Novel method for measurement of outer membrane permeability to new beta-lactams in intact Enterobacter cloacae cells

Donnan Potential across the Outer Membrane of Gram-Negative Bacteria and Its Effect on the Permeability of Antibiotics

The cell envelope structure of Gram-negative bacteria is unique, composed of two lipid bilayer membranes and an aqueous periplasmic space sandwiched in between. The outer membrane constitutes an extra barrier to limit the exchange of molecules between the cells and the exterior environment. Donnan potential is a membrane potential across the outer membrane, resulted from the selective permeability of the membrane, which plays a pivotal role in the permeability of many antibiotics. In this review, we discussed factors that affect the intensity of the Donnan potential, including the osmotic strength and pH of the external media, the osmoregulated periplasmic glucans trapped in the periplasmic space, and the displacement of cell surface charges. The focus of our discussion is the impact of Donnan potential on the cellular permeability of selected antibiotics including fluoroquinolones, tetracyclines, β-lactams, and trimethoprim.

The Whole Is Bigger than the Sum of Its Parts: Drug Transport in the Context of Two Membranes with Active Efflux

Cell envelope plays a dual role in the life of bacteria by simultaneously protecting it from a hostile environment and facilitating access to beneficial molecules. At the heart of this ability lie the restrictive properties of the cellular membrane augmented by efflux transporters, which preclude intracellular penetration of most molecules except with the help of specialized uptake mediators. Recently, kinetic properties of the cell envelope came into focus driven on one hand by the urgent need in new antibiotics and, on the other hand, by experimental and theoretical advances in studies of transmembrane transport. A notable result from these studies is the development of a kinetic formalism that integrates the Michaelis-Menten behavior of individual transporters with transmembrane diffusion and offers a quantitative basis for the analysis of intracellular penetration of bioactive compounds. This review surveys key experimental and computational approaches to the investigation of transport by individual translocators and in whole cells, summarizes key findings from these studies and outlines implications for antibiotic discovery. Special emphasis is placed on Gram-negative bacteria, whose envelope contains two separate membranes. This feature sets these organisms apart from Gram-positive bacteria and eukaryotic cells by providing them with full benefits of the synergy between slow transmembrane diffusion and active efflux.

Emerging Strategies to Combat β-Lactamase Producing ESKAPE Pathogens

Since the discovery of penicillin by Alexander Fleming in 1929 as a therapeutic agent against staphylococci, β-lactam antibiotics (BLAs) remained the most successful antibiotic classes against the majority of bacterial strains, reaching a percentage of 65% of all medical prescriptions. Unfortunately, the emergence and diversification of β-lactamases pose indefinite health issues, limiting the clinical effectiveness of all current BLAs. One solution is to develop β-lactamase inhibitors (BLIs) capable of restoring the activity of β-lactam drugs. In this review, we will briefly present the older and new BLAs classes, their mechanisms of action, and an update of the BLIs capable of restoring the activity of β-lactam drugs against ESKAPE (Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) pathogens. Subsequently, we will discuss several promising alternative approaches such as bacteriophages, antimicrobial peptides, nanoparticles, CRISPR (clustered regularly interspaced short palindromic repeats) cas technology, or vaccination developed to limit antimicrobial resistance in this endless fight against Gram-negative pathogens.

Novel Cassette Assay To Quantify the Outer Membrane Permeability of Five β-Lactams Simultaneously in Carbapenem-Resistant Klebsiella pneumoniae and Enterobacter cloacae

