Different patterns of bacterial DNA synthesis during postantibiotic effect

In Vitro and In Vivo Antibacterial Activities of Patchouli Alcohol, a Naturally Occurring Tricyclic Sesquiterpene, against Helicobacter pylori Infection

This study further evaluated the in vitro and in vivo anti-Helicobacter pylori activities and potential underlying mechanism of patchouli alcohol (PA), a tricyclic sesquiterpene. In the in vitro assay, the capacities of PA to inhibit and kill H. pylori were tested on three standard strains at different pH values and on 12 clinical isolates. The effects of PA on H. pylori adhesion (and its alpA, alpB, and babA genes), motility (and its flaA and flaB genes), ultrastructure, and flagellation were investigated. Moreover, the H. pylori resistance to and postantibiotic effect (PAE) of PA were determined. Furthermore, the in vivo effects of PA on H. pylori eradication and gastritis were examined. Results showed that MICs of PA against three standard strains (pH 5.3 to 9) and 12 clinical isolates were 25 to 75 and 12.5 to 50 μg/ml, respectively. The killing kinetics of PA were time and concentration dependent, and its minimal bactericidal concentrations (MBCs) were 25 to 75 μg/ml. In addition, H. pylori adhesion, motility, ultrastructure, and flagellation were significantly suppressed. PA also remarkably inhibited the expression of adhesion genes (alpA and alpB) and motility genes (flaA and flaB). Furthermore, PA treatment caused a longer PAE and less bacterial resistance than clarithromycin and metronidazole. The in vivo study showed that PA can effectively eradicate H. pylori, inhibit gastritis, and suppress the expression of inflammatory mediators (COX-2, interleukin 1β, tumor necrosis factor alpha, and inducible nitric oxide synthase [iNOS]). In conclusion, PA can efficiently kill H. pylori, interfere with its infection process, and attenuate gastritis with less bacterial resistance, making it a potential candidate for new drug development.

A New Synthetic Peptide Having Two Target of Antibacterial Action in E. coli ML35

The increased resistance of microorganisms to the different antimicrobials available to today has highlighted the need to find new therapeutic agents, including natural and/or synthetic antimicrobial peptides (AMPs). This study has evaluated the antimicrobial activity of synthetic peptide 35409 (RYRRKKKMKKALQYIKLLKE) against Staphylococcus aureus ATCC 29213, Pseudomonas aeruginosa ATCC 15442 and Escherichia coli ML 35 (ATCC 43827). The results have shown that peptide 35409 inhibited the growth of these three bacterial strains, having 16-fold greater activity against E. coli and P. aeruginosa, but requiring less concentration regarding E. coli (22 μM). When analyzing this activity against E. coli compared to time taken, it was found that this peptide inhibited bacterial growth during the first 60 min and reduced CFU/mL 1 log after 120 min had elapsed. This AMP permeabilized the E. coli membrane by interaction with membrane phospholipids, mainly phosphatidylethanolamine, inhibited cell division and induced filamentation, suggesting two different targets of action within a bacterial cell. Cytotoxicity studies revealed that peptide 35409 had low hemolytic activity and was not cytotoxic for two human cell lines. We would thus propose, in the light of these findings, that the peptide 35409 sequence should provide a promising template for designing broad-spectrum AMPs.

d-Methionine reduces tobramycin-induced ototoxicity without antimicrobial interference in animal models

BACKGROUND

Tobramycin is a critical cystic fibrosis treatment however it causes ototoxicity. This study tested d-methionine protection from tobramycin-induced ototoxicity and potential antimicrobial interference.

METHODS

Auditory brainstem responses (ABRs) and outer hair cell (OHC) quantifications measured protection in guinea pigs treated with tobramycin and a range of d-methionine doses. In vitro antimicrobial interference studies tested inhibition and post antibiotic effect assays. In vivo antimicrobial interference studies tested normal and neutropenic Escherichia coli murine survival and intraperitoneal lavage bacterial counts.

RESULTS

d-Methionine conferred significant ABR threshold shift reductions. OHC protection was less robust but significant at 20kHz in the 420mg/kg/day group. In vitro studies did not detect d-methionine-induced antimicrobial interference. In vivo studies did not detect d-methionine-induced interference in normal or neutropenic mice.

CONCLUSIONS

d-Methionine protects from tobramycin-induced ototoxicity without antimicrobial interference. The study results suggest d-met as a potential otoprotectant from clinical tobramycin use in cystic fibrosis patients.

