The Biology of Pichia membranifaciens Killer Toxins

Abstract

The killer phenomenon is defined as the ability of some yeast to secrete toxins that are lethal to other sensitive yeasts and filamentous fungi. Since the discovery of strains of Saccharomyces cerevisiae capable of secreting killer toxins, much information has been gained regarding killer toxins and this fact has substantially contributed knowledge on fundamental aspects of cell biology and yeast genetics. The killer phenomenon has been studied in Pichia membranifaciens for several years, during which two toxins have been described. PMKT and PMKT2 are proteins of low molecular mass that bind to primary receptors located in the cell wall structure of sensitive yeast cells, linear (1→6)-β-d-glucans and mannoproteins for PMKT and PMKT2, respectively. Cwp2p also acts as a secondary receptor for PMKT. Killing of sensitive cells by PMKT is characterized by ionic movements across plasma membrane and an acidification of the intracellular pH triggering an activation of the High Osmolarity Glycerol (HOG) pathway. On the contrary, our investigations showed a mechanism of killing in which cells are arrested at an early S-phase by high concentrations of PMKT2. However, we concluded that induced mortality at low PMKT2 doses and also PMKT is indeed of an apoptotic nature. Killer yeasts and their toxins have found potential applications in several fields: in food and beverage production, as biocontrol agents, in yeast bio-typing, and as novel antimycotic agents. Accordingly, several applications have been found for P. membranifaciens killer toxins, ranging from pre- and post-harvest biocontrol of plant pathogens to applications during wine fermentation and ageing (inhibition of Botrytis cinerea, Brettanomyces bruxellensis, etc.).

Effect of Sequential Inoculation with Non-Saccharomyces and Saccharomyces Yeasts on Riesling Wine Chemical Composition

“…It has previously been shown that yeast growth can be inhibited by yeast metabolites such as ethanol [40,41], medium chain fatty acids [42], and acetaldehyde [43]. Killer toxins produced during the exponential phase by specific strain can also have an inhibitory impact on some yeast growth [44][45][46]. More recently, it was found that S. cerevisiae could secrete peptides who inhibit non-Saccharomyces yeast growth [47].…”

Use of Autochthonous Yeasts and Bacteria in Order to Control Brettanomyces bruxellensis in Wine

“…Mehlomakulu and coworkers [15] identified two killer toxins, CpKT1 and CpKT2, from the wine-isolated yeast Candida pyralidae. A similar action was described for the killer toxins isolated from Kluyveromyces wickerhamii and Pichia anomala [38], Pichia membranifaciens [26], Torulaspora delbrueckii [25] and Ustilago maydis [39]. Furthermore, an early inoculation with a selected LAB culture has proven to be a useful tool for controlling the proliferation of B. bruxellensis in wine.…”

“…The main types of interaction between Saccharomyces spp. and non-Saccharomyces yeasts are: nutritional limitation or competition [23,24] and the release of toxic compounds as killer toxins into the environment [15,25,26]. On the other hand, there are three types of interaction between yeast and LAB.…”

Natural Antifungal Peptides/Proteins as Model for Novel Food Preservatives

“…The binding of the toxin to the yeast cell wall is recognized as the first step of the antifungal action. However, other mechanisms have been discovered, including perturbation of membrane integrity or cell functioning (Belda, Ruiz, Alonso, Marquina, & Santos, 2017;Hatoum, Labrie, & Fliss, 2012;Izgu & Altinbay, 2004). A vast majority of killer toxins have a limited spectrum of activity, display poor stability, and are only active in a narrow range of pH (Santos, San Mauro, Bravo, & Marquina, 2009;Soares & Sato, 2000).…”