Screening and evaluation of filamentous fungi potential for protease production in swine plasma and red blood cells-based media: qualitative and quantitative methods

Many fungi excrete proteases to the medium when stimulated. We aimed to submit filamentous fungi cultures to a screening so as to evaluate their proteolytic capacity. Spore solution of these fungi was inoculated in the center of plates, each one containing medium composed of dehydrated UHT milk, plasma or red blood cells. We qualitatively evaluated the enzyme index (EI) measuring the ratio of hydrolysis halo to mycelium diameters within 22 cultures of Penicillium, Aspergillus, Cenococcum, Cochliobolus, and Rhizopu genera. Proteolysis halo has occurred in 14 of the 22 evaluated strains (64%) using a traditional medium with milk. We observed 5 strains (23%) and 8 strains (36%) expressing proteolytic activity by the formation of proteolysis halo in the medium containing plasma and red blood cells, respectively. Regarding the EI, the pure strains of A. brasiliensis, P. citrinum, A. niger, A. rhizopodus as well as 3 strains of Penicillium sp. stood out. The cultures which presented the most promising results were used in solid-state fermentation in a medium composed of malt residue, plasma, and red blood cells. The enzyme activity was between 150 and 2,383 U.mL(-1). Overall, the qualitative method is a viable alternative to select stimulated cultures to produce protease by the presence of the protein sources. Additionally, plasma or red blood cells are promising for the high activity proteases production by different genera and species of fungi.