Bi-phasic growth of Listeria monocytogenes in chemically defined medium at low temperatures

Nikolaos A Tyrovouzis; Apostolos S Angelidis; Nikolaos G Stoforos

doi:10.1016/j.ijfoodmicro.2014.06.021

Bi-phasic growth of Listeria monocytogenes in chemically defined medium at low temperatures

Int J Food Microbiol. 2014 Sep 1:186:110-9. doi: 10.1016/j.ijfoodmicro.2014.06.021. Epub 2014 Jun 27.

Authors

Nikolaos A Tyrovouzis¹, Apostolos S Angelidis², Nikolaos G Stoforos³

Affiliations

¹ Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece.
² Laboratory of Milk Hygiene and Technology, Department of Food Hygiene and Technology, School of Veterinary Medicine, Faculty of Health Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece.
³ Department of Food Science and Human Nutrition, Agricultural University of Athens, 11855 Athens, Greece. Electronic address: stoforos@aua.gr.

PMID: 25016210
DOI: 10.1016/j.ijfoodmicro.2014.06.021

Abstract

The present work reports a novel observation regarding the growth of L. monocytogenes in modified Welshimer's broth (MWB) at low temperatures. Specifically, the direct monitoring of the growth of L. monocytogenes Scott A using plate count data revealed that the pathogen displays a bi-phasic growth pattern in MWB at 7 °C. This bi-phasic growth pattern is masked (not observed) when optical density (OD) measurements are used to monitor growth due to the inability of OD readings to detect L. monocytogenes population density increases up to 10(7) CFU/mL. This bi-phasic growth phenomenon was further investigated as a function of growth temperature (4 °C, 7 °C, 10 °C, 14 °C and 18 °C), medium composition (by altering the MWB composition by ten-fold increases in different sets of medium constituents), inoculum level (10(2), 10(3), 10(4), 10(5), 10(6), and 10(7) CFU/mL) and L. monocytogenes strain (10 strains). The growth of L. monocytogenes Scott A in MWB at 7 °C, 10 °C and 14 °C was consistently bi-phasic and independent of growth rate; at 18 °C, growth was consistently mono-phasic (single-phase, typical sigmoid growth curves), whereas no growth was observed at 4 °C. The tested modifications in the composition of MWB did not influence the bi-phasic nature of L. monocytogenes Scott A growth at 7 °C, and, overall, we could not point out any strain-, or serotype-specific effects. On the other hand, the initial inoculum level appears to affect the form of the growth curve, as there was a shift towards mono-phasic growth in trials with increasing initial inocula. A mathematical model, based on a stepwise response and described through two sequential sigmoid curves, was used to describe bi-phasic growth and estimate the kinetic parameters of L. monocytogenes growth. An alternative hypothesis, based on the assumption of the existence of two subpopulations, possessing different growth kinetics, materialized under the stress imposed on L. monocytogenes cells due to the combined effect of three factors (defined medium, low temperature and low initial inoculum) was also proposed and formulated.

Keywords: Inoculum level effects; Modeling; Modified Welshimer's broth; Refrigeration; Strain variation; Temperature effects.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Cold Temperature*
Colony Count, Microbial
Culture Media
Listeria monocytogenes / growth & development*
Models, Theoretical*
Species Specificity

Substances

Culture Media