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We have proposed a cooperative quinolone-DNA binding model for the inhibition of DNA gyrase. The essential feature of the model is that bound gyrase induces a specific quinolone binding site in the relaxed DNA substrate in the presence of ATP. The binding affinity and specificity are derived from two unique and equally important functional features: the specific conformation of the proposed single-stranded DNA pocket induced by the enzyme and the unique self-association phenomenon (from which the cooperativity is derived) of the drug molecules to fit the binding pocket with a high degree of flexibility. Supporting evidence for and implications of this model are provided.

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Infectious diseases are of ancient origin, and mankind has a venerable history of use of higher plant extracts for the therapy of such infections. Some such agents survive in use from earlier times--quinine, emetine, and sanguinarine, for example--but the modern use of fermentation-based antibiotics has greatly overshadowed work on agents from other sources. After a brief review of the present status of the field of antibiotics, this review focuses upon the present status of antimicrobial agents from higher plants with particular reference to agents from plants with a folkloric reputation for treatment of infections. In particular, recent work on the tropical genus Erythrina is emphasized. The use of modern microbiological techniques demonstrates that higher plants frequently exhibit significant potency against human bacterial and fungal pathogens, that many genera are involved, that many folkloric uses can be rationalized on this basis, that the active constituents are readily isolated by bioassay-directed techniques, that their chemical structures are types uncommon amongst fermentation-based agents but are familiar to natural product chemists, that their antimicrobial spectra are comparatively narrow but that their potency is often reasonable, that they are comparatively easy to synthesize and the unnatural analogues so produced can possess enhanced therapeutic potential and, thus, it is concluded that such work generates a gratifying number of novel lead structures and that the possibility of finding additional agents for human or agricultural use based upon higher plant agents is realistic.

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Antibiotic susceptibility testing of the rickettsial Q fever agent Coxiella burnetii was performed by using persistently infected L929 fibroblast cells. The efficacies of a variety of antibiotics with different metabolic targets were tested and compared. -The most effective antibiotics in bringing about the elimination of the parasite from infected cells included several quinolone compounds and rifampin. Of the' quinolone compounds tested, 'difloxacin (A-56619) was the most effective, followed by ciprofloxacin and oxolinic' acid. These three quinolones were apparently rickettsiacidal. After 48 h of exposure to micrQgram amounts of the compounds (ranging from 2 ,,g of difloxacin per ml to'5 jig of the other two antibiotics per ml), the number of intracellular parasites markedly declined; after' 10 days of treatment, very few intracellular rickettsiae were detected.Rifampin (1 ,Ig/ml) was also very effective in eliminating the parasites. Some of the 13 other antibiotics tested that were somewhat effective included chloramphemcol, doxycycline, and trimetloprim. The persistently infected L929 cells were found-to provide a convenient system for the relatively rapid determination of the susceptibility of C. burnetii to antibiotics.Chronic endocarditis and hepatosplenomegaly are occasionally associated with persistent and relapsing infection caused by the obligate intracellular procaryotic rickettsial agent of Q fever, Coxiella burnetii (8,12,18,19). Antibiotic treatment of chronic Q fever has not been consistently effective (8,13,18); in spite of treatment, the disease can continue or relapse, resulting in life-threatening endocarditis,'usually affecting the aortic and mitral valves (13,17,18). We have developed an in vitro model of persistent Q fever consisting of cell lines persistently infected with C. burnetii. These chronically infected cell lines include L929, J774, and P388D)1 (3,4). The persistently infected L929 cells have been maintained in continuous'culture for over 3 years without the addition of normal'cells; both the host cells and parasites divide, with C. burnetii proliferating within phagolysosomes (2). Recently, we discovered that heavily infected cells are capabl,e of division; those possessing one large parasite-containing vacuole give rise to both infected and uninfected daughter cells (15). This chronically infected model system was employed in this study for examining the efficacies of several antibiotics, including several DNA gyrase inhibitors (16) of the quinolone family. In this report we provide evidence that rifampin and several quinolones are effective in rapidly eliminating the parasite from chronically infected L929 cells. Unlike other host systems (embryonated eggs, guinea pigs) that have been used previously by other investigators (7, 9, 10, 17) to determine the' antibiotic susceptibility of C. burnetii, the system we describe is more convenient and precise and it allows one to obtain drug efficacy data in a relatively short time. Rocky Mountain Laboratory, Hamilton, Mont. The plaque...