Poor penetration through the outer membrane (OM) of Gram-negative bacteria is a major barrier of antibiotic development. While β-lactam antibiotics are commonly used against Klebsiella pneumoniae and Enterobacter cloacae, there are limited data on OM permeability especially in K. pneumoniae Here, we developed a novel cassette assay, which can simultaneously quantify the OM permeability to five β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Both clinical isolates harbored a blaKPC-2 and several other β-lactamases. The OM permeability of each antibiotic was studied separately ("discrete assay") and simultaneously ("cassette assay") by determining the degradation of extracellular β-lactam concentrations via multiplex liquid chromatography-tandem mass spectrometry analyses. Our K. pneumoniae isolate was polymyxin resistant, whereas the E. cloacae was polymyxin susceptible. Imipenem penetrated the OM at least 7-fold faster than meropenem for both isolates. Imipenem penetrated E. cloacae at least 258-fold faster and K. pneumoniae 150-fold faster compared to aztreonam, cefepime, and ceftazidime. For our β-lactams, OM permeability was substantially higher in the E. cloacae compared to the K. pneumoniae isolate (except for aztreonam). This correlated with a higher OmpC porin production in E. cloacae, as determined by proteomics. The cassette and discrete assays showed comparable results, suggesting limited or no competition during influx through OM porins. This cassette assay allowed us, for the first time, to efficiently quantify the OM permeability of multiple β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Characterizing the OM permeability presents a critical contribution to combating the antimicrobial resistance crisis and enables us to rationally optimize the use of β-lactam antibiotics.IMPORTANCE Antimicrobial resistance is causing a global human health crisis and is affecting all antibiotic classes. While β-lactams have been commonly used against susceptible isolates of Klebsiella pneumoniae and Enterobacter cloacae, carbapenem-resistant isolates are spreading worldwide and pose substantial clinical challenges. Rapid penetration of β-lactams leads to high drug concentrations at their periplasmic target sites, allowing β-lactams to more completely inactivate their target receptors. Despite this, there are limited tangible data on the permeability of β-lactams through the outer membranes of many Gram-negative pathogens. This study presents a novel, cassette assay, which can simultaneously characterize the permeability of five β-lactams in multidrug-resistant clinical isolates. We show that carbapenems, and especially imipenem, penetrate the outer membrane of K. pneumoniae and E. cloacae substantially faster than noncarbapenem β-lactams. The ability to efficiently characterize the outer membrane permeability is critical to optimize the use of β-lactams and combat carbapenem-resistant isolates.

β-Lactams and β-Lactamase Inhibitors: An Overview

β-Lactams are the most widely used class of antibiotics. Since the discovery of benzylpenicillin in the 1920s, thousands of new penicillin derivatives and related β-lactam classes of cephalosporins, cephamycins, monobactams, and carbapenems have been discovered. Each new class of β-lactam has been developed either to increase the spectrum of activity to include additional bacterial species or to address specific resistance mechanisms that have arisen in the targeted bacterial population. Resistance to β-lactams is primarily because of bacterially produced β-lactamase enzymes that hydrolyze the β-lactam ring, thereby inactivating the drug. The newest effort to circumvent resistance is the development of novel broad-spectrum β-lactamase inhibitors that work against many problematic β-lactamases, including cephalosporinases and serine-based carbapenemases, which severely limit therapeutic options. This work provides a comprehensive overview of β-lactam antibiotics that are currently in use, as well as a look ahead to several new compounds that are in the development pipeline.

Cefepime

Cefepime (Maxipime), Maxcef, Cepimax, Cepimex, Axepim, a parenteral fourth-generation cephalosporin, is active against many organisms causative in pneumonia. Cefepime has in vitro activity against Gram-positive organisms including Staphylococcus aureus and penicillin-sensitive, -intermediate and -resistant Streptococcus pneumoniae similar to that of cefotaxime and ceftriaxone. Cefepime also has good activity against Gram-negative organisms, including Pseudomonas aeruginosa, similar to that of ceftazidime. Importantly, cefepime is stable against many of the common plasmid- and chromosome-mediated beta-lactamases and is a poor inducer of AmpC beta-lactamases. As a result, it retains activity against Enterobacteriaceae that are resistant to third-generation cephalosporins, such as derepressed mutants of Enterobacter spp. Cefepime may be hydrolyzed by the extended-spectrum beta-lactamases produced by some members of the Enterobacteriaceae, but to a lesser extent than the third-generation cephalosporins. Monotherapy with cefepime 1 or 2g, usually administered intravenously twice daily, was as effective for clinical and bacteriological response as ceftazidime, ceftriaxone or cefotaxime monotherapy (1 or 2g two or three times daily) in a number of randomized, clinical trials in hospitalized adult, or less commonly, pediatric, patients with generally moderate to severe community-acquired or nosocomial pneumonia. More limited data indicated that monotherapy with cefepime 2g three times daily was also as effective in treating patients with nosocomial pneumonia as imipenem/cilostatin 0.5g four times daily, and when combined with amikacin, cefepime was as effective as ceftazidime plus amikacin. Patients with pneumonia who failed to respond to previous antibacterial therapy with penicillins or other cephalosporins responded to treatment with cefepime. Cefepime is generally well tolerated, with a tolerability profile similar to those of other parenteral cephalosporins. In clinical trials, the majority of adverse events experienced by cefepime recipients were mild to moderate and reversible. The most common adverse events with a causal relationship to cefepime reported in clinical trials included rash and diarrhea. Other, less common, adverse events included pruritus, urticaria, nausea, vomiting oral candidiasis, colitis, headache, fever, erythema and vaginitis.

