Extended spectrum beta lactamases (ESBLs): enzymes made by bacteria that cause the bacteria to be resistant to a group of antibiotics known as beta-lactam antibiotics. This means that the antibiotic is not effective in fighting an infection caused by these bacteria. Beta-lactam antibiotics that can be ineffective against ESBL-producing bacteria include penicillins, cephalosporins, and aztreonam. Infections with bacteria that produce ESBL organisms have been associated with poor outcomes. Several different kinds of bacteria are able to produce ESBL, and there are at least 200 types of ESBL.