Staphylococcus aureus (SA) is a major cause of infection in humans, including the Methicillin resistant strain, MRSA. However, very little is known about the mechanical properties of these cells. The investigations presented in this thesis use Atomic Force Microscopy (AFM) to examine live SA cells to quantify their mechanical properties. These were explored using force spectroscopy with different trigger forces, allowing the properties to be extracted at different indentation depths. A value obtained with sharp silicon tips of (0.0134 +/-� 0.0068) Nm^�1 for the cell wall stiffness has been extracted, along with a second, higher value of (0.2062 �+/- 0.0039) Nm^�1 which is found upon indenting at higher forces. This
higher value drops by a factor of more than 4 when the cells are exposed to high salt, sugar and detergent concentrations, implying that this measurement contains
a contribution from the internal turgor pressure of the cell.
These properties were monitored as the cells progressed through the cell cycle. Force maps were taken over the cells at different stages of the growth process
to identify changes in the mechanics throughout the progression of growth and division. These experiments revealed no measurable change in the turgor pressure
of the cell, as measured in this way, but it was found that newly formed areas of cell wall exposed for the first time during the separation of daughter cells gave a
higher measured stiffness. This can be related to the structure of the cell wall, which changes as the newly formed cell wall matures during the development of
the daughter cells.
The effect of a b-lactam antibiotic, Oxacillin, has also been studied, in an attempt to better understand its mechanism of action. This class of drug is understood to work during division, preventing successful reproduction by inhibiting cell wall production leading to cell lysis. Images were taken throughout the process of b-lactam induced cell death for the �first time.
Finally mutant strains of SA and a second species, Bacillus subtilis (BS), have been used to link the mechanical properties of the cell walls to structural alterations. Both of these groups of cells contain varied cell wall glycan chain lengths, allowing the study of the effect of chain length on the measured mechanical properties
