Food Science and Technology Department


Date of this Version



Banjara, N. (2014). Debaryomyces hansenii: a foodborne yeast that produces anti-Candida killer toxin. MS Thesis. University of Nebraska-Lincoln


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Food Science and Technology, Under the Supervision of Professor Heather Hallen-Adams. Lincoln, Nebraska: May, 2014

Copyright (c) 2014 Nabaraj Banjara


Candida yeasts are commensal members of the gastrointestinal, mucosal, oral and vaginal microbiota. Candida albicans and C. tropicalis can be found as a part of the normal human commensal flora, especially in all sections of the gastrointestinal tract. However, when the host defense system and microbiota are disturbed, Candida can become pathogenic and cause severe infection or candidiasis. Antifungal drugs used to treat candidiasis have been shown to result in treatment failures due to drug toxicity and/or development of resistance during long term antifungal therapy and, in recent years, the incidence of Candida infections has increased dramatically due to the rise in the number of immunocompromised patients. Many yeast species can produce toxic proteins or glycoproteins called killer toxins or mycocins which can kill sensitive yeasts. Debaryomyces hansenii is the most common yeast species found in cheese, and can produce killer toxins. In order to examine the diversity and killer toxin profiles of D. hansenii, 48 types of cheeses were collected in 5 sampling periods for fungal isolation. Yeasts and molds were identified and 42 strains of D. hansenii were isolated and screened for killer activity against C. albicans and C. tropicalis using the streak-plate agar diffusion bioassay. Killer activity of crude toxin isolated from D. hansenii strains was quantified at different pH values (4.5, 5, 5.5, 6.0) and temperatures (20 C, 25 C, 30 C and 35 C) by agar diffusion well bioassay. The effect of D. hansenii killer toxin on C. albicans and C. tropicalis growth kinetics was also studied. More than 50% of cheese examined contained the yeast species Debaryomyces hansenii, with Galactomyces geotrichum being the second most abundant yeast species, while Penicillium roqueforti was the most frequently isolated mold. More than 50% of D. hansenii strains demonstrated killer activity against C. albicans and C. tropicalis, killer toxin activity differed among the D. hansenii strains, and killer susceptibility differed between C. albicans and C. tropicalis. D. hansenii killer toxin was active against C. albicans up to pH 5.5 but against C. tropicalis to pH 6.0. Killer activity was higher at low temperature and low pH. Killer toxin activity against C. albicans was detected as high as 35 C. These results confirmed that the same killer toxin from D. hansenii can act differently in different species and correlates with temperature and pH condition; killer toxins which are active at physiological temperature may have medical application.

Advisor: Heather Hallen-Adams