Evolutionary trade-off between heat shock resistance, growth at high temperature, and virulence expression in Salmonella Typhimurium.

Understanding the evolutionary dynamics of foodborne pathogens throughout our food production chain is of utmost importance. In this study, we reveal that Salmonella Typhimurium can readily and reproducibly acquire vastly increased heat shock resistance upon repeated exposure to heat shock. Counterintuitively, this boost in heat shock resistance was invariantly acquired through loss-of-function mutations in the dnaJ gene, encoding a heat shock protein that acts as a molecular co-chaperone of DnaK and enables its role in protein folding and disaggregation. As a trade-off, however, the acquisition of heat shock resistance inevitably led to attenuated growth at 37°C and higher temperatures. Interestingly, loss of DnaJ also downregulated the activity of the master virulence regulator HilD, thereby lowering the fraction of virulence-expressing cells within the population and attenuating virulence in mice. By connecting heat shock resistance evolution to attenuation of HilD activity, our results confirm the complex interplay between stress resistance and virulence in Salmonella Typhimurium. IMPORTANCE: Bacterial pathogens such as Salmonella Typhimurium are equipped with both stress response and virulence features in order to navigate across a variety of complex inhospitable environments that range from food-processing plants up to the gastrointestinal tract of its animal host. In this context, however, it remains obscure whether and how adaptation to one environment would obstruct fitness in another. In this study, we reveal that severe heat stress counterintuitively, but invariantly, led to the selection of S. Typhimurium mutants that are compromised in the activity of the DnaJ heat shock protein. While these mutants obtained massively increased heat resistance, their virulence became greatly attenuated. Our observations, therefore, reveal a delicate balance between optimal tuning of stress response and virulence features in bacterial pathogens.

Protein aggregates act as a deterministic disruptor during bacterial cell size homeostasis.

Mechanisms underlying deviant cell size fluctuations among clonal bacterial siblings are generally considered to be cryptic and stochastic in nature. However, by scrutinizing heat-stressed populations of the model bacterium Escherichia coli, we uncovered the existence of a deterministic asymmetry in cell division that is caused by the presence of intracellular protein aggregates (PAs). While these structures typically locate at the cell pole and segregate asymmetrically among daughter cells, we now show that the presence of a polar PA consistently causes a more distal off-center positioning of the FtsZ division septum. The resulting increased length of PA-inheriting siblings persists over multiple generations and could be observed in both E. coli and Bacillus subtilis populations. Closer investigation suggests that a PA can physically perturb the nucleoid structure, which subsequently leads to asymmetric septation.

Gene Erosion Can Lead to Gain-of-Function Alleles That Contribute to Bacterial Fitness.

Despite our extensive knowledge of the genetic regulation of heat shock proteins (HSPs), the evolutionary routes that allow bacteria to adaptively tune their HSP levels and corresponding proteostatic robustness have been explored less. In this report, directed evolution experiments using the Escherichia coli model system unexpectedly revealed that seemingly random single mutations in its tnaA gene can confer significant heat resistance. Closer examination, however, indicated that these mutations create folding-deficient and aggregation-prone TnaA variants that in turn can endogenously and preemptively trigger HSP expression to cause heat resistance. These findings, importantly, demonstrate that even erosive mutations with disruptive effects on protein structure and functionality can still yield true gain-of-function alleles with a selective advantage in adaptive evolution.