In vitro studies of pharmacodynamic properties of vancomycin against Staphylococcus aureus and Staphylococcus epidermidis

The bactericidal activities of vancomycin against two reference strains and two clinical isolates of Staphylococcus aureus and Staphylococcus epidermidis were studied with five different concentrations ranging from 2x to 64x the MIC. The decrease in the numbers of CFU at 24 h was at least 3 log10 CFU/ml for all strains. No concentration-dependent killing was observed. The postantibiotic effect (PAE) was determined by obtaining viable counts for two of the reference strains, and the viable counts varied markedly: 1.2 h for S. aureus and 6.0 h for S. epidermidis. The determinations of the PAE, the postantibiotic sub-MIC effect (PA SME), and the sub-MIC effect (SME) for all strains were done with BioScreen C, a computerized incubator for bacteria. The PA SMEs were longer than the SMEs for all strains tested. A newly developed in vitro kinetic model was used to expose the bacteria to continuously decreasing concentrations of vancomycin. A filter prevented the loss of bacteria during the experiments. One reference strain each of S. aureus and S. epidermidis and two clinical isolates of S. aureus were exposed to an initial concentration of 10x the MIC of vancomycin with two different half-lives (t1/2s): 1 or 5 h. The post-MIC effect (PME) was calculated as the difference in time for the bacteria to grow 1 log10 CFU/ml from the numbers of CFU obtained at the time when the MIC was reached and the corresponding time for an unexposed control culture. The difference in PME between the strains was not as pronounced as that for the PAE. Furthermore, the PME was shorter when a t1/2 of 5 h (approximate terminal t1/2 in humans) was used. The PMEs at t1/2s of 1 and 5 h were 6.5 and 3.6 h, respectively, for S. aureus. The corresponding figures for S. epidermidis were 10.3 and less than 6 h. The shorter PMEs achieved with a t1/2 of 5 h and the lack of concentration-dependent killing indicate that the time above the MIC is the parameter most important for the efficacy of vancomycin.     

Variation in postantibiotic effect of clindamycin against clinical isolates of Staphylococcus aureus and implications for dosing of patients with osteomyelitis

Initial measurements of postantibiotic effect (PAE) were made by a standard laboratory method (exposure to 1 mg of clindamycin per liter for 1 h). The range of PAE for 21 strains of Staphylococcus aureus isolated from osteomyelitis patients was 0.4 to 3.9 h, which markedly exceeded the coefficient of variation for the method (6 to 19%). Exposure of S. aureus to three doses of clindamycin at 8-h intervals had no consistent effect on either PAE or MIC. The PAE was dependent on both concentration and duration of exposure to clindamycin: for example, the PAEs for one strain were 1.7 h after exposure to 1 mg/liter for 1 h, 2.4 h after exposure to 4 mg/liter for 1 h, and 5.9 h after exposure to 4 mg/liter for 3 h. Pharmacokinetic simulations showed that the dose required to maintain free serum clindamycin concentrations above the MIC was 300 mg 6 hourly after oral administration (95% confidence interval, 243 to 301 mg) and 1.2 g 6 hourly (95% confidence interval, 305 to 1,145 mg) after intravenous (i.v.) administration. The duration of PAE would have to be at least 2.4 h to allow an increase in the oral dose interval to 8 h or to allow i.v. administration of 300 mg 6 hourly. Additional PAE experiments were performed with the three strains for which PAEs are the shortest after exposure to 1 mg/liter for 1 h (0.4 to 1.2 h). The PAE for these three strains increased markedly to 4.4 to 6.7 h following exposure to 2 mg/liter for 6 h (to mimic the area under the concentration-time curve from 0 to 6 h after a 300-mg dose). These data suggest that oral clindamycin could be administered at 300 mg 8 hourly in the treatment of S. aureus infection, whereas the i.v. dose interval should be 6 h. These suggestions should be confirmed by performing clinical trials.     