CONCLUSION

Cefepime is an established and generally well tolerated parenteral drug with a broad spectrum of antibacterial activity which, when administered twice daily, provides coverage of most of the pathogens that may be causative in pneumonia. In randomized clinical trials in hospitalized patients with generally moderate to severe community-acquired or nosocomial pneumonia, cefepime monotherapy exhibited good clinical and bacteriological efficacy. Cefepime may become a preferred antibacterial agent for infections caused by Enterobacter spp. With prudent use in order to prevent the emergence of resistant organisms, cefepime will continue to be a suitable option for the empiric treatment of pneumonia.

Studies on the mechanisms of β-lactam resistance in Bordetella bronchiseptica

OBJECTIVES

Little is currently known about beta-lactam resistance in Bordetella bronchiseptica. So far, only a single beta-lactamase gene, bla(BOR-1), has been identified. In a previous study, high MICs of ampicillin, cefalotin and ceftiofur were determined among 349 porcine B. bronchiseptica isolates. The aim of this study was to identify genes associated with elevated MICs of beta-lactams and their transferability.

METHODS

Selected isolates were investigated by PCR for commonly found bla genes and class 1 integrons; selected amplicons were sequenced. Plasmid location of resistance genes was confirmed by conjugation. Beta-lactamases were characterized by SDS-PAGE and isoelectric focusing. The genomic relatedness of the isolates was investigated by XbaI macrorestriction analysis. Inhibition studies with efflux pump inhibitors were conducted. The permeability of cephalosporins into intact cells was measured exemplarily for one isolate.

RESULTS

Of the 349 B. bronchiseptica isolates, eight isolates carried a class 1 integron with a bla(OXA-2) cassette on a conjugative plasmid of ca. 50 kb. In addition, one plasmid-free isolate also carried this class 1 integron. Besides bla(BOR-1), no other beta-lactamase gene was detected in the remaining isolates with high MICs of ampicillin of >or= 32 mg/L. Inhibition experiments suggested that efflux does not play a role in beta-lactam resistance. Instead, membrane permeability for cephalosporins was reduced as shown for B. bronchiseptica isolate B543.

CONCLUSIONS

This is to the best of our knowledge the first report of a mobile bla gene in B. bronchiseptica. Reduced membrane permeability of B. bronchiseptica seems to decrease susceptibility against cephalosporins.

Method for estimation of low outer membrane permeability to beta-lactam antibiotics

The outer membrane of gram-negative bacteria plays a major role in beta-lactam resistance as it slows down antibiotic entry into the periplasm and therefore acts in synergy with beta-lactamases and efflux systems. Up to now, the quantitative estimation of low outer membrane permeability by the method of Zimmermann and Rosselet was difficult because of the secreted and cell surface-associated beta-lactamases. The method presented here uses the acylation of a highly sensitive periplasmic penicillin-binding protein (PBP) (BlaR-CTD) to assess the rate of beta-lactam penetration into the periplasm. The method is dedicated to measurement of low permeability and is only valid when the diffusion rate through the outer membrane is rate limiting. Cytoplasmic membrane associated PBPs do not interfere since they are acylated after the very sensitive BlaR-CTD. This method was used to measure the permeability of beta-lactamase-deficient strains of Enterobacter cloacae and Enterobacter aerogenes to benzylpenicillin, ampicillin, carbenicillin, cefotaxime, aztreonam, and cephacetrile. Except for that of cephacetrile, the permeability coefficients were equal to or below 10(-7) cm/s. For cephacetrile, carbenicillin, and benzylpenicillin, the outer membrane of E. cloacae was 20 to 60 times less permeable than that of Escherichia coli, whereas for cefotaxime, aztreonam, and ampicillin it was, respectively, 400, 1,000, and 700 times less permeable. The permeability coefficient for aztreonam is the lowest ever measured (P = 3.2 x 10(-9) cm/s). Using these values, the MICs for a beta-lactamase-overproducing strain of E. cloacae were successfully predicted, demonstrating the validity of the method.

