The effects of modified atmospheres on the growth of psychrotrophic pseudomonads on a surface in a model system

Abstract

Atmospheres containing concentrations of CO 2 as low as 20% (balance nitrogen) inhibited the growth of Pseudomonas fluorescens and Pseudomonas putida on the surface of buffered Brain Heart Infusion agar plates, pH 6.8, incubated at 5 or 15°C in flexible packages. The modified atmospheres decreased the growth rates and reduced the populations attained at the end of the exponential phase of growth, but had no substantial effect on the lag phase. P. fluorescens was less tolerant of CO 2 than P. putida. The inhibitory effect of CO 2 increased with its concentration and inhibition was greater at 5 than at 15°C. Growth occurred in packages flushed with 20, 40 and 100% CO 2 and 100% N 2 at 15°C and 20 and 40% CO 2 and 100% N 2 at 5°C. The residual O 2 concentration in teh packages after flushing was 0.2–0.5%. Storage of pseudomonads in CO 2 under conditions that prevented growth (e.g., 100% CO 2, 5°C) did not cause substantial loss of viability. There was no detectable residual effect of CO 2. If cultures were incubated in air after storage for up to 70 days in CO 2-containing atmospheres 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 stationary 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 reduction in the pH of the medium to 5.5 substantially increased the inhibitory effect of CO 2. At 5°C and pH 5.5, substantial growth of P. fluorescence was not observed in any of the CO 2 concentrations tested, nor in 40 or 100% CO 2 for P. putida.