Reducing the copies of one gene in the bubonic plague bacterium, Yersinia pestis, made it less deadly but potentially more transmissible. A small genetic change makes the bacterium that caused the plague less fatal but possibly more transmissible, allowing for greater disease spread in smaller populations, a study in Science reports.
The bacterium Yersinia pestis caused the Black Death — killing up to 50 million people in the mid-fourteenth century — as well as an earlier plague across the Mediterranean in the sixth century ad . The bacteria still circulate in low levels in parts of the United States, Africa and Asia, and are typically transmitted to humans by infected fleas carried by rats or other rodents.
Previous research found that some strains of Y. pestis had reduced amounts of pla, a gene associated with disease severity, but it was unclear why, says Ravneet Sidhu, a palaeogeneticist at McMaster University in Hamilton, Canada, and a co-author of the latest work. “Our findings in this study characterize a case in which a pandemic-causing pathogen has independently evolved to cause what we believe is a slightly weaker form of the disease,” she says.