Category Archives: reservoirs

Reservoirs of Salt Adapted Yersinia pestis

The Arab Maghreb is one of the most arid environments to host plague reservoirs. The most recent study on the area highlights the proximity of plague foci to salt water, either the Mediterranean Sea, Atlantic Ocean or importantly inland salt lakes (Malek et al, 2016). These inland salt springs, called chotts, are saltier than the ocean. They were specifically able to cultivate Y. pestis from high salt soil and isolate a high salt tolerant strain of Yersinia pestis from Algeria. Plague foci across North Africa were found at an average of 0.89 km from salt water, while the average distance from fresh water is 4.6 km.

Plague sites 1940-2015. Red = plague sites, Blue = salt water lakes, Yellow = fresh water. Cropped from Malek et al, 2016.

They also note the importance of L-form Yersinia pestis in their environmental samples. L-form bacteria are an understudied cell wall deficient state that quite a few bacteria, including Yersinia pestis, use for long term survival. The L-form of Y. pestis may be important in environmental persistence. Because they are believed to have a slower reproduction rate,  the L-form may also play a role in altering the molecular clock of some strains. To date,  publications that focus on L-form Y. pestis have been in either Russian or Chinese. It seems clear that the L-form is found in some instances in Asia as well. Importantly, some L-form bacteria can regain their cell wall and return to active ‘normal’ growth.

Soil osmolarity is the key feature that allows (or requires) the L-form to persist. Withstanding osmotic tensions is the primary role of the cell wall. Without the cell wall, the cell loses its ‘normal’ shape, taking on a spherical shape determined by hydrophobic-hydrophilic interactions (like oil and water). As the cell membrane is primarily made of phospholipid, its the L-form shape resembles a sturdy oil globule or a liposome (B below). This was apparent by gram stain when the normal individual short rod-shaped (coccobacilli) cells transformed into clusters of completely round (cocci) cells. This was confirmed under the electron microscope where the change is very apparent.


They also isolated a strain, Algeria3, a descendant of the third pandemic, from soil containing 4% salt, that can grow in a 15% salt broth. Other Algerian isolates that were not found in high salt soils experimentally survived as well in high salt media if the salt content was ramped up in a step-wise fashion. Growth in high salt conditions altered their protein production to increase those related to osmoregulation, metabolism, outer membrane proteins and others of unknown function.  Osmoregulation genes changes are a direct response to the higher salt concentration. The L-form cells are clearly still metabolically active.

Taken together these protein profiles suggest that it has adapted to survive in the salty soil with the ability to adjust its structure and function as necessary to persist.  They note that other plague reservoirs are in regions of the world with salt lakes or other salty sources, but more environmental sampling will be necessary to determine if this is a universal Y. pestis capability. This all has obviously important implications for plague ecology.


Malek, M. A., Bitam, I., Levasseur, A., Terras, J., Gaudart, J., Azza, S., et al. (2016). Yersinia pestis halotolerance illuminates plague reservoirs. Scientific Reports, 7, 1–10.

Plague Dialogues: Monica Green and Boris Schmid on Plague Phylogeny (II)

Monica H. Green (,@MonicaMedHist) is a historian of medieval medicine. An elected Fellow of the Medieval Academy of America, she teaches both global history and the global history of health. She was the editor in 2014 of Pandemic Disease in the Medieval World: Rethinking the Black Death, the inaugural issue of a new journal, The Medieval Globe.

Boris Schmid (@BorisVSchmid) is a theoretical biologist at the University of Oslo, Norway, and specializes in disease ecology and epidemiology. He recently described a link between climate fluctuations in medieval Central Asia and what looks like repeated introductions of plague into Europe’s harbors, a hypothesis that can be tested by the analysis of ancient DNA samples of Y. pestis. He works in a multidisciplinary team of theoreticians, archeologists, microbiologists and historians, led by Nils Chr. Stenseth.


In our previous blog post, Monica and I discussed how different lineages of plague – Yersinia pestis – collected their own genetic signature (SNP profile) as they diversified from a common ancestor. Monica also summarized in broad terms what ancient DNA samples of Y. pestis (extracted from plague victims) are now available from the initial Black Death outbreak and how they are related, using the latest plague studies of Haensch, Bos and Spyrou.

In this blog post Monica will delve into the nitty gritty details of these aDNA plague studies, and give an example of how to transform those details into a new understanding of the past rodent reservoirs and global mobility of plague, one of the deadliest diseases of our collective past. And I close the post by reflecting on the potential of aDNA to connect the fields of history and biology.


