  The dynamics of biofilm lifecycle are deeply influenced by the surrounding
environment and the interactions between sessile and planktonic phenotypes.
Bacterial biofilms typically develop in three distinct stages: attachment of
cells to a surface, growth of cells into colonies, and detachment of cells from
the colony into the surrounding medium. The attachment of planktonic cells
plays a prominent role in the initial phase of biofilm lifecycle as it
initiates the colony formation. During the maturation stage, biofilms harbor
numerous microenvironments which lead to metabolic heterogeneity. Such
microniches provide conditions suitable for the growth of new species, which
are present in the bulk liquid as planktonic cells and can penetrate the porous
biofilm matrix. We present a 1D continuum model on the interaction of sessile
and planktonic phenotypes in biofilm lifestyle which considers both the initial
attachment and colonization phenomena. The model is formulated as a
hyperbolic-elliptic free boundary value problem with vanishing initial value.
Hyperbolic equations reproduce the transport and growth of sessile species,
while elliptic equations model the diffusion and conversion of planktonic cells
and dissolved substrates. The attachment is modelled as a continuous,
deterministic process which depends on the concentrations of the attaching
species. The growth of new species is modelled through a reaction term in the
hyperbolic equations which depends on the concentration of planktonic species
within the biofilm. Existence and uniqueness of solutions are discussed and
proved for the attachment regime. Finally, some numerical examples show that
the proposed model correctly reproduces the growth of new species within the
biofilm and overcomes the ecological restrictions characterizing the
Wanner-Gujer type models.
