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Chapter 8.5 : Examination and Evaluation of Primary Cultures
Primary plates are read daily for the ﬁrst week, every other day for the second week, and twice weekly for the remaining 2 weeks. The use of 4 weeks of incubation has been challenged by some groups because few new positive cultures develop in the fourth week. I have noted that clinically signiﬁcant positive cultures are sometimes seen in the fourth week, suggesting the need to retain this incubation period. In areas of endemicity of systemic dimorphic pathogens, incubation for 5 weeks should be considered, as occasional isolates of Histoplasma capsulatum and Blastomycesdermatitidis may require that much time to form evident colonies. In cases of eumycetoma, the etiologic agent may not be evident on culture until the ﬁfth or sixth week. When growth appears, differentiate between yeast and ﬁlamentous forms (moulds) that may require microscopic examination. Use wet mounts or stain with lactophenol cotton blue (LPCB) (seeitem V.B below). If the isolate suggests an actinomycete, examine with Gram stain and with a modiﬁed acid-fast stain. (Procedures for the identiﬁcation of aerobic actinomycetes are given in procedure 6.1, items V.D.1 and V.D.2.)