Abstract:
Traditionally, growth and morphology studies of bacteria have focused on individual cells.
When microbiologists subsequently became interested in bacterial colony growth, they
investigated the dynamics of its height as well as those of its diameter. The models they
proposed, however, were little more than empirical. In recent decades there has been a shift
in focus, as researchers became interested instead in the morphology changes that colonies
undergo when exposed to stressful environments such as nutrient and moisture limitation.
As a result of this shift, models of colony growth in three dimensions have remained
underdeveloped and rudimentary.
My first task in this thesis was to assemble a sufficiently comprehensive data set from which
diameter, height and cell-number growth trends in colonies could be properly analysed. This
was achieved by studying the growth of many colonies of two bacterial species, Serratia
marcescens and Esherichia coli, on high-nutrient non-selective agar, over a range of incubation
temperatures, over periods ranging from two hours to one week.
When graphed and analysed, colony diameter and colony height growth turned out to be
most economically described as power-law in time, with exponent < 1. This contrasts with
the claims of previous researchers, who had described both growth trends as linear, with
diameter switching to a slower yet still linear growth after a certain time, and height growth
ceasing altogether.
From my results, I proposed a simple conceptual model, an extension of a model developed
by Pirt in 1967. My hypothesis was that, in colonies growing on high-nutrient surfaces,
diffusion was the dominant factor in colony growth. Ron Keam transformed my conceptual
model into a mathematical one, from which I have developed one-dimensional and twodimensional
numerical simulations. In all simulations to date, in both one and two
dimensions, a power-law growth phase emerges as a consequence of nutrient-controlled
growth, preceded by an “accelerating” phase during which colony growth overtakes
diffusive processes, and succeeded by a slow transition towards growth cessation as nutrient
becomes exhausted.
In addition to successful demonstration of the power laws, the model in its final form yields
realistic colony profiles and exhibits other features consistent with experimental results
reported in the literature.