Comparison of inhibitory and bactericidal activities and postantibiotic effects of LY333328 and ampicillin used singly and in combination against vancomycin-resistant Enterococcus faecium

One hundred ninety-five individual vancomycin-resistant Enterococcus faecium (VRE) isolates from five upstate New York hospitals were studied for antimicrobial susceptibilities to LY333328, quinupristin-dalfopristin, teicoplanin, ampicillin, and gentamicin. LY333328 was the most active antibiotic against VRE. The effect of media and methods on the antibacterial activity of LY333328, its synergy with ampicillin, and the postantibiotic effects (PAE) of LY333328 and ampicillin were evaluated. In microdilution tests, the MIC of LY333328 at which 90% of the isolates were inhibited (MIC90) was 2 microg/ml in Mueller-Hinton II (MH II) broth and 1 microg/ml in brain heart infusion (BHI) broth. In contrast, on MH II agar the MIC90 was 4 microg/ml and on BHI agar it was >16 microg/ml. Bactericidal activity was observed for most strains at concentrations from 8 to >/=133 times the MIC of the tube macrodilution in MH II broth. A bactericidal effect of LY333328 plus ampicillin was demonstrated in time-kill studies, but there was great strain-to-strain variability. By the MH II agar dilution method, bacteristatic synergy (defined as a fractional inhibitory concentration of <0.5) with LY333328 and ampicillin was demonstrated for 61% of the strains tested. Under similar conditions, there was synergy with LY333328 and quinupristin-dalfopristin or gentamicin for 27 and 15% of the strains tested, respectively. The PAE of LY333328 was prolonged (23.0 h at 10 times the MIC). However, 50% normal pooled human serum decreased the PAE to 12.2 h at 10 times the MIC. Test conditions and media had a considerable effect on VRE susceptibilities to LY333328. The prolonged PAE of LY333328, a potent new bactericidal glycopeptide, and its synergy with ampicillin in a large proportion of strains suggest that further evaluation of this drug in pharmacokinetic studies and experimental infections, including those with VRE, is warranted.     

Duration and clinical relevance of postantibiotic effect in relation to the dosing interval

The influence of half-life on the postantibiotic effect (PAE) of tobramycin against Pseudomonas aeruginosa and Staphylococcus aureus was investigated during one dosing interval. Tobramycin half-lives of 0.5 to 2.5 h were simulated in an in vitro model, and the PAE was determined by an enzymatic inactivation method at different time points, i.e., when the tobramycin concentrations were 20x, 5x, and 1x the MIC. At the time point during therapy when the tobramycin concentrations had declined to 1x the MIC, at a tobramycin half-life of 0.5 h, the times of the PAEs were approximately 0.7 and 1.7 h for P. aeruginosa and S. aureus, respectively, and the PAE disappeared completely at half-lives corresponding to those found in humans (i.e., 2 to 2.5 h). The PAE itself cannot be fully explained by the presence of free intrabacterial tobramycin or the emergence of resistant subpopulations. The explanation for the disappearance of the PAE during the dosing interval may therefore be explained by the repair of sublethal damage. Since the standard method of determining the PAE in animal models is somewhat different from the method used for measurement of the PAE in vitro, the conditions under which the PAE is measured in vivo were also simulated in the in vitro model. This resulted in PAEs longer than those found by the standard method of obtaining in vitro PAE measurements. We conclude that the PAE of tobramycin, as determined by conventional in vitro methods, has virtually no clinical importance. PAEs determined in vivo may have some clinical relevance, but they are probably primarily caused by sub-MIC effects.     

The postantibiotic effect of imipenem: relationship with drug concentration, duration of exposure, and MIC

The postantibiotic effect (PAE) of imipenem against Escherichia coli was measured at a wide variety of drug concentrations and times of exposure. We observed that the area under the concentration-time curve of drug exposure (AUC), the product of time of exposure and concentration of drug, is a much better predictor of the duration of the PAE than either parameter alone. We also measured the PAE of imipenem against strains of gram-positive and gram-negative bacteria for which MICs varied widely. The E50, the AUC required to produce 50% of the maximum PAE, is correlated with the MIC and is independent of species. This may explain why the duration of the PAE differs for bacteria of the same species for which MICs are different.     

