Prime Healthcare Services, the troubled Claifornia-based for-profit hospital chain, announced yesterday it has purchased Saint Michael's Medical Center in Newark. This is the second New Jersey hospital Prime has acquired in the last four months, and is the latest in a series of acquisitions the company has made as they aggressively expand across the country. But concerns have been raised in other states over Prime's business practices because of pending investigations into alleged Medicare fraud and violations of patient confidentiality. The company has denied any wrongdoing.