, intraocular specimens should be treated with extreme care and expediency. With Staphylococcus aureus and Pseudomonas aeruginosa infections, the eye may be destroyed within twenty four hours. Every effort should be made to quickly identify and perform susceptibility testing on isolated organisms. A brief history from the physician, including exposure to dogs and potentially rodents, was very helpful in this particular case.
The original clinical Gram stain was not at all diagnostic for Capnocytophaga, but simply detecting and reporting a Gram-negative bacilli greatly assisted the physician in his therapy choices. The Gram stain from the cultured growth was very helpful in the identification. Such a Gram stain would be generally consistent with either Capnocytophaga, or Streptobacillus moniliformis.
With this organism, routine efforts at identification failed, and a presumptive identification was made based on a few key observations and tests. The identification was confirmed by the State Public Health Laboratories. All of the other answers and their sources could have potentially caused this infection but the growth rate, colonial and Gram stain morphology, need for CO2, positive oxidase and catalase tests, and negative nitrate test are sufficient for a presumptive identification of C. canimorsus. Although Streptobacillus moniliformis may exhibit extreme pleomorphism similar to that of Capnocytophaga canimorsus, and although both are esculin hydrolysis positive and nitrate negative, the positive catalase and oxidase tests rule out Streptobacillus moniliformis. Capnocytophaga canimorsus, as the species name suggests, is a component of the normal flora of dog mouths. In this patient, recurrence may have been due to re-infection from the dog, or failure to eradicate the original infection.
Based on the physician request for susceptibility testing after recurrence, the most helpful approach would be to report minimum inhibitory concentration (MIC) values without interpretation. As a side note, many fastidious organisms including the one in this case do not grow well in liquid MIC media and thus E-test on Mueller Hinton blood or chocolate agar in CO2 is preferred. Performing disk diffusion without interpretations is relatively meaningless because a zone size cannot be directly correlated to an MIC for organisms with no CLSI interpretive guidelines. Using the CLSI Pseudomonas interpretive criteria is not acceptable as they are not intended for fastidious gram negative bacilli. CLSI is currently developing interpretive criteria for fastidious organisms other than Haemophilus, Neisseria gonorrhoeae, and Streptococcus. Although referring a physician to published references would be somewhat helpful, not performing susceptibility testing on such a critical isolate would be a disservice to both patient and physician, especially in light of the recurrent infection. Vancomycin, surprisingly has acceptable activity against some of the more fastidious gram negative rods and cocci including Capnocytophaga and Moraxella respectively, and thus would be reasonable to test and report in this case. A standard regimen for treatment of intraocular infections is ceftazidime or amikacin, either agent in combination with vancomycin to cover both gram negative and gram positive bacteria.
Capnocytophaga Infections -- Vancomycin is used for the treatment of infections caused by Capnocytophaga†. Optimum regimens for the treatment of infections caused by Capnocytophaga have not been identified; however, some clinicians recommend use of penicillin G or, alternatively, a third generation cephalosporin (cefotaxime, ceftizoxime, ceftriaxone), a carbapenem (imipenem and cilastatin sodium, meropenem), vancomycin, a fluoroquinolone, or clindamycin.