Postantibiotic effect of disinfection treatment by photolysis of hydrogen peroxide

The purpose of the present study was to evaluate the postantibiotic effect (PAE) of the disinfection treatment by photolysis of H2O2. Postantibiotic effect was induced in Staphylococcus aureus and Streptococcus salivarius by exposing the bacteria to H2O2 at concentrations of 250-1000 mmol/l, laser irradiation at a wavelength of 405 nm, and the combination of both (photolysis of H2O2) for 10-30 seconds. The photolysis of H2O2 induced significantly longer PAE than other treatments. The PAE was augmented dependently on not only the concentration of H2O2 but the laser irradiation time. Electron spin resonance analysis showed that the hydroxyl radical was also generated dependently on both the concentration of H2O2 and the laser irradiation time, suggesting that the hydroxyl radicals contribute to the PAE. These results suggest that the disinfection treatment by photolysis of H2O2 induces PAE in S. aureus and S. salivarius even though they were treated for only 10-30 seconds.

Antifungal activity of Coriandrum sativum essential oil, its mode of action against Candida species and potential synergism with amphotericin B

The increasing incidence of drug-resistant pathogens and toxicity of existing antifungal compounds has drawn attention towards the antimicrobial activity of natural products. The aim of the present study was to evaluate the antifungal activity of coriander essential oil according to classical bacteriological techniques, as well as with flow cytometry. The effect of the essential oil upon germ tube formation, seen as an important virulence factor, and potential synergism with amphotericin B were also studied. Coriander essential oil has a fungicidal activity against the Candida strains tested with MLC values equal to the MIC value and ranging from 0.05 to 0.4% (v/v). Flow cytometric evaluation of BOX, PI and DRAQ5 staining indicates that the fungicidal effect is a result of cytoplasmic membrane damage and subsequent leakage of intracellular components such as DNA. Also, concentrations bellow the MIC value caused a marked reduction in the percentage of germ tube formation for C. albicans strains. A synergetic effect between coriander oil and amphotericin B was also obtained for C. albicans strains, while for C. tropicalis strain only an additive effect was observed. This study describes the antifungal activity of coriander essential oil on Candida spp., which could be useful in designing new formulations for candidosis treatment.

Active starvation responses mediate antibiotic tolerance in biofilms and nutrient-limited bacteria

Bacteria become highly tolerant to antibiotics when nutrients are limited. The inactivity of antibiotic targets caused by starvation-induced growth arrest is thought to be a key mechanism producing tolerance. Here we show that the antibiotic tolerance of nutrient-limited and biofilm Pseudomonas aeruginosa is mediated by active responses to starvation, rather than by the passive effects of growth arrest. The protective mechanism is controlled by the starvation-signaling stringent response (SR), and our experiments link SR-mediated tolerance to reduced levels of oxidant stress in bacterial cells. Furthermore, inactivating this protective mechanism sensitized biofilms by several orders of magnitude to four different classes of antibiotics and markedly enhanced the efficacy of antibiotic treatment in experimental infections.

Norfloxacin and ursolic acid: in vitro association and postantibiotic effect against Staphylococcus aureus

AIMS

We investigated the effectiveness in vitro of the association between norfloxacin (NOR) and ursolic acid (UA) against Staphylococcus aureus.

METHODS AND RESULTS

The minimal inhibitory concentrations (MICs), the minimal bactericidal concentrations, the bacterial killing and the postantibiotic effect (PAE) of NOR and UA were determined both singly and in combination. A synergistic interaction was observed against Staph. aureus ATCC 29213: the mean PAEs were 3 h for NOR, -1.2 h for UA (1 × MIC) and 2.0 h for UA (2 × MIC). Synergism was observed with longer PAEs and postantibiotic sub-MIC effects after NOR/UA exposure. UA was also active against clinical isolates and methicillin-resistant Staph. aureus.

CONCLUSIONS

The application of antimicrobial combinations may address the rising resistance to established classes of both systemic and topical agents.

SIGNIFICANCE AND IMPACT OF THE STUDY

In vitro interactions between NOR and UA may contribute to the development of novel topical agents for the treatment of skin infections as well as for topical formulations.

