Selection of variants of Gram-negative bacteria with elevated production of type 1 beta-lactamase

Variants of Pseudomonas aeruginosa, Citrobacter freundii and Enterobacter cloacae were isolated that produced elevated levels of type 1 beta-lactamase. The variants were readily isolated on antibiotic-containing agar and showed resistance to a wide range of beta-lactam antibiotics including some regarded as being relatively stable to the beta-lactamase produced.