Monotherapy versus combination therapy for bacterial infections

The authors discuss the latest findings regarding the use of one or more antimicrobial drugs for a variety of infections. They offer suggestions for treatment based on a host of considerations, including the synergy and antagonism of specific drugs, type of infection, potential toxicities, and cost.

Quantitative determination of cefepime in plasma and vitreous fluid by high-performance liquid chromatography

An isocratic reversed-phase HPLC method was developed to determine cefepime levels in plasma and vitreous fluid. Cefepime and the internal standard cefadroxil were separated on a Shandon Hypersil BDS C18 column by using a mobile phase of 25 mM sodium dihydrogen phosphate monohydrate (pH 3) and methanol (87:13, v/v). Ultraviolet detection was carried out at 270 nm. The retention times were 4.80 min for cefepime and 7.70 min for cefadroxil. This fast procedure which involves an efficient protein precipitation step (addition of HClO4), allows a quantification limit of 2.52 microg ml(-1) and a detection limit of 0.83 microg ml(-1). Recoveries and absolute recoveries of cefepime from plasma were 96.13-99.44% and 94-102.5% respectively. The intra-day and inter-day reproducibilities were less than 2% for cefepime at 10, 30, 50 microg ml(-1) (n=10). The method was proved to be suitable for determining cefepime levels in human plasma and was modified to measure vitreous fluid samples.

Evaluation of several dosing regimens of cefepime, with various simulations of renal function, against clinical isolates of Pseudomonas aeruginosa in a pharmacodynamic infection model

The objectives of this study were as follows: (i) to examine the killing activity of 2-g doses of cefepime against two clinical isolates (mucoid and nonmucoid) of Pseudomonas aeruginosa in a pharmacodynamic in vitro infection model, (ii) to compare the percentage of time above the MIC (T > MIC) for each of the regimens against P. aeruginosa, and (iii) to evaluate the area under the bactericidal curve for each regimen. Cefepime was administered at intervals of 8, 12, and 24 h with and without tobramycin, and two different levels of renal function were simulated: normal (creatinine clearance [CLCR] = 90 ml/min) and decreased (CRCL = 60 ml/min). Also, the killing activity of cefepime with and without tobramycin was compared to the killing activity of ceftazidime (2 g every 8 h) with and without tobramycin. The T > MIC was 100% in the central chamber except for the regimen in which cefepime was administered every 12 h and the CLCR was 90 ml/min, which provided concentrations above the MIC for 92% of the dosing interval against the C31 (mucoid; MIC of cefepime, 4 microg/ml) isolate and for 75% of the interval against the C34 (nonmucoid; MIC of cefepime, 8 microg/ml) isolate. All cefepime and ceftazidime monotherapy simulations resulted in 99.9% killing of the nonmucoid isolate within 4 to 8 h and within 4 to 6 h, respectively. Against the mucoid isolate, 99.9% killing was achieved only with combination therapy. The results of this study indicate that cefepime dosed at 2 g every 12 h under conditions of normal renal function and every 24 h with decreased creatinine clearance (60 ml/min) is effective both as monotherapy and in combination therapy against a nonmucoid strain of P. aeruginosa. With cefepime MICs of 4 and 8 microg/ml, the single-agent regimens provided T > MIC values in the central chamber for 92 and >/=75% of the dosing interval against the mucoid and nonmucoid isolates, respectively. Cefepime dosed at 2 g every 12 h, with a creatinine clearance of 90 ml/min, and every 24 h, with a creatinine clearance of 60 ml/min, resulted in killing activity equivalent to that of ceftazidime dosed at 2 g every 8 h. None of the monotherapies provided adequate killing of the mucoid strain of P. aeruginosa despite drug concentrations being above the MIC for >/=92% of all dosing intervals. Finally, combination therapy with tobramycin and either cefepime or ceftazidime enhanced the killing of both the mucoid and nonmucoid P. aeruginosa isolates.