Postantibiotic effects and postantibiotic effects of subinhibitory levels of quinolines on Pseudomonas aeruginosa

The postantibiotic effect (PAE) (PA phase induced by 2x or 4x MIC) and postantibiotic effect of subinhibitory concentrations (PA SME) of three quinolones-enoxacin, norfloxacin and pefloxacin against Pseudomonas aeruginosa were determined. It was observed that the tested antibiotics at suprainhibitory concentrations 2x MIC induced PAEs of 2.1-6.7 h. Longer PAEs were observed after treatment with antibiotics at concentration 4x MIC (4.1-9.0 h). Subinhibitory concentrations of quinolones added to bacterial suspensions in the postantibiotic phase (2x or 4x MIC) displayed longer delay of bacterial regrowth (PA SMEs) compared to PAEs. No regrowth of bacterial culture was observed even at 48 h in the case of pefloxacin (2x MIC + 0.3x MIC, 4x MIC +0.2x MIC, 4x MIC + 0.3x MIC) as well as norfloxacin (4x MIC + 0.2x MIC, 4x MIC + 0.3x MIC).     

Sensitivity to antibiotics in coagulase-negative staphylococci isolated from patients with central venous catheters

We examined 25 coagulase-negative staphylococci isolated from children, of whom 17 with leukaemia and 8 with terminal renal failure. Strain identification performed by api Staph system revealed the presence of S. epidermidis in 21 children, S. hominis in 3 patients and S. haemolyticus in 1 patient. By diffusion method we examined the activity of penicillin, methicillin, cephalexin, cephtriaxon, lincomycin, erythromycin, vancomycin, co-trimoxasol, gentamicin, amikacin, chloramphenicol, rifampicin and fusidic acid. MICs of seven antibiotics were obtained by agar dilution method. MIC50 and MIC90 were as follows: /ml Methicillin 3.13 mg/ml and 50 mg/ml, lincomycin 100 mg/ml and 100 mg/ml, gentamicin 25 mg/ml and 100 mg/ml, chloramphenicol 6.25 mg/ml and 50 mg/ml, amikacin 1.56 mg/ml and 100 mg/ml, rifampicin 0.09 mg/ml and 12.5 mg/ml, fusidic acid 6.25 mg/ml and 12.5 mg/ml, vancomycin 1.56 mg/ml and 3.13 mg/ml. These data show that the examined strains are highly resistant to numerous antibiotics. Thirty six percent of all strains were resistant to methicillin, 88% to lincomycin, 60% to gentamicin, 52% to chloramphenicol, 24% to amikacin, 52% to rifampicin and 56% to fuscidic acid. All the examined strains were sensitive to vancomycin.     

Prospects of therapeutic intervention in drug addiction in prisons

The biochemical mechanisms of resistance to fusidic acid in Staphylococcus aureus were investigated. Organisms possessing plasmid genes for resistance showed a high basal level of resistance, but could be induced to higher levels after pre-incubation with fusidic acid. This induction occurred rapidly and probably did not depend on gene dosage effects. Mutants resistant to fusidic acid, obtained from plasmid-negative cultures, expressed resistance constitutively. Protein synthesis in cell-free extracts from staphylococci with plasmid-mediated resistance to fusidic acid was as sensitive to fusidic acid as was synthesis in preparations from sensitive organisms; whereas protein synthesis in preparations from a spontaneous fusidic acid resistant mutant was resistant to the antibiotic. None of the resistant strains caused detectable inactivation of fusidic acid and no new derivative of fusidic acid was found in culture extracts of plasmid-possessing organisms grown in the presence of radioactive antibiotic. Expression of plasmid-mediated resistance to fusidic acid was associated with a decrease in the molar ratio of phosphatidylglycerol to lysylphosphatidylglycerol, but the cardiolipin content remained constant.     

Postantibiotic effect and postantibiotic sub-MIC effect of levofloxacin compared to those of ofloxacin, ciprofloxacin, erythromycin, azithromycin, and clarithromycin against 20 pneumococci

The postantibiotic effect (PAE) (10 times the MIC of quinolones, 5 times the MIC of macrolides) and postantibiotic sub-MIC effect (PAE-SME) at 0.125, 0.25, and 0.5 times the MIC were determined for levofloxacin, ciprofloxacin, ofloxacin, erythromycin, azithromycin, and clarithromycin against 20 pneumococci. Quinolone PAEs ranged between 0.5 and 6.5 h, and macrolide PAEs ranged between 1 and 6 h. Measurable PAE-SMEs (in hours) at the three concentrations were 1 to 5, 1 to 8, and 1 to 8, respectively, for quinolones and 1 to 8, 1 to 8, and 1 to 6, respectively, for macrolides.     

