RESUMO
Polyalcohols such as arabitol are among the main targets of biorefineries aiming to upcycle wastes and cheap substrates. In previous works Wickerhamomyces anomalus WC 1501 emerged as an excellent arabitol producer utilizing glycerol. Arabitol production by this strain is not growth associated, therefore, in this study, pre-grown cells were entrapped in calcium alginate beads (AB) and utilized for glycerol transformation to arabitol. Flasks experiments aimed to assess the medium composition (i.e., the concentration of inorganic and organic nitrogen sources and phosphates) and to establish the appropriate carrier-to-medium proportion. In flasks, under the best conditions of ammonium limitation and the carrier:medium ratio of 1:3 (w/v), 82.7 g/L glycerol were consumed in 168 h, yielding 31.2 g/L arabitol, with a conversion of 38% and volumetric productivity of 186 mg/mL/h. The process with immobilized cells was transferred to laboratory scale bioreactors with different configurations: stirred tank (STR), packed bed (PBR), fluidized bed (FBR), and airlift (ALR) bioreactors. The STR experienced oxygen limitation due to the need to maintain low stirring to preserve AB integrity and performed worse than flasks. Limitations in diffusion and mass transfer of oxygen and/or nutrients characterized also the PBR and the FBR and were partially relieved only in ALR, where 89.4 g/L glycerol were consumed in 168 h, yielding 38.1 g/L arabitol, with a conversion of 42% and volumetric productivity of 227 mg/mL/h. When the ALR was supplied with successive pulses of concentrated glycerol to replenish the glycerol as it was being consumed, 117 g/L arabitol were generated in 500 h, consuming a total of 285 g/L glycerol, with a 41% and 234 mg/L/h. The study strongly supports the potential of W. anomalus WC 1501 for efficient glycerol-to-arabitol conversion using immobilized cells. While the yeast shows promise by remaining viable and active for extended periods, further optimization is required, especially regarding mixing and oxygenation. Improving the stability of the immobilization process is also crucial for reusing pre-grown cells in multiple cycles, reducing dead times, biomass production costs, and enhancing the economic feasibility of the process.
RESUMO
Indole and p-cresol are precursors of the most important uremic toxins, generated from the fermentation of amino acids tryptophan and tyrosine by the proteolytic community of intestinal bacteria. The present study focused on the relationship between the microbiome composition, the fecal levels of indole and p-cresol, and their kinetics of generation/degradation in fecal cultures. The concentration of indole and p-cresol, the volatilome, the dry weight, and the amount of ammonium and carbohydrates were analyzed in the feces of 10 healthy adults. Indole and p-cresol widely differed among samples, laying in the range of 1.0-19.5 µg/g and 1.2-173.4 µg/g, respectively. Higher fecal levels of indole and p-cresol were associated with lower carbohydrates and higher ammonium levels, that are markers of a more pronounced intestinal proteolytic metabolism. Positive relationship was observed also with the dry/wet weight ratio, indicator of prolonged intestinal retention of feces. p-cresol and indole presented a statistically significant negative correlation with OTUs of uncultured Bacteroidetes and Firmicutes, the former belonging to Bacteroides and the latter to the families Butyricicoccaceae (genus Butyricicoccus), Monoglobaceae (genus Monoglobus), Lachnospiraceae (genera Faecalibacterium, Roseburia, and Eubacterium ventriosum group). The kinetics of formation and/or degradation of indole and p-cresol was investigated in fecal slurries, supplemented with the precursor amino acids tryptophan and tyrosine in strict anaerobiosis. The presence of the precursors bursted indole production but had a lower effect on the rate of p-cresol formation. On the other hand, supplementation with indole reduced the net rate of formation. The taxa that positively correlated with fecal levels of uremic toxins presented a positive correlation also with p-cresol generation rate in biotransformation experiments. Moreover other bacterial groups were positively correlated with generation rate of p-cresol and indole, further expanding the range of taxa associated to production of p-cresol (Bacteroides, Alistipes, Eubacterium xylanophylum, and Barnesiella) and indole (e.g., Bacteroides, Ruminococcus torques, Balutia, Dialister, Butyricicoccus). The information herein presented contributes to disclose the relationships between microbiota composition and the production of uremic toxins, that could provide the basis for probiotic intervention on the gut microbiota, aimed to prevent the onset, hamper the progression, and alleviate the impact of nephropaties.