Antibiotic dosing: do we dose to cure the individual or do we treat the greater societal needs?

Killing of Streptococcus pneumoniae by azithromycin, clarithromycin, erythromycin, telithromycin and gemifloxacin using drug minimum inhibitory concentrations and mutant prevention concentrations

Streptococcus pneumoniae continues to be a significant respiratory pathogen, and increasing antimicrobial resistance compromises the use of β-lactam and macrolide antibiotics. Bacterial eradication impacts clinical outcome, and bacterial loads at the site of infection may fluctuate. Killing of two macrolide- and quinolone-susceptible clinical S. pneumoniae isolates by azithromycin, clarithromycin, erythromycin, telithromycin and gemifloxacin against varying bacterial densities was determined using the measured minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC). For kill experiments, 10(6)-10(9) CFU/mL were exposed to the drug and were sampled at 0, 0.5, 1, 2, 3, 4, 6, 12 and 24 h following drug exposure. The log(10) reduction and percent reduction (kill) of viable cells was recorded. MICs and MPCs (mg/L) for azithromycin, clarithromycin, erythromycin, telithromycin and gemifloxacin were 0.063-0.125/0.5-1, 0.031-0.063/0.25-0.5, 0.063/0.25-0.5, 0.008/0.016 and 0.031/0.25, respectively. Killing 10(6)-10(9) CFU/mL of bacteria by the drug MIC yielded incomplete killing, however log10 reductions occurred by 12 h and 24 h for all drugs. Exposure of 10(6)-10(9) CFU/mL to MPC drug concentrations resulted in the following log(10) reduction by 6h of drug exposure: azithromycin, 1.3-3.9; clarithromycin, 1.9-5.8; erythromycin, 0.8-4.7; telithromycin, 0.3-1.7; and gemifloxacin, 1.8-4.2. Bacterial loads at the site of infection may range from 10(6) to 10(9), and kill experiments utilising a higher bacterial inoculum provided a more accurate measure of antibiotic performance in high biomass situations. Killing was slower with telithromycin. Kill was greater and fastest with MPC versus MIC drug concentrations.

Comparative minimal inhibitory and mutant prevention drug concentrations of four fluoroquinolones against ocular isolates of Haemophilus influenzae

PURPOSE

Haemophilus influenzae is a known ocular pathogen, particularly in the pediatric population. The purpose of this study was to determine the minimal inhibitory concentration (MIC) and mutant prevention concentration (MPC) values of third-generation (i.e., ciprofloxacin and ofloxacin) and fourth-generation (i.e., gatifloxacin and moxifloxacin) fluoroquinolone antibiotics against clinical ocular isolates of H. influenzae.

METHODS

Twenty-six clinical isolates of H. influenzae were evaluated. For MIC testing, approximately 10(5) colony-forming units/mL of test organism was added to an H. influenzae test medium containing twofold dilution increments of each fluoroquinolone. Cultures were incubated for 18 to 24 hours at 35 degrees C to 37 degrees C in 5% CO2. The lowest drug concentration that prevented growth was recorded as the MIC. The lowest drug concentration that inhibited 90% of strains tested was recorded as the MIC90. To determine MPC values, more than 10(10) bacteria were inoculated to agar plates containing varying drug concentrations and incubated for 24 to 48 hours at 35 degrees C to 37 degrees C in 5% CO2. To confirm MPC values, bacteria isolated from drug-containing plates were subcultured to drug-free agar, incubated, and then subcultured to a plate with the same drug concentration. The lowest drug concentration that allowed no growth was recorded as the MPC.

RESULTS

All tested fluoroquinolones were highly active against H. influenzae. The MIC90 values were 0.016 microg/mL for gatifloxacin and ciprofloxacin and 0.031 microg/mL for moxifloxacin and ofloxacin. The MPC90 values were 0.125 microg/mL for gatifloxacin and 0.5 microg/mL for moxifloxacin, ofloxacin, and ciprofloxacin.

CONCLUSIONS

All evaluated fluoroquinolones were effective against H. influenzae. The MIC90 and MPC90 values for gatifloxacin were consistently lower than those for moxifloxacin.

Application of two methods to determine killing of Streptococcus pneumoniae by various fluoroquinolones

Minimum inhibitory concentration (MIC) testing measures the lowest drug concentration that prevents microbial growth using an inoculum of 10(5) colony forming units/ml (cfu/ml) whereas the mutant prevention concentration (MPC) (inoculum approximately 10(10) cells) defines the antimicrobial drug concentration threshold that would require an organism to possess two simultaneous mutations for continued growth in the presence of the drug. The rates at which multidrug-resistant Streptococcus pneumoniae [MDRSP] were killed by the respiratory fluoroquinolones, gatifloxacin, gemfloxacin, levofloxacin and moxifloxacin, were compared based on the MIC and MPC drug concentrations and at inocula ranging from 10(6)-10(9) cfu/ml. The MIC drug concentration failed to eradicate all viable cells whereas the MPC drug concentration resulted in 99.9% to 100% cellular reduction following 12-24 hours of drug exposure. MPC values against S. pneumoniae were different for each fluoroquinolone. The MPC drug concentration prevents the selection of multidrug-resistant or fluoroquinolone-resistant S. pneumoniae. The value of dosing of antimicrobial agents based on MPC thresholds results in a rapid reduction in viable cells--even at higher inocula which are more reflective of organism burden in pneumonia. The rapid reduction in viable cells observed at MPC drug concentrations may not only have an impact on preventing the selection of resistant mutants but may also help explain the rapid symptom resolution seen with new fluoroquinolones since these agents lead to little or low release of cell contents which are known to drive the inflammatory response.

Antibacterial drug discovery—Then, now and the genomics future

Drug discovery research in the area of infectious diseases, in particular that dealing with antibacterial/antibiotic susceptibility and resistance, is in a process of continuing evolution. Steeped in the history of the highly successful intervention with chemotherapeutic agents to treat human infections, the emergence of drug-resistant pathogens worldwide presents a serious unmet medical need, if not a pending catastrophe. Research in both academia and industry over the past 30 years using molecular biology, genetics and more recently--bacterial genomics--has assembled key enabling technologies to increase productivity and success rates in the discovery and development of novel antibacterial agents. However genomics is not limited only to antibacterial target selection but provides the opportunity to further understand key interactions in the use of antibacterial compounds as therapeutic agents (such as resistance emergence, susceptibility, efflux, interactions between compound and pathogen, etc.). Genomics also offers the potential for insights into: bacterial niche adaptation, host susceptibility, treatment regimens, antibiotic resistance, pharmacokinetics (e.g., host metabolism differences), safety and the microbial genesis of chronic diseases (e.g., gastric ulceration).