The yeasts constitute a large and heterogeneous group of microorganisms that are currently attracting increased attention from scientists and industry. Numerous and diverse biological activities make them promising candidates for a wide range of applications not limited to the food sector. In addition to their major contribution to flavor development in fermented foods, their antagonistic activities toward undesirable bacteria and fungi are now widely known. These activities are associated with their competitiveness for nutrients, acidification of their growth medium, their tolerance of high concentrations of ethanol, and release of antimicrobial compounds such as antifungal killer toxins or “mycocins” and antibacterial compounds. While the design of foods containing probiotics (microorganisms that confer health benefits) has focused primarily on Lactobacillus and Bifidobacterium, the yeast Saccharomyces cerevisiae var. boulardii has long been known effective for treating gastroenteritis. In this review, the antimicrobial activities of yeasts are examined. Mechanisms underlying this antagonistic activity as well as recent applications of these biologically active yeasts in both the medical and veterinary sectors are described.

/pdf/antimicrobial-and-probiotic-properties-of-yeasts-from-4yoao5nrcf.pdf

Antimicrobial and Probiotic Properties of Yeasts: From Fundamental to Novel Applications

The killer phenomenon in yeasts has been revealed to be a multicentric model for molecular biologists, virologists, phytopathologists, epidemiologists, industrial and medical microbiologists, mycologists, and pharmacologists. The surprisingly widespread occurrence of the killer phenomenon among taxonomically unrelated microorganisms, including prokaryotic and eukaryotic pathogens, has engendered a new interest in its biological significance as well as its theoretical and practical applications. The search for therapeutic opportunities by using yeast killer systems has conceptually opened new avenues for the prevention and control of life-threatening fungal diseases through the idiotypic network that is apparently exploited by the immune system in the course of natural infections. In this review, the biology, ecology, epidemiology, therapeutics, serology, and idiotypy of yeast killer systems are discussed.

Yeast killer systems.

Two linear deoxyribonucleic acid plasmids, designated pGK11 and pGK12, were isolated from the yeast Kluyveromyces lactis IFO 1267. pGK11 and pGK12 had molecular weights of 5.4 X 10(6) and 8.4 X 10(6), respectively. Both plasmids possessed the same density of 1.687 g/cm3, lighter than the densities of mitochondrial (1.692 g/cm3) and nuclear (1.699 g/cm3) deoxyribonucleic acids. A restriction map of pGK11 was constructed from digestions by EcoRI, HindIII, PstI, and BamHI. pGK12 was cleaved by EcoRI into seven fragments and by BamHI into two fragments K. lactis IFO 1267 killed Saccharomyces cerevisiae sensitive and killer strains and certain strains of Saccharomyces italicus, K. lactis, Kluyveromyces thermotolerans, and K. vanudenii. All K. lactis strains lacking the pGK1 plasmids were nonkillers. A hybrid was constructed between K. lactis IFO 1267 and a nonkiller K. lactis strain lacking the plasmids and subjected to tetrad analysis after sporulation. The killer character was extrachromosomally transmitted in all tetrads in association with the pGK1 plasmids. The double-stranded ribonucleic acid killer plasmid could not be detected in any K. lactis killer strains. It is thus highly probable that the killer character is mediated by the linear deoxyribonucleic acid plasmids. A single chromosomal gene was found which was responsible for the resistance to the K. lactis killer.

Isolation and characterization of linear deoxyribonucleic acid plasmids from Kluyveromyces lactis and the plasmid-associated killer character.

https://escholarship.umassmed.edu/cgi/viewcontent.cgi?article=1030&context=maps_pubs

Double-stranded ribonucleic acid killer systems in yeasts.

The ψ factor of yeast: A problem in inheritance

1. First- and second-generation crosses between killer, neutral and sensitive strains of yeast have been carried out in all combinations. 2. The results of this analysis indicated that the killer character is under the control of two types of cytoplasmic genetic determinant. One type, (k), determines killing, and the other, (n), neutrality. The absence, (o), of both types of determinants confers the sensitive phenotype. 3. That both types of cytoplasmic determinant require the same dominant nuclear allele, M , for their maintenance has been indicated in two ways. First, both types are lost when the nuclear genotype is changed from M to m . Secondly, cells of genotype m (k) or m (n), which have been shown to occur among the first formed cells arising from spores of Mm (k) and Mm (n) diploids respectively, are unable to maintain their cytoplasmic determinants. On the other hand, spore cultures of M (k) and M (n) genotype derived from these same diploide continue to maintain the determinants. 4. Thus genotype of killer cells is M (k), of neutrals M (n), and of sensitivities either M (o) or m (o). 5. Cells maintaining both types of cytoplasmic determinant (i.e. of genotype M (k)(n) or M −(k)(n)) have been obtained by appropriate crosses, and shown to be of killer phenotype. 6. Alternative hypotheses to account for the results of this genetic analysis have been discussed.

/pdf/the-inheritance-of-the-killer-character-in-yeast-gbvcpu8ind.pdf

The inheritance of the killer character in yeast.

When killer and neutral strains of Saccharomyces cerevisiae are crossed the resulting diploid clones possess a killer phenotype and when spored yield a complete range of tetrad ratios.The combined results of analysing tetrads and vegetative cells of diploid clones derived from two different neutral × killer crosses (K1 × N1 and K2 × N1) demonstrate that the range of tetrad ratios can be accounted for by the occurrence of somatic segregation of killer (k) cytoplasmic determinants prior to sporulation. Such results support the genetic model for the inheritance of the killer character in yeast already proposed (Somers & Bevan, 1969).During the course of these studies a correlation was found between the strengths of the killer phenotypes of diploid colonies and the proportions of killer spore cultures obtained after sporulation of their cells.

Somatic segregation of the killer (k) and neutral (n) cytoplasmic genetic determinants in yeast.

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The inheritance of the killer character in yeast.

Somatic segregation of the killer (k) and neutral (n) cytoplasmic genetic determinants in yeast.