Comparison of strategies using cefpirome and ceftazidime for empiric treatment of pneumonia in intensive care patients. The Cefpirome Pneumonia Study Group

In an international, multicenter, open-label, randomized comparative study, adult patients in intensive care units were enrolled to receive cefpirome intravenously at 2 g twice daily or ceftazidime intravenously at 2 g three times daily for the empiric treatment of pneumonia. Randomization was performed after a double stratification according to the investigator's initial choice of monotherapy or combination therapy and then on the basis of the severity of disease. The primary endpoint was the clinical response at the end of treatment in the intent-to-treat population. Data for all patients were reviewed by a blinded observer. Of the 400 enrolled patients, 201 received cefpirome (monotherapy, 56%) and 199 received ceftazidime (monotherapy, 51%). Pneumonia was hospital acquired for 75% of the patients. Clinical failures rates were 34 versus 36% (odds ratio = 0.922; upper bound of 90% confidence interval = 1.301) in the intent-to-treat analysis for cefpirome and ceftazidime, respectively. For the cefpirome and ceftazidime groups, there were 35 versus 30% clinical failures among monotherapy-stratified patients, respectively, and 34 versus 42% clinical failures among combination therapy-stratified patients, respectively. The rates of clinical failures in the per-protocol analysis were 38 and 42%, respectively. In the population of patients evaluable for bacteriologic efficacy, eradication or presumed eradication was obtained for 71% (172 of 241) and 70% (162 of 230) of the pathogens isolated from the patients receiving cefpirome and ceftazidime, respectively. The mortality rates within 2 weeks after the end of treatment were similar (cefpirome group, 31%; ceftazidime group, 26%), as were the percentages of patients with at least one treatment-related adverse event (17 and 19%, respectively). An empiric treatment strategy with cefpirome at 2 g twice daily is equivalent in terms of efficacy and tolerance to ceftazidime at 2 g three times daily for the treatment of pneumonia in patients in intensive care units.

Cefpirome. A review of its antibacterial activity, pharmacokinetic properties and clinical efficacy in the treatment of severe nosocomial infections and febrile neutropenia

Cefpirome is an injectable extended-spectrum or 'fourth generation' cephalosporin. Its antibacterial activity encompasses many of the pathogens involved in hospital-acquired infections such as Enterobacteriaceae, methicillin-susceptible Staphylococcus aureus, coagulase-negative staphylococci and viridans group streptococci. Cefpirome also has in vitro activity against Streptococcus pneumoniae regardless of penicillin susceptibility. It is stable against most plasmid- and chromosome-mediated beta-lactamases, with the exception of the extended-spectrum plasmid-mediated SHV enzymes. Intravenous cefpirome 2g twice daily has shown clinical efficacy comparable to that of ceftazidime 2g 3 times daily in the treatment of hospitalised patients with moderate to severe infections. Clinical response and bacteriological eradication rates were similar in patients with severe pneumonia or septicaemia treated with either cefpirome or ceftazidime. Cefpirome appeared more effective than ceftazidime in the eradication of bacteria in patients with febrile neutropenia in 1 study; however, clinical response rates were similar in the 2 treatment groups. The tolerability of cefpirome appears similar to that of ceftazidime and other third generation cephalosporins, diarrhoea being the most frequently observed event. Thus, cefpirome is likely to be a valuable extended-spectrum agent for the treatment of severe infections. Cefpirome offers improved coverage against some Gram-positive pathogens and Enterobacteriaceae producing class I beta-lactamases compared with the third generation cephalosporins, although this has yet to be demonstrated in clinical trials.

