Emergence of resistance during beta-lactam therapy of gram-negative infections. Bacterial mechanisms and medical responses

Growth of cell-wall-deficient variants of Enterobacter cloacae facilitates beta-lactamase derepressed mutants

The degree to which cell-wall-deficient bacteria (CWDB) are involved in the generation of beta-lactamase derepressed mutants (DM) was measured using Enterobacter cloacae 3624. The frequency of DM in non-permissive isotonic ticarcillin medium was compared with their frequency in hypertonic ticarcillin medium that supports CWDB growth. DM were resistant to extended spectrum penicillins and cephalosporins and had a basal beta-lactamase activity of >300 units/mg protein. Anaerobic growth of CWDB increased the relative risk of DM 2 x 10(6)-fold. Aerobic incubation produced fewer CWDB colonies but the risk of DM was still increased 400-fold over non-permissive controls. These results define a new role for CWDB as intermediaries in the emergence of resistance.

Differences in the resistant variants of Enterobacter cloacae selected by extended-spectrum cephalosporins

The rates of development of resistance to ceftriaxone, ceftazidime, cefepime, and cefpirome in 10 strains of Enterobacter cloacae were determined by daily transfer for 7 days to fresh medium containing twofold serial dilutions of the antibiotics. Development of resistance to ceftriaxone was the most rapid; this was followed by ceftazidime, cefpirome, and cefepime. Resistant variants selected by ceftriaxone and ceftazidime were cross-resistant and produced very high levels of beta-lactamase. On the other hand, resistant variants selected by cefepime and cefpirome often had moderately high levels of beta-lactamase and diminished levels of the 39- to 40-kDa porin protein.

Resistance to third generation cephalosporins: the current situation

Newer beta-lactam antibiotics, notably the third generation cephalosporins (3 GC) have been designed for providing high intrinsic potency against a large variety of microorganisms. Bacterial resistance can occur however, and nowadays, clinicians are concerned by novel situations where even most recently developed compounds can be ineffective. A first situation is generated by bacteria which produce great amounts of chromosomal cephalosporinase. The resistance emerges during therapy, in hospital isolates which are classified as susceptible with conventional susceptibility testing. The prevalence of 3 GC resistance among these gram-negative rods with inducible beta-lactamase seems to increase in some institutions but the significance of susceptibility testing in this regard is doubtful. It is probably more important to note that the prevalence of gram-negative rods with inducible beta-lactamases remains stable. A second problem arose with the abrupt development of plasmid mediated beta-lactamases markedly active against 3 GC. This resistance is underestimated because some strains fall into susceptibility range of 3 GC as determined by MICs or inhibition zone sizes. These extended spectrum enzymes are now distributed over four continents and represent a growing threat.