Activity of a new cephalosporin antibiotic, Ro 13-9904 against dense populations of selected enterobacteria.

Ro 13-9904 is a new cephalosporin antibiotic with a wide spectrum of activity similar to that of cefotaxime and some other newer beta-lactam compounds. In the present study, the response to Ro 13-9904 of 13 strains of enterobacteria, selected on the basis of differential patterns of susceptibility to beta-lactam agents, was investigated. The new cephalosporin displayed high intrinsic activity against most of the enterobacteria tested, but a group of Klebsiella and Hafnia alvei strains was identified against which Ro 13-9904 (and cefotaxime) exhibited reduced activity. Investigation of these strains revealed several different types of response to Ro 13-9904 and cefotaxime. Although such "difficult" strains represent a small minority of clinical isolates, it is likely that it is this group of strains against which the new generation of highly potent, broad-spectrum beta-lactam antibiotics will display significant differential activity.