Gram negative organisms with enzymes that confer resistance to 3rd generation cephalosporins in addition to lower order beta-lactams.
ESBLs vs. AmpC: Most ESBLs are plasmid-encoded, whereas AmpCs are usually inducible and chromosomal.
In the lab, ESBLs test susceptible to cefoxitin and cefotetan, and are inhibited by beta-lactamse inhibitors such as tazobactam.