Killer Toxins of the Yeasts; Candida utilis 22 and Kluyveromyces marxianus and their Potential Applications as Biocontrol Agents

Abstract

Objectives of the present work were to investigate optimal conditions for production and characterization of the killer toxins produced by Kluyveromyces marxianus and Candida utilis 22, as the most killer yeasts, against Candida famata var. famata and Cryptococcus albidus as the most sensitive strains and to conduct a primary investigation on the potential usefulness of these toxins in the biological control and biotechnological applications. Toxin secretion patterns followed the growth curves and reached their maximum activities at the early stationary phase. At pH 6, optimal killing activities were observed at 25°C in the presence of glucose as the best carbon source besides yeast extract and peptone together as the best nitrogen sources in case of C. utilis 22 strain, and ammonium sulphate in case of K. marxianus. Toxin activity decreased significantly with increasing the NaCl concentrations in culture medium. High stability of the toxins was observed at different temperatures up to 100oC for 1h, pH at a broad range (2.5-7) for 1 h and at salt concentration up to 3 and 4 M for 12 h for K. marxianus and C. utilis 22 strains, respectively. Low molecular weight was observed and chromosomal gene(s) might be responsible for both killer toxins. The toxins of both killer yeasts showed a wide range of antimicrobial activity against plant and human pathogens. These characteristics proved that such toxins are promising candidates for several biotechnological applications, such as biological control and medical purposes.