Abstract
Biological experiments performed on living bacterial colonies have demonstrated the microbial capability to develop finger-like shapes and highly irregular contours, even starting from an homogeneous inoculum. In this work, we study from the continuum mechanics viewpoint the emergence of such branched morphologies in an initially circular colony expanding on the top of a Petri dish coated with agar. The bacterial colony expansion, based on either a source term, representing volumetric mitotic processes, or a nonconvective mass flux, describing chemotactic expansion, is modeled at the continuum scale. We demonstrate that the front of the colony is always linearly unstable, having similar dispersion curves to the ones characterizing branching instabilities. We also perform finite element simulations, which not only prove the emergence of branching, but also highlight dramatic differences between the two mechanisms of colony expansion in the nonlinear regime. Furthermore, the proposed combination of analytical and numerical analysis allowed studying the influence of different model parameters on the selection of specific patterns. A very good agreement has been found between the resulting simulations and the typical structures observed in biological assays. Finally, this work provides a new interpretation of the emergence of branched patterns in living aggregates, depicted as the results of a complex interplay among chemical, mechanical and size effects.
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We are grateful to Davide Ambrosi for helpful discussions.
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This work was partially supported by the “Start-up Packages and Ph.D. Program” project, co-funded by Regione Lombardia through the “Fondo per lo sviluppo e la coesione 2007–2013—formerly FAS” and by the “Progetto Giovani GNFM 2014”, funded by the National Group of Mathematical Physics (GNFM-INdAM).
Appendix
Appendix
In Sect. 3.2, the linear stability analysis applied to the quasi-stationary problem leads to the definition of the dispersion equation in the compact form (26), as a function of the unperturbed and perturbed pressure fields. Here, we report some details on how Eqs. (25) and (26) have been obtained and the specific expressions for the perturbed pressure and the dispersion equations in the chemotactic growth model (Table 3) and in the bulk growth model (Table 4). The boundary conditions (25) for the perturbed pressure at the interface can be easily obtained, provided that (6) should hold, therefore
Computing the curvature of the perturbed interface and considering on both sides only the first-order terms, the following relation holds
The derivation of (25) is then straightforward.
In a similar way, the dispersion equation (26) can be retrieved imposing the boundary condition (7) at the perturbed interface and neglecting the terms of order higher than the first, in the series expansion.
The coefficients \(A,\, A_0,\, A_1\) can be found in Table 2, and they are the same for both models. As it is evident from Tables 3 and 4, the dispersion equations link the time-growth mode \(\lambda \) to the wavenumber k in an implicit way, as a function of the four dimensionless parameters \(\beta _i,\, \sigma ,\, R^*\) and \(R_\mathrm{out}\).
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Giverso, C., Verani, M. & Ciarletta, P. Emerging morphologies in round bacterial colonies: comparing volumetric versus chemotactic expansion. Biomech Model Mechanobiol 15, 643–661 (2016). https://doi.org/10.1007/s10237-015-0714-9
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DOI: https://doi.org/10.1007/s10237-015-0714-9