Synergism and postantibiotic effect of tobramycin and Melaleuca alternifolia (tea tree) oil against Staphylococcus aureus and Escherichia coli

The application of antimicrobial combinations may address the rising resistance to established classes of both systemic and topical agents and their clinical relevance is related to the presence of a significant postantibiotic effect (PAE). We investigated the effectiveness in vitro of the association between tobramycin and tea tree oil (TTO) against Gram-positive and Gram-negative bacteria. The minimal inhibitory concentrations, the bacterial killing and the PAE of tobramycin and TTO were determined both singly and in combination against Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 29213. A synergistic interaction was observed against both strains tested: the mean PAEs were 1.3 and 1.7h for tobramycin against E. coli and S. aureus respectively, 10.8h for tobramycin and TTO (0.05%) against E. coli, 10.4h and 17.4h against S. aureus for tobramycin and TTO (0.25 and 0.50%, respectively). Longer PASMEs were observed with S. aureus after TTO/tobramycin exposure. In vitro interactions can improve the antimicrobial effectiveness of the antibiotic and may contribute for the development of novel topical agents for the treatment of skin lesions including conjunctiva and respiratory infections by inhalation.

Time course of microbiologic outcome and gene expression in Candida albicans during and following in vitro and in vivo exposure to fluconazole

Pharmacodynamics (PD) considers the relationship between drug exposure and effect. The two factors that have been used to distinguish the PD behaviors of antimicrobials are the impact of concentration on the extent of organism killing and the duration of persistent microbiologic suppression (postantibiotic effect). The goals of these studies were (i) to examine the relationship between antimicrobial PD and gene expression and (ii) to gain insight into the mechanism of fluconazole effects persisting following exposure. Microarrays were used to estimate the transcriptional response of Candida albicans to a supra-MIC F exposure over time in vitro. Fluconazole at four times the MIC was added to a log-phase C. albicans culture, and cells were collected to determine viable growth and for microarray analyses. We identified differential expression of 18% of all genes for at least one of the time points. More genes were upregulated (n=1,053 [16%]) than downregulated (174 [3%]). Of genes with known function that were upregulated during exposure, most were related to plasma membrane/cell wall synthesis (18%), stress responses (7%), and metabolism (6%). The categories of downregulated genes during exposure included protein synthesis (15%), DNA synthesis/repair (7%), and transport (7%) genes. The majority of genes identified at the postexposure time points were from the protein (17%) and DNA (7%) synthesis categories. In subsequent studies, three genes (CDR1, CDR2, and ERG11) were examined in greater detail (more concentration and time points) following fluconazole exposure in vitro and in vivo. Expression levels from the in vitro and in vivo studies were congruent. CDR1 and CDR2 transcripts were reduced during in vitro fluconazole exposure and during supra-MIC exposure in vivo. However, in the postexposure period, the mRNA abundance of both pumps increased. ERG11 expression increased during exposure and fell in the postexposure period. The expression of the three genes responded in a dose-dependent manner. In sum, the microarray data obtained during and following fluconazole exposure identified genes both known and unknown to be affected by this drug class. The expanded in vitro and in vivo expression data set underscores the importance of considering the time course of exposure in pharmacogenomic investigations.

Influence on Enterobacter cloacae metabolism, cell-surface hydrophobicity and motility of suprainhibitory concentrations of carbapenems

The impact of postantibiotic effect (PAE) of carbapenems (imipenem, meropenem) on the metabolism (biosynthesis of macromolecules, respiration), cell-surface hydrophobicity and motility of a clinical isolate of Enterobacter cloacae was examined. The metabolism was evaluated after 16 h and after 1 d of cultivation using 2x and 4x minimum inhibitory concentrations (MIC) of both antibiotics for the induction of PAE. Imipenem at 4 x MIC did not induce PAE. After a 16-h cultivation (in the postantibiotic phase of both carbapenems), inhibition of nucleosynthesis and protein synthesis was found; after a 1-d cultivation, during regrowth stimulation of mainly 14C-leucine incorporation was found. The presence of the exogenous intermediates of citrate cycle, viz. 2-oxoglutarate, increased the respiratory activity of the cells. The cell-surface hydrophobicity (evaluated by three methods--bacterial adhesion to hydrocarbon, nitrocellulose-filter test and salt-aggregation test) decreased after PAE of both carbapenems; meropenem was more effective. Motility (an important virulence factor) was inhibited in the postantibiotic phase of both carbapenems; the 4 x MIC caused a higher inhibition.

