Metatranscriptome Profiling of a Specialized Microbial Consortium during the Degradation of Nixtamalized Maize Pericarp.

Lignocellulose degradation by microbial consortia is multifactorial; hence, it must be analyzed from a holistic perspective. In this study, the temporal transcriptional activity of consortium PM-06, a nixtamalized maize pericarp (NMP) degrader, was determined and related to structural and physicochemical data to give insights into the mechanism used to degrade this substrate. Transcripts were described in terms of metabolic profile, carbohydrate-active enzyme (CAZyme) annotation, and taxonomic affiliation. The PM-06 gene expression pattern was closely related to the differential rates of degradation. The environmental and physiological conditions preceding high-degradation periods were crucial for CAZyme expression. The onset of degradation preceded the period with the highest degradation rate in the whole process, and in this time, several CAZymes were upregulated. Functional analysis of expressed CAZymes indicated that PM-06 overcomes NMP recalcitrance through modular enzymes operating at the proximity of the insoluble substrate. Increments in the diversity of expressed modular CAZymes occurred in the last stages of degradation where the substrate is more recalcitrant and environmental conditions are stressing. Taxonomic affiliation of CAZyme transcripts indicated that Paenibacillus macerans was fundamental for degradation. This microorganism established synergistic relationships with Bacillus thuringiensis for the degradation of cellulose and hemicellulose and with Microbacterium, Leifsonia, and Nocardia for the saccharification of oligosaccharides. IMPORTANCE Nixtamalized maize pericarp is an abundant residue of the tortilla industry. Consortium PM-06 efficiently degraded this substrate in 192 h. In this work, the temporal transcriptional profile of PM-06 was determined. Findings indicated that differential degradation rates are important sample selection criteria since they were closely related to the expression of carbohydrate-active enzymes (CAZymes). The initial times of degradation were crucial for the consumption of nixtamalized pericarp. A transcriptional profile at the onset of degradation is reported for the first time. Diverse CAZyme genes were rapidly transcribed after inoculation to produce different enzymes that participated in the stage with the highest degradation rate in the whole process. This study provides information about the regulation of gene expression and mechanisms used by PM-06 to overcome recalcitrance. These findings are useful in the design of processes and enzyme cocktails for the degradation of this abundant substrate.

Degradation profile of nixtamalized maize pericarp by the action of the microbial consortium PM-06.

The nixtamalized maize pericarp (NMP) is a plentiful by-product of the tortilla industry and an important source of fermentable sugars. The aim of this study was to describe the degradation profile of NMP by the action of a consortium (PM-06) obtained from the native microbial community of this residue. The degradation was analyzed in terms of the changes in the community dynamics, production of enzymes (endo-xylanase and endo-cellulase), physicochemical parameters, and substrate chemical and microstructural characteristics, to understand the mechanisms behind the process. The consortium PM-06 degraded 86.8 ± 3.3% of NMP after 192 h of growth. Scanning electron microscopy images, and the composition and weight of the residual solids, showed that degradation was sequential starting with the consumption of hemicellulose. Xylanase was the highest enzyme activity produced, with a maximum value of 12.45 ± 0.03 U mL-1. There were fluctuations in the pH during the NMP degradation, starting with the acidification of the culture media and finishing with a pH close to 8.5. The most abundant species in the consortium, at the moment of maximum degradation activity, were Aneurinibacillus migulanus, Paenibacillus macerans, Bacillus coagulans, Microbacterium sp. LCT-H2, and Bacillus thuringiensis. The diversity of PM-06 provided metabolic abilities that in combination helped to produce an efficient process. The consortium PM-06 generated a set of different tools that worked coordinated to increase the substrate availability through the solubilization of components and elimination of structural diffusion barriers. This is the first report about the degradation of NMP using a microbial consortium.