In March, Spirit Airlines came out of bankruptcy protection in less than four months and entered a worsening landscape. Consumers were holding off booking flights and U.S. planes were awash in empty seats. Even the most profitable airlines cut the rosy financial forecasts they had issued at the start of the year.

But Spirit, an airline with bright yellow planes that has become synonymous with budget travel in the U.S., now appears on even shakier ground. Last week, five months after getting out of bankruptcy, Spirit warned it might not be able to survive a year without more cash and that its credit card processor was seeking more collateral.

On Thursday, Spirit said it borrowed the entire $275 million available under its revolver. It also reached a two-year extension on

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