Cysteine Hydropersulfide Inactivates ß-Lactam Antibiotics with Formation of Ring-Opened Carbothioic S-Acids in Bacteria.

Hydrogen sulfide (H2S) formed during sulfur metabolism in bacteria has been implicated in the development of intrinsic resistance to antibacterial agents. Despite the conversion of H2S to hydropersulfides greatly enhancing the biochemical properties of H2S such as antioxidant activity, the effects of hydropersulfides on antibiotic resistance have remained unknown. In this work, we investigated the effects of H2S alone or together with cystine to form cysteine hydropersulfide (CysSSH) on the activities of antibacterial agents. By using the disc diffusion test, we found that CysSSH treatment effectively inactivated ß-lactams of the penicillin class (penicillin G and ampicillin) and the carbapenem class (meropenem). These ß-lactams were resistant to treatment with H2S alone or cystine alone. In contrast, cephalosporin class ß-lactams (cefaclor and cefoperazone) and non-ß-lactam antibiotics (tetracycline, kanamycin, erythromycin, and ofloxacin) were stable after CysSSH treatment. Chromatographic and mass spectrometric analyses revealed that CysSSH directly reacted with ß-lactams to form ß-lactam ring-opened carbothioic S-acids (BL-COSH). Furthermore, we demonstrated that certain bacteria (e.g., Escherichia coli and Staphylococcus aureus) efficiently decomposed ß-lactam antibiotics to form BL-COSH, which were transported to the extracellular space. These data suggest that CysSSH-mediated ß-lactam decomposition may contribute to intrinsic bacterial resistance to ß-lactams. BL-COSH may become useful biomarkers for CysSSH-mediated ß-lactam resistance and for investigation of potential antibacterial adjuvants that can enhance the antibacterial activity of ß-lactams by reducing the hydropersulfides in bacteria.

Novel ß-Lactam/ß-Lactamase inhibitor combinations vs alternative antibiotics in the treatment of complicated urinary tract infections: A meta-analysis of randomized controlled trials.

OBJECTIVES: This meta-analysis assessed the efficacy and safety of novel ß-lactam/ß-lactamase inhibitor combinations in the treatment of complicated urinary tract infection (cUTI)/acute pyelonephritis (APN). METHODS: PubMed, Web of Science, EBSCO (Elton B. Stephens Co.), Cochrane Library, Ovid MEDLINE, and Embase databases were accessed until November 21, 2019. In this meta-analysis, only randomized controlled trials comparing the treatment efficacy of novel ß-lactam/ß-lactamase inhibitor combinations with other antibiotics for cUTI/APN in adult patients were included. The outcomes included the clinical and microbiological responses, and risk of adverse events (AEs). RESULTS: Overall, the experimental group treated with a novel ß-lactam/ß-lactamase inhibitor combination and the control group comprised 1346 and 1376 patients, respectively. No significant difference in the clinical response rate at test-of-cure was observed between the novel ß-lactam/ß-lactamase inhibitor combination and comparators among the microbiological modified intent-to-treat population (89.1% vs 88.3%, OR, 1.04; 95% confidence interval [CI], 0.76-1.42; I = 28%) and the microbiologically evaluable population (95.2% vs 94.7%, OR, 1.12; 95% CI, 0.68-1.84; I = 0%). Additionally, the novel ß-lactam/ß-lactamase inhibitor combination was associated with a better microbiological response at test-of-cure than the comparators among the microbiological modified intent-to-treat population (74.4% vs 68.5%, OR, 1.34; 95% CI, 1.04-1.72; I = 45%) and microbiologically evaluable population (80.1% vs 72.5%, OR, 1.49; 95% CI, 1.06-2.10; I = 58%). Finally, the risk of AEs associated with the novel ß-lactam/ß-lactamase inhibitor combination was similar to that associated with the comparators (treatment-emergent adverse events [TEAE], OR, 1.04; 95% CI, 0.87-1.23; I = 19%; serious AEs, OR, 1.21; 95% CI, 0.82-1.76; I = 0%; treatment discontinuation for drug-related TEAE, OR, 077; 95% CI, 0.38-1.56, I = 5%). The all-cause mortality did not differ between the novel ß-lactam/ß-lactamase inhibitor combination and comparators (OR, 1.19; 95% CI, 0.37-3.81; I = 0%). CONCLUSIONS: The clinical and microbiological responses of novel ß-lactam/ß-lactamase inhibitor combinations in the treatment of cUTI/APN are similar to those of other available antibiotics. These combinations also share a safety profile similar to that of other antibiotics.

