the contact lens exam
Culturing Contact Lenses for Presumed Microbial Keratitis
BY LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
Up to 50 percent of presumed microbial keratitis is culture negative. Thus, if you encounter a situation in which a patient presents with a corneal ulcer that you're suspicious about, consider culturing the contact lens as well as the cornea. You may be surprised by what you find. Often you'll observe large numbers of bacteria or yeast adhering to the surface of the contact lens, which can guide you in your treatment.
Performing the Culture
If a patient has already removed the lens(es) and brought them in to the appointment, there is sure to be some contamination from lens handling, a dirty lens case or from non-sterile solutions. Nonetheless, the lens culture results may still point you in the right direction if the corneal cultures are negative.
The technique for lens culturing is simple. If you have agar plates directly accessible, simply remove the lens from its container with sterile forceps. If the patient is still wearing the lenses, remove them aseptically using sterile gloves.
If possible, cut the lens in half with a sterile blade. Place one half of the lens concave side down on a chocolate agar plate and lightly smear it across the surface. This will allow the lab to assess if the microbes were present on the posterior lens surface, which was against the cornea. Place the other half of the lens on a Sabouraud's plate to assess fungal growth.
Place about 0.5ml of the transport saline on multiple culture plates as well.
An Alternate Method
In research microbiology labs used for many extended wear clinical trials, an agar overlay technique is used that is probably not readily available in clinical microbiology laboratories. In this technique, the lens is removed aseptically from the patient's eye and placed in about 1ml to 2ml of sterile, non-preserved saline. Upon arrival at the microbiology laboratory, each vial is shaken for one minute. The lens is then aseptically removed and placed concave side up on a chosen agar plate, covered with 10ml of molten agar and incubated.
Figure 1. A contact lens incubated using the agar overlay technique.
Researchers also assess the transport saline for contamination by streaking 0.4ml onto various agar plates.
Figure 1 shows a lens incubated using the agar overlay technique. Note recovery of a mixture of Pseudomonas aeruginosa and Serratia marcescens organisms on the culture plate.
Culture Even After Disinfection
Don't be swayed if a patient has already disinfected the lenses. Culturing the lens(es) may still prove practical and useful in cases of suspected microbial keratitis.
Studies show that soaking lenses in one cycle of some multipurpose solutions and even some hydrogen peroxide disinfecting solutions is not always able to totally eradicate micro-organisms associated with contact lens-related infection and inflammation. It's thought that biofilm formation on the surfaces of lenses and lens cases is responsible for the resistance of micro-organisms against disinfection. CLS
Dr. Szczotka-Flynn is an associate professor at Case Western Reserve University Dept. of Ophthalmology and is director of the Contact Lens Service at University Hospitals of Cleveland.