Determination of in vitro postantibiotic effects in Staphylococcus aureus and Escherichia coli by [3H]thymidine incorporation

Postantibiotic effects (PAE) and control-related effective regrowth time (CERT) of dicloxacillin, vancomycin, rifampin and gentamicin in Staphylococcus aureus and imipenem, gentamicin, tobramycin, doxycycline and rifampin in Escherichia coli were measured by standard viability counting and [3H]thymidine incorporation. For PAE determination, the two methods correlated well; r2=0.821 for S. aureus and r2=0.939 for E. coli. For viable counts below the detection limits of 105 to 106 log10 CFU/mL, the PAE was overestimated by the [3H]thymidine method. Quantitation of CERT by both methods showed a good correlation, r2=0.867 for S. aureus and r2=0.997 for E. coli. Measuring [3H]thymidine incorporation in bacteria is a novel alternative method for the determination of PAE and CERT.

Postantibiotic leukocyte enhancement of meropenem against gram-positive and gram-negative strains

The postantibiotic leukocyte enhancement (PALE) of meropenem in vitro in comparison with that of imipenem was evaluated with 24 recently isolated gram-positive and gram-negative strains. In general, pre-exposure to carbapenems (at four times the MIC for 2 h) led to increased polymorphonuclear cell phagocytic killing. The PALE of imipenem was generally significantly less than that observed with meropenem.

Structures of existing and new quinolones and relationship to bactericidal activity against Streptococcus pneumoniae

The in-vitro bactericidal profiles of a number of quinolones against Streptococcus pneumoniae were investigated. Tosufloxacin was found to be the most bactericidal quinolone at the optimum bactericidal:MIC ratio (OBMR), followed by levofloxacin, ciprofloxacin and sparfloxacin, in order of potency. After exposure at the OBMR of each quinolone for 2 h, tosufloxacin showed a post-antibiotic effect (PAE) about 2.3-2.6 times longer than the other quinolones. Compounds with a 2,4-difluorophenyl group at the N-1 position in the quinolone nucleus had the greatest bactericidal activity and PAE. This activity exceeded that found with substitution of the quinolone nucleus at the C-7 position. Although the bactericidal activities of the quinolones correlated well with their PAEs, they were not always consistent with their MICs. These results suggest that bactericidal activity and PAE are governed by factors other than those which determine the MIC values, and a 2,4-difluorophenyl group at the N-1 position in the quinolone nucleus may play an important role in the expression of bactericidal activity and PAE against S. pneumoniae.

Effects of NorA inhibitors on in vitro antibacterial activities and postantibiotic effects of levofloxacin, ciprofloxacin, and norfloxacin in genetically related strains of Staphylococcus aureus

NorA is a membrane-associated multidrug efflux protein that can decrease susceptibility to fluoroquinolones in Staphylococcus aureus. To determine the effect of NorA inhibition on the pharmacodynamics of fluoroquinolones, we evaluated the activities of levofloxacin, ciprofloxacin, and norfloxacin with and without various NorA inhibitors against three genetically related strains of S. aureus (SA 1199, the wild-type; SA 1199B, a NorA hyperproducer with a grlA mutation; and SA 1199-3, a strain that inducibly hyperproduces NorA) using susceptibility testing, time-kill curves, and postantibiotic effect (PAE) methods. Levofloxacin had the most potent activity against all three strains and was minimally affected by addition of NorA inhibitors. In contrast, reserpine, omeprazole, and lansoprazole produced 4-fold decreases in ciprofloxacin and norfloxacin MICs and MBCs for SA 1199 and 4- to 16-fold decreases for both SA 1199B and SA 1199-3. In time-kill experiments reserpine, omeprazole, or lansoprazole increased levofloxacin activity against SA 1199-3 alone by 2 log10 CFU/ml and increased norfloxacin and ciprofloxacin activities against all three strains by 0.5 to 4 log10 CFU/ml. Reserpine and omeprazole increased norfloxacin PAEs on SA 1199, SA 1199B, and SA 1199-3 from 0.9, 0.6, and 0.2 h to 2.5 to 4.5, 1.1 to 1.3, and 0.4 to 1.1 h, respectively; similar effects were observed with ciprofloxacin. Reserpine and omeprazole increased the levofloxacin PAE only on SA 1199B (from 1.6 to 5.0 and 3.1 h, respectively). In conclusion, the NorA inhibitors dramatically improved the activities of the more hydrophilic fluoroquinolones (norfloxacin and ciprofloxacin). These compounds may restore the activities of these fluoroquinolones against resistant strains of S. aureus or may potentially enhance their activities against sensitive strains.