Stability Studies of Antipyocyanic Beta-Lactam Antibiotics Used in Continuous Infusion.

In intensive care, beta-lactams can be reconstituted in 50 mL polypropylene syringes with NaCl 0.9 % and administered for 8 to 12 h at various concentrations with motor-operated syringe pumps. The feasibility and/or the stability of these antibiotic therapies are often poorly known by clinicians. The purpose of this study was to determine the stability of seven antipyocyanic beta-lactam antibiotics and cilastatin under real-life conditions. Stability indicating HPLC methods allowing quantification in pharmaceutical preparations and subsequent stability studies were performed. The stability studies showed that continuous infusion of piperacillin/tazobactam 80/10 mg/mL, of cefepime 20 and 40 mg/mL and of aztreonam 40 and 120 mg/mL can be used over 12 h. Moreover, continuous infusion of cefepime 120 mg/mL can be used over 10 h, whereas meropenem 10 and 20 mg/mL and ceftazidime 40 mg/mL remained stable only over 8 h, and meropenem 40 mg/mL was significantly degraded after 6 h. Finally, imipenem/cilastatin 5/5 mg/mL and piperacillin/tazobactam 320/40 mg/mL should not be used as continuous infusion. These data allow the establishment of protocols of administration of antipyocyanic beta-lactams by continuous infusion. Some of them are not appropriate to this mode of administration (imipenem/cilastatin, piperacillin/ tazobactam 320/40 mg/mL) or must be avoided if possible (ceftazidime 40 mg/mL).

Real-time monitoring ß-lactam/ß-lactamase inhibitor (BL/BLI) mixture towards the bacteria porin pathway at single molecule level.

Multidrug-resistant bacteria are a great concern and a problem that must be addressed. Extended-spectrum ß-lactamases are a common defence mechanism of bacteria to make ß-lactam (BL) antibiotics ineffective. ß-Lactamase inhibitors (BLIs) are consequently designed and are often clinically prescribed with a BL antibiotic to hinder degradation. Current studies focusing on how BL antibiotics or BLIs interact solely with the bacterial outer membrane nanopores (porins) on reaching the periplasmic side using a nanopore-based sensing technique. In electrochemical studies, the bias voltage allows real-time monitoring of BL antibiotics, BLIs and their mixture through the porin pathway at the single-molecule level. Here we consider the most abundant membrane protein from Escherichia coli (i.e. OmpF), purify and reconstitute the membrane protein in an artificial lipid bilayer and then study its ex vivo electrochemical behaviour. We show the piperacillin/tazobactam mixture interacts with OmpF, whereas the substrate interacts under the maximum bandwidth. The power spectrum analysis of the ionic current trace demonstrates the ampicillin/sulbactram mixture requires more energy than ampicillin alone to pass through the porin pathway. Our results demonstrate that clinically relevant combinations (e.g. piperacillin/tazobactam and ampicillin/sulbactam) interact more strongly with OmpF than either the BL antibiotic or the BLI alone. We suggest a quick and relatively cheap screening method to test the ability of BL antibiotics/BLIs to cross the bacterial cellular membrane.

Assessment of Antipiperacillin IgG Binding to Structurally Related Drug Protein Adducts.

The risk of developing hypersensitivity to alternative antibiotics is a concern for penicillin hypersensitive patients and healthcare providers. Herein we use piperacillin hypersensitivity as a model to explore the reactivity of drug-specific IgG against alternative ß-lactam protein adducts. Mass spectrometry was used to show the drugs (amoxicillin, flucloxacillin, benzyl penicillin, aztreonam, and piperacillin) bind to similar lysine residues on the protein carrier bovine serum albumin. However, the hapten-specific IgG antibodies found in piperacillin hypersensitive patient plasma did not bind to other ß-lactam protein conjugates. These data outline the fine specificity of piperacillin-specific IgG antibodies that circulate in patients with hypersensitivity.

Effect of High N-Acetylcysteine Concentrations on Antibiotic Activity against a Large Collection of Respiratory Pathogens.

