Detection of loading effects in bacterial stress response using biochemical stochasticity

Bacteria have the ability to sense environmental stress signals and translate them into the appropriate response necessary for survival. One recurring mechanism used for this are two-component systems composed of membrane-bound sensory proteins and response regulator proteins. The regulator proteins are transcription factors that activate a set of genes necessary to trigger the global response leading to phenotypic switching. Measuring the activity of these regulators is essential to the understanding of the stress response and is typically done through the insertion of non-native fluorescent reporter proteins. Introduction of these reporters inside the cell can cause loading as they interfere with the native cell response and regulator activity. In this paper we use mathematical modeling and analysis of a typical response mechanism to show how loading can bias the measurements and cause misidentification of important characteristics of the stress-induced response, such as its qualitative nature and the thresholds that cause phenotypic switching. We establish an approach for determining the presence of loading effects using single cell measurements and biochemical stochasticity of the reporter mechanism. The fluorescence measurements of the loaded system are shown to have different noise characteristics compared to unloaded systems, with larger loads creating more significant differences. The approach depends only on knowing the structural properties of the response model and the load, and it does not rely on the identification of model parameter values.