Characteristics and dynamics of bacterial populations during postantibiotic effect determined by flow cytometry

Changes in bacterial ultrastructure after antibiotic exposure and during the postantibiotic effect (PAE) have been demonstrated by electron microscopy (EM). However, EM is qualitative and subject to individual interpretation. In contrast, flow cytometry gives qualitative and quantitative information. The sizes and nucleic acid contents of Escherichia coli and Pseudomonas aeruginosa were studied during antimicrobial exposure as well as during the PAE period by staining the organisms with propidium iodide and analyzing them with flow cytometry and fluorescence microscopy. The effects of ampicillin, ceftriaxone, ciprofloxacin, gentamicin, and rifampin were studied for E. coli, whereas for P. aeruginosa imipenem and ciprofloxacin were investigated. After exposure of E. coli to ampicillin, ceftriaxone, and ciprofloxacin, filamentous organisms were observed by fluorescence microscopy. These changes in morphology were reflected by increased forward light scatter (FSC) and nucleic acid content as measured by flow cytometry. For the beta-lactams the extent of filamentation increased in a dose-dependent manner after drug removal, resulting in formation of distinct subpopulations of bacteria. These changes peaked at 20 to 35 min, and bacteria returned to normal after 90 min after drug removal. In contrast, the subpopulations induced by ciprofloxacin did not return to normal until > 180 min after the end of the classically defined PAE. Rifampin resulted in formation of small organisms with low FSC, whereas no distinctive characteristics were noted after gentamicin exposure. For P. aeruginosa an identifiable subpopulation of large globoid cells and increased nucleic acid content was detected after exposure to imipenem. These changes persisted past the PAE, as defined by viability counting. Swollen organisms with increased FSC were detected after ciprofloxacin exposure, even persisting during bacterial growth. In summary, for beta-lactam antibiotics and ciprofloxacin, the PAE is characterized by dynamic formation of enlarged cell populations of increased nucleic acid content, whereas rifampin induces a decrease in size and nucleic acid content in the organisms. Flow cytometry is an ideal method for future studies of bacterial phenotypic characteristics during the PAE.

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.

Postantibiotic effects and postantibiotic sub-MIC effects of ciprofloxacin, pefloxacin and amikacin on the biological properties of Salmonella strains

The postantibiotic effect (PAE) and the postantibiotic sub-MIC effect (PASME) of ciprofloxacin, pefloxacin and amikacin were studied for Salmonella typhimurium and S. enteritidis strains. PAE was induced by 2 x and 4 x MIC of antibiotics studied for 0.5 h. After PAE and PASME their effect on prophage induction of a lysogenic S. typhimurium strain and on Congo red binding for both strains as a marker of their surface hydrophobicity was examined. The longest PAE was found after treatment with ciprofloxacin, higher values being observed with S. typhimurium. PAEs of pefloxacin and amikacin were much lower, except for the suprainhibitory concentration 4 x MIC of amikacin with S. enteritidis (6.9h). PASMEs of ciprofloxacin did not allow any regrowth of either strain. For other antibiotics the PASMEs were different while concentrations of 2 x MIC + 0.2 x MIC and 0.3 x MIC, and of 4 x MIC + 0.1 x MIC, 0.2 x MIC and 0.3 x MIC of amikacin did not allow any regrowth of S. enteritidis. PAEs of the antibiotics tested did not affect the Congo red binding by both Salmonella strains, but the PAEs of ciprofloxacin and pefloxacin expressively induced a prophage of lysogenic S. typhimurium strain. We noted the influence of Congo red binding after applying 4 x MIC + 0.1 x MIC, 0.2 x MIC and 0.3 x MIC of amikacin for S. typhimurium and 2 x MIC + 0.1 x MIC for S. enteritidis.

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.

