Dactimicin, a new aminoglycoside: in vitro activity, post-antibiotic effect and interaction with other antibiotics

Synergistic interaction of phenylpropanoids with antibiotics against bacteria

Phenylpropanoids constitute a large part of our daily diet and there is a possibility that they might interact with synthetic drugs. The present work was aimed at studying the interaction of seven phenylpropanoids (cinnamic, p-coumaric, caffeic, chlorogenic, ferulic, 3,4-dimethoxycinnamic and 2,4,5-trimethoxycinnamic acid) with five antibiotics (amikacin, ampicillin, ciprofloxacin, erythromycin and vancomycin) against Gram-negative (Escherichia coli, Enterobacter aerogenes and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria. The interaction studies were performed by chequerboard and time–kill curve assays. Both assays revealed that cinnamic, p-coumaric and ferulic acids were the most active. They combined synergistically with the majority of the antibiotics and exhibited enhanced activity against all the micro-organisms. The time–kill curve parameters were better (P<0.05) for the combinations of amikacin with ferulic, cinnamic or p-coumaric acid than for the individual treatments. Amikacin was the most favourable antibiotic and S. aureus was the most sensitive microbe to most of the combinations. These phenylpropanoids damaged the bacterial membrane as assessed by the LIVE/DEAD BacLight kit, and structure–activity relationship studies indicated that hydrophilic groups enhanced this activity.

Postantibiotic effect of ceftriaxone and gentamicin alone and in combination on Klebsiella pneumoniae, Pseudomonas aeruginosa and Streptococcus viridans

A persistent suppression of bacterial growth following limited exposure to an antimicrobial agent, the postantibiotic effect (PAE), has been described for a variety of antibiotics and microorganisms. In this study the PAE of ceftriaxone and gentamicin was determined in vitro on three strains each of Klebsiella pneumoniae, Pseudomonas aeruginosa and Streptococcus viridans. The strains were exposed to the substances for 2 h at varying concentrations. Ceftriaxone was used at the minimal inhibitory concentration (MIC) and 1/2 MIC and gentamicin at 1/2 MIC, 1/4 MIC, and 1/8 MIC, each alone and in combination. Antibiotic concentrations were reduced by 1,000-fold dilution, bacterial regrowth was consequently monitored by viable count. The PAE of ceftriaxone alone reached up to 145 min (MIC) and 50 min (1/2 MIC), that of gentamicin alone up to 170 min (1/2 MIC), 135 min (1/4 MIC) and 70 min (1/8 MIC), depending on the bacterial species. Combinations of the antibiotics produced longer PAEs than one substance alone; the longest PAE was produced by the combination of ceftriaxone (MIC) and gentamicin (1/2 MIC) lasting up to 320 min (S. viridans). It may be important to take the PAE into account when evaluating dosing intervals.

The in vivo and in vitro postantibiotic effect of aminoglycosides using a clinically isolated micro-organism

We reported a case of abscess in which Serratia marcescens was isolated as the causative organism. We measured the postantibiotic effect (PAE) of dibekacin (DKB) and gentamicin (GM) against S. marcescens and studied the relationship between the clinical effect and the PAE. The minimal inhibitory concentration (MIC) of DKB against S. marcescens was 6.25 micrograms/mL and the serum concentration 30 min after infusion of 100 mg DKB was 5.99 micrograms/mL. The abscess was cured by the administration of DKB every 12 h. The PAE in vivo was 2.5, 2.9 and 3.3 h when DKB was administered at 50 mg/kg, 100 mg/kg and 200 mg/kg, respectively. This PAE is one of the reasons that infection can be effectively treated with intermittent administration, even if the serum concentration is below the MIC.