Studies of the killing kinetics of benzylpenicillin, cefuroxime, azithromycin, and sparfloxacin on bacteria in the postantibiotic phase

Most antibiotics are known to be incapable of killing nongrowing or slowly growing bacteria with few exceptions. Bacterial cell division is inhibited during the postantibiotic phase (PA phase) after short exposure to antibiotics. Only scarce and conflicting data are available concerning the ability of antibiotics to kill bacteria in the PA phase. The aim of the present study was to investigate the killing effect of four different antibiotics on bacteria in the PA phase. A postantibiotic effect (PAE) was induced by exposing Streptococcus pyogenes and Haemophilus influenzae to 10x MICs of benzylpenicillin, cefuroxime, sparfloxacin, and azithromycin. The bacteria were thereafter reexposed to a 10x MIC of the same antibiotic used for the induction of the PAE at the beginning of and after 2 and 4 h in the PA phase. Due to a very long PAE, the bacteria in PA phase induced by azithromycin were also exposed to 10x MICs after 6 and 8 h. A previously unexposed culture exposed to a 10x MIC was used as a control. The results seem to be dependent on both the antibiotic used and the bacterial species. The antibiotics exhibiting a fork bactericidal action gave significantly reduced killing of the bacteria in PA phase (cefuroxime with S. pyogenes, P < 0.01, and sparfloxacin with H. influenzae, P < 0.001), which was restored at 4 h for cefuroxime with S. pyogenes. There was a tendency to restoration of the bactericidal activity also with sparfloxacin and H. influenzae, but there was still a significant difference in killing between the control and the test bacteria in PA phase at 4 h. However, in the combinations with a lesser bactericidal effect (benzylpenicillin with S. pyogenes and sparfloxacin with S. pyogenes), there was no difference in killing between the control and the test bacteria in PA phase. Azithromycin induced long PAEs in both S. pyogenes and H. influenzae and exhibited a slower bactericidal action on both the control and the bacteria in PA phase especially at the end of the PAE, when the killing was almost bacteriostatic. Our findings in this study support the concept that a long interval (> 12 h) between doses of azithromycin, restoring full bactericidal action, may be beneficial to optimize efficacy of this drug but is not necessary for the other antibiotics evaluated, since the bactericidal effect seems to be restored already at 4 h.     

Utilization of a new strength citrate anticoagulant during centrifugal plateletpheresis. I. Assessment of donor effects

The minimal inhibitory concentrations of 32 antimicrobial agents were established for 73 strains of Propionibacterium acnes and four related species (P. granulosum, P. avidum, Corynebacterium minutissimum, and C. parvum). Most strains showed good susceptibility to those agents usually considered active against gram-positive organisms. With the exception of C. minutissimum, the strains tested revealed more or less identical susceptibility ranges. The lowest minimal inhibitory concentrations were observed with benzylpenicillin, ampicillin, cephalothin, rifampin, erythromycin, clindamycin, and minocycline. C. minutissimum was more susceptible to gentamicin, sisomicin, tobramycin, and fusidic acid but more resistant to most other drugs than were the other species examined.     

MIC and time-kill study of antipneumococcal activities of RPR 106972 (a new oral streptogramin), RP 59500 (quinupristin-dalfopristin), pyostacine (RP 7293), penicillin G, cefotaxime, erythromycin, and clarithromycin against 10 penicillin-susceptible and -resistant pneumococci

