The study measures the plasma and bronchoalveolar lavage fluid concentrations of linezolid in septic shock patients comparing with non-septic shock patients to confirm the impact of septic shock on PK/PD of linezolid.

A wide array of pathophysiological changes occurring in severe sepsis and septic shock patients may influence antibiotics' pharmacokinetic (PK) properties. The pharmacokinetics and tissue distributions of linezolid achieving effective concentrations are key factors of successful clinical outcomes. To the best of our knowledge, no observational clinical studies are available on the plasma and bronchoalveolar lavage fluid concentrations of linezolid in septic shock patients. Severe pneumonia patients known or suspected to be caused by Gram-positive pathogens will be considered eligible for the study when the attending physician prescribed linezolid as treatment. The plasma and bronchoalveolar lavage fluid concentrations of linezolid are measured in each group at steady state. In an in vitro PK Model, concentration-time curve is drawn to calculate %T>MIC or AUC0-24h/MIC. Linezolid pharmacodynamics evaluation should be based on bacterial eradication and clinical outcomes.

Inclusion Criteria:

1. between 18 years old and 75 years old;
2. Admitted to the ICU;
3. Expectation, in the opinion of the investigator, that the patients' infection will require ICU stay more than 3 days;
4. Patients diagnosed as severe pneumonia requiring mechanical ventilatory support;
5. Severe pneumonia known or suspected to be caused by Gram-positive pathogens;
6. Expected to treat with linezolid.

Exclusion Criteria:

1. Unable to take bronchoalveolar lavage via bronchofiberscopy;
2. Allergy, hypersensitivity or a serious reaction to linezolid;
3. Treatment with linezolid during the previous 72 hours;
4. Pregnancy.

Patients diagnosed as severe pneumonia requiring mechanical ventilatory support.