Bacteriologic and clinical applications of a new extended-spectrum parenteral cephalosporin

Although third-generation cephalosporins have been considered the backbone of antibiotic therapy for the treatment of many kinds of serious infections, including those in hospitalized patients, lack of activity against some important pathogens still exists among currently available drugs. In addition, increasing accounts of antibiotic resistance, particularly in the hospital environment, are of deep concern and have thus led to the need for the development of newer antimicrobial agents. Cefepime is a now parenteral cephalosporin with an extended spectrum of antibacterial activity that includes both aerobic gram-negative and gram-positive bacteria. It is also active against many gram-negative organisms resistant to ceftriaxone and cefotaxime, as well as many strains of Enterobacter and Citrobacter resistant to ceftazidime. Cefepime appears to be less likely to select out resistant organisms, and it may be less likely to change hospital flora than currently available antimicrobials. Cefepime has been shown to be very well tolerated and effective in the treatment of a variety of infections including moderate-to-severe pneumonia (including cases associated with concurrent bacteremia), complicated and uncomplicated urinary tract infections (also including cases associated with concurrent bacteremia), and skin and skin-structure infections. Clinical response rates are > or = 75% for most infections and have been comparable to ceftazidime in comparative trials. In addition, pretreatment susceptibility testing indicates that >94% of organisms isolated in patients enrolled in clinical trials were susceptible to cefepime.

Comparative activity of eight antimicrobial agents against clinical bacterial isolates from the United States, measured by two methods

In a surveillance study conducted during 1992-1993 at 83 medical institutions of different types and sizes (e.g., laboratories, community hospitals, teaching hospitals) and from different geographical areas of the United States, clinical bacterial isolates were tested for their susceptibility to eight comparative antimicrobial agents (cefepime, ceftazidime, cefotaxime, ceftriaxone, ciprofloxacin, gentamicin, imipenem, and piperacillin). A total of 12,574 isolates were tested by either the Etest method (AB Biodisk) or a microdilution method (MicroScan) in the participating laboratories; 11.8% of these isolates were subsequently retested for quality assurance purposes by both methods in a central laboratory. The results obtained in the central laboratory were essentially the same as the results obtained in the participating laboratories. This article presents data for gram-negative and gram-positive isolates other than Streptococcus pneumoniae, the results of which have been previously published. Antimicrobial susceptibility results obtained with the two different minimum inhibitory concentration (MIC) methods--MicroScan and Etest--showed that most isolates of Enterobacteriaceae were susceptible to cefepime, exceeding the activity of ceftazidime, ceftriaxone, and cefotaxime, principally because of the greater activity of cefepime against the species that produce Bush group 1 beta-lactamases (predominantly Enterobacter cloacae, Enterobacter aerogenes, and Citrobacter freundii). In addition, the activity of cefepime against Pseudomonas aeruginosa isolates was essentially equivalent to that of ceftazidime and greater than that of third-generation cephalosporins. Most methicillin-susceptible Staphylococcus aureus were susceptible to all the cephalosporins, whereas methicillin-resistant S. aureus and enterococci were resistant. Overall, the most active antimicrobials in this study were imipenem, ciprofloxacin, and cefepime, but the activity of all the antimicrobials varied with different species. Categorically, the results from the microdilution and Etest methods were equivalent.

Role of efflux pump(s) in intrinsic resistance of Pseudomonas aeruginosa: active efflux as a contributing factor to beta-lactam resistance

Wild-type strains of Pseudomonas aeruginosa are more resistant to various beta-lactam antibiotics as well as other agents than most enteric bacteria. Although resistance to compounds of earlier generations is explained by the synergism between the outer membrane barrier and the inducible beta-lactamase, it was puzzling to see significant levels of resistance to compounds that do not act as inducers or are not hydrolyzed rapidly by the chromosomally encoded enzyme. This intrinsic-resistance phenotype becomes enhanced in those strains with the so-called intrinsic carbenicillin resistance. In the accompanying paper (X.-Z. Li, D. M. Livermore, and H. Nikaido, Antimicrob. Agents Chemother. 38:1732-1741, 1994), we showed that active efflux played a role in the resistance, to various non-beta-lactam agents, of P. aeruginosa strains in general and that the efflux was enhanced in intrinsically carbenicillin-resistant strains. We show in this paper that, in comparison with the drug-hypersusceptible mutant K799/61, less benzylpenicillin was accumulated in wild-type strains of P. aeruginosa and that the accumulation levels were even lower in intrinsically carbenicillin-resistant strains. Deenergization by the addition of a proton conductor increased the accumulation level to that expected for equilibration across the cytoplasmic membrane. In intrinsically carbenicillin-resistant isolates, there was no evidence that either nonspecific or specific permeation rates of beta-lactams across the outer membrane were lowered in comparison with those of the more susceptible isolates. Furthermore, these carbenicillin-resistant isolates were previously shown to have no alteration in the level or the inducibility of beta-lactamase and in the affinity of penicillin-binding proteins. These data together suggest the involvement of an active efflux mechanism also in the resistance to beta-lactams. Hydrophilic beta-lactams with more than one charged group did not cross the cytoplasmic membrane readily. Yet one such compound, ceftriaxone, appeared to be extruded from the cells of more-resistant strains, although with this compound effects of proton conductors could not be shown. We postulate that wild-type strains of P. aeruginosa pump out such hydrophilic beta-lactams either from the periplasm or from the outer leaflet of the lipid bilayer of the cytoplasmic membrane, in a manner analogous to that hypothesized for multidrug resistance protein of human cancer cells (M.M. Gottesman and I. Pastan, Annu. Rev. Biochem. 62:385-427, 1993).

