Metschnikowia pulcherrima and Related Pulcherrimin-Producing Yeasts: Fuzzy Species Boundaries and Complex Antimicrobial Antagonism

Abstract

Yeasts affiliated with the Metschnikowia pulcherrima clade (subclade) of the large ascomycetous genus Metschnikowia frequently turn out to produce the characteristic maroon-red pulcherrimin when tested for pigment production and prove to exert antagonistic effects on many types of microorganisms. The determination of the exact taxonomic position of the strains is hampered by the shortage of distinctive morphological and physiological properties of the species of the clade and the lack of rDNA barcode gaps. The rDNA repeats of the type strains of the species are not homogenized and are assumed to evolve by a birth-and-death mechanism combined with reticulation. The taxonomic division is further hampered by the incomplete biological (reproductive) isolation of the species: certain type strains can be hybridized and genome sequencing revealed chimeric genome structures in certain strains that might have evolved from interspecies hybrids (alloploid genome duplication). Various mechanisms have been proposed for the antimicrobial antagonism. One is related to pulcherrimin production. The diffusible precursor of pulcherrimin, the pulcherriminic acid is secreted by the cells into the environment where it forms the insoluble pulcherrimin with the ferric ions. The lack of free iron caused by the immobilization of ferric ions inhibits the growth of many microorganisms. Recent results of research into the complexity of the taxonomic division of the pulcherrimin-producing Metschnikowia yeasts and the mechanism(s) underlying their antimicrobial antagonism are discussed in this review.

Keywords: Metschnikowia; antagonism; barcodes; bioprotection; genome; iron-depletion; pulcherrimin; reticulation; taxonomy.

Figure 1

Pigment production and antagonism. (A) Pigmented halos around the colonies of the Metschnikowia strain 02.11.1.21. (B) Inhibition zone around the colony of the same strain (1: Botrytis cinerea 3318, 2: Saccharomyces cerevisiae S288c, 3: Metschnikowia 02.11.1.21). (C) Coincidence of pigmented halo and inhibition zone around the colony of Metschnikowia 02.11.1.21 on a medium supplemented with 0.005 mg/mL FeCl₃ and flooded with conidia of B. cinerea 3318. (D) Inhibition of rotting caused by Botrytis (1: untreated, 2: dipped in a suspension Metschnikowia 02.11.1.21 cells, 3: dipped in a mixed suspension of B. cinerea 3318 conidia and Metschnikowia 02.11.1.21 cells, 4: dipped in a suspension of B. cinerea 3318 conidia). See reference [6] for the description of the strains.

Figure 2

A phylogenetic tree derived from the neighbor-joining analysis of the cloned D1/D2 sequences of the type strains of six pulcherrimin-producing Metschnikowia species and the D1/D2 sequences of the type strains available in databases. Outgroup: Candida (Metschnikowia) picachoensis (Cpicach). Type-strain sequences are highlighted by arrows. GenBank accession numbers are listed in the supplementary Table S1.

Figure 3

Processes implicated in the antimicrobial antagonism of strains of the M. pulcherrima clade.

Figure 4

General scheme of pulcherrimin biosynthesis in bacteria and yeasts. Gene names are in italics. For the explanation of symbols and the references, see Section 3.1.

Figure 5

Hyphal lysis and inhibition of germination of Botrytis conidia in the inhibition zones surrounding Metschnikowia colonies. (A and B) Lysing and dying hyphal tips. (C) Inhibition of the germination of conidia. 1: Metschnikowia colony; 2: conidia.