Atmospheric analysis and redox potentials of culture media in the GasPak System

Oxygen and carbon dioxide concentrations, internal atmospheric pressure, catalyst temperature, and time of appearance of water condensate were monitored for various time intervals at ambient (20 to 25 degrees C) temperature in a GasPak 100 Anaerobic System (BBL Microbiology Systems, Cockeysville, Md). Simultaneously, the redox potential (Eh) of various plated culture media in the system was also measured. The oxygen concentration was reduced to less than 0.4% in 100 min. The Eh of the media, corrected for hydrogen ion, reached -100 mV within 60 to 100 min, and the carbon dioxide concentration increased to between 4 and 7% in 60 min, depending on the number of plates of media present. Condensate appeared generally between 10 and 15 min, and the temperature of the lid reached a maximum between 20 and 40 min. Condensate time and lid temperature increase are important early indicators of a correctly functioning GasPak System. A characteristic pressure-vacuum-pressure profile is produced as a result of controlled release of hydrogen and carbon dioxide gases and the reaction of hydrogen and oxygen to produce water. Anaerobic conditions were achieved well before the methylene blue anaerobic indicator became decolorized, which required more than 6 h at 20 to 25 degrees C. At this time the Eh of media in the jar was well below -200 mV. Since the indicator is reduced within 5 h at 35 degrees C, the Eh of media in the jar would also be expected to decrease more rapidly at the higher temperature.