Ampicillin killing curve patterns of Haemophilus influenzae type b isolates by agar dilution plate count method

Synthesis and Evaluation of Troponoids as a New Class of Antibiotics

Novel antibiotics are urgently needed. The troponoids [tropones, tropolones, and α-hydroxytropolones (α-HT)] can have anti-bacterial activity. We synthesized or purchased 92 troponoids and evaluated their antibacterial activities against Staphylococcus aureus, Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa. Preliminary hits were assessed for minimum inhibitory concentrations (MIC₈₀) and cytotoxicity (CC₅₀) against human hepatoma cells. Sixteen troponoids inhibited S. aureus/E. coli/A. baumannii growth by ≥80% growth at <30 μM with CC₅₀ values >50 μM. Two selected tropolones (63 and 285) inhibited 18 methicillin-resistant S. aureus (MRSA) strains with similar MIC₈₀ values as against a reference strain. Two selected thiotropolones (284 and 363) inhibited multidrug-resistant (MDR) E. coli with MIC₈₀ ≤30 μM. One α-HT (261) inhibited MDR-A. baumannii with MIC₈₀ ≤30 μM. This study opens new avenues for development of novel troponoid antibiotics to address the critical need to combat MDR bacterial infections.

Comparison of two techniques for measurement of in vitro killing kinetics of five antibiotics against Pseudomonas aeruginosa

The in vitro activity of ticarcillin, imipenem, gentamicin, amikacin and ciprofloxacin against six Pseudomonas aeruginosa isolates was evaluated by two time-killing curve methods: the conventional broth technique, and another method previously described which employs a transfer filter membrane. The patterns of the killing curves obtained over a 5 h period with the two techniques were similar. In contrast to the results obtained for beta-lactam agents, the reduction of inoculum was great and increased with the concentration for aminoglycosides and ciprofloxacin. After 5 h, regrowth of the six strains tested was observed in broth with all antibiotics, whereas a bactericidal effect was observed over 24 h with the filter membrane method. Further studies are warranted to determine the reasons for this difference.

Ampicillin killing curve patterns for ampicillin-susceptible nontypeable Haemophilus influenzae strains by the agar dilution plate count method

Ampicillin killing curve patterns for 20 strains of ampicillin-susceptible nontypeable Haemophilus influenzae were determined by the agar dilution plate count method. The paradoxical effect was detected in the 24-h killing curve patterns for each strain. For the biphasic effect, minimum survivor percentages (maximum killing) occurred over a narrow range of ampicillin concentrations immediately above the MIC, with survivor percentages then rising rapidly to peak at approximately 1-log10-unit increment higher. The 24-h minimum survivor percentages for the 20 strains ranged from approximately 0.01% (rapid killing) to greater than 10% (slow killing). In comparison with the previous results for typeable strains, the present findings suggest that nontypeable stains are, on average, killed much more slowly. Based on the initial 24-h killing curve patterns for the 20 strains, 4 strains were selected as putative representatives of the range of bactericidal responses encountered. These strains were then studied to examine the reproducibility of the 24-h patterns and to determine sequential killing curves. These patterns were found to be reproducible and served to characterize the relative killing responses of the strains. In the sequential studies of three of the four strains, tiny colonies having the gross and microscopic characteristics of L-forms were found to be present on the agar dilution plate count plates prior to the application of penicillinase at 48 and 72 h. Such colonies reverted to vegetative forms within 24 to 48 h after application of penicillinase to the panels. Of particular interest was the observation that the paradoxical effect was manifested both by the L-form colonies and by the reverted vegetative colonies. The late development of L-forms was observed for both rapidly and slowly killed strains.

Paradoxical activity of beta-lactam antibiotics against Proteus vulgaris in experimental infection in mice

In previous papers (Y. Ikeda and T. Nishino, Antimicrob. Agents Chemother. 32:1073-1077, 1988; Y. Ikeda, T. Nishino, and T. Tanino, Antimicrob. Agents Chemother. 31:865-869, 1987), we reported that many of the 7-aminothiazolyl cephalosporins, such as cefmenoxime, showed paradoxically reduced activity against Proteus vulgaris at higher concentrations, whereas these paradoxical effects were not observed for other types of cephalosporins, such as cefbuperazone and cefoperazone. In this study, we compare the therapeutic effect of cefmenoxime with that of cefbuperazone and explore the in vivo paradoxical effect of cefmenoxime by using an experimental infection model in mice. In an intraperitoneal infection with P. vulgaris 11, the survival rate with cefmenoxime was increased to 43% at 3.13 mg/kg but was lower at higher doses. On the other hand, cefbuperazone did not show such a paradoxical therapeutic effect. In mice infected with P. vulgaris 11, cefmenoxime levels in both serum and peritoneal washings were rapidly reduced and beta-lactamase activities in the peritoneal cavity were increased at higher cefmenoxime doses. These findings suggested that high levels of cefmenoxime at the infection site induced increased production of beta-lactamase, which then rapidly inactivated the antibiotic. We conclude that the paradoxical therapeutic effect of cefmenoxime against P. vulgaris occurs by the same mechanisms as the in vitro effect and that the high beta-lactamase inducibility and low beta-lactamase stability may account for the paradoxical therapeutic effect of cefmenoxime against P. vulgaris.