Chemical kinetics and bacterial growth
Along with his other achievements, Hinsh
pioneered the application of the principles of chemical kinetics to the
behaviour of biological systems, especially bacterial cells, although studies
were also carried out with yeasts and fungi. The work started in the
Balliol-Trinity laboratories in the late 1930s in a small way, but soon
expanded to involve a large number of research workers in the PCL. The earlier
work was discussed in Chemical Kinetics of the Bacterial Cell (OUP,
1946). Alastair Dean joined him in 1949 and collaborated closely with him until
Hinshelwood's death in 1967.
The formulation of the basic equations of
bacterial growth was followed by detailed kinetic studies of the development of
resistance by bacterial cells to a wide range of antibacterial agents and of
their adaptation to new sources of carbon and nitrogen as nutrients. Many of
these results are summarized in Growth, Function and Regulation in Bacterial
Cells (Dean and Hinshelwood, Clarendon Press, 1966), in which the results
are interpreted kinetically and a kinetic model proposed which applies to
biological systems in general. It postulates a principle of total integration,
a network theorem and the concept of the spatial map of the cell. It does not
argue against the mutational origin of resistant bacteria but implies that
there is another way in which cells can become resistant to antibacterial
agents or adapt to utilise new substrates, that is, by a change in their
reaction pattern.
The various adaptive responses encountered
in this work and on which the theoretical treatment was based have also been
found in plant and animal cell cultures and appear to be central to the
processes of cell differentiation and neoplastic transformation.
The formation of bacterial colonies on agar
plates was also studied kinetically. Hinshelwood was for long fascinated by the
physico-chemical mechanisms involved in colony development and indeed was
continuing investigations in this field at Imperial College when he died.
Batch culture was used in all the
experiments noted above, but in the early 1960s Dean became interested in
continuous culture, whereby true steady states of growth can be maintained for
long periods in an unchanging environment. Two methods are available – the
turbidostat and chemostat techniques: both, but particularly the chemostat
technique (due to the wide range of growth rates achievable in a series of
unique environments) have been used to study the control mechanisms involved in
the synthesis of a number of microbial enzymes, and to elucidate the mode of
action of various antimicrobial agents.
In a new departure, investigations into the
accumulation of heavy metal ions by bacteria were undertaken since it seemed
likely that if an organism with a high uptake could be found, it might form the
basis of a biological process to remove these ions from polluted waste streams.
After much searching a suitable organism was isolated from polluted soil and
was identified as a Citrobacter sp. It accumulates appreciable amounts
of cadmium (and other heavy metals) during growth in their presence and even
more when cells pre-grown in Cd-free medium are subjected to the metal in the
non-growing (resting) state. Typically this is achieved by immobilizing the
cells in a polyacrylamide gel and packing the shredded gel into a column to act
as a filter through which contaminated solution can be passed. Much effort has
gone into optimizing the accumulation process (which is enzyme-mediated), and
Lynne Macaskie and Dean have shown that as much as 9 g metal per gram of
bacteria (dry weight) can be removed from solution. The process has been
assigned to Isis Innovation Ltd (a company wholly owned by the University) for
possible commercial development.
