How Escherichia coli bacteria develop a particular colonial,
3-D biofilm morphological pattern is still a poorly understood process. Recently,
we reported a new E. coli K-12 morphotype
exhibited by old macrocolonies described as volcano-like. The formative developmental
process of this morphotype has been presented as a suitable experimental model for
the study of 3D patterning in macrocolony biofilms. Here, we report the optical
microscopy observations and genetic analysis that have unveiled the existence of a novel autoaggregative
behaviour which generates massive lumpiness over the surface of the volcano-like
macrocolonies. These lumpy formations are generated by the autoaggregation and strong interaction
of tightly packed bacterial cells in structures with a chondrule-like appearance
which give the colony’s surface its characteristic microscopic lumpy phenotype.
Furthermore, they exhibit different levels of maturation from the edge to the center
of the colony. Hence, its generation appears to follow a spatiotemporal program
of development during
the macrocolony’s morphogenesis. Interestingly, the agar’s hardness influences the
morphology exhibited by these formations, with high agar concentration (1.5%, 15
g/L) suppressing its
development. This new auto-aggregative E.
coli’s behaviour does not require the activity of the biofilm master regulator CsgD, the
adhesiveness of flagella, pili type 1, adhesin Ag43, β-1,6-N-acetyl-D-glucosamine polymer-PGA, cellulose or colanic acid,
but it is under glucose repression and the control of
cAMP receptor protein (CRP). The possible physiological role of these chondrule-like
formations in the adaptability of the colony to different stressful environmental
conditions is discussed.