Comparison of Growth of a Combined Strain Listeria monocytogenes Challenge Study Inoculum in Different Chloride Salt Solutions

Introduction:   Reduced sodium food formulations are a current focus of product development and redevelopment.  However, reduction of sodium in foods that are time and temperature controlled for safety may impact the growth of psychrotrophic bacterial pathogens, such as Listeria monocytogenes. Potassium chloride (KCl) is the most popular choice as a replacement for NaCl, but comparisons of these and other chloride salts against growth of L. monocytogenes are needed.

Purpose:   This study compared growth of L. monocytogenesin broth at equal chloride ion concentrations achieved using five different chloride salts and at unequal chloride ion concentrations made with 2% of each of the same five salts

Methods:   A five-strain culture of L. monocytogenes was inoculated at ca. 3 log CFU/ml into tryptic soy broth with 0.4% yeast extract (TSBYE) supplemented with CaCl₂, KCl, LiCl, MgCl₂, or NaCl at chloride equivalent concentrations standardized to 2% (w/v) NaCl, or in TSBYE with the same salts at a 2% (w/v) concentration.  Broths were incubated at 35 °C and populations were enumerated after 48 h by plating on modified Oxford medium (MOX) and tryptic soy agar with yeast extract (TSAYE).  Average log populations were compared with repeated measures ANOVA and Tukey’s test.

Results:   As determined by plating on TSAYE, populations of L. monocytogenes in equal chloride ion concentrations increased by 6.13, 5.86, 5.83, 5.56, and 5.22 log, for NaCl, MgCl₂, KCl, LiCl, and CaCl₂, respectively.  However, counts on MOX were significantly (P < 0.05) lower than TSAYE counts for NaCl, MgCl₂, and CaCl₂, indicating possible cellular stress.  TSAYE counts showed that LiCl at 2% (w/v) was significantly more suppressive to L. monocytogenes growth compared to other salts, allowing only a 5.19-log increase while NaCl and KCl permitted 6.21- and 6.18-log increases, respectively.  At 2%, CaCl₂similarly caused the greatest difference between TSAYE and MOX plate counts.

Significance:   The enhanced inhibition and apparent stress of L. monocytogenes caused by growing in the presence of CaCl₂ compared to the chloride equivalents of other salts should be explored further.  This work provides the foundation for further research on inhibition of the pathogen by chloride salts in combinations with other antimicrobials in food systems.