Thanks, Boris. It might be important to remind readers that we don’t have any aDNA evidence from past rodent populations yet. All the samples to date have been retrieved from human victims. But the SNP study that Seifert et al. published earlier this year from samples in Brandenburg, an inhumation from the time of the 30 Years War (1618-1648); the whole genome study that Bos and Herbig et al. also published this year, reporting on the samples from 18th-century Marseille; and the sample from Ellwangen included in the new study by Spyrou et al., all document that Branch 1A (see tree in our previous post) “focalized,” that is, it set up shop in some rodent population(s) and happily continued to proliferate for another 400 years. But all that happened, it is clear, separately from what was going on with Branch 1B, what I have taken to call the pestis secunda.

In their most recent study, the Tübingen/Jena team headed by Krause give us further insight into the early stages of Branch 1B. The beginning of Branch 1B was first documented in the 2011 London study, though it was only earlier this year that I realized that London sample 6330 likely dates from the 1360s and does not come from the initial Black Death outbreak. (It comes from a different burial ground, St Mary Graces.) In Bos and Herbig et al. 2016, it was reported that sample 6330 differed from the 1348-50 London Y. pestis genome by two SNPs. In the present study, interestingly, Spyrou et al. report something slightly different. Sample 6330 does indeed differ from the London Black Death genome by two SNPs (p3 and p4), but a third SNP in sample 6330 they are reporting here for the first time (p5) seems to be unique, a ‘G’ to ‘T’ switch at position 4,301,295 not found in any other historic genome or in the reference strain, CO92. (Spyrou et al. did not include London 6330 on their Table 1, so we offer a modified version of it here in fig. 3.)

CORRECTED fig03 for Contagions blog, (a) Spyrou et al 2016 fig02B Y pestis phylogenetic tree (detail of origins of Branches 1A and 1B), (b) Table 1 with 6330 SNPs added (06272016)
Fig. 3: (a) Detail of Spyrou et al. 2016, fig 2B: Yersinia pestis phylogeny – SNPs distinguishing Branches 1A and 1B; (b) Spyrou et al. 2016, Table 1, modified with data on London sample 6330 drawn from Spyrou et al. 2016, Table S4, SNP table. The SNPs unique to London 6330 and Bolgar City are highlighted in yellow.


First of all, we might say that those three SNPs are significant for the time gap they suggest between the Black Death and the 2nd wave of plague to hit London in 1361-63; for the sake of argument, we’ll say “21 years,” to use your formula (3 x 7 years), Boris. But think about the implications of that: plague arrives in London at the end of 1348 as a new disease, and a new strain (with 3 new SNPs) is causing a major new outbreak in 1361-63, which is when this burial seems to date from. 21 years have not passed since the previous outbreak. So what gives? Obviously, the “time-to-SNP” calculus we’re using is an average, not an absolute. But it does make us stop and wonder: did all this really happen so fast? And did it really happen in western Europe?

Which brings us to the Bolgar City sample. It, clearly, is a “descendant” of the same strain as London 6330: it has the two new SNPs, p3 and p4. (It doesn’t have that unique p5 SNP of London 6330, but that may have arisen as little as three days before this person died. We cannot attach any evolutionary significance to it until we see it documented somewhere else.) But note this: the Bolgar City strain has evolved further. It now has the p6 SNP that will define all the rest of Branch 1B and it has its own unique SNP, p7. And again, we have a problem of time compression: the Bolgar City sample (if we can trust the dating of the coins which were said to have been found with the body) may date as early as the late 1360s.

Remember what we need to have an outbreak in humans from a new lineage of Y. pestis: not simply does that new lineage have to arise from a single change in a single cell of Y. pestis, but that new SNP needs to proliferate enough in a reservoir rodent population to cause a new epidemic in humans. So looking over all these SNPs, p1-p7, we can see that they cluster into two “founder effect” phenomena: one that creates the initial Black Death lineage (Branch 1A) and one that creates the pestis secunda lineage (Branch 1B).

Where did those two lineage foundations happen? Let’s go back to Caffa, the “hurling bodies over the walls” scenario. Clearly, if we can believe that story (and remember, we have only one account of it, and that from a non-eye-witness), it tells of an already proliferating plague outbreak. By October 1346, Y. pestis was multiplying by the millions in rats and mice and rodents of whatever kind that lived in and around Caffa.

