Dec 15 (Reuters) - Merck & Co Inc isapproaching banks about an $8 billion, 364-day bridge loan thatwill back the drugmaker's plans to acquire CubistPharmaceuticals Inc, sources told Thomson Reuters LPC.

JP Morgan and Deutsche Bank are leading the deal.

The loan has a drawn cost of LIB+75 if Merck keeps itscurrent rating of AA/A2, sources said.

Further details were not available by press time.

Merck announced December 8 that it would buy Cubist for $9.5billion. The transaction will include $1.1 billion of net debt.

The Cubist purchase will give Merck access to antibioticCubicin, Reuters said. Cubist also has a drug in development forcomplicated urinary tract infections.

The deal signals that large pharmaceutical companies areturning their attention back to antibiotics after decades of lowinvestment, according to Reuters.

The Cubist acquisition is Merck's second-largest acquisitionthis year. The company bought Idenix Pharmaceuticals for $3.85billion in June. (Editing By Michelle Sierra and Jon Methven)