The Lon protease of Pseudomonas aeruginosa is induced by aminoglycosides and is involved in biofilm formation and motility

Pseudomonas aeruginosa is an important nosocomial opportunistic human pathogen and a major cause of chronic lung infections in individuals with cystic fibrosis. Serious infections by this organism are often treated with a combination of aminoglycosides and semi-synthetic penicillins. Subinhibitory concentrations of antibiotics are now being recognized for their role in microbial persistence and the development of antimicrobial resistance, two very important clinical phenomena. An extensive screen of a P. aeruginosa PAO1 luciferase gene fusion library was performed to identify genes that were differentially regulated during exposure to subinhibitory gentamicin. It was demonstrated that subinhibitory concentrations of gentamicin and tobramycin induced a set of genes that are likely to affect the interaction of P. aeruginosa with host cells, including the gene encoding Lon protease, which is known to play a major role in protein quality control. Studies with a lon mutant compared to its parent and a complemented strain indicated that this protein was essential for biofilm formation and motility in P. aeruginosa.

Involvement of nitric oxide in biofilm dispersal of Pseudomonas aeruginosa

Bacterial biofilms at times undergo regulated and coordinated dispersal events where sessile biofilm cells convert to free-swimming, planktonic bacteria. In the opportunistic pathogen Pseudomonas aeruginosa, we previously observed that dispersal occurs concurrently with three interrelated processes within mature biofilms: (i) production of oxidative or nitrosative stress-inducing molecules inside biofilm structures, (ii) bacteriophage induction, and (iii) cell lysis. Here we examine whether specific reactive oxygen or nitrogen intermediates play a role in cell dispersal from P. aeruginosa biofilms. We demonstrate the involvement of anaerobic respiration processes in P. aeruginosa biofilm dispersal and show that nitric oxide (NO), used widely as a signaling molecule in biological systems, causes dispersal of P. aeruginosa biofilm bacteria. Dispersal was induced with low, sublethal concentrations (25 to 500 nM) of the NO donor sodium nitroprusside (SNP). Moreover, a P. aeruginosa mutant lacking the only enzyme capable of generating metabolic NO through anaerobic respiration (nitrite reductase, DeltanirS) did not disperse, whereas a NO reductase mutant (DeltanorCB) exhibited greatly enhanced dispersal. Strategies to induce biofilm dispersal are of interest due to their potential to prevent biofilms and biofilm-related infections. We observed that exposure to SNP (500 nM) greatly enhanced the efficacy of antimicrobial compounds (tobramycin, hydrogen peroxide, and sodium dodecyl sulfate) in the removal of established P. aeruginosa biofilms from a glass surface. Combined exposure to both NO and antimicrobial agents may therefore offer a novel strategy to control preestablished, persistent P. aeruginosa biofilms and biofilm-related infections.

Three-year analysis of microbial aetiology and antimicrobial susceptibilities of PD peritonitis

The first-line antibiotic treatment of peritoneal dialysis (PD) peritonitis has to cover the most common causative microorganisms. Our aim was to analyse antimicrobial sensitivities of different empirical protocols for initial therapy of PD peritonitis. We analysed the aetiological microorganisms of PD peritonitis and their antimicrobial sensitivities during a 36-month period. Clinical characteristics of the cases were recorded. Altogether 86 PD peritonitides were diagnosed during the study period. In 58 cases, microbial cultures were positive with 72 different causative agents. 28 cases (33%) were culture-negative. Over-representation of icodextrin users was noted among the culture-negative cases. Staphylococcus aureus was the most frequent causative agent, often leading to severe course of illness. Of antimicrobial protocols for initial treatment of peritonitis tested in vitro, the combination of a first-generation cephalosporin and an aminoglycoside was superior to the combination of a first-generation cephalosporin and ceftazidime as well as to fluoroquinolone monotherapy but similar to the combination of vancomycin and ceftazidime. Based on antimicrobial sensitivities we continue using an aminoglycoside in the empirical treatment of PD peritonitis. In the present material, users of icodextrin PD fluid were over-represented among patients with culture-negative peritonitis.

Controlling the rate of organic reactions: rational design of allosteric Diels-Alderase ribozymes

Allosteric mechanisms are widely used in nature to control the rates of enzymatic reactions, but little is known about RNA catalysts controlled by these principles. The only natural allosteric ribozyme reported to date catalyzes an RNA cleavage reaction, and so do almost all artificial systems. RNA has, however, been shown to accelerate a much wider range of chemical reactions. Here we report that RNA catalysts for organic reactions can be put under the stringent control of effector molecules by straight-forward rational design. This approach uses known RNA sequences with catalytic and ligand-binding properties, and exploits weakly conserved sequence elements and available structural information to induce the formation of alternative, catalytically inactive structures. The potential and general applicability is demonstrated by the design of three different systems in which the rate of a catalytic carbon–carbon bond forming reaction is positively regulated up to 2100-fold by theophylline, tobramycin and a specific mRNA sequence, respectively. Although smaller in size than a tRNA, all three ribozymes show typical features of allosteric metabolic enzymes, namely high rate acceleration and tight allosteric regulation. Not only do these findings demonstrate RNA's power as a catalyst, but also highlight on RNA's capabilities as signaling components in regulatory networks.

Detection of Methicillin/Oxacillin Resistance and Typing in Aminoglycoside-Susceptible Methicillin-Resistant and Kanamycin-Tobramycin-Resistant Methicillin-SusceptibleStaphylococcus aureus

Eighty-five atypical isolates of Staphylococcus aureus divided into 73 aminoglycoside-susceptible methicillinresistant (AS-MRSA) and 12 kanamycin-tobramycin-resistant methicillin-susceptible (KTR-MSSA) were phenotypically and genotypically examined for methicillin resistance. Among these tests, the diffusion method using the oxacillin and cefoxitin disks on Mueller-Hinton agar with and without NaCl, the incubation at 35 degrees C or 30 degrees C for 24 or 48 hr, respectively, and the determination of oxacillin MICs by E-test were performed. We also examined the presence of the mecA gene by PCR and its product PBP 2a by the Slidex MRSA Detection test after induction by cefoxitin disk. All of the AS-MRSA strains (100%) were detected by the cefoxitin disk in all conditions and by the oxacillin disk on Mueller-Hinton agar with 2% of NaCl at 35 degrees C. Without NaCl, the sensitivity fell to 97.2% by oxacillin disk. The oxacillin MICs for these isolates ranged from 2 to 128 mg/L. The mecA gene determinant and its product PBP 2a were detected in all AS-MRSA strains. All KTR-MSSA strains were phenotypically methicillin-susceptible and oxacillin MICs were below or borderline of breakpoint (< or =2 mg/L). The mecA gene determinant and its product were detected in one strain. Pulsed-field gel electrophoresis (PFGE) was applied and revealed the presence of two major patterns A (36.9%) and B (46.2%) in AS-MRSA isolates and seven patterns in the KTR-MSSA strains.

Enhancement of antimicrobial activity against pseudomonas aeruginosa by coadministration of G10KHc and tobramycin

Pseudomonas aeruginosa is a common opportunistic human pathogen that is associated with life-threatening acute infections and chronic airway colonization during cystic fibrosis. Previously, we converted the wide-spectrum antimicrobial peptide novispirin G10 into a selectively-targeted antimicrobial peptide (STAMP), G10KHc. Compared to novispirin G10, the STAMP had an enhanced ability to kill Pseudomonas mendocina. In this study, we explored the activity of G10KHc against P. aeruginosa. G10KHc was found to be highly active (as active as tobramycin) against P. aeruginosa clinical isolates. Most interestingly, we observed a synergistic-like enhancement in killing activity when biofilms and planktonic cultures of P. aeruginosa were cotreated with G10KHc and tobramycin. The data indicate that the mechanism of enhanced activity may involve increased tobramycin uptake due to G10KHc-mediated cell membrane disruption. These results suggest that G10KHc may be useful against P. aeruginosa during acute and chronic infection states, especially when it is coadministered with tobramycin.

