Mode of Action
Inhibits the formation of cross-links in the peptidoglycan layer (which prvides rigidity to the cell wall). Most effctive against cells in log phase growth (since this is when new cross-links are being formed), and has little effect on cells in stationary phase.
Mechanism of Resistance
Expression of β-lactamase neutralizes ampicillin. When this enzyme is expressed on a high-copy number plasmid there is significant diffusion into the extracellular medium. As a result non-resistant satellite colonies may form around larger resistant colonies.
(would be nice to have a picture of this here)
Molecular Cloning: A Laboratory Manual, Vol 1.