Resistance to β-lactamase-stable cephalosporin antibiotics in strains of Enterobacteriaceae and Pseudomonas aeruginosa isolated from blood cultures

A total of 224 strains of Enterobacteriaceae and Pseudomonas aeruginosa isolated from blood cultures were screened for resistance to cefuroxime and cefotaxime. Seven of 200 strains of Enterobacteriaceae were resistant to either cefuroxime or cefotaxime, while five of 24 strains of Ps. aeruginosa were resistant to cefotaxime. A further 32 resistant variants were isolated by use of a single step selection procedure, without prior mutagenesis, from strains which were sensitive on initial screening. From the total of 44 resistant strains and variants isolated, 31 were found to produce elevated levels of chromosomal beta-lactamases. Of these 31, five strains of Proteus sp. produced significant amounts of beta-lactamase only when induced in the presence of cefuroxime or cefotaxime, while the remaining 26 strains also produced high levels of beta-lactamase in the absence of an inducer. Thirteen resistant strains and variants (9 Ps. aeruginosa, 3 P. mirabilis, 1 K. aerogenes) did not produce detectable levels of beta-lactamase in either the presence or absence of induction and presumably owed their resistance to another mechanism.