Mutant prevention concentrations of levofloxacin alone and in combination with azithromycin, ceftazidime, colistin (Polymyxin E), meropenem, piperacillin-tazobactam, and tobramycin against Pseudomonas aeruginosa

The mutant prevention concentrations (MPCs) of levofloxacin alone and in combination with ceftazidime, colistin (polymyxin E), meropenem, piperacillin-tazobactam, and tobramycin were established against Pseudomonas aeruginosa. Antibiotic combinations using levofloxacin with any antibiotic with individual activity against P. aeruginosa resulted in synergistic lowering (an at-least-fourfold reduction) of the isolate's MPC.

Once-daily tobramycin in cystic fibrosis: better for clinical outcome than thrice-daily tobramycin but more resistance development?

OBJECTIVES

Once-daily administration of aminoglycosides in cystic fibrosis (CF) patients is considered equally efficacious and potentially less nephrotoxic than dosing three times a day. However, the choice of the most suitable PK/PD index (C(max)/MIC versus AUC(24)/MIC) to ensure optimum clinical outcome in this patient population is not clear.

PATIENTS AND METHODS

In a single-centre, open, randomized, controlled, non-blinded study 33 adult CF patients (20 females, 19-37 years) were treated with intravenous tobramycin (10 mg/kg/day) for 14 days given either as single dose once a day (Q24; 17 patients) or divided into three equal doses every 8 h (Q8; 16 patients). Tobramycin serum concentrations and MICs for Pseudomonas aeruginosa were determined on days 1 and 14. The clinical outcome parameter, correlated to PK/PD indices, was the percentage predicted forced expiratory volume in 1 s (FEV(1)% pred.).

RESULTS

FEV(1)% pred. improved significantly for both treatments. There was a log-linear relationship between C(max)/MIC and FEV(1)% pred. and AUC/MIC and FEV(1)% pred. for both treatments. For equal values of AUC24/MIC, however, Q24 treatment provided better improvement in lung function than Q8 dosing, whereas C(max)/MIC did not show any dosing interval dependence. A statistically significant increase was observed for MIC (day 1) versus MIC (day 14) for Q24 treatment, however, no such difference was observed for Q8 treatment.

CONCLUSIONS

The most important PK/PD parameter for clinical outcome in CF patients was C(max)/MIC. Outcome prediction of AUC(24)/MIC was dependent on the regimen. The increase of P. aeruginosa resistance after once-daily administration is linked to a long dosing interval. More and larger studies are needed to optimize the dosing regimen for maximum clinical outcome with minimum resistance development.

Efficacy of two chondroitin sulfate ophthalmic solutions in the therapy of spontaneous chronic corneal epithelial defects and ulcerative keratitis associated with bullous keratopathy in dogs

OBJECTIVE

To determine the efficacy of two antimicrobial-chondroitin sulfate ophthalmic solutions in the therapy of spontaneous chronic corneal epithelial defects (SCCED) and ulcerative keratitis associated with bullous keratopathy in dogs.

ANIMALS STUDIED

Eighty dogs with SCCED and 14 dogs with ulcerative keratitis associated with bullous keratopathy.

PROCEDURE

Following manual debridement of nonadherent epithelium, dogs were treated topically with a chondroitin sulfate ophthalmic solution containing either tobramycin or ciprofloxacin. Patients were re-evaluated at 2-week intervals for 4 weeks.

RESULTS

After 2 weeks of treatment, 53.6% of eyes with SCCED and 17.6% of eyes with ulcerative keratitis associated with bullous keratopathy had healed. After 4 weeks of treatment, 81.0% of eyes with SCCED and 23.5% of eyes with ulcerative keratitis associated with bullous keratopathy had healed. There were no statistically significant differences in healing percentages between the tobramycin-chondroitin sulfate solution treatment groups and the ciprofloxacin-chondroitin sulfate solution treatment groups. Two dogs with SCCED, one treated with the tobramycin-chondroitin sulfate solution and the other treated with the ciprofloxacin-chondroitin sulfate solution, developed sterile corneal stromal abscesses during the study.

CONCLUSIONS

Topical therapy with an antimicrobial-chondroitin sulfate ophthalmic solution combined with manual debridement of nonadherent epithelium compares favorably with other published medical and surgical therapies for SCCED; however, these compounds are only equivocally more effective than therapy with manual debridement alone. These solutions appear to be ineffective in the treatment of ulcerative keratitis associated with bullous keratopathy. The significance of the two cases of corneal stromal abscessation is unknown at this time and warrants further investigation.

Evaluation of MRSA Select, a new chromogenic medium for the detection of nasal carriage of methicillin-resistant Staphylococcus aureus

MRSA Select is a new chromogenic medium for the detection from clinical specimens of methicillin-resistant Staphylococcus aureus (MRSA) growing as pink colonies. Screening samples (n=666) taken from patients were spread onto MRSA Select, oxacillin resistance screening agar base (ORSAB), mannitol salt agar and an in-house medium containing tobramycin-colistin-fungizone (TCF). Ninety-nine samples were positive for MRSA on at least one selective medium. Sensitivity and specificity were >94% for all media provided that incubation was extended to 48 h for ORSAB and that a coagulase test was performed for ORSAB and TCF. By contrast, on the single criterion of the colour of colonies for MRSA positivity, the specificity and sensitivity of MRSA Select were high (99.8% and 99%, respectively).

Inhalation solutions: which one are allowed to be mixed? Physico-chemical compatibility of drug solutions in nebulizers

Therapy of chronic respiratory diseases often involves inhalation therapy with nebulizers. Patients often attempt to shorten the time consuming administration procedure by mixing drug solutions/suspensions for simultaneous inhalation. This article considers the issue of physico-chemical compatibility of admixtures of drug solutions/suspensions in nebulizers. A search of databases, prescribing information and primary literature was conducted to locate literature concerning the physico-chemical compatibility of inhalation solutions/suspensions. This was supplemented by telephone interviews. Admixtures of albuterol with ipratropium and/or cromolyn, of albuterol and budesonide, or tobramycin, or colistin are physico-chemically compatible. Physico-chemical compatibility has been demonstrated for admixtures of cromolyn with albuterol and/or ipratropium and for admixtures of cromolyn and budesonide. Admixtures of budesonide with ipratropium and/or fenoterol, and admixtures of budesonide and albuterol, or cromolyn are physico-chemically compatible. Both cromolyn and colistin are incompatible with benzalkonium chloride. Admixtures should be prepared from inhalation solutions/suspensions formulated without preservatives. Besides studies of the physico-chemical compatibility, the aerodynamic behaviour of physico-chemical mixtures needs to be studied before a final recommendation of simultaneous nebulization of compatible admixtures can be made.

In-vivo impact of the MexXY efflux system on aminoglycoside efficacy in an experimental model of Pseudomonas aeruginosa pneumonia treated with tobramycin

Aminoglycosides are of major importance in treating Pseudomonas aeruginosa pneumonia (PAP). However, their efficacy may be compromised by low-level resistance caused by the inducible MexXY multidrug efflux pump. In the present study, the impact of the MexXY efflux pump was investigated in vivo in an experimental model of PAP in rabbits treated with intravenous tobramycin. Three strains were used to induce PAP in rabbits: PAO1 (wild-type strain; MIC 1 mg/L), mutant 11B (mexX::Tn501; no expression of MexXY; MIC 0.5 mg/L) and mutant MutGR1 (MexZ null; constitutive expression of MexXY; MIC 2 mg/L). Five hours after inoculation, treatment with tobramycin (10 mg/kg) was implemented (peak serum concentration 30 mg/L). The animals were killed humanely 48 h after inoculation, and the residual pulmonary bacterial concentration was determined. Selection of bacteria expressing MexXY was determined by plating lung homogenates on agar plates containing antibiotic. Mean bacterial counts (log(10) CFU/g) for treated vs. untreated rabbits were 6.26 and 8.13 (p < 0.0001), 6.00 and 8.38 (p < 0.001), and 7.25 and 8.79 (p 0.04) for PAO1, 11B and MutGR1, respectively, with an overall mortality rate of 0% vs. 8.9% (p < 0.01). MexXY-overexpressing bacteria were recovered from three (21%) treated rabbits. The C(max)/MIC ratio was the parameter that was best associated with tobramycin efficacy. The bacteria overexpressing MexXY, recovered from lung, occurred with a C(max)/MIC window of 19-26. It was concluded that the experimental PAP model highlights poor tobramycin bacteriological efficacy in vivo, contrasting with survival gain, and that the contribution of the MexXY system to this low level of tobramycin efficacy is modest. Finally, this model appears to be suitable for the investigation of new anti-pseudomonal therapeutic strategies.

