  An explicit proposal for experiments leading to abrupt transitions in
spatially extended bacterial populations in a Petri dish is presented on the
basis of an exact formula obtained through an analytic theory. The theory
provides accurately the transition expressions in spite of the fact that the
actual solutions, which involve strong nonlinearity, are inaccessible to it.
The analytic expressions are verified through numerical solutions of the
relevant nonlinear equation. The experimental set-up suggested uses opaque
masks in a Petri dish bathed in ultraviolet radiation as in Lin et al.,
Biophys. J. {\bf 87}, 75 (2004) and Perry, J. R. Soc. Interface {\bf 2}, 379
(2005) but is based on the interplay of two distances the bacteria must
traverse, one of them favorable and the other adverse. As a result of this
interplay feature, the experiments proposed introduce highly enhanced
reliability in interpretation of observations and in the potential for
extraction of system parameters.
