No abstract

A procedure was developed which allows direct identification and antimicrobial susceptibility testing of fermentative and nonfermentative gram-negative bacilli from positive blood cultures. A 10-ml sample was removed from turbid blood culture bottles, and the bacteria were washed and concentrated by centrifugation. The bacterial pellet was used to inoculate an Enterobacteriaceae Plus Identification Card and a Gram-Negative General Susceptibiity Card of the Auto-Microbic system. Results with these cards were compared with results obtained with standard techniques for 196 blood cultures seeded with recent clinical isolates. Identification of most cultures was available in 8 h, whereas the antimicrobial susceptibility results were available in an average of 4.7 h for all organisms. Direct identification was correct for 95% of the cultures, whereas the antimicrobial susceptibility data had an average agreement of 87% with 3.8% very major and 1.4% major errors. In using this procedure it was possible to provide accurate preliminary identification and results of antimicrobial susceptibility tests for gram-negative bacilli on the same day that a blood culture was determined to be positive.

Netilmicin, a new semisynthetic aminoglycoside derived by ethylation of the 1-N position of the deoxystreptamine ring of'sisomicin, was tested in vitro with 4,070 strains of gram-negative bacilli isolated at the UCLA Medical Center during 1975 to 1976, using the agar dilution technique and an inoculum of approximately 104 organisms. Results were compared with those simultaneously obtained for amikacin, gentamicin, and tobramycin. Using Mueller-Hinton medium, inhibitory concentrations in broth correlated with those obtained by the agar dilution method except for Pseudomonas aeruginosa, where a 2-to 16-fold difference in susceptibility was noted. For most clinically significantEnterobacteriaceae and P. aeruginosa, the activity of netilmicin in vitro was comparable or superior to that of gentamicin, tobramycin, and amikacin with respect to potency by weight and achievable blood levels. Against gentamicin-resistant strains (MIC > 16 ,ug/ml), the activity of netilmicin paralleled that of amikacin with the exception ofProvidencia stuartii, which was inhibited by amikacin but not by netilmicin.Gram-negative bacilli are responsible for an increasing number of the serious infections occurring in hospitalized patients (3). Gentamicin and other aminoglycoside antibiotics have been used successfully to treat many such infections. Nonetheless, the emergence of resistant gramnegative bacilli in certain clinical settings (5, 7, 9, 13, 16) and toxic side effects of these agents present a serious problem. There are increasing reports of bacterial resistance to gentamicin on the basis of either selection of naturally occurring resistant organisms or spread of highly resistant strains that possess resistance transfer factors (2). This changing pattern of antimicrobial susceptibility serves as a constant stimulus for the development of new antimicrobial agents and for structural modification of existing compounds. For these reasons, the evaluation of a new aminoglycoside antibiotic with a broadened anti-gram-negative bacillary spectrum but with increased potency and relatively less toxicity is an important goal of laboratory and clinical research.Netilmicin (Sch 20569) is a new semisynthetic aminoglycoside antibiotic derived by ethylation of the 1-N position of the deoxystreptamine ring of sisomicin. This synthetic modification has resulted in a compound that is resistant to inactivation by gentamicin adenyltransferase 1 and relatively resistant to gentamicin acetyltransferase 1. Netilmicin possesses no active site for kanamycin phosphotransferase inactivation, but the compound is inactivated by kanamycin acetyltransferase and gentamicin acetyltransferase 2. Preliminary reports (12, 15) have indicted that netilmicin has a broad spectrum of activity against most Enterobacteriaceae and Pseudomonas aeruginosa. In this respect, it resembles the parent compound, sisomicin. Because the biosynthetic reaction that has yielded netilmicin is similar to the process by which amikacin was derived from kanamycin A, we were particularly interes...