Efficacy and potential for resistance selection of antipseudomonal treatments in a mouse model of lung infection by hypermutable Pseudomonas aeruginosa

Hypermutable Pseudomonas aeruginosa strains are found with high frequency in the lungs of patients with chronic infections and are associated with high antibiotic resistance rates. The in vivo consequences of hypermutation for treatment in a mouse model of lung infection using strain PAO1 and its hypermutable derivative PAOdeltamutS are investigated. Groups of 30 mice were treated for 3 days with humanized regimens of ciprofloxacin (CIP), tobramycin (TOB), CIP plus TOB, or placebo, and mortality, total lung bacterial load, and 4x- and 16x-MIC mutants were recorded. The rates of mutation and the initial in vivo frequencies of mutants (at the onset of treatment) were also estimated and the in vitro- and in vivo-selected mutants characterized. Since both strains had identical MICs, the same pharmacokinetic/pharmacodynamic (PK/PD) parameters were obtained: area under the 24-h concentration-time curve (fAUC)/MIC = 385 for CIP and maximum concentration of drug in serum (fC(max))/MIC = 19 for TOB. Despite adequate PK/PD parameters, persistence of high bacterial numbers and amplification (50,000-fold) of resistant mutants (MexCD-OprJ hyperexpression) were documented with CIP treatment for PAOdeltamutS, in contrast to complete resistance suppression for PAO1 (P < 0.01), showing that conventional PK/PD parameters may not be applicable to infections by hypermutable strains. On the other hand, the efficacy of TOB monotherapy in terms of mortality reduction and bacterial load was very low regardless of the strain but not due to resistance development, since mutants were not selected for PAO1 and were only modestly amplified for PAOdeltamutS. Finally, the CIP-plus-TOB combination was synergistic, further reducing mortality and bacterial load and completely preventing resistance even for PAOdeltamutS (P < 0.01 compared to monotherapy), showing that it is possible to suppress resistance selection in infections by hypermutable P. aeruginosa using appropriate combined regimens.

Arginine or nitrate enhances antibiotic susceptibility of Pseudomonas aeruginosa in biofilms

Arginine enhanced the killing of Pseudomonas aeruginosa by ciprofloxacin and tobramycin under anaerobic, but not aerobic, growth conditions. Arginine or nitrate also enhanced the killing by these antibiotics in mature biofilms, reducing viable cell counts by a factor of 10 to 100 beyond that achieved by antibiotics alone.

Effects of N-trimethylchitosan on transcellular and paracellular transcorneal drug transport

The effects of N-trimethylchitosan (TMC) on the transcorneal transport of dexamethasone, taken as a marker of the transcellular penetration route, and of tobramycin, a marker of the paracellular route, were studied by assessing the TMC effect on the intraocular pharmacokinetics of each marker. The drugs were topically applied via erodible inserts (weight, 20 mg; diameter, 6 mm; drug dose, 0.3 mg) based on poly(ethylene oxide), containing 10% w/w medicated TMC microspheres (diameter < 2.5 microm). Before application, drug release and insert erosion kinetics, and release mechanism were studied in vitro. With either drug, introduction of 10% TMC into insert did not substantially alter the release and erosion rates, hence this formulation was apt to isolate the transcorneal penetration enhancing effect of TMC. Ocular pharmacokinetics were determined in the rabbit model. TMC produced significant increases of dexamethasone Cmax (5.69 +/- 0.49 vs. 3.07 +/- 0.31 microg/ml) and AUC (619.3 +/- 32.5 vs. 380.5 +/- 32.0 microg min/ml) in the aqueous with respect to the reference TMC-free insert. On the other hand, TMC was unable to yield tobramycin concentrations in the aqueous exceeding the determination limit (0.5 microg/ml). In conclusion, TMC enhances transcorneal transport via the transcellular route, whereas it is unable to effectively open the tight junctions between corneal cells.

Mucin-Pseudomonas aeruginosa interactions promote biofilm formation and antibiotic resistance

Pseudomonas aeruginosa is an opportunistic pathogen that causes chronic lung infections in people suffering from cystic fibrosis (CF). In CF airways, P. aeruginosa forms surface-associated communities called biofilms. Compared with free-swimming cultures, biofilms resist clearance by the host immune system and display increased resistance to antimicrobial agents. In this study we developed a technique to coat surfaces with molecules that are abundant in CF airways in order to investigate their impact on P. aeruginosa biofilm development. We found that P. aeruginosa biofilm development proceeds differently on surfaces coated with the glycoprotein mucin compared with biofilm development on glass and surfaces coated with actin or DNA. Biofilms formed on mucin-coated surfaces developed large cellular aggregates and had increased tolerance to the antibiotic tobramycin compared with biofilms grown on glass. Analysis of selected mutant backgrounds in conjunction with time-lapse microscopy revealed that surface-associated motility was blocked on the mucin surface. Furthermore, our data suggest that a specific adhesin-mucin interaction immobilizes the bacterium on the surface. Together, these experiments suggest that mucin, which may serve as an attachment surface in CF airways, impacts P. aeruginosa biofilm development and function.

Evidence-based recommendations for antimicrobial nasal washes in chronic rhinosinusitis

BACKGROUND

Chronic rhinosinusitis (CRS) refractory to medical and surgical therapy is a difficult problem for patients and physicians. Topical antimicrobial nasal irrigations are commonly used for treatment with great variation in methodology and without clear scientific support for current treatment formulations. The purpose of this study was to develop a scientific rationale for creating standardized recommendations for clinical practice in the use of topical antimicrobial washes for CRS.

METHODS

An extensive review of basic science and clinical literature on the treatment of CRS with topical antimicrobial washes was completed. Pharmacokinetics of and organism susceptibility to appropriate topically applied antimicrobial agents were reviewed.

RESULTS

The most common organisms associated with CRS were identified. The relevant pharmacokinetics of drugs targeted at these organisms are presented. Susceptibility breakpoints set by the National Committee for Clinical Laboratory Standards are identified to help establish the most effective concentration of the identified drugs. Recommendations for agent selection, agent concentration, length of treatment, dosing schedule, and methods of irrigation are presented.

CONCLUSION

Antimicrobial nasal washes provide a potentially effective treatment for the growing population of patients who remain symptomatic after appropriate medical and surgical intervention. This study establishes the basic principles supporting this treatment option and offers rational, evidence-based treatment guidelines. The study has identified additional areas that need to be investigated before prospective clinical trials can be effectively undertaken.

[Antimicrobial susceptibility of phenotypically extended spectrum beta-lactamase producing Pseudomonas aeruginosa and Acinetobacter baumannii isolates]

The aim of this study was to investigate the antimicrobial susceptibilities of Pseudomonas aeruginosa and Acinetobacter baumannii strains that were identified as producers of extended spectrum beta-lactamases (ESBL) phenotypically. Fifty three P. aeruginosa and 53 A. baumannii strains that were isolated from patients with hospital infections followed-up by Infectious Diseases Unit of Hacettepe University Adult Hospital in 2002-2004, were included in the study. The susceptibilities of the isolates to ceftazidime, cefotaxime, cefepime, tobramycin, ciprofloxacin, piperacillin-tazobactam, imipenem and meropenem were determined by Etest (AB Biodisk, Solna, Sweden) method. The susceptibility rates of A. baumannii isolates against tobramycin and carbapenem group were 54.7% and 49.1%, respectively; whereas for the rest of the antimicrobial agents tested, these rates were lower than 25 percent. The susceptibility rates of P. aeruginosa isolates against ciprofloxacin, carbapenem group, piperacillin-tazobactam and tobramycin differed between 35-38 percent. Of the P. aeruginosa isolates, 37.5%, and of the A. baumannii isolates 37.7% were resistant against imipenem/meropenem, piperacillin-tazobactam, ciprofloxacin and tobramycin simultaneously. As a result, phenotypically ESBL producing P. aeruginosa and A. baumannii isolates were found highly resistant not only against the third and fourth generation cephalosporins, but also to the antibiotics currently used in the therapy including carbapenemes.

