The Antimicrobial Effect of Nitric Oxide on the Bacteria That Cause Nosocomial Pneumonia in Mechanically Ventilated Patients in the Intensive Care Unit

BACKGROUND: Nosocomial pneumonia is the second most frequent nosocomial infection and the leading cause of death from hospital-acquired infection. Endogenously produced nitric oxide is an important component of the body's natural defense mechanism. Recent studies have demonstrated that exogenous gaseous nitric oxide (gNO) is bactericidal and that inhaled gNO is beneficial to bacterial clearance.

OBJECTIVE: Determine the antimicrobial effect of exogenous gNO in vitro against organisms from culture collections and pathogens derived from tracheal aspirates of mechanically ventilated patients with pneumonia in an intensive care unit.

METHODS: Using bacterial isolates in pure culture, a 0.5 McFarland standard (10⁸ colony-forming-units [cfu] per mL) was prepared and further diluted to 1:1,000 with saline, to 10⁵ cfu/mL. For each isolate tested, 3 mL was pipetted into each well of a 6-well plate, and placed in a specially designed incubator with compartments for both a treatment arm and a control arm. Both chambers received a continuous flow of heated, humidified gas. The treatment chamber had 200 ppm of gNO in the gas flow, which is higher than the clinically accepted concentration for gNO. Samples were drawn off at time intervals, plated onto Columbia agar base with 5% sheep blood, and placed in a traditional incubator at 35°C for a minimum of 24 h. All tests were performed in duplicate. The colony-forming units were visually counted to determine percentage kill.

RESULTS: There was total kill (100% of all colony-forming units) of each bacterial strain subjected to the test conditions at between 2 and 6 h of exposure to 200 ppm gNO.

CONCLUSION: gNO is bactericidal against various strains of bacteria suspended in saline, including both Gram-positive and Gram-negative organisms, and those that commonly cause nosocomial pneumonia in mechanically ventilated patients. Future work should focus on developing strategies that maximize the antimicrobial effect while minimizing the effect of these same interventions on host cells.