We studied 21 strains of amikacin-resistant Serratia marcescens from three different U.S. cities, Twenty of the 21 strains contained conjugative R plasmids mediating gentamicin and tobramycin resistance. Amikacin-resistant S. marcescens from two cities predominated in protracted outbreaks. Conversely, the amikacin-resistant Charleston strain (serotype 02/03:nonmotile) was isolated from only four patients during an outbreak of gentamicin- and tobramycin resistant, amikacin-susceptible S. marcescens (serotype O19:O17). Five different representative amikacin-resistant S. marcescens, each containing a single conjugative plasmid, elaborated a nontransferable aminoglycoside (6')-N-acetyltransferase [AAC(6')] with similar substrate profiles in addition to other transferable aminoglycoside-modifying enzymes. One amikacin-resistant S. marcescens cured of its plasmid and another naturally occurring plasmid-free amikacin-resistant S. marcescens elaborated only AAC(6')-1. These data support the concept of a chromosomal locus in S. marcescens for AAC(6')-1 which commonly coexists with plasmid-mediated genes for aminoglycoside-modifying enzymes.

Eight cephalosporins were tested for their activity against methicillin-susceptible and methicillin-resistant, coagulase-negative staphylococci and for their resistance to ,B-lactamase from methicillin-resistant, coagulase-negative staphylococci. Susceptibility testing by the agar plate method was evaluated for the effect of inoculum size and duration of incubation. Methicillin-susceptible, coagulase-negative staphylococci were highly susceptible to the cephalosporins, with cephapirin and cephalothin showing the greatest activity, followed by cefazolin and cefamandole. Methicillin-resistant, coagulase-negative staphylococci displayed nearly total cross-resistance to the cephalosporins. Resistance increased with increasing inoculum size. fB-Lactamases produced by methicillin-resistant, coagulase-negative staphylococci had a minimal hydrolytic effect on cephalothin, cephapirin, cefazolin, and cefamandole and no measurable effect on cefoxitin. There was no correlation between the anti-staphylococcal activity and resistance to ,8-lactamases.Coagulase-negative staphylococci cause not only infections of prosthetic devices (12, 13) but also postoperative wound infections (4), peritonitis related to chronic peritoneal dialysis (8)

Amifloxacin (WIN 49375) activity against a well-defined group of gentamicin-resistant gram-negative bacilli was compared with the activity of 11 other antimicrobial agents. For all strains, amifloxacin and norfloxacin were the most active agents, followed by cefotaxime and moxalactam. For Acinetobacter sp. only amifloxacin had an achievable MIC for 90% of the strains. Amifloxacin joins other newly developed DNA gyrase inhibitors as potentially useful agents for infections due to aminoglycoside-resistant gram-negative bacilli.The proliferation of antibiotic resistance, particularly among gram-negative bacteria, has prompted a continuing search for new agents unaffected by existing mechanisms of resistance. The quinolones are a group of synthetic antimicrobial agents whose prototype, nalidixic acid, was developed over 30 years ago (5,7,10 Fig. 1

No abstract