The in vitro antimicrobial activity of two new aryl-fluoroquinolone antibiotics, A-56619 and A-56620, was compared with those of norfloxacin and several other antibiotics against 448 bacterial isolates. A-56620 was the most active agent tested. The usual 90% MIC of A-56620 was .2 ,ug/ml, except for enterococci, gentamicinresistant Serratia marcescens, and gentamicin-resistant Pseudomonas aeruginosa, for which the 90% MIC was 4 Fg/ml. A-56619 and porfloxacin were generally severalfold less active than A-56620. Cross resistance was observed between the quinolope antibiotics and other unrelated antibiotic classes. Antimicrob. Agents Chemother., abstr. no. 77, 1984). In this study, the antimicrobial spectrum of potency of A-56619 and A-56620 were studied further and compared those of with norfloxacin and several broad-spectrum cephalosporins, penicillins, and other relevant antimicrobial agents.Antimicrobial compounds of known potency were provided as follows: A-56619 and A-56620 (Abbott Laborato- The comparative in vitro antimicrobial activity for A-56619, A-56620, and the other agents evaluated is presented in Table 1. A-56619 and A-56620 had similar activity against grampositive cocci. Both antimicrobial agents were more active than norfloxacin against methicillin-resistant Staphylococcus aureus, while A-56620 was more active against the enterococci and methicillin-susceptible Staphylococcus aureus. A-56619 and'A-56620 were generally more active than the other agents tested against the gram-positive cocci. Only ampicillin was more active than A-56619 and A-56620 against the enterococci and vancomycin was of about equal activity against methicillin-resistant Staphylococcus aureus.For the gentamicin-susceptible members of the family Enterobacteriaceae, cefotaxime was generally the most active beta-lactam agent, with an MIC for 90% of strains tested (MICgo) of .0.25 ,ug/ml for all species tested, except Providencia spp., Citrobacter freundii, Enterobacter sp., and Citrobacter diversus. A-56620 was the most active quinolone carboxylic acid tested, with MIC90s ranging from 0.25 to 1.0 ,ug/ml for most of these same bacteria. However,A-56620 did demonstrate exceptional potency against Escherichia coli, Citrobacter diversus, Salmonella enteritis, and Shigella sonnei. Norfloxacin was slightly more active than A-56620 against Serratia marcescens and Proteus mirabilis. A-56619 was generally two-to fourfold less active than A-56620 against these Enterobacteriaceae and was less active than norfloxacin against Aeromonas hydrophilia, Citrobacter diversus, Citrobacter freundii, Morganella morganii, Proteus mirabilis, and Serratia marcescens.The quinolone carboxylic acids showed substantially diminished activity against gentamicin-resistant strains of Escherichia coli, Serratia