One, and only one, of those gazillions of Caffese offspring gave rise to Branch 1B. It, too, needed to find a place to set up shop and proliferate to make gazillions of (nearly) identical copies. And where was that place? Was it (as Krause seemed to imply in his April lecture) in London? Maybe it was in or near Bergen op Zoom (NL), where we find a sample with the same SNP profile as the London 6330 sample (Haensch et al. 2010)? Or was it near the same place where Branch 1A had already established its original home, before it reached the Black Sea? Haensch et al. had already proposed in 2010 a “northern” route for the introduction of the pestis secunda strain that reached the Netherlands. I’ll admit, I was skeptical for the longest time. But now I see that this possibility might bear more analysis. At the very least, the question shifts our focus away from western Europe and back to the areas around the Black and Caspian Seas. And that’s exactly where our Bolgar City sample is from, the one that is already showing two SNPs of further evolution beyond London 6330 but might not be a whole lot younger than it. As we said, jetting out of Heathrow wasn’t yet an option in the 14th century. But there was plenty of activity in these central Eurasian areas dominated by the Mongol Golden Horde to connect lots of rodent reservoirs to a bacterium looking for a new place to call home.


Thanks Monica! The amount of information that follows from a few different nucleotides between aDNA samples is quite amazing, and learning how to interpret this data historically is rightly one of the transformative processes now happening in Biology (and if I say so, in Medical History as well).

Monica’s interpretation of plague’s past mobility is based on the same genetic data as the one sketched out in Spyrou 2016, and highlights the challenge of interpreting ancient DNA, given that the ancient DNA sequences of plague are still so sparsely sampled across time and space. One thing that strikes me as especially important is how much the argument of “favor the most simple, parsimonious explanation” changes based on whether you think of plague largely in terms of a human epidemic (which Wagner 2014, and by extension Spyrou 2016 appear to do), or as a disease that spread through human and wildlife both, as Monica and I do. If you include the possibility of new wildlife reservoirs of plague (and plague has created numerous new wildlife reservoirs in time), say near Bolgar City, the logic of how plague moved across Eurasia changes.

As more aDNA data becomes available, it will be very interesting to see the geographic range that a lineage of plague bacteria can spread without collecting changes in its SNP profile. Once we have a good idea of that, and a more complete view of the SNP profiles that existed during the past pandemics, SNP profiles might be used to shed light on the actual source of a historic plague outbreak, and thus offer an independent way of checking the reliability of historic sources that blame particular smugglers, ships, refugees or clothing as the source of a plague outbreak.

Wow, thanks, Monica, for this great discussion. This is an example on how history and biology can intertwine, and while we are all waiting for more revelations from aDNA and historic sources, it seems prudent to start more interactions between historians and biologists. There is an inherent bias to doubt your own data too much, and trust another fields’ data too blindly, leading to mistakes at both sides: we blindly pick some historic report as authoritative, or put too much faith in a report on the (in-) efficiency of plague transmission by different flea species, whilst a single mutation that causes the loss of a gene can have drastic effects on how well the disease transmits (Hinnebusch, 2016). The only practical way to avoid falling into such pitfalls is by investing in cross-talk between scholars of the humanities and natural sciences!


Thank you, Boris. This was great. And very special thanks to Michelle Ziegler, for hosting our discussion on her super blog, Contagions.


Bos, K. I., Herbig, A., Sahl, J., Waglechner, N., Fourment, M., Forrest, S. A., et al. (2016). Eighteenth century Yersinia pestis genomes reveal the long-term persistence of an historical plague focus. eLife, 5, 17837.

Cui, Y., Yu, C., Yan, Y., Li, D., Li, Y., Jombart, T., et al. (2012). Historical variations in mutation rate in an epidemic pathogen, Yersinia pestis. Proceedings of the National Academy of Sciences, 110(2), 577-582.

Haensch, S., Bianucci, R., Signoli, M., Rajerison, M., Schultz, M., Kacki, S., et al. (2010). Distinct Clones of Yersinia pestis Caused the Black Death. PLoS Pathogens, 6(10), e1001134.

Hinnebusch, B. J., Chouikha, I., & Sun, Y.-C. (2016). Ecological Opportunity, Evolution, and the Emergence of Flea-borne Plague. Infection and Immunity, IAI.00188–16–31.

