Effect of cell immobilization on the growth dynamics of Salmonella Typhimurium and Escherichia coli at suboptimal temperatures

Abstract

Predictive microbiology has recently acknowledged the impact of the solid(like) food structure on microbial behavior. The presence of this solid(like) structure causes microorganisms to grow as colonies and no longer planktonically as in liquid. In this paper, the growth dynamics of Salmonella Typhimurium and Escherichia coli were studied as a function of temperature, considering different growth morphologies, i.e., (i) planktonic cells, (ii) immersed colonies and (iii) surface colonies. For all three growth morphologies, both microorganisms were grown in petri dishes. While E. coli was grown under optimal pH and water activity (aw), for S. Typhimurium pH and aw were adapted to 5.5 and 0.990. In order to mimic a solid(like) environment, 5% (w/v) gelatin was added. All petri dishes were incubated under static conditions at temperatures in the range [8.0°C-22.0°C]. Cell density was determined via viable plate counting. This work demonstrates that the growth morphology (planktonic vs. colony) has a negligible effect on the growth dynamics as a function of temperature. The observation of almost equal growth rates for planktonic cultures and colonies is in contrast to literature where, mostly, a difference is observed, i.e., μplanktonic cells≥μimmersed colonies≥μsurface colonies. This difference might be due to shaking of the liquid culture in these studies, which results in a nutrient and oxygen rich environment, in contrast to the diffusion-limited gel system. Experiments also indicate that lag phases for solid(like) systems are similar to those for the planktonic cultures, as can be found in literature for similar growth conditions. Considering the maximum cell density, no clear trend was deducted for either of the microorganisms. This study indicates that the growth parameters in the suboptimal temperature range do not depend on the growth morphology. For the considered experimental conditions, models previously developed for liquid environments can be used for solid(like) systems.

Influence of food intrinsic complexity on Listeria monocytogenes growth in/on vacuum-packed model systems at suboptimal temperatures

“…However, in this study, L. monocytogenes growth between the two systems was similar, neglecting the influence of the growth morphology. Similar microbial dynamics in liquids and on aqueous gels have also been observed in literature (Smet et al, 2015). Nevertheless, growth parameters will be discussed one by one.…”

“…This observation indicates that stressing temperature favours planktonic microbial growth, although at more optimal temperature levels microbial growth on solid surfaces is more favourable, as also reported in Smet et al (2015). It has been reported that μ max in liquid systems is higher than μ max on solid surfaces Meldrum et al, 2003;Theys et al, 2008).…”

“…19 has been found by other studies (Robins and Wilson, 1994;Smet et al, 2015). In early stationary phase, bacteria influence their own environment, with their metabolites production, which inhibits growth (Robins and Wilson, 1994).…”

“…It has been reported that μ max in liquid systems is higher than μ max on solid surfaces Meldrum et al, 2003;Theys et al, 2008). Significant differences at μ max values between these two systems were induced when liquid systems were continuously shaken during incubation (Smet et al, 2015).…”

“…When liquid cultures are shaken, nutrients, metabolites and oxygen are homogeneously transported and cell sedimentation is avoided, which also influences the distribution of the previously mentioned compounds (Smet et al, 2015). Therefore, when liquid systems are incubated without shaking exhibit negligible differences in μ max values or even lower than μ max values on solid surfaces.…”