Inclusion Criteria:

• Male or female subjects aged 18 years or older
• Isolation of one of the following multi-antibiotic resistant bacteria: vancomycin-resistant Enterococcus faecium, vancomycin-resistant Enterococcus faecalis, or methicillin-resistant Staphylococcus aureus, alone or as part of a polymicrobial infection
• Have a confirmed diagnosis of a serious infection (eg, bacteremia [unless due to an excluded infection], complicated intra-abdominal infection, complicated skin and skin structure infection, or pneumonia) requiring administration of intravenous (IV) antibiotic therapy

Exclusion Criteria:

• Subjects with any concomitant condition or taking any concomitant medication that, in the opinion of the investigator, could preclude an evaluation of a response or make it unlikely that the contemplated course of therapy or follow-up assessment will be completed or that will substantially increase the risk associated with the subject's participation in this study
• Anticipated length of antibiotic therapy less than 7 days
• For subjects with VRE, known or suspected hypersensitivity to tigecycline or linezolid, or other compounds related to these classes of antibacterial agents (eg, oxazolidinones, tetracyclines, minocycline, doxycycline). For subjects with MRSA, known or suspected hypersensitivity to tigecycline or vancomycin, or other compounds related to these classes of antibacterial agents (eg, tetracyclines, minocycline, doxycycline)