Notable fibrolytic enzyme production by Aspergillus spp. isolates from the gastrointestinal tract of beef cattle fed in lignified pastures

Abstract

Fungi have the ability to degrade vegetal cell wall carbohydrates, and their presence in the digestive tract of ruminants can minimize the effects of lignified forage on ruminal fermentation. Here, we evaluated enzyme production by Aspergillus spp. isolates from the digestive tracts of cattle grazed in tropical pastures during the dry season. Filamentous fungi were isolated from rumen and feces by culture in cellulose-based medium. Ninety fungal strains were isolated and identified by rDNA sequence analysis, microculture, or both. Aspergillus terreus was the most frequently isolated species, followed by Aspergillus fumigatus. The isolates were characterized with respect to their cellulolytic, xylanolytic, and lignolytic activity through qualitative evaluation in culture medium containing a specific corresponding carbon source. Carboxymethyl cellulase (CMCase) activity was quantified by the reducing sugar method. In the avicel and xilan degradation test, the enzyme activity (EA) at 48 h was significantly higher other periods (P < 0.05). Intra- and inter-specific differences in EA were verified, and high levels of phenoloxidases, which are crucial for lignin degradation, were observed in 28.9% of the isolates. Aspergillus terreus showed significantly higher EA for avicelase (3.96 ±1.77) and xylanase (3.13 ±.091) than the other Aspergillus species at 48 h of incubation. Isolates AT13 and AF69 showed the highest CMCase specific activity (54.84 and 33.03 U mg-1 protein, respectively). Selected Aspergillus spp. isolates produced remarkable levels of enzymes involved in vegetal cell wall degradation, suggesting their potential as antimicrobial additives or probiotics in ruminant diets.

Fig 1. Detection rate of cellulolytic fungi (%) in samples obtained from the gastrointestinal tract of beef cattle that grazed on tropical lignified pastures.

Lowercase letters indicate significant difference between organ origins and uppercase letters indicate significant difference between categories as determined by χ² test with a 5% significance cutoff.

Fig 2. Distribution of Aspergillus species (%) from the rumen and rectal ampulla of cattle that grazed on Brachiaria sp. during the dry season.

Fig 3. Avicel degradation at 48 h by Aspergillus terreus isolated from cow rumen fluid.

Fig 4. Linear increase of the avicelase activity for five selected Aspergillus terreus isolates from rumen fluid collected from Nellore cows.

Avicelase activity = hydrolysis zone diameter/colony diameter.