An upset digestive system is also a disturbed habitat. Alongside the animal’s own tissues, the gut contains communities of bacteria and fungi. Some help process food or interact with the intestinal lining. Their surroundings can change during illness, and changes in the community might also affect the animal. Working out which comes first is part of the scientific problem.
Aoyun Li, Bingxian Liu and colleagues investigated that relationship in giraffes in a 2021 study in Frontiers in Microbiology. Instead of looking only for one infectious agent, they compared the bacterial and fungal communities of healthy giraffes with those of animals experiencing diarrhoea. Their work revealed a difference in the community as a whole, while leaving the direction of cause and effect open.
Twelve giraffes, two health states
The animals lived at Yichang zoo in China. All twelve were four years old, and each group contained three males and three females. Veterinarians assessed their condition before sampling; the six giraffes with diarrhoea had not received medicinal treatment before the samples were collected.
The researchers describe shared husbandry conditions and diets. That made the comparison more informative than comparing unrelated animals from zoos with very different feeding practices. It could not remove every difference between individuals, but age, sex balance and treatment history were less obvious alternative explanations for the result.
The study analysed twelve faecal samples. DNA markers allowed the team to recognise groups of bacteria and fungi without having to grow each organism in a laboratory. Similar bacterial marker sequences were grouped into microbial types; fungal markers supplied a separate view of the fungal community.
This approach describes what microbial DNA was present and its share of the recovered sequences. It does not directly measure what every organism was doing inside the giraffe, or establish that an organism detected in a dropping was causing disease. The animal sample also remained twelve giraffes, however many DNA sequences the equipment produced.
Fewer bacterial types is only one kind of change
There are several ways for two microbial communities to differ. One may contain fewer kinds of organisms. Another may contain a similar number of kinds but be dominated by different members. Those possibilities matter because a single word, “diversity”, can hide quite different biological results.
The giraffes with diarrhoea had lower estimated bacterial richness: fewer microbial types according to the study’s richness estimator. The group averages were approximately 740 compared with 835 in the healthy animals. These are estimates based on grouped DNA sequences, not a complete count of named bacterial species.
Two other bacterial measures, the Shannon and Simpson indices, take account of how sequences are distributed among the types as well as which types occur. Neither showed a statistically significant difference between the health groups. The supported finding is therefore more specific than a general collapse of bacterial diversity.
The identities and proportions of bacteria differed too. When samples were arranged according to the similarity of their communities, the healthy and diarrhoea groups separated. This was evidence that illness was associated with a different mixture of organisms, beyond the lower richness estimate.
Proportions need careful interpretation. If one microbial group occupies a larger share of the recovered DNA, its absolute number of cells need not have increased: other groups may have declined. The study identifies differences in relative composition, rather than measuring the total population of each microbe throughout the gut.
Fungi changed without becoming less diverse
The fungal results make the distinction especially clear. None of the fungal diversity measures differed significantly between the two groups. Yet the composition of their fungal communities did.
For example, Aspergillus and Microascus represented larger proportions in the diarrhoea group. Several other fungal groups were less represented. A community can retain a broadly similar level of diversity while its membership changes, just as two woodlands can contain similar numbers of tree species without containing the same trees.
That does not mean these fungi caused the giraffes’ symptoms. The researchers discuss organisms that can be associated with disease in other settings, but identifying a broad microbial group is different from demonstrating an infection by a particular strain. The fungal comparison supplies leads for further investigation; it does not turn each changed organism into a diagnosis.
By including fungi alongside bacteria, the team made this distinction visible. A bacterial-only account would have missed a second community whose composition differed even when its summary diversity measures did not.
Diet and illness are different windows into the gut
A separate study of seven giraffes’ rumen microbes, by Michael Roggenbuck and colleagues, found that community composition grouped according to recent feeding conditions. Three animals had recently eaten natural browse alone, while four continued a mixture including pellets, hay and browse. Our account of that dietary comparison explains the result.
That investigation sampled the rumen, the fore-stomach where much plant fermentation occurs. Li and colleagues examined faeces and a difference in health state. The two studies therefore concern different digestive samples and different comparisons. Together, they show why a microbial profile needs context: a changed community is not automatically a disease signature.
Following a community through recovery
The decisive missing information is time. Comparing sick and healthy animals on one occasion cannot establish whether the microbial differences developed before diarrhoea, followed it, or reflected another influence on both health and microbes. Even the absence of prior medication leaves those possibilities open.
Repeated samples from the same giraffes, collected as health changed, could test whether particular microbial shifts consistently appeared early or accompanied recovery. Feeding changes and veterinary treatment would need to be recorded, because either could affect the community. A repeat study at other institutions could then ask whether the same pattern held beyond one zoo.
The finding is a starting point for those comparisons: diarrhoea was associated with a different bacterial and fungal community, and the two groups of microbes did not change in identical ways. Understanding that pattern requires knowing which organisms change, what they do, and when the change occurs.
Sources
Aoyun Li, Bingxian Liu, Feiran Li, Yuanyuan He, Lei Wang, Muhammad Fakhar-e-Alam Kulyar, Huade Li, Yuhang Fu, Huaisen Zhu, Yaping Wang and Xiong Jiang. “Integrated Bacterial and Fungal Diversity Analysis Reveals the Gut Microbial Alterations in Diarrheic Giraffes.” Frontiers in Microbiology 12, 712092 (2021). Paper and full text.
Michael Roggenbuck and colleagues. “The giraffe (Giraffa camelopardalis) rumen microbiome.” FEMS Microbiology Ecology 90, 237–246 (2014). Paper and full text.
