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.
A 1990 paper by Bill Studier discusses how the secreted β-lactamase can quickly consume all the ampicillin in a culture (even at 20μg/ml Amp). A stationary culture of ampicillin resistant cells can have such a concentration of β-lactamase that even a 1/200 to 1/1000 dilution will still contain enough β-lactamase to consume all the fresh ampicillin before all the non-resistant cells from the stationary phase culture have been killed.
The authors reccommend not allowing cultures to reach stationary phase if you need a high proportion of cells to contain your plasmid.
Molecular Cloning: A Laboratory Manual, Vol 1.