Food Science and Technology Department


Date of this Version

Spring 4-2016


Crespo Ramírez, R.A. (2016). Studies of Debaryomyces hansenii killer toxin and its effect on pathogenic bloodstream Candida isolates. 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 & Technology, Under the Supervision of Professor Heather E. Hallen-Adams. Lincoln, Nebraska: April, 2016.

Copyright © 2016 Rhaisa A. Crespo-Ramírez


Candida species are normal commensal organisms found in the skin, mouth, and gastrointestinal and genitourinary tracts of humans. Candida albicans is a normal component of the gut flora, and the oral and genital cavities. However, this organism is an opportunistic pathogen and it is one of the most frequently isolated species from patients with nosocomial fungal infections. Candida infections have grown in recent years due to antimycotics resistance and the extensive use of antibiotics. This has pushed scientists to look for alternative therapeutic agents to treat fungal infections.

Yeasts can produce toxic proteins called “mycocins” or “killer toxins” that can kill sensitive yeast species. The production of killer toxins is a phenomenon widely spread among yeast species. Debaryomyces hansenii, a yeast commonly found in cheeses, produces killer toxins that are able to kill the strains of Candida albicans SC5314 and Candida tropicalis NRRL-10985 at optimum conditions of 20°C and pH 4.5. The use of Debaryomyces hansenii as a biotechnological agent has been suggested, but the amount of toxins produced, and the genetic basis and mechanism of action of these toxins, remains a mystery.

To study the extent of pathogenic Candida species affected by this toxin, nineteen strains of Debaryomyces hansenii were tested for killer activity against Candida species, previously isolated from patients with bloodstream infections, by streak-plate agar diffusion bioassay. The killer activity of Debaryomyces hansenii killer strains was also tested against Debaryomyces hansenii non-killer strains. Additionally, to understand the nature of these toxins, 2x250 paired end Illumina sequencing was performed in 6 killer and 3 non-killer strains of Debaryomyces hansenii previously isolated from different types of cheeses. Numerous differences among the strains were found. Primers and protocols were designed to knockout presumptive genes and confirm their relationship with toxicity.

Adviser: Heather E. Hallen-Adams