Extended Data Fig. 4: Population-level observations of growth and expansion by confocal microscopy.
From: Chemotaxis as a navigation strategy to boost range expansion
Densities of bacteria growing in soft agar were determined at various times using confocal microscopy and fluorescently labelled cells; see Supplementary Text 1.4. a, Calibration of fluorescence intensity. Known numbers of cells were transferred from a batch culture to a fresh but cold soft-agar plate. After agar solidification (<10 min), intensity was measured. Fitted line gives relation between observed intensity (fluorescence integrated along the agar thickness) and cell density measured in batch culture (OD600). b, Example of experiment to obtain growth rates in agar. Data are for strain HE274 (wild-type) grown in 40 mM glycerol and 200 μM aspartate. A small number of cells was mixed uniformly into fresh soft-agar plates. After agar solidification (<10 min), fluorescence intensity was observed over time. c, Derived growth curves in soft agar based on experiments as in b. Typically, there is a fast growth regime followed by a slower regime related to oxygen consumption and limitation for OD > 0.1: with oxygen running out, cells accumulate towards the agar surface and growth becomes slower. In this work, the population was always kept in the first aerobic regime. Growth rates in the first regime (coloured lines, Supplementary Table 2) were obtained by an exponential fit of the data and are comparable to those obtained in batch culture (inset). In b, c the experiment was conducted once. d, Photograph and spatiotemporal density profiles (linear intensity scale) for population expansion under reference conditions (glycerol + 100 μM aspartate; same data as in Figs. 1a, 2b). Scale bar, 2 cm. The confocal observations under reference conditions were repeated twice with similar results. e–i, Spatiotemporal density profiles (logarithmic density scale) for population expansion under different conditions, similar to those observed for the reference conditions (glycerol + 100 μM aspartate; d). Conditions are glycerol + 100 μM serine (e), glycerol + 100 μM aspartate + 100 μM serine (f), glycerol + 0.05% CAA (g), 1% tryptone broth (h), and glucose + 100 μM aspartate (i), all with strain HE274 (wild type). Colour scale bar applies to all panels. In e–i, the experiments were conducted once; expansion speeds were highly comparable to those measured manually.