[Evaluation of aminoglycoside resistance phenotypes and genotyping of acetyltransferase in Escherichia coli]

OBJECTIVE

To investigate the prevalence of aminoglycoside resistance and genotyping of acetyltransferase in Escherichia coli.

METHODS

Resistance phenotypes to 12 antibiotics of 44 Escherichia coli isolates were analyzed using agar dilution method and 3 aminoglycoside resistance genes aac(3)-I, II and aac(6')-I were determined by PCR method.

RESULTS

In 44 clinical isolates, the occurrence of ESBLs was 45.45%, resistance rates were discrepant for amikacin (18.18%), gentamicin (56.82%) and tobramycin (61.36%), the prevalence of phenotype TG (tobramycin and gentamicin) indicative of aac(3)-II production and TGA (tobramycin, gentamicin and amikacin) indicative of aac(6')-I production were 36.36% and 18.18%, respectively. The most common aminoglycoside resistance genotype of acetyltransferase was aac(3)-II (52.27%) and aac(6')-I was lower (29.55%), but no aac(3)-I was detected.

CONCLUSION

At least 2 acetyltransferase genes exist in this area i.e. aac(3)-II and aac(6')-I.

[The role of aminoglycoside--modifying enzymes in resistance of Gram-negative rods]

The aim of the study was to evaluate the aminoglycoside resistance of Gram-negative bacilli isolated from patients. To the examination 35 strains of Enterobacteriaceae and 18 of non-fermentative bacteria were included. Resistance to aminoglycosides (gentamicin (G), netilmicin (Nt), tobramycin (T), amikacin (A), kanamycin (K), neomycin (N)) was established by disk diffusion method. Interpretation of enzymatic mechanisms was performed by Livermore. The most common enzymes AAC(6')I were found in Enterobacteriaceae group (mostly in E. cloaceae and P. mirabilis) and AAC(3') and in non-fermentative bacteria: AAC(6')I in P. aeruginosa and APH(3')VI and AAC(3')I in A. baumanii. The most frequent phenotype was resistance to six antibiotics (G, Nt, T, A, K, N) Resistance rates were high for gentamicin (>70 %) in both groups and amikacin (88,89 %) in non-fermentatives.

Tobramycin-impregnated calcium sulfate prevents infection in contaminated wounds

Open fractures often are associated with increased rates of infection and nonunion, multiple surgical procedures, and delayed return to preinjury activity. Antimicrobial bone graft substitutes used as an alternative to antibiotic cement beads and/or delayed autologous bone grafting may provide a useful adjunct in patients with open fractures. A stable, unicortical defect was created and contaminated with 30 microL of 5 x 10 colony-forming units/mL of Staphylococcus aureus in the proximal tibial metaphysis of Spanish goats. The negative control group received no treatment, the carrier group received synthetic bone graft alone, the positive control group received tobramycin antibiotic cement, and the treatment group received tobramycin antimicrobial synthetic bone graft (calcium sulfate). After a 3-week evaluation period, intraosseous microbiologic specimens were obtained. The Staphylococcus aureus contaminant was recovered in 11 of 12 animals (mean = 6.9 x 10 colony-forming units/g marrow) in the negative control group and in all animals (mean = 2.2 x 10 colony-forming units/g marrow) in the carrier group. Bacteria were not found in the antibiotic-treated groups. The tobramycin-impregnated calcium sulfate was effective in preventing infection in a contaminated defect. It could be beneficial in reducing the number of surgeries and recovery time because it is bioabsorbable and osteoconductive.

In vitro efficacy of the successive or staggered use of eardrops

Chronic suppurative otitis media is still a significant health problem. Several topical agents such as antibacterials, steroids and acid media eardrops are widely used alone or in combination to control active otorrhea and obtain a dry ear. Patients frequently ask if there is any benefit or loss in the effect of these due to their use in a successive or staggered manner. This question led us to design this in vitro study to investigate the effect of the acetic acid solution and dexamethasone sodium phosphate on the antimicrobial effects of some ototopical antibiotics during their successive or staggered usage. Staphylococcus aureus, Pseudomonas aeruginosa, Proteus mirabilis and Escherichia coli were used for testing the antimicrobial activities of the selected antibiotic drops. Six microliters of acetic acid solution and dexamethasone sodium phosphate were used alone or together with 6 microl each of ciprofloxacin HCl, tobramycin and ofloxacin solution. Drops were placed on Whatman no. 1 paper disks with a diameter of 6 mm, and the disks were placed onto Mueller-Hinton agar plates on which bacteria had been swabbed over the entire surface. After overnight incubation of the plates at 35 degrees C, the diameters of the inhibition zones were recorded. There was no antibacterial effect in response to either acid solution or dexamethasone used alone. The inhibition zones of the antibiotics did not change with either steroid or acid media solution added onto the antibiotic disks alone or together. Acid solution or steroids should be preferred to use in combination with antibacterial otic drugs in in-vivo settings; no negative or positive effects were seen in in-vitro conditions. Therefore, prescribed eardrops can be used in a successive or staggered manner without any influence on the treatment results.

Structure of the Hepatitis C Virus IRES Bound to the Human 80S Ribosome: Remodeling of the HCV IRES

Initiation of translation of the hepatitis C virus (HCV) polyprotein is driven by an internal ribosome entry site (IRES) RNA that bypasses much of the eukaryotic translation initiation machinery. Here, single-particle electron cryomicroscopy has been used to study the mechanism of HCV IRES-mediated initiation. A HeLa in vitro translation system was used to assemble human IRES-80S ribosome complexes under near physiological conditions; these were stalled before elongation. Domain 2 of the HCV IRES is bound to the tRNA exit site, touching the L1 stalk of the 60S subunit, suggesting a mechanism for the removal of the HCV IRES in the progression to elongation. Domain 3 of the HCV IRES positions the initiation codon in the ribosomal mRNA binding cleft by binding helix 28 at the head of the 40S subunit. The comparison with the previously published binary 40S-HCV IRES complex reveals structural rearrangements in the two pseudoknot structures of the HCV IRES in translation initiation.

Effect of oxygen limitation on the in vitro antimicrobial susceptibility of clinical isolates of Pseudomonas aeruginosa grown planktonically and as biofilms

Pseudomonas aeruginosa, the predominant causative pathogen of chronic lung infection in patients with cystic fibrosis, may grow under anaerobic conditions as biofilms in the lungs of cystic fibrosis patients. To determine if growth under anaerobic conditions affects the antimicrobial susceptibility of P. aeruginosa, the susceptibility of clinical isolates of P. aeruginosa grown planktonically and as biofilms to a range of antibiotics was determined. Growth under anaerobic conditions did not reduce the ability of ceftazidime, meropenem, aztreonam, piperacillin, or piperacillin/tazobactam to inhibit planktonic growth, with MIC50 values for these antibiotics remaining unchanged or decreasing. However, tobramycin was less effective at inhibiting planktonic bacterial growth under anaerobic conditions, with the MIC50 of tobramycin increasing twofold. Growth under anaerobic conditions also decreased the bactericidal activity of tobramycin, with the MBC50 of tobramycin increasing fourfold. The killing kinetics of tobramycin was also examined under aerobic and anaerobic conditions for selected isolates. When isolates 6A and 12A were grown aerobically, concentration-dependent decreases in total viable count were apparent with tobramycin. In contrast, when these isolates were grown anaerobically, tobramycin at the same concentrations did not decrease the total viable count. When isolates were grown as biofilms under both aerobic and anaerobic conditions, isolate- and concentration-dependent differences in killing of the biofilms by tobramycin were apparent. However, tobramycin at concentrations up to 128 mg/l was unable to eradicate biofilms of any of the isolates tested, whether biofilms were grown aerobically or anaerobically. These results show that oxygen limitation may reduce, in a strain-dependent manner, the susceptibility to tobramycin of P. aeruginosa grown planktonically and as biofilms.