Cefepime. A review of its antibacterial activity, pharmacokinetic properties and therapeutic use

Cefepime is a 'fourth' generation cephalosporin that has a broader spectrum of antibacterial activity than the third generation cephalosporins and is more active in vitro against Gram-positive aerobic bacteria. The fact that cefepime is stable to hydrolysis by many of the common plasmid- and chromosomally-mediated beta-lactamases, and that it is a poor inducer of type I beta-lactamases, indicates that cefepime may be useful for treatment of infections resistant to earlier cephalosporins. In comparative trials, cefepime 1 to 2 g, usually administered intravenously twice daily, was as effective as ceftazidime 1 to 2 g, usually administered 3 times daily, for treatment of bacteraemia and infections of the lower respiratory tract, urinary tract, pelvis and skin and skin structures. Furthermore, cefepime was as effective as ceftazidime and piperacillin or mezlocillin in combination with gentamicin when administered as empirical treatment for fever in patients with neutropenia. A limited number of trials have found cefepime to be as effective as cefotaxime for the treatment of gynaecological and lower respiratory tract infections. Similarly, cefepime 2 g twice daily intravenously (alone or in combination with metronidazole) was as effective as gentamicin in combination with mezlocillin or clindamycin, respectively, for the treatment of intra-abdominal infection. Cefepime has a linear pharmacokinetic profile, an elimination half-life of approximately 2 hours and is primarily excreted by renal mechanisms as unchanged drug. Cefepime has a tolerability profile similar to that of other parenteral cephalosporins; adverse events are primarily gastrointestinal in nature. A total of 1.4 and 2.9% of patients receiving cefepime < or = 2 g/day and > 2 g/day, respectively, required treatment withdrawal as a result of any adverse event. Thus, cefepime has the advantage of an improved spectrum of antibacterial activity, and is less susceptible to hydrolysis by some beta-lactamases, compared with third generation cephalosporins. Despite these advantages, cefepime has not been found to be more effective than ceftazidime and cefotaxime in clinical trials, although most trials selected patients with organisms sensitive in vitro to both comparator agents. Further trials, particularly in areas of widespread bacterial resistance, are required to confirm the positioning of cefepime for treatment of serious infection, and in particular to further explore whether its potential advantages result in clinical benefits.