Krause, Johannes (4-12-2016)  Oral Presentation #S577:  Ancient pathogen genomics: what we learn from historic pandemics. European Congress  of Clinical Microbiology and Infectious Diseases

Seifert, L., Wiechmann, I., Harbeck, M., Thomas, A., Grupe, G., Projahn, M., et al. (2016). Genotyping Yersinia pestis in Historical Plague: Evidence for Long-Term Persistence of Y. pestis in Europe from the 14th to the 17th Century. PLoS ONE, 11(1), e0145194-8.

Spyrou, M. A., Tukhbatova, R. I., Feldman, M., Drath, J., Kacki, S., de Heredia, J. B., et al. (2016). Historical Y. pestisGenomes Reveal the European Black Death as the Source of Ancient and Modern Plague Pandemics. Cell Host and Microbe, 19(6), 874–881.

Wagner, D. M., Klunk, J., Harbeck, M., Devault, A., Waglechner, N., Sahl, J. W., et al. (2014). Yersinia pestis and the Plague of Justinian 541–543 AD: a genomic analysis. The Lancet Infectious Diseases, 14(4), 1–8.

Plague in Surat: 20 Years Later

I can’t let 2014 pass in a few weeks without mentioning that this fall was the twentieth anniversary of the plague outbreak in Surat, India — a major turning point in modern plague history and in the development of the (re)emerging infectious disease paradigm.

In the final accounting, 53 people died of plague, mostly pneumonic, but there are over 5000 cases classified as suspected and at least half a million people fled across India. Compared to other pneumonic plague outbreaks in Africa within the last twenty years, the number of deaths was small and the mortality rate tiny (1% of suspected cases). The government response was not only woefully inadequate but also exacerbated the damage within India and scared the rest of the world.

The lessons learned from Surat are really what is important.

  • The need for a national database to keep track of seemingly isolated cases and the need for surveillance of rodents, even when there haven’t been any human cases in many years. Better surveillance established since 1994 has identified several more plague outbreaks in India and enough evidence of enduring plague foci in the country.
  • The need for transparency, willingness to accept foreign help and the futility of trying to hide the epidemic from the press.
  • The costs of unsupported allegations of biological warfare or terrorism are too high to make unless there is certainty. It ultimately does not deflect responsibility away from the government for the response. The political costs for governments who make official erroneous allegations are greater than accepting responsibility for the outbreak.

In this month’s issue of the Indian Journal of Microbiology, the full genomic sequence of Yersinia pestis collected at Surat in 1994 and at a 2002 outbreak in India was released. Four samples were sequenced and they are all four different strains. Unfortunately, they did not do a phylogenetic analysis to indicate where they fit on the Y. pestis tree.

Twenty years ago it was the double hit of plague in Surat in 1994 and the discovery of antibiotic resistant plague in Madagascar in 1995 that raised concern about re-emerging infection diseases. Antibiotic resistant strains of Yersinia pestis have continued to appear in Madagascar and now insecticide resistant fleas are a problem as well. While public health processes and surveillance are better than in 1994, there has been no improvement plague incidence or concerning resistant strains.

Ebola is currently extracting the toll that was feared of plague in Surat two decades ago. If Surat was the warning that acute pandemics are still possible, Ebola is showing how far we still have to go 20 years later. Both plague in Surat and Ebola in 2014 are also reminding us that knowing what to do to stop an epidemic is not enough, execution is everything.

Further reading on Surat:

Barrett, Ron. (2008) “The 1994 Plague in Western India: Human Ecology and the Risks of Misattribution” p. 49-71 in Terrorism, War, or Disease? Unraveling the Use of Biological Weapons. Edited by A.L. Clunan, P.B. Lavoy, and S. B. Martin. Stanford Security Studies. Stanford University Press. This is the best analysis of the Surat outbreak that I have found.

Ziegler, Michelle (2014) The Black Death and the Future of the Plague. The Medieval Globe, 1: 183-199 for an overview of plague since 1994.

Mahale, K. N., Paranjape, P. S., Marathe, N. P., Dhotre, D. P., Chowdhury, S., Shetty, S. A., et al. (2014). Draft Genome Sequences of Yersinia pestis Strains from the 1994 Plague Epidemic of Surat and 2002 Shimla Outbreak in India. Indian Journal of Microbiology, 54(4), 480–482. doi:10.1007/s12088-014-0475-7

For more on antibiotic resistant plague, see this past post and on the most recent reported pneumonic plague outbreak in Madagascar.