Influence of gentamicin and tobramycin on binary biofilm formation by co-cultures ofBurkholderia cepacia andPseudomonas aeruginosa

The aim of this study was to investigate the stability and dynamics of binary culture biofilm formation following antibiotic treatment. Pseudo steady-state biofilm cultures of clinical strains of P. aeruginosa and B. cepacia concurrently isolated from a single CF patient were established in separate Constant Depth Film Fermenters (CDFFs). Pans, containing established biofilms, were swapped between CDFFs. Biofilms were treated either for 5 days with tobramycin (0.3 mg/ml) prior to pan-swapping, or with gentamicin (1 mg/ml) immediately following pan-swapping. In both instances stable binary biofilms were formed. In addition, fresh un-colonised surfaces added at the time of pan-swapping and exposed simultaneously to biofilm derived P. aeruginosa and B. cepacia and subjected to antibiotic challenge also established stable binary communities. Treatment of P. aeruginosa and B. cepacia with tobramycin or gentamicin, either prior to or immediately after surface colonisation had little effect on the establishment of stable binary biofilms. Such treatment also had little effect on the immigration of one organism into an established biofilm of a second organism.

Molecular and phenotypic features for identification of the opportunistic pathogens Ochrobactrum spp

Among the six species characterized within the genus Ochrobactrum, Ochrobactrum anthropi and Ochrobactrum intermedium are currently reported as opportunistic pathogens in humans. Since the species identification is mainly based on 16S rDNA analysis, the aim of this study was to search for other characteristics useful for Ochrobactrum species discrimination. Ribotyping, morphological and biochemical analyses, and antimicrobial susceptibility testing were performed for a panel of 35 clinical isolates, first identified to the species level using 16S rDNA sequencing. Type and reference strains of five Ochrobactrum species were comparatively analysed. Commercial identification systems such as API 20NE and VITEK 2 were tested for their ability to identify Ochrobactrum anthropi and to detect other members of the genus Ochrobactrum. An improved protocol for the identification of Ochrobactrum spp. by routine medical microbiology practices is proposed: isolation of a non-fastidious non-fermenting oxidase-positive Gram-negative rod resistant to all beta-lactams except imipenem indicates the genus Ochrobactrum, and the API 20NE system confirms the genus identification for most strains, whereas the VITEK 2 system using ID-GNB cards was less powerful. Urease activity, the mucoidy of the colonies, growth at 45 degrees C on tryptic soy agar, and susceptibility to colistin, tobramycin and netilmicin should be considered as differential characteristics for identification of O. anthropi and O. intermedium to the species level. However, definitive identification depends on genotyping methods.

Kinetic mechanism of streptomycin adenylyltransferase from a recombinant Escherichia coli

Bacterial resistance to the aminoglycoside antibiotics is manifested primarily by enzymic modification of these drugs. One important mechanism of streptomycin modification is through ATP-dependent O-adenylation, catalyzed by streptomycin adenylyltransferase. Initial velocity patterns deduced from steady state kinetics indicate a sequential mechanism. Dead-end inhibition by tobramycin and neomycin is non-competitive versus streptomycin and uncompetitive versus ATP, indicative of ordered substrate binding where ATP binds first and then streptomycin. These results surmise that streptomycin adenylyltransferase follows an ordered, sequential kinetic mechanism in which one substrate (ATP) binds prior to the antibiotic and pyrophosphate is released prior to formation of AMP-streptomycin.

Aminoglycoside antibiotics induce bacterial biofilm formation

Biofilms are adherent aggregates of bacterial cells that form on biotic and abiotic surfaces, including human tissues. Biofilms resist antibiotic treatment and contribute to bacterial persistence in chronic infections. Hence, the elucidation of the mechanisms by which biofilms are formed may assist in the treatment of chronic infections, such as Pseudomonas aeruginosa in the airways of patients with cystic fibrosis. Here we show that subinhibitory concentrations of aminoglycoside antibiotics induce biofilm formation in P. aeruginosa and Escherichia coli. In P. aeruginosa, a gene, which we designated aminoglycoside response regulator (arr), was essential for this induction and contributed to biofilm-specific aminoglycoside resistance. The arr gene is predicted to encode an inner-membrane phosphodiesterase whose substrate is cyclic di-guanosine monophosphate (c-di-GMP)-a bacterial second messenger that regulates cell surface adhesiveness. We found that membranes from arr mutants had diminished c-di-GMP phosphodiesterase activity, and P. aeruginosa cells with a mutation changing a predicted catalytic residue of Arr were defective in their biofilm response to tobramycin. Furthermore, tobramycin-inducible biofilm formation was inhibited by exogenous GTP, which is known to inhibit c-di-GMP phosphodiesterase activity. Our results demonstrate that biofilm formation can be a specific, defensive reaction to the presence of antibiotics, and indicate that the molecular basis of this response includes alterations in the level of c-di-GMP.

Breakpoints for predicting Pseudomonas aeruginosa susceptibility to inhaled tobramycin in cystic fibrosis patients: use of high-range Etest strips

Inhaled administration of tobramycin assures high concentrations in cystic fibrotic lungs, improving the therapeutic ratio over that of parenteral tobramycin levels, particularly against Pseudomonas aeruginosa. Conventional Clinical and Laboratory Standards Institute (CLSI; formerly National Committee for Clinical Laboratory Standards) breakpoints only consider parenteral levels and do not take into account these high antimicrobial concentrations. The Spanish Antibiogram Committee (The MENSURA Group) has tentatively defined specific breakpoint values for inhaled tobramycin when testing P. aeruginosa isolates from cystic fibrosis (CF) patients (susceptible, < or =64 microg/ml; resistant, > or =128 microg/ml). The antimicrobial susceptibilities of 206 prospectively collected CF P. aeruginosa isolates were determined by the reference agar dilution method. For tobramycin, the performance of high range tobramycin Etest strips (AB Biodisk, Solna, Sweden) and conventional tobramycin disks were assessed with the same collection. Applying MENSURA proposed breakpoints, 95.1% of the strains were categorized as susceptible to tobramycin, either using agar dilution or Etest high-range strips (99% categorical agreement between both methods). With CLSI breakpoints, susceptibility rates decreased to 79.1 and 81.1% for agar dilution and Etest strips, respectively (83.5% categorical agreement). Minor, major, and very major errors for Etest strips (CLSI criteria) were 13.6, 1.2, and 14.8%, respectively. Upon applying the new proposed criteria for inhaled tobramycin, only one major and one very major error were observed with Etest strips. Whenever inhaled tobramycin is considered for therapy, we suggest that P. aeruginosa strains from CF patients categorized as intermediate or resistant to tobramycin according to the CLSI criteria should be retested with high-range Etest strips and recategorized using MENSURA interpretive criteria. CLSI breakpoints should still be followed when intravenous tobramycin is used in CF patients, particularly during the course of exacerbations.

Nocardia cyriacigeorgica is a significant pathogen responsible for nocardiosis in Japan and Thailand

Nocardia cyriacigeorgica is a recently described species. During routine diagnostic testing of 121 clinical isolates, we found that about one fourth of the strains from Japan (19 isolates) and Thailand (8 isolates), which were identified in our laboratories as N. asteroides, in fact belong to N. cyriacigeorgica. To our knowledge, this is the first report of infection due to N. cyriacigeorgica in Japan and Thailand, and the third report of infection anywhere in the world. Although N. cyriacigeorgica is usually differentiated from other Nocardia species by utilization of glucose and gluconate, we found that it can also be differentiated by a characteristic synergistic effect between imipenem (IPM) and tobramycin (TOB).

