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How to make culturing a part of your practice
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[main article: Culturing still valuable despite increasingly effective antibiotics]
To add culturing capabilities to a primary care optometry office, doctors should maintain supplies of the three media used in plating culture specimens: blood, chocolate and Sabouraud’s agar medium, said Joseph P. Shovlin, OD, in private group practice in Scranton, Pa. Blood and chocolate can be maintained for a maximum of 3 months, and some have shelf lives of only 6 to 8 weeks.
For practices that do not obtain cultures regularly, it may not be economically feasible to maintain culture plates, he said. However, practitioners should be aware of the limitations of transport media such as a thioglycolate broth. Frequently, the laboratory will not retrieve the sample from the transport media and put it on a plate in a timely fashion, he said.
Another option for offices that lack the space and resources to maintain culture media is to collect a scraping from the cornea and transfer it directly to a microculturette or culturette swab, which can then be transported to the laboratory, said Christopher Quinn, OD, in private group practice in Iselin, N.J. While this will not produce as high a yield as using culture medium, practices have to balance what is practical with what might be done in an academic institution.
Practices also should have an anesthetic, preferably an unpreserved lidocaine or preserved proparacaine, available to prepare the eye for culturing and a heat-sterilized platinum spatula or number 12 blade to obtain the sample, Dr. Shovlin said. Wet dacryon calcium alginate swabs also can be used, but optometrists should avoid using cotton swabs because of their fatty acid content.
Collecting a usable sample
When obtaining culture material, practitioners should be certain to scrape both the bed and the edges of ulcerative lesions, because different organisms inhabit different areas, Dr. Shovlin said. For example, Moraxella is generally found at the base, while Pseudomonas thrives in jagged edges and is usually found in the margin.
Culture plates must be refrigerated, but, to increase the yield, Dr. Shovlin recommends removing the plates prior to inoculating them with the culture sample. Bacteria may not survive at 4º C, so if he knows he will likely be obtaining a culture from a given patient he will set the plates out about 30 minutes before the exam. When plating the sample, the practitioner should be careful not to press too hard because the organism can become impaled into the media and will not grow.
“About 70% of all true infections are culture positive, so if you have no growth, it doesn’t mean you don’t have an infection, it just may mean you couldn’t harvest something on those plates,” Dr. Shovlin said. “The opposite is true as well. If you do have growth, it could be a contaminant from the lids or elsewhere, so you really have to look at these things very closely.”
When a doctor collects a sample, the area being scraped is only about 4 to 9 mm², so it is common to not see growth, agreed William D. Townsend, OD, in private practice in Canyon, Texas.
Dr. Townsend recommended asking the laboratory, which more frequently deals with cultures from the throat or intestines and the large growths expected there, to report even small growth, as is recovered from the eye.