title: 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.
authors.affiliation: CPMF(2) - Flemish Cluster Predictive Microbiology in Foods, Belgium(1); BioTeC - Chemical and Biochemical Process Technology and Control, Department of Chemical Engineering, KU Leuven, Leuven, Belgium. Electronic address: cindy.smet@cit.kuleuven.be.
authors.forename: Cindy
authors.initials: C
authors.lastname: Smet
authors.affiliation: BioTeC - Chemical and Biochemical Process Technology and Control, Department of Chemical Engineering, KU Leuven, Leuven, Belgium. Electronic address: eva.vanderlinden@cit.kuleuven.be.
authors.forename: Eva
authors.initials: E
authors.lastname: Van Derlinden
authors.affiliation: BioTeC - Chemical and Biochemical Process Technology and Control, Department of Chemical Engineering, KU Leuven, Leuven, Belgium. Electronic address: laurence.mertens@cit.kuleuven.be.
authors.forename: Laurence
authors.initials: L
authors.lastname: Mertens
authors.affiliation: CPMF(2) - Flemish Cluster Predictive Microbiology in Foods, Belgium(1); BioTeC - Chemical and Biochemical Process Technology and Control, Department of Chemical Engineering, KU Leuven, Leuven, Belgium. Electronic address: estefania.noriegafernandez@cit.kuleuven.be.
authors.forename: Estefanía
authors.initials: E
authors.lastname: Noriega
authors.affiliation: CPMF(2) - Flemish Cluster Predictive Microbiology in Foods, Belgium(1); BioTeC - Chemical and Biochemical Process Technology and Control, Department of Chemical Engineering, KU Leuven, Leuven, Belgium. Electronic address: jan.vanimpe@cit.kuleuven.be.
authors.forename: Jan F
authors.initials: JF
authors.lastname: Van Impe
mesh_terms: D018914:Cells, Immobilized; D015169:Colony Count, Microbial; D004926:Escherichia coli / Q000254:growth & development*; D005516:Food Microbiology; D012486:Salmonella typhimurium / Q000187:drug effects / Q000254:growth & development*; D013696:Temperature; D014867:Water / Q000378:metabolism
chemical_list: D014867:Water
keywords: Growth morphology; Immersed colony; Microbial interaction; Planktonic cells; Refrigeration temperature; Solid food structure; Surface colony