Pharmacodynamics of tobramycin in patients with cystic fibrosis

Relationships between pharmacodynamic indices (PI), such as the area under the concentration-time curve (AUC)/MIC ratio and time > MIC (T(>MIC)), and efficacy have been described for antimicrobial drugs. The use of these quantitative relationships may increase the power to demonstrate significant effects of drugs, obviating the need to include large numbers in comparative trials. Patients with cystic fibrosis (CF) hospitalized for treatment of an infectious exacerbation due to Pseudomonas aeruginosa were eligible for the study. They received tobramycin 3.3 mg/kg tid as initial therapy in combination with ticarcillin 125 mg/kg qid. Blood samples were drawn at t = 0-8 h after infusion. Pharmocokinetic parameters and PI were calculated for every individual and correlated to the relative improvement in forced expiratory volume during the first second (FEV1) and forced vital capacity (FVC) between pretreatment and days 9-11 as a measure of efficacy. The 3 PI fAUC/MIC, f Peak/MIC, and T(>MIC) of tobramycin showed a significant correlation with effect and was the highest for the fAUC/MIC relationships with FEV1 and FVC as determined both by the Emax model as well as Spearman correlations (r = 0.77, P = 0.002 and 0.58, P = 0.036 for FEV1 and FVC). Pharmacokinetic parameters AUC and Peak as such showed no significant correlation with effect, nor did the MIC values. There is a significant relationship between PI of aminoglycosides and efficacy parameter (increase in FEV1 and FVC) in patients with CF. This study demonstrates the applicability of pharmacodynamic relationships in determining efficacy of antimicrobial therapy, by demonstrating a strong PI-effect relationship in a group of only 13 patients.

Antibiotic tolerance induced by lactoferrin in clinical Pseudomonas aeruginosa isolates from cystic fibrosis patients

Lactoferrin-induced cell depolarization and a delayed tobramycin-killing effect on Pseudomonas aeruginosa cells were correlated. This antibiotic tolerance effect (ATE) reflects the ability of a defense protein to modify the activity of an antibiotic as a result of its modulatory effect on bacterial physiology. P. aeruginosa isolates from cystic fibrosis patients showed higher ATE values (< or = 6-fold) than other clinical strains.

In vitro comparison of four aminoglycoside antibiotics: sisomicin, gentamicin, tobramycin, and BB-K8

The activity of sisomicin, gentamicin, and tobramycin against 273 clinical bacterial isolates was about equal (except for greater activity of tobramycin against Pseudomonas) and was greater than that of BB-K 8 as determined by antibiotic dilution methods in agar and broth.

Sub-inhibitory concentrations of ceftazidime and tobramycin reduce the quorum sensing signals of Pseudomonas aeruginosa

AIM

Concentrations of antimicrobials below minimum inhibitory concentration (subMIC) may reduce the production by Pseudomonas aeruginosa of virulence factors such as elastase. We sought to determine whether the reduction in elastase production may be mediated by a reduction in acyl-homoserine lactones.

METHODS

Pseudomonas aeruginosa in broth was exposed to three conditions for ceftazidime and tobramycin: control, 6% MIC and 25% MIC. Elastase was assayed using elastin congo red. N-(3-Oxododecanoyl)-homoserine lactone (C12-HSL) and N-butyryl-homoserine lactone (C4-HSL) were assayed using biosensor Escherichia coli.

RESULTS

Elastase was unchanged with ceftazidime. Elastase was reduced by 16% at 6% MIC tobramycin and reduced by 70% at 25% MIC tobramycin (P<0.0001). As a percentage of control, C12-HSL was mean 69.4% (SEM 7.3%) at 6% MIC tobramycin, and 31.7% (3.3%) at 25% MIC tobramycin (P=0.0001). C12-HSL was 78.9% (5.3%) at 6% MIC ceftazidime and was 29.7% (1.8%) at 25% MIC ceftazidime (P=0.0001). Both ceftazidime and tobramycin were associated with reduced C4-HSL at 6% MIC and 25% MIC (P<0.03).

CONCLUSIONS

SubMIC tobramycin but not ceftazidime reduced elastase production by P. aeruginosa. In contrast, subMIC concentrations of both antimicrobials reduced C12-HSL and C4-HSL. It is unlikely that reduced HSL is the sole explanation for the reduction in elastase.

Evaluation on the use of beta-lactamase and aminoglycoside modifying enzyme gene sequences as markers for the early detection of antibiotic resistance profile of Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the major causes of infections including the hospital acquired (Nosocomial) infections. Detection of them and their antibiotic resistance profile by conventional method takes about three days. Recently, DNA based diagnostic methods are being used for the identification of the pathogens. Hence we have tested a rapid and sensitive method using DNA sequences as markers for detecting the presence of three genes coding for the enzymes that inactivate the two most commonly used Anti-pseudomonadal drugs such as beta-lactam antibiotics (Penicillin, and its derivatives) and Aminoglycosides such as Gentamicin, Tobramycin, Amikacin, Streptomycin. The internal region of these genes were used for designing and synthesizing primers and these primers were used in Polymerase Chain Reaction (PCR) to screen for the presence of these genes in the clinical isolates and to label them non-radioactively with Biotin. They in turn were used to detect the presence of the antibiotic resistance genes in the clinical isolates by hybridization. The specificity (ratio of positive results obtained in both methods and the sensitivity (the minimum amount of sample DNA and the labeled probe required for the tests) were evaluated.

Resistance of Pseudomonas aeruginosa to gentamicin and related aminoglycoside antibiotics

This study was undertaken to investigate biochemical, genetic, and epidemiological aspects of resistance to aminoglycoside antibiotics among 650 consecutive isolates of Pseudomonas aeruginosa from Parkland Memorial Hospital, Dallas, Tex. In 364 strains, minimal inhibitory concentrations were 25 mug/ml or greater for gentamicin (G), tobramycin (T) or kanamycin (K). Four patterns of resistance were noted: (A) G, T, K (four strains), (B) G, K (23 strains), (C) T, K (one strain), and (D) K (336 strains). Gentamicin acetyltransferase (GAT) activities were associated with resistance to gentamicin in strains of groups A and B, whereas kanamycin phosphotransferase activity was found in strains of group D. The GAT from group B strains acetylates both gentamicin and tobramycin. Resistance to gentamicin and susceptibility to tobramycin may reflect the fact that the K(m)'s for tobramycin (25 to 44 mug/ml) of GAT activities in these group B strains are much greater than the K(m)'s for gentamicin (1.9 to 2.7 mug/ml) and exceed the minimal inhibitory concentrations for tobramycin (1.25 to 7.5 mug/ml). GAT from strains of group A was associated with resistance to G, T, and K. Gentamicin acetyltransferases can be distinguished by their specificities for aminoglycoside substrates. The substrate specificity of GAT from group B strains is similar to that reported for GAT(I), but the specificity of GAT from group A strains differs from those described for GAT(I) and GAT(II). Conjugal transfer of gentamicin or tobramycin resistance from our strains of P. aeruginosa to various potential recipient strains was not observed. Pyocin typing showed that many, but not all, of the strains resistant to gentamicin were similar, and retrospective epidemiological investigation revealed that these strains were isolated almost exclusively from patients in the adult and pediatric burn intensive care units and geographically continguous areas of the hospital.

Isolation of an amikacin-resistant Escherichia coli strain after tobramycin treatment of previous recurrent episodes of respiratory tract infections caused by Pseudomonas aeruginosa

Amikacin-resistant Escherichia coli strains are isolated rarely from clinical samples. In the present study, investigation of an amikacin-resistant clinical isolate of E. coli demonstrated the presence of two class 1 integrons carrying the aacA4 gene plus the aacA7 gene, and the dfrA17 gene plus the aadA5 gene, respectively. Resistance to amikacin in this E. coli isolate was related to the presence of both aacA4 and aacA7.

Pseudomonas aeruginosa tolerance to tobramycin, hydrogen peroxide and polymorphonuclear leukocytes is quorum-sensing dependent

The opportunistic human pathogen Pseudomonas aeruginosa is the predominant micro-organism of chronic lung infections in cystic fibrosis (CF) patients. P. aeruginosa colonizes the CF lungs by forming biofilm structures in the alveoli. In the biofilm mode of growth the bacteria are highly tolerant to otherwise lethal doses of antibiotics and are protected from bactericidal activity of polymorphonuclear leukocytes (PMNs). P. aeruginosa controls the expression of many of its virulence factors by means of a cell-cell communication system termed quorum sensing (QS). In the present report it is demonstrated that biofilm bacteria in which QS is blocked either by mutation or by administration of QS inhibitory drugs are sensitive to treatment with tobramycin and H2O2, and are readily phagocytosed by PMNs, in contrast to bacteria with functional QS systems. In contrast to the wild-type, QS-deficient biofilms led to an immediate respiratory-burst activation of the PMNs in vitro. In vivo QS-deficient mutants provoked a higher degree of inflammation. It is suggested that quorum signals and QS-inhibitory drugs play direct and opposite roles in this process. Consequently, the faster and highly efficient clearance of QS-deficient bacteria in vivo is probably a two-sided phenomenon: down regulation of virulence and activation of the innate immune system. These data also suggest that a combination of the action of PMNs and QS inhibitors along with conventional antibiotics would eliminate the biofilm-forming bacteria before a chronic infection is established.

