Incubation and Processing of Cultures in the Laboratory

Once the specimen has been inoculated on the media by the ophthalmologist, and received in the microbiology laboratory, the plates and tubes or broths must be placed in the appropriate atmosphere and temperature for isolation. Occasionally it is necessary to process specimens that are not set up at the bedside but are submitted to the microbiology laboratory on swabs or in syringes. Given below is the description for processing of specimens for fungal cultures.

INCUBATION REQUIREMENTS 

Conditions (Fig. 6.1) 

Temperature: 25°C to 30°C 

Atmosphere: Sabouraud’s dextrose agar or potato dextrose agar for fungi is incubated in a regular non - CO2 incubator.

Fig. 6.1: Mycological incubator: Maintains temperature at 26°C

Length of Incubation: 7 days to 21 days, but varies depending on the specimen and or culture.

PROCESSING OF CULTURES 

Fungal Cultures (Fig. 6.2)

Fungal plates/tubes are to be observed daily for the first week, because Aspergillus spp may be positive within 24 to 48 hours, as may Fusarium spp the two most common causes of mycotic keratitis. Significant growth (i.e., all fungi growing in the inoculated area of the agar or true pathogen) should be immediately informed to the physician with a written preliminary report and with the note “Final report to follow.”

Cultures showing no growth should be followed by a written report at 7 days and final report “No growth” at 3 weeks. Lacto phenol cotton blue wet preparations are recommended for initial observation of fungi for spores.

Figs 6.2A to D: Various form of fungal growth in blood agar of corneal scraping specimen culture. A. Profuse growth of hyaline fungus in all C streaks. B. Profuse growth of dematiaceous fungus in all C streaks. C. Growth of A.flavus. D. Mixed growth of fungus and bacteria