Wet bottom: koalas decimated by bacteria

Chlamydia infections have been decimating Australian populations of koalas for several years. But other bacteria could be involved.


“Wet bottom” counts as one of the most serious threats hanging over koalas. Although the name may appear harmless, in reality it is a fatal disease which has already decimated numerous marsupial populations. It is now urgent to understand this disease better, if there is to be any hope of saving the species.

Chlamydia exonerated

Reddened wet bottoms, cystitis and urinary incontinence: such are the symptoms of “wet bottom”, which kills the animals affected in only a few months. The research conducted at the beginning of the epidemic discovered the culprit rapidly: Chlamydia pecorum (whose presence is also responsible for a drop in the diversity of the urogenital microbiome). This bacterium is however different from the Chlamydia responsible for sexually transmitted infections (STI) in humans, and unfortunately the antibiotics used in human medicine are ineffective against C. pecorum. Studies published in 2015 revitalized research into this disease. In fact, analyses of a large sample of koalas showed that many were not carriers of C. pecorum although they presented with a moderate form of wet bottom disease. The scientists then suggested that other microbes were involved in the disease. To verify this, they used samples taken in 2011 from 10 female koalas free from C. pecorum (half did not present any clinical sign of the disease) living on an island near Melbourne, French Island. They then analyzed the microbial DNA extracted from these smears and compared the species observed in the two groups.

A “microbial signature” in the sick koalas?

In total, 254 species were identified. Only 112 were present in both the healthy and sick animals, and 75 were only present in the koalas with wet bottom disease. In the two groups, the family present in the greatest numbers was that of the Aerococcaceae, with the most prevalent genus being Aerococcus, which was however significantly less present in the sick koalas. Conversely, the Tissierellaceae family was more prevalent in sick animals. These results therefore confirm that “wet bottom” is accompanied by a modification of the urogenital microbiome in female koalas, even in the absence of Chlamydia. The authors emphasize the importance of continuing this research in other populations of koalas in order to specify the bacterial groups which could be involved in this disease.



Legione AR, Amery-Gale J, Lynch M, Haynes L, Gilkerson JR, Sansom FM, Devlin, JM. Variation in the microbiome of the urogenital tract of Chlamydia-free female koalas (Phascolarctos cinereus) with and without 'wet bottom'. PLoS One. 2018 Mar 26;13(3):e0194881. doi: 10.1371/journal.pone.0194881. eCollection 2018