WASHINGTON, May 19 -- Giant pandas eat almost nothing but bamboo, but the endangered species appears not to have evolved a gut microbiota to efficiently digest bamboo, a Chinese study said Tuesday.

These bamboo-eating animals actually harbor a carnivore-like gut microbiota predominated by bacteria such as Escherichia/ Shigella and Streptococcus, according to the study published in the online U.S. journal mBio.

"Unlike other plant-eating animals that have successfully evolved, anatomically specialized digestive systems to efficiently deconstruct fibrous plant matter, the giant panda still retains a gastrointestinal tract typical of carnivores," said lead study author Zhihe Zhang, director of the Chengdu Research Base of Giant Panda Breeding, in southwest China's Sichuan Province, in a statement. "The animals also do not have the genes for plant- digesting enzymes in their own genome. This combined scenario may have increased their risk for extinction."

Giant pandas evolved from bears that ate both plants and meat, and started eating bamboo exclusively about two million years ago, the researchers said.

The animals spend up to 14 hours daily consuming up to 12.5 kilograms of bamboo leaves and stems but can digest only about 17 percent of them and their feces is mainly composed of undigested bamboo fragments.

In this study, the researchers used a laboratory technique called 16S rRNA sequencing to evaluate 121 fecal samples from 45 giant pandas living in Zhang's Research Base, including 24 adults, 16 juveniles and five unweaned cubs. Samples were obtained during the spring, summer and late autumn of one year.

They found that despite their diet, these giant pandas, together with nine captive and seven wild individuals investigated previously, showed extremely low gut microbiota diversity and an overall structure that diverged from non-panda plant-eaters but was similar to carnivorous and omnivorous bears.

The giant panda gut did not harbor plant-degrading bacteria such as Ruminococcaceae and Bacteroides that are typically enriched in other herbivores, but instead was predominated by Escherichia/Shigella and Streptococcus.

Panda gut microbiota also varied by season, with late autumn being quite different from spring and summer. The researchers said the lack of bamboo shoots in late autumn could be an important factor.

"This result is unexpected and quite interesting, because it implies the giant panda's gut microbiota may not have well adapted to its unique diet, and places pandas at an evolutionary dilemma," added study coauthor Xiaoyan Pang, an associate professor in the School of Life Sciences and Biotechnology at Shanghai Jiao Tong University.