A primary purpose of this column is to educate practitioners in order to help prevent microbial keratitis, a condition that most frequently occurs in contact lens wearers (Sankaridurg et al, 2013). In general, all cases of microbial keratitis are assumed to be bacterial until proven otherwise (Kanski, 2007). Also, with the availability of highly effective antibacterial medications, more than half of all clinicians treat most cases of bacterial keratitis empirically (McDonnell 1992; American Academy of Ophthalmology [AAO], 2013).
With that said, there are times that we should attempt to better characterize the offending organism prior to initiating treatment. Specifically, the AAO recommends that microbial keratitis should be cultured when a patient presents with a large, central infiltrate (AAO, 2013). Culturing should also be performed when a patient is unresponsive to treatment or when the case resembles fungal or amoebic keratitis (AAO, 2013).
Culture sample collection is generally performed by scraping the base and edges of the ulcer with a spatula or spud; a swab can also be used (AAO, 2013). Corneal biopsies may be performed in high-risk, hard-to-culture cases, and aqueous or vitreous taps might be performed when endophthalmitis is suspected (AAO, 2013). Samples are then transferred to a number of different devices for analysis.
Traditionally, multiple scrapes/swabs are performed and used to create slide smears for gram staining (bacteria, fungi, Acanthamoeba) and potassium hydroxide staining (fungi) as well as to inoculate a thioglycolate broth, a blood agar plate, a chocolate agar plate, and a Sabouraud dextrose agar plate (all grow different types of aerobic and facultative anaerobic bacteria) (AAO, 2013). A non-nutrient agar with Escherichia coli plate can also be added when Acanthamoeba is suspected. Samples are then transferred to a laboratory where slides are immediately analyzed and cultures grown for up to one or two weeks (AAO, 2013). With that done, positive cultures (~50% of cultures) are analyzed as quickly as possible to determine the offending organism(s) (AAO, 2013). Sensitivity testing can also be performed to help determine the best medical treatment (AAO, 2013).
Collected samples may also be analyzed with a “one touch” culture system or with a polymerase chain reaction (PCR). The “one touch” method employs a single sample collection with a swab that is used to inoculate a transport broth; this broth can then subsequently be split by the laboratory and used for multiple identification tests (Pakzad-Vaezi et al, 2015).
PCR is an enzymatic nucleic acid amplification system that uses DNA-specific primers to identify organisms (Sharma, 2012). PCR is advantageous because it is relatively quick, which allows for faster identification of slow-growing organisms and those that cannot be cultured (Sharma, 2012). PCR is also emerging as the method of choice for identifying viruses (Sharma, 2012).
Furthermore, confocal microscopy can be used to directly and repeatedly visualize organisms within a living eye (Sharma 2012). Confocal microscopy is not inhibited by corneal haze, a feature that allows confocal microscopy to be able to accurately diagnose Acanthamoeba and fungal keratitis.
Identifying an offending organism can be an integral part of treating some forms of keratitis (AAO, 2013). While it is unlikely that you will need to culture on a daily basis, you will need to do some planning if you wish to employ these methods in your office. A relationship with a laboratory must be formed, and some culture supplies need to be regularly replaced (Pakzad-Vaezi et al, 2015).
Alternatively, you could refer high-risk patients to a specialist who has the required equipment for performing these advanced procedures. Either way, it is your job to ensure that testing is performed when a high-risk ulcer patient presents at your office (AAO, 2013). CLS
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