The effect of high N-acetylcysteine (NAC) concentrations (10 and 50 mM) on antibiotic activity against 40 strains of respiratory pathogens was investigated. NAC compromised the activity of carbapenems (of mostly imipenem and, to lesser extents, meropenem and ertapenem) in a dose-dependent fashion. We demonstrated chemical instability of carbapenems in the presence of NAC. With other antibiotics, 10 mM NAC had no major effects, while 50 mM NAC sporadically decreased (ceftriaxone and aminoglycosides) or increased (penicillins) antibiotic activity.

Loss of a Class A Penicillin-Binding Protein Alters ß-Lactam Susceptibilities in Mycobacterium tuberculosis.

Recent studies have renewed interest in ß-lactam antibiotics as a potential treatment for Mycobacterium tuberculosis infection. To explore the opportunities and limitations of this approach, we sought to better understand potential resistance mechanisms to ß-lactam antibiotics in M. tuberculosis. We identified mutations in the penicillin-binding protein (PBP) ponA2 that were able to confer some degree of resistance to the cephalosporin subclass of ß-lactams. Surprisingly, deletion of ponA2 also confers resistance, demonstrating that ß-lactam resistance can spontaneously arise from PBP loss of function. We show that ponA2 mutants resistant to the cephalosporin subclass of ß-lactams in fact show increased susceptibility to meropenem, a carbapenem that is known to target l,d-transpeptidases, thereby suggesting that in the absence of PonA2, an alternative mode of peptidoglycan synthesis likely becomes essential. Consistent with this hypothesis, a negative genetic selection identified the l,d-transpeptidase ldtMt2 as essential in the absence of ponA2. The mechanism of ß-lactam resistance we outline is consistent with emerging models of ß-lactam killing, while the investigation of ponA2 downstream and synthetic lethal genes sheds light on the mechanism of cell wall biosynthesis and the interaction between conventional PBPs and l,d-transpeptidases.

Discovery of potent wall teichoic acid early stage inhibitors.

The widespread emergence of methicillin-resistant Staphylococcus aureus (MRSA) has dramatically eroded the efficacy of current ß-lactam antibiotics and created an urgent need for novel treatment options. Using an S. aureus phenotypic screening strategy, we have identified small molecule early stage wall teichoic acid (WTA) pathway-specific inhibitors predicted to be chemically synergistic with ß-lactams. These previously disclosed inhibitors, termed tarocins, demonstrate by genetic and biochemical means inhibition of TarO, the first step in WTA biosynthesis. Tarocins demonstrate potent bactericidal synergy in combination with broad spectrum ß-lactam antibiotics across diverse clinical isolates of methicillin-resistant Staphylococci. The synthesis and structure-activity relationships (SAR) of a tarocin series will be detailed. Tarocins and other WTA inhibitors may provide a rational strategy to develop Gram-positive bactericidal ß-lactam combination agents active against methicillin-resistant Staphylococci.

In vitro susceptibility of characterized ß-lactamase-producing strains tested with avibactam combinations.

Avibactam, a broad-spectrum ß-lactamase inhibitor, was tested with ceftazidime, ceftaroline, or aztreonam against 57 well-characterized Gram-negative strains producing ß-lactamases from all molecular classes. Most strains were nonsusceptible to the ß-lactams alone. Against AmpC-, extended-spectrum ß-lactamase (ESBL)-, and KPC-producing Enterobacteriaceae or Pseudomonas aeruginosa, avibactam lowered ceftazidime, ceftaroline, or aztreonam MICs up to 2,048-fold, to ≤4 µg/ml. Aztreonam-avibactam MICs against a VIM-1 metallo-ß-lactamase-producing Enterobacter cloacae and a VIM-1/KPC-3-producing Escherichia coli isolate were 0.12 and 8 µg/ml, respectively.

Structure of rhomboid protease in complex with ß-lactam inhibitors defines the S2' cavity.

Rhomboids are evolutionarily conserved serine proteases that cleave transmembrane proteins within the membrane. The increasing number of known rhomboid functions in prokaryotes and eukaryotes makes them attractive drug targets. Here, we describe structures of the Escherichia coli rhomboid GlpG in complex with ß-lactam inhibitors. The inhibitors form a single bond to the catalytic serine and the carbonyl oxygen of the inhibitor faces away from the oxyanion hole. The hydrophobic N-substituent of ß-lactam inhibitors points into a cavity within the enzyme, providing a structural explanation for the specificity of ß-lactams on rhomboid proteases. This same cavity probably represents the S2' substrate binding site of GlpG. We suggest that the structural changes in ß-lactam inhibitor binding reflect the state of the enzyme at an initial stage of substrate binding to the active site. The structural insights from these enzyme-inhibitor complexes provide a starting point for structure-based design for rhomboid inhibitors.

