ReviewPenicillium roqueforti: an overview of its genetics, physiology, metabolism and biotechnological applications
Introduction
Penicillium roqueforti is a saprophytic fungus, found in natural environments (forest soil and wood), although its natural reservoir remains unknown. Fungal colonies usually reach 40–70 mm within 14 d on Czapek yeast extract agar (CYA) and malt extract agar (MEA) media, and present a typical blue–green color associated with conidia (asexual spore) production. Noteworthy, high macroscopic variability has been observed for strains of this species (Gillot et al., 2015). On PDA medium, colony color and texture as well as margin width can vary (Fig 1). Color ranges from light to dark greenish gray and can include gray, yellowish and olive green shades; while colony texture can vary from velvety to fasciculate. Margin size can also largely vary according to the considered strain, going from very thin up to representing one third of colony diameter. Exudates, as liquid droplets, over the colony surface have also been reported. The mycelium harbors a dense and velutinous felt of conidiophores with stipes (100–200 μm long) with characteristic rough walls. Terminal penicilli that include the phialides (spore producing cells) are typically terverticillate while quaterverticillate, and more rarely biverticillate, can be observed. Flask-shaped phialides produce loose columns of spherical, smooth and dark green conidia with a diameter ranging from 3 to 4.5 μm. P. roqueforti is a well-known food and feed spoiler (i.e. grated cheeses, rye bread, silage …) (Samson et al., 2010), but is best known for its technological use for blue-veined cheese production and ripening worldwide. In this context, it is used as an adjunct culture for the production of many emblematic cheeses (e.g. Roquefort, Fourme d'Ambert and Bleus in France, Cabrales and Valdeón in Spain, Danablu in Denmark, Gorgonzola in Italy, Stilton and Dorset Blue in the UK, Niva in Czech Republic, Bleuchâtel in Switzerland). Blue-veined cheese is produced from different milks and its manufacture follows specific process steps leading to distinctive organoleptic traits. Noteworthy, blue-veined cheese production has been recorded for centuries (i.e. Italian Gorgonzola (879), French Roquefort (1070), English Stilton (1785) and Danish Danablu (1870s)), but their production is thought to be much older (Dumay, 1982). This has raised questions about possible domestication footprints in P. roqueforti. This species is also well known for its ability to produce a wide range of secondary metabolites, some of them being of interest in the medical field (e.g. mycophenolic acid and potentially andrastin A) and it has even been hypothesized to be linked to the “French paradox” (Petyaev and Bashmakov, 2012), a term generated in 1992 based on epidemiological data from French people who had a low incidence of coronary diseases despite the consumption of high saturated fat meals (Renaud and De Lorgeril, 1992). During the past decade, several studies have been performed on this species on different aspects (taxonomy, population genetics, genetic diversity, physiology and metabolism) making it one of the best, if not the best studied, food-associated mold.
Section snippets
Taxonomy of P. roqueforti
Penicillium roqueforti belongs, within the Penicillium genus, to the Roquefortorum section that also includes the P. carneum, P. paneum, P. psychrosexualis (Houbraken et al., 2016) and P. mediterraneum (Guevara-Suarez et al., 2020) species. While the three first species were separated based on different extrolite productions and β-tubulin sequence polymorphisms (Boysen et al., 1996, Samson et al., 2004), P. psychrosexualis was included in this section based on a unique phylogenetic positioning
Impact of abiotic factors
P. roqueforti has very specific physiological traits that explain its ecology, and especially its ability to colonize and develop in blue-veined cheese (Chávez et al., 2011). While the impact of abiotic factors has been studied for a long time, one of the most recent advances corresponds to the use of mathematical models to describe P. roqueforti behavior (growth, conidial germination and metabolism) according to variations of the considered abiotic factor (e.g. temperature, pH, water activity -
Proteolytic and lipolytic activities
P. roqueforti enzymes have been widely studied and, in particular, proteases and lipases due to their crucial role during the cheese making process (Cantor et al., 2004). Concerning proteolytic activities, P. roqueforti produces several enzymes. Two extracellular proteases, an aspartyl protease and a metalloprotease have been described (Cerning et al., 1987). Moreover, 2 extracellular peptidases, an alkaline metalloaminopeptidase, an acid carboxypeptidase, as well as several intracellular
Biotechnological potential
Obviously, as previously stated, the main use of P. roqueforti as a biotechnological agent corresponds to its use as an adjunct culture for the production of emblematic blue-cheeses in Europe (e.g. Roquefort, Fourme d'Ambert, Bleu des Causses and Bleu de Gex in France, Gorgonzola in Italy, Cabrales and Picón de Valdeón in Spain, Stilton in the UK). This usage is actually historical as, as previously stated, mentions of blue cheese have been found in the literature from 879 for Gorgonzola, 962
Conclusion
Due in particular to its use as an adjunct culture for the production of emblematic blue-veined cheeses around the world, P. roqueforti is among the most studied, if not the most, food-associated mold. Major advances in research on different aspects have been recently done on this species. They clarify its phylogenetic position, revealed subpopulations associated with domestication events and adaptive traits. These studies have also provided a better understanding of its physiology and
Conflict of interest
The authors have no conflicts of interest to declare.
Acknowledgments
This work was funded by the French National Research Agency -ANR- in the framework of the FUNGADAPT project (ANR-19-CE20-0002). The authors are thankful to the editor and the reviewers for their in-depth analyses of the manuscript and their constructive suggestions.
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