Broth MICs and time-kill studies were used to test the activity of RP 59500 (quinupristin-dalfopristin), RPR 106972, pyostacine (RP 7293), erythromycin, clarithromycin, and cefotaxime for four penicillin-susceptible (MICs of 0.008 to 0.03 microgram/ml), two penicillin-intermediate (MIC of 0.25 microgram/ml), and four penicillin-resistant (MIC of 2.0 to 4.0 micrograms/ml) strains of pneumococci: 6 of 10 strains were resistant to macrolides (MICs of > or = 0.5 microgram/ml). MICs of RP 59500 (0.5 to 1.0 microgram/ml), RPR 106972 (0.125 to 0.25 microgram/ml), and pyostacine (0.125 to 0.25 microgram/ml) did not alter with the strain's penicillin or macrolide susceptibility status. Three penicillin-susceptible strains and one penicillin-intermediate strain were susceptible to macrolides (MICs of < or = 0.25 microgram/ml); the macrolide MICs for the remaining strains were > or = 4.0 micrograms/ml. Cefotaxime MICs rose with those of penicillin G, but all strains were inhibited at MICs of < or = 2.0 micrograms/ml. RP 59500 was bactericidal for all strains after 24 h at 2 x MIC and yielded 90% killing of all strains at 6 h at 2 x MIC; at 8 x MIC, RP 59500 showed 90% killing of six strains within 10 min (approximately 0.2 h). In comparison, RPR 106972 was bactericidal for 9 of 10 strains at 2 x MIC after 24 h and yielded 90% killing of all strains at 2 x MIC after 6 h; 90% killing of six strains was found at 8 x MIC at 0.2 h. Results for pyostacine were similar to those of RPR 106972. Erythromycin and clarithromycin were bactericidal for three of four macrolide-susceptible strains after 24 h at 4 x MIC. Clarithromycin yielded 90% killing of three strains at 8 x MIC after 12 h. Cefotaxime was bactericidal for all strains after 24 h at 4 x MIC, yielding 90% killing of all strains after 6 h at 4 x MIC. All three streptogramins yielded rapid killing of penicillin- and erythromycin-susceptible and -resistant pneumococci and were the only compounds which killed significant numbers of strains at 0.2 h.     

Non-alcoholic steatohepatitis (NASH): a disease of emerging identity and importance

The agar dilution MIC method was used to test the activity of cefminox, a beta-lactamase-stable cephamycin, compared with those of cefoxitin, cefotetan, moxalactam, ceftizoxime, cefotiam, cefamandole, cefoperazone, clindamycin, and metronidazole against 357 anaerobes. Overall, cefminox was the most active beta-lactam, with an MIC at which 50% of isolates are inhibited (MIC50) of 1.0 microg/ml and an MIC90 of 16.0 microg/ml. Other beta-lactams were less active, with respective MIC50s and MIC90s of 2.0 and 64.0 microg/ml for cefoxitin, 2.0 and 128.0 microg/ml for cefotetan, 2.0 and 64.0 microg/ml for moxalactam, 4.0 and > 128.0 microg/ml for ceftizoxime, 16.0 and > 128.0 microg/ml for cefotiam, 8.0 and >128.0 microg/ml for cefamandole, and 4.0 and 128.0 microg/ml for cefoperazone. The clindamycin MIC50 and MIC90 were 0.5 and 8.0 microg/ml, respectively, and the metronidazole MIC50 and MIC90 were 1.0 and 4.0 microg/ml, respectively. Cefminox was especially active against Bacteroides fragilis (MIC90, 2.0 microg/ml), Bacteroides thetaiotaomicron (MIC90, 4.0 microg/ml), fusobacteria (MIC90, 1.0 microg/ml), peptostreptococci (MIC90, 2.0 microg/ml), and clostridia, including Clostridium difficile (MIC90, 2.0 microg/ml). Time-kill studies performed with six representative anaerobic species revealed that at the MIC all compounds except ceftizoxime were bactericidal (99.9% killing) against all strains after 48 h. At 24 h, only cefminox and cefoxitin at 4x the MIC and cefoperazone at 8x the MIC were bactericidal against all strains. After 12 h, at the MIC all compounds except moxalactam, ceftizoxime, cefotiam, cefamandole, clindamycin, and metronidazole gave 90% killing of all strains. After 3 h, cefminox at 2 x the MIC produced the most rapid effect, with 90% killing of all strains.     

Pharmacodynamic effects of antibiotics and acid pump inhibitors on Helicobacter pylori

Pharmacodynamic studies of Helicobacter pylori exposed to amoxicillin, clarithromycin, metronidazole, omeprazole, and lansoprazole were performed with microscopy, viable count determination, and bioluminescence assay of intracellular ATP. The pharmacodynamic parameters determined were change in morphology, change in cell density, postantibiotic effect (PAE), and control-related effective regrowth time (CERT). The PAE is delayed regrowth after brief exposure to antibiotics or acid pump inhibitors. CERT was defined as the time required for the bacteria to resume logarithmic growth and return to the pre-exposure inoculum in the test culture minus the corresponding time for the control culture. CERT measures the combined effect of initial killing and PAE. There was a good concordance between the bioluminescence assay and viable counts for determining CERT, which makes this parameter useful for pharmacodynamic studies of the effects of antibiotics and acid pump inhibitors on H. pylori. Amoxicillin and metronidazole produced a strong, concentration-dependent initial decrease in CFU per milliliter, but there was a less prominent initial change in intracellular ATP in these cultures. Amoxicillin caused a long PAE when assayed by the bioluminescence assay but no PAE or a negative PAE when assayed by viable count determination. However, amoxicillin showed similar long CERTs with both methods. The pharmacodynamic effects of amoxicillin were concentration dependent up to a maximum response, indicating that concentrations above this level do not increase the antibiotic effect. The PAEs and CERTs of clarithromycin and metronidazole were concentration dependent with no maximum response. With omeprazole and lanzoprazole, there was no PAE or CERT.     