[Novel inhibitors against the bacterial signal peptidase I].

New antibiotics with novel modes of action and structures are urgently needed to combat the emergence of multidrug-resistant bacteria. Bacterial signal peptidase I (SPase I) is an indispensable enzyme responsible for cleaving the signal peptide of preprotein to release the matured proteins. Increasing evidence suggests that SPase I plays a crucial role in bacterial pathogenesis by regulating the excretion of a variety of virulent factors, maturation of quorum sensing factor and the intrinsic resistance against beta-lactams. Recently, breakthrough has been achieved in the understanding of three-dimensional structure of SPase I as well as the mechanism of enzyme-inhibitors interaction. Three families of inhibitors are identified, i.e. signal peptide derivatives, beta-lactams and arylomycins. In this article, we summarize the recent advance in the study of structure, activity and structure-activity relationship of SPase I inhibitors.

Synthetic lethal compound combinations reveal a fundamental connection between wall teichoic acid and peptidoglycan biosyntheses in Staphylococcus aureus.

Methicillin resistance in Staphylococcus aureus depends on the production of mecA, which encodes penicillin-binding protein 2A (PBP2A), an acquired peptidoglycan transpeptidase (TP) with reduced susceptibility to ß-lactam antibiotics. PBP2A cross-links nascent peptidoglycan when the native TPs are inhibited by ß-lactams. Although mecA expression is essential for ß-lactam resistance, it is not sufficient. Here we show that blocking the expression of wall teichoic acids (WTAs) by inhibiting the first enzyme in the pathway, TarO, sensitizes methicillin-resistant S. aureus (MRSA) strains to ß-lactams even though the ß-lactam-resistant transpeptidase, PBP2A, is still expressed. The dramatic synergy between TarO inhibitors and ß-lactams is noteworthy not simply because strategies to overcome MRSA are desperately needed but because neither TarO nor the activities of the native TPs are essential in MRSA strains. The "synthetic lethality" of inhibiting TarO and the native TPs suggests a functional connection between ongoing WTA expression and peptidoglycan assembly in S. aureus. Indeed, transmission electron microscopy shows that S. aureus cells blocked in WTA synthesis have extensive defects in septation and cell separation, indicating dysregulated cell wall assembly and degradation. Our studies imply that WTAs play a fundamental role in S. aureus cell division and raise the possibility that synthetic lethal compound combinations may have therapeutic utility for overcoming antibiotic-resistant bacterial infections.

Propenylamide and propenylsulfonamide cephalosporins as a novel class of anti-MRSA beta-lactams.

Novel C(3) propenylamide and propenylsulfonamide cephalosporins have been synthesized and tested for their ability to inhibit the penicillin-binding protein 2' (PBP2') from Staphylococcus epidermidis and the growth of a panel of clinically relevant bacterial species, including methicillin-resistant Staphylococcus aureus (MRSA). The most potent compounds inhibited the growth of MRSA strains with minimum inhibitory concentrations (MIC) as low as 1 microg/mL. The structure-activity relationship revealed the potential for further optimization of this new cephalosporin class.

Carbapenem-hydrolysing beta-lactamase KPC-2 in Klebsiella pneumoniae isolated in Rio de Janeiro, Brazil.

OBJECTIVES: The aim of this study was to characterize the KPC-type carbapenem-hydrolysing beta-lactamase, extended-spectrum beta-lactamases (ESBLs) and class 1 integrons among nosocomial Klebsiella pneumoniae isolated in Rio de Janeiro, Brazil. METHODS: MICs were determined and isolates were screened for ESBLs, metallo-beta-lactamases (MBLs) and class A carbapenemase-producing phenotypes. The main beta-lactamases resistance genes (bla(TEM), bla(SHV), bla(CTX-M), bla(KPC), bla(IMP) and bla(VIM)) and class 1 integrons were detected by PCR followed by DNA sequencing. The genetic relatedness of isolates was determined by PFGE. RESULTS: All K. pneumoniae isolates were positive for ESBL and class A carbapenemase production and negative for MBL production. All isolates were resistant to all beta-lactam antibiotics, ciprofloxacin and gentamicin, being susceptible only to tigecycline and polymyxin B. The bla(KPC-2), bla(CTX-M-1), bla(CTX-M-2), bla(CTX-M-8) and bla(SHV-11) genes were detected. PFGE analysis revealed two clonal types among KPC-producing isolates, both identified in the same hospital. CONCLUSIONS: Our findings should alert medical authorities to implement stringent methods for the detection and spread control of emerging KPC-2 carbapenemases in the hospital setting in Brazil.

