New evidence against kissing toads

Toads on the road, on the sidewalk, on the doorstep, and even on other toads! It can seem like they are everywhere in the spring after emerging from hibernation to travel to their breeding ponds – males often riding atop an accessible female. Researchers in Switzerland studied the European toad (Bufo bufo) to test an old paradigm explaining the spatial distribution of microorganisms in the environment (1). Back in 1934, Dutch microbiologist and botanist Lourens Baas Becking proposed that in microbial biogeography, “everything is everywhere, but the environment selects”. (2)

The researchers wondered whether the theory applies to the so-called Chlamydia-like organisms (CLOs), families of obligate intracellular bacteria usually isolated from environmental sources. These bacteria live in a range of hosts – from amoeba to fish and amphibians, to reptiles and terrestrial animals to humans – and include emerging pathogens relevant to human health. But studies have been limited, because it turns out they don’t like to grow in convenient model systems. (3)

The team tested B. bufo tadpoles collected from 41 different ponds in the Geneva area for CLOs and then looked for distribution patterns and environmental associations in the results. They analyzed DNA from the tadpole tails using a qPCR assay that amplifies the 16S ribosomal RNA gene from the entire Chlamydiales order. Samples with threshold cycle values (Ct) less than 35 were sequenced, so the researchers could identify which CLO families were present.

Of the tadpoles analyzed, 38.7% tested positive, and 36 of the 41 ponds sampled contained CLO-positive tadpoles. But there were no patterns in CLO occurrence or any relationship with the type of land cover or the density of people living near the ponds. The results support Baas Becking’s hypothesis, confirming that CLOs are distributed randomly and ubiquitously.

In addition to discovering B. bufo as a new animal host for CLO bacteria, the researchers found particular CLO families in the tadpoles that might be relevant to the conservation of other toad species threatened by disease. (4) They also identified two CLO genera that are recognized as emerging pathogens in fish, cats, and humans, (3) but they doubt that the bacteria’s presence in the city ponds is any concern to public health. Although some fairy tales talk of kissing frogs and toads (5), in light of these new findings, perhaps there is a better way to find a prince.


  1. Vajana E., Widmer I., Rochat E., Duruz S., Selmoni O., Vuilleumier S., Aeby S., Greub G., Joost S. (2019) Indication of spatially random occurrence of Chlamydia-like organisms in Bufo bufo tadpoles from ponds located in the Geneva metropolitan area. New Microbe and New Infect. 27:54–63.
  2. Baas-Becking, L.G.M. (1934) Geobiologie of inleiding tot de milieukunde. The Hague, the Netherlands: W.P. Van Stockum & Zoon.
  3. Taylor-Brown A., Vaughan L., Greub G., Timms P., Polkinghorne A. (2015) Twenty years of research into Chlamydia-like organisms: a revolution in our understanding of the biology and pathogenicity of members of the phylum Chlamydiae. Pathog Dis. 73:1–15.
  4. Bates K.A., Clare F.C., O’Hanlon S., Bosch J., Brookes L., Hopkins K. (2018) Amphibian chytridiomycosis outbreak dynamics are linked with host skin bacterial community structure. Nat Commun. 9:693.
  5. Heiner, H.A. (2010) The frog prince and other frog tales from around the world: fairy tales, fables and folklore about frogs. CreateSpace Independent Publishing Platform. ISBN 9781453626573.


Photo: European toads in De Maasduinen National Park, by Sebastian Kaiser