Validity of the time-to-threshold calibration at very low starting concentrations

Determine the validity and correct form of the calibration relation underpinning the time-to-threshold method for estimating initial transconjugant concentrations at very low starting concentrations (on the order of 10 cells per milliliter), and develop an appropriate analytical treatment if the linear calibration fails in this regime.

Background

The paper extends and analyzes the stochastic aspects of the time-to-threshold method used to quantify transconjugant concentrations. While a linear calibration between log initial concentration and time-to-threshold holds over the experimental range, simulations and theory indicate potential breakdown at very low starting concentrations, where stochastic growth effects dominate.

Clarifying the behavior of the calibration in this limit is important for accurate quantification in dilute samples and for defining the methodological limits of detection and inference.