A convenient microbiological assay employing cell-free extracts for the rapid characterization of Gram-negative carbapenemase producers.

OBJECTIVES: The dissemination of metallo and serine carbapenem-hydrolysing beta-lactamases among Gram-negative nosocomial bacteria represents an acute problem worldwide. Here, we present a rapid and sensitive assay for the characterization of carbapenemase producers to aid in infection control and prevention. METHODS: The assay involves a rapid disruption of bacterial isolates with silicon dioxide microbeads, followed by the testing in cell-free extracts of hydrolytic activity towards various beta-lactams including two carbapenems (imipenem and meropenem) and a cephalosporin (ceftazidime). A parallel testing of the effects of selective beta-lactamase inhibitors such as EDTA and clavulanic acid allows differentiation of metallo carbapenemases from serine carbapenemases, and also clavulanic-acid-sensitive from -resistant enzymes among the latter. RESULTS: The efficiency of bacterial disruption using silicon dioxide microbeads was identical to that of ultrasonic treatment. The subsequent microbiological assay aimed to evaluate both substrate specificity and inhibitor profile of carbapenem-hydrolysing enzymes present in the extracts and allowed an accurate differentiation of A, B and D types, as judged by the analysis of 24 well-characterized clinical strains that included metallo-beta-lactamase producers (i.e. VIM-, IMP- and SPM-type Pseudomonas producers; an L1 Stenotrophomonas maltophilia producer; and a GOB-18 Elizabethkingia meningoseptica producer) as well as serine carbapenemase producers (i.e. an SME-type Serratia marcescens producer, a GES-2 Pseudomonas aeruginosa producer, Klebsiella pneumoniae and Citrobacter freundii KPC-2 producers and OXA-type Acinetobacter baumannii producers). CONCLUSIONS: We have developed a convenient microbiological assay aimed to more accurately and in a short time characterize carbapenem-hydrolysing enzymes produced by Gram-negative bacteria. The assay possesses broad applicability in the clinical setting.

Inhibitory effect of zinc on the absorption of beta-lactam antibiotic ceftibuten via the peptide transporters in rats.

Zinc is an essential metal ion for the body, and is widely used for nutritional and clinical purposes. Previously, we showed that zinc inhibits the transport of glycylsarcosine via the intestinal peptide transporter PEPT1 in the human intestinal cell line Caco-2. In this study, we examined the effect of zinc on the activity of peptide transporters in rats using the oral beta-lactam antibiotic ceftibuten as a model drug. The plasma ceftibuten concentration after intraintestinal administration was decreased in the presence of zinc. The maximum plasma concentration (C(max)) was significantly decreased and the time required to reach C(max) (T(max)) was prolonged by zinc coadministration. The plasma ceftibuten concentration after iron coadministration or two hours after zinc administration was not affected. The in situ loop technique revealed 50% inhibition of ceftibuten absorption by zinc. In conclusion, zinc inhibits the transport activity of PEPT1 in vivo as well in vitro.

Tratamiento de las infecciones no complicadas del tracto urinario inferior / Treatment of uncomplicated lower urinary tract infections

