Diversity of acid stress resistant variants of Listeria monocytogenes and the potential role of ribosomal protein S21 encoded by rpsU

Abstract

The dynamic response of microorganisms to environmental conditions depends on the behavior of individual cells within the population. Adverse environments can select for stable stress resistant subpopulations. In this study, we aimed to get more insight in the diversity within Listeria monocytogenes LO28 populations, and the genetic basis for the increased resistance of stable resistant fractions isolated after acid exposure. Phenotypic cluster analysis of 23 variants resulted in three clusters and four individual variants and revealed multiple-stress resistance, with both unique and overlapping features related to stress resistance, growth, motility, biofilm formation, and virulence indicators. A higher glutamate decarboxylase activity correlated with increased acid resistance. Whole genome sequencing revealed mutations in rpsU, encoding ribosomal protein S21 in the largest phenotypic cluster, while mutations in ctsR, which were previously shown to be responsible for increased resistance of heat and high hydrostatic pressure resistant variants, were not found in the acid resistant variants. This underlined that large population diversity exists within one L. monocytogenes strain and that different adverse conditions drive selection for different variants. The finding that acid stress selects for rpsU variants provides potential insights in the mechanisms underlying population diversity of L. monocytogenes.

Modeling and Validation of the Ecological Behavior of Wild-Type Listeria monocytogenes and Stress-Resistant Variants

“…In a large fraction of the acid stress-selected variants, a mutation in rpsU, encoding ribosomal protein S21, appeared to be responsible for the stress-resistant phenotype (12). An interesting observation in all these variants is that increased stress resistance seems to be at the cost of another feature.…”

“…monocytogenes stress-resistant variants have been shown to comprise a wide range of phenotypic features that are different from those of the main wild-type (WT) population. Apart from resistance toward the selection stress, numerous variants showed a multiple-stress-resistant phenotype (12,13). It was shown that different types of stress lead to selection for different types of variants.…”

“…It was shown that different types of stress lead to selection for different types of variants. Although many phenotypic features are overlapping, the genetic basis for the increased resistance was shown to be partially selection stress dependent (12).A large fraction of the heat-and HHP-selected variants had a mutation in the class III heat shock repressor gene ctsR (11, 13), while this mutation was not found in the acid stress-selected variants. In a large fraction of the acid stress-selected variants, a mutation in rpsU, encoding ribosomal protein S21, appeared to be responsible for the stress-resistant phenotype (12).…”

“…It was shown that different types of stress lead to selection for different types of variants. Although many phenotypic features are overlapping, the genetic basis for the increased resistance was shown to be partially selection stress dependent (12).…”

Impact of microbial variability on food safety and quality

“…Although the variability in T min between variants is larger than strain variability, this does not mean that the population diversity is more important than strain variability. The  max of most variants, especially when they were grown at 7C, was lower than that of the wild type (Metselaar et al, 2015). These slower growing variants, if present in a population, might be easily overgrown by the wild type, and therefore have a limited impact on the final concentration of the microorganism in food products.…”