Determining biomass in solid state fermentation cultures of aspergillus terreus during lovastatin production

Abstract

Lovastatin, a class of secondary metabolite typically produced by Aspergillus terreus strains, has become the focus of great attention due to its ability to block the de novo synthesis of endogenous cholesterol, which is nearly 2/3 of human total body cholesterol. Additionally, statins present potential applications for the treatment cancer, Parkinson and Alzheimer diseases, as well as viral and fungal infections, because of their capability to inhibit mevalonate derivatives biosynthesis. High lovastatin production yields may be microbiologically obtained by implementing culture systems in solid state. Since biomass levels are closely related to lovastatin productivity, appropriate methods for the determination of fungal growth are usually required. However, one of the main drawbacks is that the direct analysis of biomass by dry weight determination is not possible in solid state fermentation systems. Therefore, this work was aimed at comparing two indirect methods for biomass determination: one based on the N-acetylglucosamine content, a component of the fungal cell wall, and the other one, based on the ergosterol content, a fungal cell membrane steroid. N-acetylglucosamine was assessed by the colorimetric method with MBTHreagent, after sample acid hydrolysis. Extracted ergosterol, previous sample saponification, was analyzed by RP-HPLC with a photodiode array detection (PDA)system. A high correlation coefficient (r2: 0.98) was found between fungal biomass dry weight and the ergosterol content as determined by HPLC (PDA),confirming the validity and reliability of this standardized technique. Additionally, the ergosterol method showed to be highly reproducible and time-efficient.On the other hand, the obtained results according to the N-acetylglucosamine method denoted that this technique would not be applicable to any solid culturesystem due to the false positive reactions (interference) observed for abiotic controls in complex systems with certain solid substrates.