Postantibiotic effect of imipenem, alone and in combination with amikacin, on Pseudomonas aeruginosa

Imipenem and amikacin, alone and in combination, were investigated for their postantibiotic effect (PAE) on Pseudomonas aeruginosa. Four clinical strains of P. aeruginosa in the logarithmic phase of growth were exposed for 1 h to antibiotics, alone and in combination. Recovery periods of test cultures were evaluated after dilution using viable counting. Imipenem produced a PAE ranging from 0.7 to 1.55 h. Similar PAEs were induced by amikacin (ranging from 0.65 to 2 h). In combination, imipenem and amikacin produced as a final PAE (ranging from 1.6 to 2.65 h), a rough mathematical sum of the individual effects. The finding of this study may have important implications for the timing of doses during therapy with antimicrobial combinations.     

Elimination of virulence plasmids of Yersinia pseudotuberculosis as affected by fluoroquinolones

Pefloxacin, ciprofloxacin and norfloxacin were shown to induce elimination of the virulence plasmids in the strains of Y. pseudotuberculosis. Pefloxacin had a more pronounced eliminating effect than ciprofloxacin and norfloxacin. The elimination of the virulence plasmids with the molecular weight of 40-50 MD in the strains of Y. pseudotuberculosis was more frequent at the subinhibitory concentrations of the fluoroquinolones or at 1/4 of the fluoroquinolone MIC., At 1/8 of the MIC it was rarer. The virulence plasmid elimination in the strains was observed in spite of their serovar but was more frequent in the serovar 1B strains.     

Postantibiotic effect of amikacin, rifampin, sparfloxacin, clofazimine and clarithromycin against Mycobacterium avium

Antimycobacterial drugs acting efficiently against Mycobacterium avium complex have in common low MICs and MBC/MIC ratios. The recently reported clinical efficacy of some of the newer drugs is also clearly linked to their pharmacokinetic properties such as higher serum level and/or intracellular concentrations and half-life. In the present investigation, comparative postantibiotic effects (PAEs) of amikacin, rifampin, sparfloxacin, clofazimine and clarithromycin were investigated. Bacteria were exposed to MIC, MIC x 4 and MIC x 8 concentrations of each drug for 2 h, the drug was removed by centrifugation and cells were thoroughly washed and resuspended in drug-free medium. Growth was compared to control organisms which underwent a similar treatment (but without drugs) and PAEs were assessed using the equation "T-C", where T equals the time required for colony counts to increase by 1 log10 in test samples after antibiotic exposure and C equals the time for 1 log10 growth in control. Our results underlined two distinct patterns concerning PAE: pattern I included drugs for which PAE (in hours) was dose-dependent and varied (for MIC, MIC x 4 and MIC x 8 concentrations) for amikacin (10.3 +/- 1.7, 14.7 +/- 1.9 and 17.7 +/- 4.1), rifampin (28.0 +/- 7.6, 62.0 +/- 18.5 and 71.0 +/- 3.2) and clarithromycin (2.6 +/- 1.0, 15.0 +/- 4.0 and 22.0 +/- 4.0), whereas pattern II included drugs with a stable PAE, relatively independent of the drug concentrations: sparfloxacin (11.0 +/- 2.5, 12.3 +/- 6.4 and 13.0 +/- 2.1) and clofazimine (26.0 +/- 2.8, 28.8 +/- 2.5 and 27.3 +/- 1.3). These results may be useful for guidance in scheduling of drug administration in M. avium-infected AIDS patients overburdened with too many drugs given for various opportunistic infections.     