Comparison of the postantibiotic and postantibiotic sub-MIC effects of levofloxacin and ciprofloxacin on Staphylococcus aureus and Streptococcus pneumoniae

The postantibiotic subminimum inhibitory concentration effect (PA SME) may simulate in vivo drug exposure more accurately than the postantibiotic effect (PAE) since subinhibitory concentrations of drug persist between antibiotic dosings. In this study, we compared the PAEs and PA SMEs of levofloxacin and ciprofloxacin for clinical isolates of fluoroquinolone-susceptible Staphylococcus aureus and Streptococcus pneumoniae. At two times the MIC, PAEs of levofloxacin were an average of 0.6 h longer than the PAEs obtained for ciprofloxacin for methicillin-susceptible and methicillin-resistant S. aureus strains. The PAEs of levofloxacin and ciprofloxacin ranged from 1.8 to 3.1 and 1.1 to 2.4 h, respectively. Continued exposure of the methicillin-resistant strain to 1/16, 1/8, and 1/4 the MIC resulted in PA SMEs of 6.5, 15.3, and >22.3 h, respectively, for levofloxacin and 3.8, 8.0, and 12.3 h, respectively, for ciprofloxacin. For isolates of S. pneumoniae, at two times the MIC of both fluoroquinolones, the average PAEs of levofloxacin and ciprofloxacin were equivalent: 1.3 h for the penicillin-susceptible isolate and 0.6 h for the penicillin-resistant isolate. Continued exposure of the penicillin-susceptible S. pneumoniae strain to 1/16, 1/8, and 1/4 the MIC resulted in average PA SMEs of 1.0, 1.4, and 2.8 h, respectively, for levofloxacin and 1.8, 2.0, and 2.5 h, respectively, for ciprofloxacin. Continued exposure of penicillin-resistant S. pneumoniae to 1/16, 1/8, and 1/4 the MIC of the same fluoroquinolones resulted in average PA SMEs of 0.6, 1.1, and 2.9 h, respectively, for levofloxacin and 0.6, 1.1, and 1.5 h, respectively, for ciprofloxacin. The PA SMEs observed demonstrate the superior activity of levofloxacin against methicillin-susceptible or methicillin-resistant S. aureus. Although PAEs were similar for the penicillin-susceptible and penicillin-resistant S. pneumoniae strains, the PA SME of levofloxacin at one-fourth the MIC was longer for penicillin-resistant S. pneumoniae.

Use of a new mouse model of Acinetobacter baumannii pneumonia to evaluate the postantibiotic effect of imipenem

Acinetobacter baumannii is responsible for severe nosocomial pneumonia. To evaluate new therapeutic regimens for infections due to multiresistant strains and to study the pharmacodynamic properties of various antibiotics, we developed an experimental mouse model of acute A. baumannii pneumonia. C3H/HeN mice rendered transiently neutropenic were infected intratracheally with 5 x 10(6) CFU of A. baumannii. The mean log10 CFU/g of lung homogenate (+/- the standard deviation) were 9 +/- 0.9, 9.4 +/- 0.8, 8.6 +/- 1.2, and 7.7 +/- 1.4 on days 1, 2, 3, and 4 postinoculation. The lung pathology was characterized by pneumonitis with edema and a patchy distribution of hemorrhages in the peribronchovascular spaces of both lungs. Abscesses formed on days 3 and 4. Four days after inoculation, subacute pneumonitis characterized by alveolar macrophage proliferation and areas of fibrosis was observed. The cumulative mortality on day 4 was 85%. This new model was used to study the effects of 1, 2, or 3 50-mg/kg doses of imipenem. Imipenem concentrations in lungs were above the MIC for 2 h after the last dose. The in vivo postantibiotic effect (PAE) was determined during the 9-h period following the last dose; it decreased in duration with the number of doses: 9.6, 6.4, and 4 h after 1, 2, and 3 50-mg/kg doses, respectively. In contrast, no in vitro PAE was observed. This model offers a reproducible acute course of A. baumannii pneumonia. The presence of a prolonged in vivo PAE supports the currently recommended dosing intervals of imipenem for the treatment of human infections due to A. baumannii, i.e., 15 mg/kg three times a day.

Pharmacodynamic parameters of aminoglycosides and their effect on exoenzymes of Pseudomonas aeruginosa

Aminoglycosides at 2x or 4x minimum inhibitory concentration induced postantibiotic effects against Pseudomonas aeruginosa lasting 3.5-4.9 h (gentamicin) and 0.5-3.7 h (selemycin). Postantibiotic effects of subinhibitory concentrations of the aminoglycosides tested were substantially longer. Some combinations of supra- and subinhibitory concentrations of antibiotics did not even allow any regrowth of the bacterial strain. The postantibiotic effects and postantibiotic effects of subinhibitory concentrations of gentamicin and selemycin were associated with changes of P. aeruginosa elastase and proteinase. Combinations of supra- and subinhibitory concentrations more pronouncedly suppressed enzymic activities than did suprainhibitory concentrations alone.