المحاضرة السادسة.pdf

09/08/1444 YEASTS By YEASTS A. Introduction • Yeasts are common causes of vaginitis and urinary tract infections (UTIs) in Women and can cause a number of other diseases in healthy and immunosuppressed individuals. • In addition, yeast can cause newborn infections and meningitis. • The most common...

09/08/1444 YEASTS By YEASTS A. Introduction • Yeasts are common causes of vaginitis and urinary tract infections (UTIs) in Women and can cause a number of other diseases in healthy and immunosuppressed individuals. • In addition, yeast can cause newborn infections and meningitis. • The most common cause of yeast infections is Candida albicans 1 09/08/1444 Methods for identification • a. Microscopic appearance • 1) Saline wet mounts and Gram stains will show budding yeast. • 2) Yeasts are discovered in routine urinalysis. • 3) India ink preparations are used to show the capsule surrounding (Cryptococcus neoformans ). Culturing 1) Yeasts are grown on Sabouraud-brain heart infusion agar at 22-30°C. 2) Yeasts will form cream-colored, mucoid to smooth colonies within several days. On blood agar, yeast colonies can resemble Staphylococcus colonies. 2 09/08/1444 Culturing 3) Cornmeal agar with Tween 80 is used to differentiate Candida spp. by enhancing the formation of fungal elements such as hyphae, pseudohyphae, and conidia. 3 09/08/1444 Culturing 4) Candida albicans will show chlamydospores with clusters of blastoconidia along the hyphae. 5) Candida tropicalis typically produces long-branched pseudohyphae. Blastoconidia are produced singly or in short chains. This species does not produce chlamydospores. 4 09/08/1444 FIGURE Candida albicans on cornmeal agar showing typical chlamydospores. FIGURE Pseudohyphae occur when the blastoconidia germinate and form a filamentous. Germ tube production • 1) Germ tubes are hyphae like extensions of young yeast cells showing parallel sides, are non - septate (showing no cell wall division), and will not constrict at their point of origin. • Pseudohyphae look like germ tubes but are septate and constricted at their point of origin. 5 09/08/1444 Germ tube production 2) Germ tube procedure: Yeasts are incubated with serum at 37°C for up to 3 hours and examined for germ tube production. 3) C. albicans is positive for germ tube production. Candida tropicalis is used for the negative control; however, some strains can produce germ tubes if incubated over 3 hours. 6 09/08/1444 Germ tube production Germ tube production 7 09/08/1444 Germ-tube production by Candida albicans. A positive germ tube has no constriction at its base . Candida tropicalis shows constriction at the base of the germ tube, called a pseudogerm tube. 8 09/08/1444 Carbohydrate assimilation test 1) Assimilation tests determine the aerobic utilization of carbohydrates. 2) Agar slants containing various carbohydrates are inoculated with yeast suspended in saline. Carbohydrate assimilation test • The medium contains the pH indicator bromcresol purple. • The tubes are incubated at room temperature and read 14 days. • Use of the carbohydrates results in the formation of yellow colonies. 9 at 7 and 09/08/1444 Carbohydrate assimilation test 3) A number of commercially prepared tests based on carbohydrate utilization and enzyme hydrolysis are also available. Urease test 1) Used to identify Cryptococcus spp., which are urease positive. 2) C. albicans is used for the negative control. 3) A positive urease is indicated by a pink to purple color. 10 09/08/1444 CHROMagars • CHROMagars allow for the identification of several species of yeasts. • The media contain a variety of substrates. • The ability to metabolize different substrates results in the production of colonies of different colors. 11 09/08/1444 12