Nosocomial infections have become a major health problem. They induced important use of antibiotics which is a preponderant factor for the development of bacterial resistance. The multi-resistance to antibiotics affects primarily Gram-negative bacteria. Some strains (as Acinetobacter or Pseudomonas) have become resistant to almost all antibiotics currently available, and the use of old molecules as Colistin may be the only alternative. However, there are few reliable data about Colistin and its PK characteristics. These data are essential to optimize its administration. The aim of the present study is to evaluate the pharmacokinetics of Colistin and its prodrug, colistimethate (CMS) after intravenous administration of Colistimethate alone or combined with inhaled Colistin in severely ill patients infected with multiresistant gram-negative bacteria

Inclusion Criteria:

• Age: 18 to 85 years
• Patients with nosocomial infection justifying Colistin.

Exclusion Criteria:

• Patients who received Colistin within 7 days prior to its inclusion, whatever the dosage or the route of administration
• Known hypersensitivity to Colistin or products of the polymyxin family
• Personal and family history for myasthenia
• Positive serology for HBV, HCV and HIV
• Positive pregnancy test or currently lactating