El tratamiento antibiótico empírico de las infecciones del tracto urinario inferior debe basarse en los datos clínicos del paciente y en las tasas locales de sensibilidad antibiótica. El aumento de las resistencias de los uropatógenos ha obligado a modificar las recomendaciones para el tratamiento empírico de las infecciones urinarias. Actualmente se desaconseja el uso empírico de cotrimoxazol, ampicilina, cefalosporinas y quinolonas, ambas de primera generación. Las fluoroquinolonas han demostrado ser muy eficaces en estudios comparativos, pero el aumento de resistencias obliga a seleccionar el tipo de paciente que se puede beneficiar de estos antimicrobianos. Las cefalosporinas de segunda y tercera generación mantienen tasas de sensibilidad elevadas, aunque se deben tener en cuenta las mayores tasas de recurrencia asociadas a su utilización y la aparición de enterobacterias productoras de betalactamasas de espectro extendido en la comunidad. La amoxicilina-ácido clavulánico tiene menor eficacia erradicadora que las quinolonas. Fosfomicina-trometamol mantiene tasas de resistencia inferiores al 2% y ha demostrado su eficacia y seguridad con una dosis única. Nitrofurantoína también es activa en la actualidad, aunque precisa una administración de 7 días y no está exenta de toxicidad. Ambos agentes se recomiendan actualmente como opciones alternativas a las fluoroquinolonas en la infección no complicada del tracto urinario inferior

Empirical antibiotic treatment of lower urinary tract infections should be based on the patient's clinical data and on local sensitivity data. Because of the increase in resistance among uropathogens, recommendations on the empirical treatment of urinary tract infections have been modified. Currently, the empirical use of co-trimoxazole, ampicillin, and first-generation cephalosporins and quinolones is not recommended. Fluoroquinolones have been demonstrated to be highly effective in comparative studies but, because of the increase in resistance, the type of patient who can benefit from these antimicrobial agents must be selected. Second- and third-generation cephalosporins still have high sensitivity rates, although the higher recurrence rates associated with their use and the emergence of extended-spectrum beta-lactamase-producing enterobacterial in the community should be taken into account. Amoxicillin-clavulanate is less effective in eradicating infections than quinolones. Fosfomycin-trometamol has resistance rates of below 2% and single-dose therapy has been demonstrated to be safe and effective. Nitrofurantoin is also currently active, although it must be administered for 7 days and can produce toxicity. Both agents are currently recommended as alternative therapeutic options to fluoroquinolones in uncomplicated infections of the lower urinary tract

In vitro antagonism between beta-lactam and macrolide in Streptococcus pneumoniae: how important is the antibiotic order?

We found that the in vitro interaction between penicillin or cefotaxime and erythromycin against Streptococcus pneumoniae varies depending on the order of antibiotic exposure. Time-kill experiments were performed with penicillin, cefotaxime, erythromycin and different order combinations of both beta-lactams with erythromycin. The mean difference between the colony count at 0 and 6h for penicillin, cefotaxime and erythromycin tested separately was 3.5 log cfu/mL, 2.4 and 1.5 respectively for susceptible strains. The mean difference for the combination of beta-lactam and erythromycin studied simultaneously was 1.8 log cfu/mL for these strains. The association of penicillin or cefotaxime with erythromycin added two hours later showed an activity similar to those of beta-lactam alone (mean difference was 3.0 for this association with penicillin and 2.5 with cefotaxime). Therefore, the antagonistic effect of macrolide activity could be less important if erythromycin was administrated after beta-lactam.

Comparative activity of piperacillin/tazobactam against clinical isolates of extended-spectrum beta-lactamase-producing Enterobacteriaceae.

beta-Lactam resistance on the part of the Enterobacteriaceae causes serious therapeutic problems in our institutions due to their production of extended-spectrum beta-lactamases (ESbetaLs). We studied the in vitro activity of beta-lactam/beta-lactamase inhibitor combinations and third-generation cephalosporins and monobactams against 71 clinically relevant Enterobacteriaceae which produced TEM- and SHV-derivative ESbetaLs. Of the single drugs and combinations tested, piperacillin/tazobactam proved to be the most effective. Piperacillin/tazobactam was highly active against Proteus mirabilis, with minimum inhibitory concentrations (MICs) ranging from 0.125 to 16 microg/ml; Escherichia coli (MICs from 2 to 16 microg/ml) and Serratia marcescens (MICs from 4 to 8 microg/ml), while its activity against Klebsiella pneumoniae ESbetaL producers turned out to be closely related to the type and the amount of enzyme produced, the MIC ranging from 1 to 128 microg/ml. The antibacterial activity of piperacillin/tazobactam was stronger than that of ticarcillin/clavulanate, ceftriaxone, cefotaxime, ceftazidime and aztreonam, and the combination shared favorable in vitro activity properties against the ESbetaL producers with imipenem which, however, should be kept as reserve product.