THE EFFECTS OF MODIFIED ATMOSPHERES ON THE GROWTH OF PSYCHROTROPHIC PSEUDOMONADS ON A SURFACE IN A MODEL SYSTEM

Atmospheres containing concentrations of CO2 as low as 20% (balance ni trogen) inhibited the growth of Pseudomonas fluorescens and Pseudomona s putida on the surface of buffered Brain Heart Infusion agar plates, pH 6.8, incubated at 5 or 15 degrees C in flexible packages. The modif ied atmospheres decreased the growth rates and reduced the populations attained at the end of the exponential phase of growth, but had no su bstantial effect on the lag phase. P. fluorescens was less tolerant of CO2 than P. putida. The inhibitory effect of CO2 increased with its c oncentration and inhibition was greater at 5 than at 15 degrees C. Gro wth occurred in packages flushed with 20, 40 and 100% CO2 and 100% N-2 at 15 degrees C and 20 and 40% CO2 and 100% N-2 at 5 degrees C. The r esidual O-2 concentration in the packages after flushing was 0.2-0.5%. Storage of pseudomonads in CO2 under conditions that prevented growth (e.g., 100% CO2, 5 degrees C) did not cause substantial loss of viabi lity. There was no detectable residual effect of CO2. If cultures were incubated in air after storage for up to 70 days in CO2-containing at mospheres which prevented growth, the subsequent growth curve did not differ noticeably from that observed when plates were incubated in air immediately after inoculation. When cultures in the exponential or st ationary phases of growth in modified atmospheres were transferred to air, growth rates increased quickly to rates similar to those observed in air and the final populations observed in air were attained. A red uction in the pH of the medium to 5.5 substantially increased the inhi bitory effect of CO2. At 5 degrees C and pH 5.5, substantial growth of P. fluorescens was not observed in any of the CO2 concentrations test ed, nor in 40 or 100% CO2 for P. putida.