Susceptibilities of penicillin- and erythromycin-susceptible and -resistant pneumococci to HMR 3647 (RU 66647), a new ketolide, compared with susceptibilities to 17 other agents

Susceptibility of 230 penicillin- and erythromycin-susceptible and -resistant pneumococci to HMR 3647 (RU 66647), a new ketolide, was tested by agar dilution, and results were compared with those of erythromycin, azithromycin, clarithromycin, roxithromycin, rokitamycin, clindamycin, pristinamycin, ciprofloxacin, sparfloxacin, trimethoprim-sulfamethoxazole, doxycycline, chloramphenicol, cefuroxime, ceftriaxone, imipenem, and vancomycin. HMR 3647 was very active against all strains tested, with MICs at which 90% of the strains were inhibited (MIC90s) of 0.03 microg/ml for erythromycin-susceptible strains (MICs, < or =0.25 microg/ml) and 0.25 microg/ml for erythromycin-resistant strains (MICs, > or =1.0 microg/ml). All other macrolides yielded MIC90s of 0.03 to 0.25 and >64.0 microg/ml for erythromycin-susceptible and -resistant strains, respectively. The MICs of clindamycin for 51 of 100 (51%) erythromycin-resistant strains were < or =0.125 microg/ml. The MICs of pristinamycin for all strains were < or =1.0 microg/ml. The MIC90s of ciprofloxacin and sparfloxacin were 4.0 and 0.5 microg/ml, respectively, and were unaffected by penicillin or erythromycin susceptibility. Vancomycin and imipenem inhibited all strains at < or =1.0 microg/ml. The MICs of cefuroxime and cefotaxime rose with those of penicillin G. The MICs of trimethoprim-sulfamethoxazole, doxycycline, and chloramphenicol were variable but were generally higher in penicillin- and erythromycin-resistant strains. HMR 3647 had the best kill kinetics of all macrolides tested against 11 erythromycin-susceptible and -resistant strains, with uniform bactericidal activity (99.9% killing) after 24 h at two times the MIC and 99% killing of all strains at two times the MIC after 12 h for all strains. Pristinamycin showed more rapid killing at 2 to 6 h, with 99.9% killing of 10 of 11 strains after 24 h at two times the MIC. Other macrolides showed significant activity, relative to the MIC, against erythromycin-susceptible strains only.     

5-HT2A receptor subtype is involved in the thermal hyperalgesic mechanism of serotonin in the periphery

Quinupristin/dalfopristin (RP59500) is a novel streptogramin and a semisynthetic derivative of pristinamycins IA and IIB. The following properties of RP59500 were investigated: (i) its in-vitro activity against 164 hospital isolates of Staphylococcus aureus, 101 of which were methicillin-resistant (MRSA); (ii) its killing effect against 24 MRSA and seven methicillin-susceptible (MSSA) isolates; (iii) its interactions with rifampicin and ciprofloxacin against 18 MRSA isolates, six susceptible to both rifampicin and ciprofloxacin and 12 resistant to both, at 1 x MIC, 2 x MIC and 4 x MIC. Rifampicin and ciprofloxacin were applied at a concentration equal to their mean serum levels in order to establish the clinical relevance of the results. The MIC50, MIC90, MBC50 and MBC90 of quinupristin/dalfopristin were, respectively, < or = 0.015, 2, 0.12 and 2 mg/L for MRSA isolates and < or = 0.015, 0.06, < or = 0.015 and 0.25 mg/L for MSSA isolates. All isolates were inhibited by quinupristin/dalfopristin. Its killing effect varied with concentration and time, being optimal at 4 x MIC and after 24 h growth. Strains surviving 24 h exposure to this agent had much higher MICs than the parent strain, but only a limited number of them became resistant. Quinupristin/dalfopristin at 2 x MIC and 4 x MIC showed in-vitro synergy with rifampicin against highly resistant isolates mainly at 6 h and 24 h of growth involving 50-83% of MRSA isolates, and showed synergy with ciprofloxacin at 24 h involving 42-75% of isolates. The MIC increase in colonies surviving at 24 h was restricted by the presence of rifampicin or ciprofloxacin. In contrast, the above combinations acted synergically over the total number of MRSA strains susceptible to both rifampicin and ciprofloxacin. The above findings show that quinupristin/dalfopristin is a very potent antistaphylococcal agent, and that its activity against MRSA isolates is enhanced when it is combined with rifampicin or ciprofloxacin.