The bacterium Yersinia pestis has been held responsible for taking lives of around 200 million or even more people. It has led to plagues in the 6th and 14th centuries. But as per researchers, genetic changes have happened owing to which the germ became so dangerous.

Researchers said on Tuesday that the bacterium Yersinia pestis has undergone minor genetic changes many centuries ago and that has turned it from milk to murderous. It has killed an estimated 200 million or more people.

In human history, Yersinia pestis has caused two pandemics. Firstly, in the 6th century Justinian Plague and in 14th century, Black death. Rats having fleas carrying the germ passed on the plague to people.

Study researchers carried out mouse experiment and retracted the genetic change in the bacterium. The researchers took an ancestral form of the bacterium and inserted into a gene called Pla, which is involved in breaking down blood clots.

The addition made in the gene long ago made Yersinia pestis, a pathogen that made a mild gastrointestinal infection, to the one that was responsible for the fatal respiratory disease known as pneumonic plague.

The researchers also found that a single mutation of the same led the bacterium to spread in the body and invade the lymph nodes. Study's lead researcher Wyndham Lathem of Northwestern University's Feinberg School of Medicine in Chicago said that the addition of the Pla gene might have taken place in more than 1,500 years ago.

Lathem said, "That's something to keep in mind when we're studying other bacterial pathogens, A small change is all that's needed and suddenly we may be faced with a new pandemic of some sort".