Cefepime clinical pharmacokinetics

Cefepime is a new parenteral cephalosporin with antimicrobial activity similar to third-generation cephalosporins. It acts against the Enterobacteriaceae family, and Pseudomonas aeruginosa, but maintains Gram-positive activity similar to that of first- or second-generation cephalosporins. Cefepime has in vitro activity against many bacterial isolates resistant to ceftazidime and cefotaxime, is stable against chromosomally mediated beta-lactamases, demonstrates lower affinity for these enzymes and shows a high resistance to enzymatic hydrolysis. Clinical uses thus far include treatment of lower respiratory tract, intra-abdominal and urinary tract infections, skin and soft tissue infections and for prophylaxis in biliary tract and prostate surgery. Pharmacokinetic studies indicate that cefepime exhibits linear pharmacokinetic behaviour. Pharmacokinetic variables are not significantly different between single- and multiple-dose administration, indicating a lack of drug accumulation in patients with normal renal function. Cefepime is not highly bound to plasma proteins, with binding values of approximately 16 to 19%. The drug is widely distributed in various biological tissues and fluids. The primary route of elimination is from the kidneys, with over 80% of the drug recovered in the urine as unchanged drug in patients with normal renal function. Total drug clearance and renal clearance are similar to creatinine clearance, and glomerular filtration is thought to be the primary mechanism of renal excretion. The elimination half-life is approximately 2 to 2.5 h in patients. Cefepime is removed by haemodialysis (over 3h) and peritoneal dialysis (over 72h) to an appreciable extent, with 40 to 68% and 26% of the drug removed, respectively. Overall, cefepime is well tolerated by patients and no significant drug interactions have been reported to date.

Molecular basis of the efficacy of cefaclor against Haemophilus influenzae

Cefaclor sustained its inhibitory activity against a beta-lactamase-producing strain of Haemophilus influenzae. Although a relatively high permeability coefficient was calculated for ampicillin compared with that calculated for cefaclor, the resulting periplasmic concentration of cefaclor was 5.7 times that of ampicillin. The efficacy of cefaclor may be due to its higher beta-lactamase resistance, which allows it to achieve a greater periplasmic concentration and adequate binding to crucial penicillin-binding proteins.

Comparison of cefepime, cefpirome, and cefaclidine binding affinities for penicillin-binding proteins in Escherichia coli K-12 and Pseudomonas aeruginosa SC8329

The relative binding affinities of the extended-spectrum cephalosporins cefepime, cefpirome, and cefaclidine for the penicillin-binding proteins (PBPs) of Escherichia coli K-12 and Pseudomonas aeruginosa SC8329 were determined. Affinities were calculated from competition experiments between these antibiotics and [3H]benzylpenicillin in isolated membrane preparations. The concentrations which reduced binding to a PBP by 50% (IC50s) were determined. For E. coli, all three antibiotics displayed good PBP 3 binding (IC50s of 0.5 microgram/ml or less), and MICs roughly correlated with these values. Cefepime had a greater than 20-fold-lower IC50 for PBP 2 of E. coli than the other antibiotics. For P. aeruginosa, all of the antibiotics bound poorly (greater than 25 micrograms/ml) to PBP 2 but showed excellent pseudomonal (less than 0.0025 microgram/ml) PBP 3 binding. No correlations were seen between IC50s and MICs for P. aeruginosa. Despite differences in PBP binding, cefepime, cefpirome, and cefaclidine all displayed similar bactericidal activity for E. coli K-12 over the initial 3 h after antibiotic addition. All three caused E. coli to form filaments at values close to the MICs. In addition, cefepime induced "bleb" formation along the filaments at concentrations greater than 10x the MIC.

Reevaluation of the factors involved in the efficacy of new beta-lactams against Enterobacter cloacae

The roles of outer membrane permeability, beta-lactamase stability, and inhibition of penicillin-binding proteins in the activity of new beta-lactams against Enterobacter cloacae were reappraised by using several methodological improvements. Outer membrane permeability in intact cells was determined by using a high-pressure liquid chromatography (HPLC)-based technique that avoided certain possible artifacts of the traditional methods. Vmax values were calculated from the numbers of enzyme molecules produced per cell and from catalytic constant (Kcat) values, which were obtained with purified beta-lactamase. Minimal periplasmic antibiotic concentrations needed to inhibit bacterial cell wall synthesis were estimated from the Zimmermann-Rosselet equation. All the beta-lactams tested formed relatively stable complexes with purified beta-lactamase. The antibiotics that exhibited low affinity for beta-lactamase apparently needed higher periplasmic concentrations to inhibit cell wall synthesis, suggesting a possible correlation between the affinity of beta-lactamase and the affinity of penicillin-binding proteins for the new beta-lactams. By using these estimates of outer membrane permeability, beta-lactamase hydrolysis, and cell wall-inhibiting concentrations, MIC could be theoretically predicted to within 1 dilution for five beta-lactams in three isogenic E. cloacae strains with differences in antibiotic susceptibility due to different porin or beta-lactamase contents.