Ninety-eight percent of the cells in a population of Escherichia coli in log-phase growth lost colony-forming ability after being exposed for 3 h to the quinolone antibiotic ciprofloxacin at four times the MIC in nutrient broth, a concentration easily reached in vivo. Flow cytometric analysis, however, demonstrated that only 68% of this bacterial population had lost membrane potential, as judged by the membrane potential-sensitive dye bis-(1,3-dibutylbarbituric acid) trimethine oxonol [DiBAC 4 (3)], and only 30% could no longer exclude the nucleic acid-binding dye propidium iodide (PI), reflecting lost membrane integrity, efflux mechanisms, or both. Subsequent removal of ciprofloxacin and resuspension in nutrient broth resulted in renewed cell division after 2 h, with a calculated postantibiotic effect (PAE) time of 57 min. The proportion of DiBAC-and PI-fluorescent cells in this recovering population remained stable for more than 4 h after antibiotic removal. Eighty percent of cells present at drug removal were filamentous. Their number subsequently decreased with time, and the increase in particle count seen at the end of the PAE resulted from the division of short cells. Exposure to ciprofloxacin in the presence of the protein synthesis inhibitor chloramphenicol increased colony-forming ability to 60% of starting population numbers. In contrast to ciprofloxacin alone, this antibiotic combination resulted in insignificant filamentation and no dye uptake. Subsequent drug removal and resuspension in nutrient broth resulted in the appearance of filaments within 1 h, with 69% of the population forming filaments at 3 h. Dye uptake was also seen, with 20% of the population fluorescing with either dye after 4 h. We were unable to relate dye uptake to the viable count. Cell division resumed 240 min after removal of both drugs, yielding a PAE calculated at 186 min. Inhibition of protein synthesis with chloramphenicol prevented ciprofloxacin-induced changes in bacterial morphology, cell membrane potential, and ability to exclude nucleic acid-binding dye. These changes persisted beyond the end of the classically defined PAE and were not a definite indicator of cell death as defined by loss of colony formation, which related at least in part to filamentation.

In recent years, new formulations of the original amphotericin B preparation (Fungizone) have been devised in order to overcome toxicity problems that frequently occur. These preparations represent an improved method of drug delivery, with an increased therapeutic index and a decrease in toxicity to mammalian cell membranes. The new formulations have different physico-chemical characteristics and differ in pharmacokinetic parameters. Their effects must be compared with conventional amphotericin B to ascertain potential roles in future antifungal therapy.

Diosquinone and plumbagin isolated from the root of Diospyros rnespiliforrnis (Hostch), a common ingredient in several folk medicines and foods, have been shown to have antibacterial activity against a wide range of organisms. The minimum inhibitory concentrations (MICs) of diosquinone against Staphylococcus aureus NCTC 6571 and S. aureus E3T ranged from 3 to 30 pglmL, while those against Escherichia coli KL16 and Pseudomoms aeruginosa NCTC 6750 ranged from 15 to 16pglmL. MICs were found to increase with the concentration of cells used in the inaculum. Bacterial studies showed that S. aureus NCTC 6571 exhibited a paradoxical biphasic response to dioquinone in nutrient broth, whereas bacterial activity against E . coli KL16 increased with concentration up to the highest concentration of dioquinone tested. Activity against E . coli KL16 was more pronounced in phosphate-buffered saline than in nutrient broth.The other active compound isolated, the naphthoquinone plumbagin, gave MIC values between 400 and 600 pg/mL.

U.V. irradiation mediated by R factor R46 has been studied in strains deficient in excision repair and recombination repair. The R factor protected wild-type bacteria and also wild-type cells in which repair had been inhibited by the substitution of bromouracil for chromosomal thymine. It increased the survival of strains defective in the endonucleolytic (uvr), repolymerizing (pol) and joining (lig) stages of the excision repair process. Recombination deficient bacteria mutant at the recB or recC loci were protected by R46, but the R factor had little effect on the survival of a recA strain or a recA recB double mutant. R46 increased the survival of cells that had been treated with chloramphenicol before U.V. irradiation, but did not protect cultures treated with chloramphenicol after irradiation. It is concluded that R46 confers resistance to the lethal effects of U.V. irradiation by a mechanism that is independent of excision repair. Resistance appears to be mediated by an inducible gene product, which is possibly a nuclease and dependent on a functional host recA gene for expression.

The recA13 mutant of Escherichia coli strain K-12, which lacks recombination and SOS error-prone DNA repair is hypersensitive to nalidixic acid and to the newer 4-quinolones ciprofloxacin, norfloxacin and ofloxacin. However, whereas recombination-proficient but SOS repair-deficient strains, such as those carrying the lexA3 or recA430 alleles are no more sensitive to nalidixic than the lexA+ recA+ parent, they are more sensitive to the newer quinolones, although not as sensitive as the recA13 derivative. Nalidixic acid possesses only bactericidal mechanism A (which requires RNA and protein synthesis and is only effective on actively dividing cells), whereas the newer 4-quinolones exhibit additional mechanisms B (which does not require RNA and protein synthesis and is effective on bacteria unable to multiply) and C (which requires RNA and protein synthesis but does not depend on cell division). Results obtained with bacteria suspended in phosphate-buffered saline, which inhibits mechanism A, and with bacteria suspended in nutrient broth plus rifampicin, which inhibits mechanisms A and C, showed that the lexA3 mutant was still more sensitive than the lexA+ parent under these conditions. The results suggest that, unlike bactericidal mechanism A, DNA damage that results from bactericidal mechanisms B and C of the newer 4-quinolones is subject to SOS error-prone (mutagenic) repair.