Masked man Bubonic plague doctor, head profile, with bird mask and hat. Vintage style. Biohazard concept.

(© lenaivanova2311 -

UNIVERSITY PARK, Pa. — The Black Death, also known as the Second Plague Pandemic, eradicated up to 60 percent of Europe’s population in the mid-14th century. Now, researchers say this plague might be responsible for our love of junk food today. Their study finds this devastating event not only altered history’s course but may also have led to changes in the survivors’ diet and hygiene — influencing the human oral microbiome’s composition.

Moreover, researchers from Penn State University and the University of Adelaide believe this shift is contributing to chronic diseases in present-day society.

“Modern microbiomes are linked to a wide range of chronic diseases, including obesity, cardiovascular disease and poor mental health,” says corresponding author Laura Weyrich, an associate professor of anthropology at Penn State, in a university release.

“Uncovering the origins of these microbial communities may help in understanding and managing these diseases.”

Prof. Weyrich explains that dietary changes over time are believed to have influenced the evolution of the oral microbiome. However, the historical trajectory of human oral microbiomes within a single population has seldom been directly examined.

Weyrich points out that some studies have used the microbiomes of contemporary indigenous people, who adhere to traditional subsistence lifestyles, as a stand-in for the microbiomes of pre-industrial populations. Yet, the researchers caution that this approach may be flawed, as the microbes in modern non-industrialized populations might not accurately reflect those found in the ancestors of industrialized societies.

“This research places unnecessary responsibilities and obligations on Indigenous communities to participate in microbiome research, where the benefits of these studies may not directly serve Indigenous peoples,” Weyrich explains.

Prof. Weyrich suggests that a more precise and ethically responsible approach to studying the evolution of the oral microbiome is to directly examine the calcified dental plaque, known as calculus, from the ancestors of industrialized people.

In the most extensive study of its kind, Weyrich and her team analyzed dental calculus from 235 individuals buried across 27 archaeological sites in England and Scotland, spanning from approximately 2200 BC to 1853 AD. To minimize contamination, they processed the samples in an ultra-sterile, ancient DNA laboratory.

The team identified 954 microbial species, categorizing them into two distinct bacterial communities: one dominated by the genus Streptococcus, common in modern industrialized humans, and the other by the genus Methanobrevibacter, now largely extinct in healthy industrialized populations.

Their research revealed that almost 11 percent of the variation in the microbiome species composition over time, including the impact of the Second Plague Pandemic, could be attributed to changes across eras.

“We know that survivors of the Second Plague Pandemic earned higher incomes and could afford higher-calorie foods,” Weyrich continues. “It’s possible that the pandemic triggered changes in people’s diets that, in turn, influenced the composition of their oral microbiomes.”

Woman eating doughnuts, junk food
(© Drobot Dean –

To investigate whether dietary changes influenced the rise of the Streptococcus group and the decline of the Methanobrevibacter group, the team compiled a list of functional differences among the bacteria in both groups, particularly those linked to dietary fiber, carbohydrate, and lactose metabolism.

The researchers discovered that the bacteria in the Streptococcus-dominated group exhibited more traits associated with low-fiber, high-carbohydrate diets, and dairy consumption — characteristics of modern Western diets. In contrast, the Methanobrevibacter-dominated group lacked traits associated with dairy and sugar consumption, which were prevalent in some ancient human diets.

Additionally, they found that the Streptococcus group was linked to periodontal disease, characterized by infections and inflammation of the gums and bones around the teeth. This condition can lead to bacteria entering the bloodstream through gum tissue, potentially causing respiratory disease, rheumatoid arthritis, coronary artery disease, and complications in diabetes.

Conversely, the Methanobrevibacter group was associated with skeletal pathologies.

“Our research suggests that modern-day oral microbiomes may reflect past changes in diet, resulting from the Second Plague Pandemic,” Weyrich concludes. “Importantly, this work helps to inform our understanding of modern-day chronic, noncommunicable diseases.”

The findings are published in the journal Nature Microbiology.

South West News Service writer Stephen Beech contributed to this report.

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