科研人员发现了帮助大熊猫以竹子为食茁壮成长的消化道微生物。
拥有类似于食肉动物的胃肠道的大熊猫的饮食习惯长久以来让科研人员感到困惑。
大熊猫是一种杂食性的熊，每天消耗至多12公斤竹子，它缺乏帮助草食动物消化纤维素和半纤维素（它们是纤维性植物饮食的主要成分）的酶。
Fuwen Wei及其同事使用了基因测序技术来发现生活在大熊猫消化道中的微生物，希望发现消化纤维素的共生体——而此前的尝试没能发现它们。
这组作者分析了5000多个核糖体RNA序列，以此作为见于野生和捕获的大熊猫的粪样本中发现的微生物的遗传条码，然后与那些见于食草动物的序列进行了比较。
这组作者报告说，7种在分类学上与梭菌有密切关系的细菌是熊猫粪独有的—梭菌已知能消化纤维素。
此外，这组作者还发现了消化纤维素和半纤维素的酶的推定的基因序列。
这组作者这说，再加上诸如伪拇指、强壮的牙齿、咀嚼肌和丰富的消化道黏液等其他适应，消化纤维素的肠道细菌可能有助于维持熊猫的在肉食动物中不寻常的饮食习惯。
doi:10.1073/pnas.1017956108
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Evidence of cellulose metabolism by the giant panda gut microbiome
Lifeng Zhu, Qi Wu, Jiayin Dai, Shanning Zhang, and Fuwen Wei
The giant panda genome codes for all necessary enzymes associated with a carnivorous digestive system but lacks genes for enzymes needed to digest cellulose, the principal component of their bamboo diet. It has been posited that this iconic species must therefore possess microbial symbionts capable of metabolizing cellulose, but these symbionts have remained undetected. Here we examined 5,522 prokaryotic ribosomal RNA gene sequences in wild and captive giant panda fecal samples. We found lower species richness of the panda microbiome than of mammalian microbiomes for herbivores and nonherbivorous carnivores. We detected 13 operational taxonomic units closely related to Clostridium groups I and XIVa, both of which contain taxa known to digest cellulose. Seven of these 13 operational taxonomic units were unique to pandas compared with other mammals. Metagenomic analysis using ∼37-Mbp contig sequences from gut microbes recovered putative genes coding two cellulose-digesting enzymes and one hemicellulose-digesting enzyme, cellulase, β-glucosidase, and xylan 1,4-β-xylosidase, in Clostridium group I. Comparing glycoside hydrolase profiles of pandas with those of herbivores and omnivores, we found a moderate abundance of oligosaccharide-degrading enzymes for pandas (36%), close to that for humans (37%), and the lowest abundance of cellulases and endohemicellulases (2%), which may reflect low digestibility of cellulose and hemicellulose in the panda's unique bamboo diet. The presence of putative cellulose-digesting microbes, in combination with adaptations related to feeding, physiology, and morphology, show that giant pandas have evolved a number of traits to overcome the anatomical and physiological challenge of digesting a diet high in fibrous matter.