Effects of aminoglycoside administration on cochlear elements in human temporal bones

OBJECTIVE

Although there have been numerous reports on the relationship between the period of aminoglycoside administration and cochlear damage in animals, to date there have been no such studies in humans. The purpose of this study is to observe the early and late cochlear effects of aminoglycoside administration on hair cells, spiral ganglion cells, stria vascularis, and spiral ligament.

METHODS

Specimens were divided into three groups. Group I included "normal" temporal bones with no histopathologic findings of otitis media and no history of otologic or ototoxic drug administration. Group II consisted of temporal bones that received aminoglycosides within 2 weeks before death and group III of temporal bones that had aminoglycosides from 2 weeks to 6 months prior to death. Patients in groups II and III received gentamycin, kanamycin or tobramycin. Temporal bones were excluded from groups II and III if patients had a history of otologic disease or other ototoxic drugs. All temporal bones were examined under light microscopy. Standard cytocochleograms and spiral ganglion cell reconstructions were done on all temporal bones. Morphometric measurements of areas of stria vascularis were made in all turns of the cochlea on mid-modiolar sections. Spiral ligament was divided into four segments according to the locations of different types of fibrocytes. The mean loss of fibrocytes in each segment was estimated.

RESULTS

The percentages of intact outer hair cells in the basal turn were significantly greater in group I compared to groups II and III. The mean area of the stria vascularis in the apical turn was significantly less in groups II and III compared to group I.

CONCLUSION

This study demonstrates that in a short period (within 2 weeks) after aminoglycoside administration, a decrease in hair cells and in the area of the stria vascularis occurred.

A non-sequence-specific requirement for SMN protein activity: the role of aminoglycosides in inducing elevated SMN protein levels

Spinal muscular atrophy (SMA) is caused by homozygous loss of the survival motor neuron (SMN1) gene. In virtually all SMA patients, a nearly identical copy gene is present, SMN2. SMN2 cannot fully compensate for the loss of SMN1 because the majority of transcripts derived from SMN2 lack a critical exon (exon 7), resulting in a dysfunctional SMN protein. Therefore, the critical distinction between a functional and a dysfunctional SMN protein is the inclusion or the exclusion of the exon 7 encoded peptide. To determine the role of the 16 amino acids encoded by SMN exon 7, a panel of synthetic mutations were transiently expressed in SMA patient fibroblasts and HeLa cells. Consistent with previous reports, the protein encoded by SMN exons 1-6 was primarily restricted to the nucleus. However, a variety of heterologous sequences fused to the C-terminus of SMN exons 1-6 allowed mutant SMN proteins to properly distribute to the cytoplasm and to the nuclear gems. These data demonstrate that the SMN exon 7 sequence is not specifically required, rather this region functions as a non-specific 'tail' that facilitates proper localization. Therefore, a possible means to restore additional activity to the SMNDelta7 protein could be to induce a longer C-terminus by suppressing recognition of the native stop codon. To address this possibility, aminoglycosides were examined for their ability to restore detectable levels of SMN protein in SMA patient fibroblasts. Aminoglycosides can suppress the accurate identification of translation termination codons in eukaryotic cells. Consistent with this, treatment of SMA patient fibroblasts with tobramycin and amikacin resulted in a quantitative increase in SMN-positive gems and an overall increase in detectable SMN protein. Taken together, this work describes the role of the critical exon 7 region and identifies a possible alternative approach for therapeutic intervention.

Development of a rapid colorimetric time-kill assay for determining the in vitro activity of ceftazidime and tobramycin in combination against Pseudomonas aeruginosa

It is standard clinical practice to use a combination of two or more antimicrobial agents to treat an infection caused by Pseudomonas aeruginosa. The antibiotic combinations are usually selected empirically with methods to determine the antimicrobial effect of the combination such as the time-kill assay rarely used as they are time-consuming and labour intensive to perform. Here, we report a modified time-kill assay, based on the reduction of the tetrazolium salt, 2,3-bis[2-methyloxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide (XTT), that allows simple, inexpensive and more rapid determination of the in vitro activity of antibiotic combinations against P. aeruginosa. The assay was used to determine the in vitro activity of ceftazidime and tobramycin in combination against P. aeruginosa isolates from cystic fibrosis patients and the results obtained compared with those from conventional viable count time-kill assays. There was good agreement in interpretation of results obtained by the XTT and conventional viable count assays, with similar growth curves apparent and the most effective concentration combinations determined by both methods identical for all isolates tested. The XTT assay clearly indicated whether an antibiotic combination had a synergistic, indifferent or antagonistic effect and could, therefore, provide a useful method for rapidly determining the activity of a large number of antibiotic combinations against clinical isolates.

Novel approach to characterization of combined pharmacodynamic effects of antimicrobial agents

There is considerable need for new modeling approaches in the study of combined antimicrobial effects. Current methods based on the Loewe additivity and Bliss independence models are associated with implicit assumptions about the interacting system. To circumvent these limitations, we propose an alternative approach to the quantification of pharmacodynamic drug interaction (PDI). Pilot time-kill studies were performed with 10(8) CFU of Pseudomonas aeruginosa/ml at baseline with meropenem or tobramycin alone. The studies were repeated with 25 concentration combinations of meropenem (0 to 64 mg/liter) and tobramycin (0 to 32 mg/liter) in a five-by-five array. The data were modeled with a three-dimensional response surface using effect summation as the basis of null interaction. The interaction index (Ii) is defined as the ratio of the volumes under the planes (VUP) of the observed and expected surfaces: VUP(observed)/VUP(expected). Synergy and antagonism are defined as Ii values of <1 and >1, respectively. In all combinations, an enhanced killing effect was seen compared to that of either drug at the same concentration. The most significant synergism was observed between 1 and 5 mg/liter of meropenem and between 1 and 4 mg/liter of tobramycin; seven out of nine combinations had a >2-log drop compared to the more potent agent. The Ii was found to be 0.76 (95% confidence interval, 0.65 to 0.91) for the concentration ranges of the agents. The results corroborate previous data indicating that meropenem is synergistic with an aminoglycoside when used in combination against P. aeruginosa. Our parametric approach to quantifying PDI appears robust and warrants further investigations.

Antibiotic combinations for serious infections caused by carbapenem-resistant Acinetobacter baumannii in a mouse pneumonia model

OBJECTIVES

Successful therapy of carbapenem-resistant Acinetobacter baumannii strains has been reported with colistin, but recently we argued against its use as monotherapy because of the poor results obtained in a mouse pneumonia model. Our aim was to identify antibiotic combinations that were valid therapeutic alternatives in the same model.

METHODS

We used two carbapenem-resistant A. baumannii strains (D and E; MICs of imipenem, 8 and 512 mg/L, respectively). MICs of tobramycin, rifampicin and colistin for both strains were 8, 8 and 0.5 mg/L, respectively.

RESULTS

In infections caused by strain D, lung bacterial counts (log(10) cfu/g, mean +/- s.d.) were: controls (10.86+/-0.25), imipenem (5.99+/-0.59, P < 0.05 versus controls), and colistin (10.43 +/- 1.09); imipenem + tobramycin was the most active combination (5.46+/-0.62, P < 0.05 versus controls). In infections caused by strain E, results were: controls (10.82+/-0.33), rifampicin (5.62+/-0.26, P < 0.05 versus controls), colistin (8.38+/-1.22, P < 0.05 versus controls), and imipenem (11.01+/-0.2); rifampicin + imipenem (3.79+/-0.99) and rifampicin + tobramycin (3.96+/-0.30) were the most active combinations (P < 0.05); results with rifampicin + colistin (5.59+/-1.17) were similar to those with rifampicin alone.

