Degeneration of penicillin titre in cultures of Penicillium chrysogenum

Rapid Phenotypic and Metabolomic Domestication of Wild Penicillium Molds on Cheese

Fermented foods provide novel ecological opportunities for natural populations of microbes to evolve through successive recolonization of resource-rich substrates. Comparative genomic data have reconstructed the evolutionary histories of microbes adapted to food environments, but experimental studies directly demonstrating the process of domestication are lacking for most fermented food microbes. Here, we show that during adaptation to cheese, phenotypic and metabolomic traits of wild Penicillium molds rapidly change to produce domesticated phenotypes with properties similar to those of the industrial cultures used to make Camembert and other bloomy rind cheeses. Over a period of just a few weeks, populations of wild Penicillium strains serially passaged on cheese had reduced pigment, spore, and mycotoxin production. Domesticated strains also had a striking change in volatile metabolite production, shifting from production of earthy or musty volatile compounds (e.g., geosmin) to fatty and cheesy volatiles (e.g., 2-nonanone, 2-undecanone). RNA sequencing demonstrated a significant decrease in expression of 356 genes in domesticated strains, with an enrichment of many secondary metabolite production pathways in these downregulated genes. By manipulating the presence of neighboring microbial species and overall resource availability, we demonstrate that the limited competition and high nutrient availability of the cheese environment promote rapid trait evolution of Penicillium molds.IMPORTANCE Industrial cultures of filamentous fungi are used to add unique aesthetics and flavors to cheeses and other microbial foods. How these microbes adapted to live in food environments is generally unknown as most microbial domestication is unintentional. Our work demonstrates that wild molds closely related to the starter culture Penicillium camemberti can readily lose traits and quickly shift toward producing desirable aroma compounds. In addition to experimentally demonstrating a putative domestication pathway for P. camemberti, our work suggests that wild Penicillium isolates could be rapidly domesticated to produce new flavors and aesthetics in fermented foods.

Sexual reproduction as the cause of heat resistance in the food spoilage fungus Byssochlamys spectabilis (anamorph Paecilomyces variotii)

Paecilomyces variotii is a common cosmopolitan species that is able to spoil various food- and feedstuffs and is frequently encountered in heat-treated products. However, isolates from heat-treated products rarely form ascospores. In this study we examined by using molecular techniques and mating tests whether this species can undergo a sexual cycle and form ascospores. The population structure of this species was examined by analyzing the nuclear ribosomal internal transcribed spacer 1 (ITS1) and ITS2 and the 5.8S rRNA gene, as well as partial beta-tubulin, actin, and calmodulin gene sequences. Phylogenetic analyses revealed that P. variotii is a highly variable species. Partition homogeneity tests revealed that P. variotii has a recombining population structure. In addition to sequence analyses, mating experiments indicated that P. variotii is able to form ascomata and ascospores in culture in a heterothallic manner. The distribution of MAT1-1 and MAT1-2 genes showed a 1:1 ratio in the progeny of the mating experiments. From the sequence analyses and mating data we conclude that P. variotii is the anamorph of Talaromyces spectabilis and that it has a biallelic heterothallic mating system. Since Paecilomyces sensu stricto anamorphs group within Byssochlamys, a new combination Byssochlamys spectabilis is proposed.

An unstable strain of Aspergillus foetidus segregating proline auxotrophs

Two basic colony types have been obtained through single conidial isolation from the Bode strain of Aspergillus foetidus as well as from mutants of this unstable strain. Type I is prototrophic whereas type II is an auxotroph requiring proline. When a type I strain is grown on complex medium it gradually becomes overwhelmed by type II sectors of growth. However, essentially pure cultures of type I can be maintained on minimal medium (lacking proline). The yield of glucoamylase from type II cultures is less than half that obtained with type I cultures. The instability of type I cultures when grown on complex medium can not be explained by heterokaryosis or the presence of virus-like particles found in the original Bode strain and its derivatives. The isolation of five stable prototrophic strains obtained as more rapidly growing sectors from type I subcultures grown on complex medium suggests that the instability most probably results from a duplicated chromosomal segment or other chromosomal aberration analogous to those described in A. nidulans.