Merck & Co Inc said it would buy Cubist Pharmaceuticals Inc for $8.4 billion plus assumption of debt, giving the major drugmaker an entry into the market for drugs that target so-called superbugs.

Merck and British rival AstraZeneca Plc have turned their attention to newer kinds of antibiotics that attack superbugs - strains of bacteria that are resistant to several types of antibiotics - after the 2013 threat report from the U.S. Centers for Disease Control and Prevention.

The CDC estimated that more than 2 million people in the United States are sickened every year by such infections, with at least 23,000 dying as a result.

Merck said on Monday that the deal, which will give it access to Cubist's antibiotic Cubicin, is expected to add more than $1 billion to revenue in 2015 after closing in the first quarter.

Cubist's third-quarter sales rose 16 percent, driven by strong sales of Cubicin.

Cubist's lead drug in development, Ceftolozane/Tazobactam, is widely expected to win marketing approval from the U.S. Food and Drug Administration later this month as a treatment for complicated urinary tract infections.

Merck will pay $102 per share for Cubist, a premium of 37 percent to the Lexington, Massachussetts-based company's closing share price of $74.36 on Friday.

The deal includes assumption of $1.1 billion in debt.

Cubist shares were trading at $101.15 premarket.

The New York Times, citing people briefed on the matter, first reported the deal on Friday.

The Cubist deal is Merck's second big acquisition this year. The company bought IdenixPharmaceuticals for $3.85 billion in June to boost its hepatitis C drug portfolio.