Success of empirical treatment today is mainly due to the frequency of dosing and effectiveness of the later generation fluoroquinolones. Risks of empirical treatment include failure to eradicate the infection, not having a clinical guide to modify treatment when things aren’t going according to plan and medico-legal issues that might later arise. Also, because there is now an apparent increase in multi-drug resistant organisms, culturing and testing for sensitivity to assorted anti-microbial agents may be the way to go.
Just last month a warning surfaced regarding a new superbug spreading widely from India and Pakistan in patients who have recently undergone surgery.1 Researchers have also reported finding a new gene that alters bacteria, making it resistant to nearly all known antibiotics. It is seen mostly in E.coli and on DNA structures that can easily be copied and passed onto other bacteria.2
The apparent increase in the number of non-bacterial corneal infections in contact lens wearers is another issue to consider when discussing whether to culture. It can be debated that non-bacterial infections are not really on the rise, but rather that the increase is a factor of a more empirical treatment of contact lens bacterial infections with later generation fluoroquinolones imparting a cure and creating less of a need for culturing.
When coupled with an acute red eye and suspicious infiltrate, these manifestations warrant culturing:3
• Any infiltrate following organic/vegetative trauma.
• Any atypical ulceration or if a rare infection is suspected (non-bacterial).
• An infiltrate/suppuration that involves 25% depth of the cornea or a 50% thinning (or a scleral extension of the infectious process).
• Any patient who is hospitalized, immuno-compromised or incompetent.
• Any infiltrate/ulceration that is not responding to seemingly appropriate therapy.
Multiple media should be used to capture a broad assortment of possible pathogens. Direct inoculation onto solid media provides the best yield for detecting organisms. For example, if a contact lens wearer is suspected of having a significant bacterial keratitis, media that include blood and chocolate should be plated directly for the best yield. When a non-bacterial or unusual infection is suspected, media such as Saboraud’s and Lowenstein-Jensen and the use of other laboratory techniques should be considered.
Co-infection in contact lens wearers is not unusual; Saboraud’s media can be used (or held for later inspection) if a fungal organism is suspected. The following is a list of media and likely organisms that can be grown from the media:
• Blood—aerobic organisms, saprophytic fungi.
• Chocolate—Nesiseria, Moraxella, Hemophilus.
• Lowenstein-Jensen—Nocardia, mycobacterium species.
Smears/stains like gram and Giemsa are additional laboratory tests that can be helpful. Keep in mind that gram stain is only 12% - 60% reliable at predicting organisms.4 Giemsa stains are able to provide better morphologic characteristics in distinguishing bacteria from fungi. Sabouraud’s media, blood agar (32 C) stains (gram, giemsa, acridine orange and calcofluor white) and polymerase chain reaction are valuable tests if fungi are suspected. When protozoan infection is suspected, confocal microscopy (although not overwhelmingly specific or sensitive) can be employed for diagnostic and therapeutic assessment in non-bacterial infections—histology and DNA analysis is a more reliable measure.
Using culture results to guide therapy may become more commonplace with the advent of new superbugs and emerging fluoroquinolone resistance. In the meantime, adhere painstakingly to the basic guidelines on when to culture contact lens wearers who present with corneal infiltrates that appear to be a response to microbial antigens.
1. Seeley M. What is NDM-1 and should I be afraid? National Living Examiner. 2010 Sep. Available at: www.examiner.com/natural-health-in-national/what-is-ndm-1-and-should-i-be-afraid. (Accessed September 2010).
2. Bagaria J, Bhargavi R, Butt F, et al. Emergence of a new antibiotic resistance mechanism in India, Pakistan, and the UK: a molecular, biological, and epidemiological study. The Lancet Infectious Disease. 2010 Sep;10(9):597-602.
3. Alfonso E, Mandelbaum S, Fox MJ, et al. Ulcerative keratitis associated with contact lens wear. Am J Ophthalmol. 1986 Apr;101(4):429-33.
4. Wilhelmus KR, Robinson NM, Font RA, et al. Fungal keratitis in contact lens wearers. Am J Ophthalmol. 1988 Dec:106(6):708-14.