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Chemotactic drift is the directed velocity associated with sensing a chemical gradient. In a simple continuum model the drift is $\chi \nabla c$; multiplying by cell density gives the chemotactic flux.

An unbiased random walk of bacteria leads, on the continuum scale, to diffusion with coefficient $D$, represented by $D{B}_{XX}$. The cell proliferation rate $\nu GB$ is proportional both to the available biological growth factor concentration and to the bacterial density. Consumption of biological growth factor at rate $\mu GB$ gives its negative reaction term.

There is no diffusion term for $G$ because the biological growth factor is attached to the substrate rather than freely transported. There is also no chemotactic drift in this model: the specified motion is random, while the biological growth factor changes cell proliferation rather than directional movement. Both reaction terms vanish if either bacteria or biological growth factor are absent.