CONCLUSIONS

Our data indicate that imipenem can still be the best alternative for carbapenem-resistant A. baumannii infections with moderate levels of imipenem resistance, preferably combined with aminoglycosides. For strains highly resistant to imipenem, a combination of rifampicin with imipenem, tobramycin or colistin may be useful, if resistance to rifampicin is only moderate.

Correction of ATM gene function by aminoglycoside-induced read-through of premature termination codons

Approximately 14% of genetic mutations in patients with ataxia-telangiectsia (A-T) are single-nucleotide changes that result in primary premature termination codons (PTCs), either UAA, UAG, or UGA. The purpose of this study was to explore a potential therapeutic approach for this subset of patients by using aminoglycosides to induce PTC read-through, thereby restoring levels of full-length ATM (A-T mutated) protein. In experiments using a modified in vitro cDNA coupled transcription/translation protein truncation test, 13 A-T cell lines carrying PTC mutations in different contexts exhibited read-through expression of ATM fragments, with three of four aminoglycosides tested. In ex vivo experiments with lymphoblastoid cell lines, we used radiosensitivity, radioresistant DNA synthesis, and irradiation-induced autophosphorylation of ATM Ser-1981 to show that the aminoglycoside-induced full-length ATM protein was functional and corrected, to various extents, the phenotype of A-T cells. These results encourage further testing of other compounds in this class, as well as follow up animal studies. Because some A-T patients with 5-20% of normal levels of ATM protein show slower neurological progression, A-T may prove to be a good model for aminoglycoside-induced read-through therapy.

Post-antibiotic effect induced by an antibiotic combination: influence of mode, sequence and interval of exposure

OBJECTIVES

The effects of mode, sequence and interval of antibiotic exposure on the post-antibiotic effect (PAE) induced by rifampicin and tobramycin were studied using Escherichia coli ATCC 25922 as the test organism.

METHODS

In triplicate, baseline PAEs were evaluated by exposing E. coli to rifampicin and tobramycin individually and simultaneously for 1 h. PAEs were further assessed in a second study, with the organism exposed first to rifampicin for 1 h, followed by a second 1 h tobramycin exposure, commencing at the beginning, middle and end of the PAE phase induced by rifampicin. The third study was similar to the above, but with the sequence of the two antibiotics reversed, i.e. tobramycin then rifampicin.

RESULTS

The PAE produced by simultaneous exposure of the combination showed an apparent additive interaction (PAE: 5.0+/-0.3 h) when compared with the PAE of individual antibiotics (rifampicin alone: 3.0+/-0.1 h; tobramycin alone: 1.5+/-0.1 h). However, an antagonistic interaction was observed in the second study, with a more pronounced degree of antagonism at the beginning, dissipating towards the end of the previous rifampicin PAE (PAE at the beginning: 2.6+/-0.3 h; the middle: 1.5+/-0.2 h; and at the end: 1.7+/-0.3 h). By subtracting the residual contribution from the first rifampicin exposure, the net average PAEs attributed to the second tobramycin exposure actually increased, from -0.4 to 1.7 h from the beginning to the end of the rifampicin PAE. For the third study, an additive interaction was again observed when the organism was exposed to tobramycin first (PAE at the beginning: 4.7+/-0.4 h; the middle: 3.7+/-0.7 h; and at the end: 3.1+/-0.4 h). The timing of the second rifampicin exposure had no impact to the interaction; after correction, the net mean PAEs attributed to the second rifampicin exposure were maintained at 3.2, 3.2 and 3.1 h.

CONCLUSIONS

The present data suggest that the expression of interaction type on PAE by an antibiotic combination was dependent on the mode, sequence and interval of exposure. The impact of these variables should not be overlooked when clinical dosing regimens are optimized.

[Antimicrobial susceptibility of Pseudomonas aeruginosa isolated at Kochi Municipal Central Hospital in the last 3 years]

Antimicrobial susceptibility of Pseudomonas aeruginosa isolated at Kochi Municipal Central Hospital between 2001 and 2003 was assessed according to the NCCLS interpretive criteria. 1. The piperacillin-susceptible rate was 92.9%. 2. Among cephem antibiotics, the ceftazidime-susceptible rate was the highest (96.0%). 3. As for aminoglycosides, susceptibility to tobramycin and amikacin remained with a susceptible rate of 93.2% and 94.8%, respectively. 4. The carbapenem-susceptibility remained high. The susceptible rate for meropenem (94.1%) was higher than that for imipenem (88.3%). 5. Acquisition of resistance was observed in urinary isolates. Four multi-drug resistant P. aeruginosa, which are resistant to all of imipenem, amikacin and ofloxacin were isolated in this study and all were isolated from urine. 6. Of 388 isolates, 34 isolates were resistant to imipenem, but no positive isolate was found in screening of metallo-beta-lactamase-producing bacteria.

In vitro activity of parenteral Beta-lactams, levofloxacin and tobramycin alone or in combination against extended-spectrum Beta-lactamase producing Klebsiella pneumoniae

MICs and time-kill studies were performed for four clinical isolates of extended-spectrum Beta-lactamase (ESBL)-producing Klebsiella pneumoniae. MICs (mg/L) were: piperacillin/tazobactam 8, cefepime 1-2, meropenem 0.03-0.06, levofloxacin 0.5-8 and tobramycin 0.25-32. For monotherapy, only meropenem maintained bactericidal activity over the 24 h for all isolates. Levofloxacin and tobramycin maintained bactericidal activity against the isolate susceptible to each drug. Piperacillin/tazobactam and cefepime did not maintain bactericidal activity against any isolate. Combination therapy with piperacillin/tazobactam or cefepime combined with levofloxacin or tobramycin were able to provide dramatic killing against ESBL K. pneumoniae, but did not always maintain bactericidal activity. Future studies should evaluate different antimicrobial combinations against pathogens producing specific ESBL enzymes to define their utility as an alternative to carbapenems.

Rapid colorimetric assay for antimicrobial susceptibility testing of Pseudomonas aeruginosa

A colorimetric assay based on the reduction of a tetrazolium salt [2,3-bis[2-methyloxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide (XTT)] for rapidly determining the susceptibility of Pseudomonas aeruginosa isolates to bactericidal antibiotics is described. There was excellent agreement between the tobramycin and ofloxacin MICs determined after 5 h using the XTT assay and after 18 h using conventional methods. The data suggests that an XTT-based assay could provide a useful method for rapidly determining the susceptibility of P. aeruginosa to bactericidal antibiotics.

Peritoneal fluid titer test for peritoneal dialysis-related peritonitis

Standard microbiological tests (i.e., MIC) do not account for the unique factors of peritoneal dialysis (PD)-related peritonitis which can significantly influence treatment response. Our goals were to develop a peritoneal fluid titer (PFT) test and to conduct a pilot study of its association with clinical outcome. The methodology was developed by using spent dialysate collected from patients with bacterial PD-related peritonitis prior to the initiation of antibiotics. Dialysate was processed and spiked with antibiotic to simulate two standard intraperitoneal regimens: cefazolin plus tobramycin and cefazolin alone. Thirty-six clinical isolates, including Staphylococcus epidermidis, Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, and Pseudomonas aeruginosa, were tested. In the pilot study, dialysate was collected from 14 patients with bacterial PD-related peritonitis. Titers were determined by using each patient's dialysate and infecting pathogen. Titers were highly reproducible, with discrepancies in only 1% of cases. Overall, PFTs were notably higher against gram-positive bacteria (P < 0.0001). The addition of tobramycin increased titers significantly from zero to values of 1/16 to 1/64 against E. cloacae and P. aeruginosa (P < 0.0001). In the pilot study, peritoneal fluid inhibitory titers were significantly associated with clinical outcome, with a median value of 1/96 for patients who were cured compared to 1/32 for those who failed treatment (P = 0.036). In conclusion, this study provides preliminary support for the PFT as a pharmacodynamic index specific to the treatment of PD-related peritonitis. With further characterization and validation in patients, the PFT test may advance the study of antibiotic therapies for PD-related peritonitis.