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

Design, synthesis and antibacterial evaluation of low toxicity amphiphilic-cephalosporin derivatives

Global public health is facing a serious problem as a result of the rise in antibiotic resistance and the decline in the discovery of new antibiotics. In this study, two series of amphiphilic-cephalosporins were designed and synthesized, several of which showed good antibacterial activity against both Gram-positive and Gram-negative bacteria. Structure-activity relationships indicated that the length of the hydrophobic alkyl chain significantly affects the antibacterial activity against Gram-negative bacteria. The best compound 2d showed high activity against drug-susceptible Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) with MICs of 0.5 and 2-4 μg/mL, respectively. Furthermore, 2d remained active in complex mammalian body fluids and had a longer post-antibiotic effect (PAE) than vancomycin. Mechanism studies indicated that compound 2d lacks membrane-damaging properties and can target penicillin-binding proteins to disrupt bacterial cell wall structure, inhibit the metabolic activity and induce the accumulation of reactive oxygen species (ROS) in bacteria. Compound 2d showed minimal drug resistance and was nontoxic to HUVEC and HBZY-1 cells with CC50 > 128 μg/mL. These findings suggest that 2d is a promising drug candidate for treating bacterial infections.

Cephalosporin resistance, tolerance, and approaches to improve their activities

Cephalosporins comprise a β-lactam antibiotic class whose first members were discovered in 1945 from the fungus Cephalosporium acremonium. Their clinical use for Gram-negative bacterial infections is widespread due to their ability to traverse outer membranes through porins to gain access to the periplasm and disrupt peptidoglycan synthesis. More recent members of the cephalosporin class are administered as last resort treatments for complicated urinary tract infections, MRSA, and other multi-drug resistant pathogens, such as Neisseria gonorrhoeae. Unfortunately, there has been a global increase in cephalosporin-resistant strains, heteroresistance to this drug class has been a topic of increasing concern, and tolerance and persistence are recognized as potential causes of cephalosporin treatment failure. In this review, we summarize the cephalosporin antibiotic class from discovery to their mechanisms of action, and discuss the causes of cephalosporin treatment failure, which include resistance, tolerance, and phenomena when those qualities are exhibited by only small subpopulations of bacterial cultures (heteroresistance and persistence). Further, we discuss how recent efforts with cephalosporin conjugates and combination treatments aim to reinvigorate this antibiotic class.

An NDM-Producing Escherichia coli Clinical Isolate Exhibiting Resistance to Cefiderocol and the Combination of Ceftazidime-Avibactam and Aztreonam: Another Step Toward Pan-β-Lactam Resistance

Background

Cefiderocol and ceftazidime-avibactam plus aztreonam (CZA-ATM) are preferred treatment regimens for New Delhi metallo-β-lactamase (NDM)-producing infections.

Methods

We report the case of a US patient who traveled to India to receive a renal transplant. He subsequently experienced pyelonephritis by an NDM-producing Escherichia coli. Broth microdilution and the broth disk elution method indicated resistance to all β-lactams, including cefiderocol and CZA-ATM. Whole-genome sequencing investigations were undertaken to identify resistance mechanisms.

Results

An E. coli isolate belonging to sequence type (ST) 167 containing a bla_NDM-5 gene was identified on a plasmid of the IncFIA/IncFIB/IncFIC replicon groups. When compared with the genome of another ST167 E. coli clinical isolate containing bla_NDM-5 and exhibiting susceptibility to cefiderocol and CZA-ATM, a 12-base pair insertion in ftsI, translating to a 4-amino acid duplication in PBP3, was identified. Moreover, a bla_CMY-59 gene was harbored on an IncI-γ replicon type, and frameshift mutations were identified in the cirA iron transport gene.

Conclusions

This is the first clinical case of a US patient harboring an NDM-producing isolate exhibiting resistance to all available β-lactam agents. The isolate's unexpected resistance to cefiderocol and CZA-ATM was likely due to a combination of (1) a modified PBP3 (increased MICs to both regimens), (2) truncated iron-binding protein (increased cefiderocol MIC), and (3) a bla_CMY gene (reduced CZA-ATM activity). E. coli ST167 clinical isolates harboring bla_NDM-5 genes are a recognized international high-risk clone. When coupled with the additional mechanisms identified in our patient's isolate, which is not uncommon for this high-risk clone, pan-β-lactam resistance may occur.

Molecular characterization of cefepime and aztreonam nonsusceptibility in Haemophilus influenzae

BACKGROUND

Cefepime and aztreonam are highly efficacious against H. influenzae, and resistant strains are rare. In this study, we isolated cefepime- and aztreonam-nonsusceptible H. influenzae strains and addressed the molecular basis of their resistance to cefepime and aztreonam.

METHODS

Two hundred and 28 specimens containing H. influenzae were screened, of which 32 isolates were enrolled and applied to antimicrobial susceptibility testing and whole-genome sequencing. Genetic variations that were detected in all nonsusceptible isolates with statistical significance by Fisher's exact tests were identified as cefepime or aztreonam nonsusceptibility related. Functional complementation assays were conducted to assess the in vitro effects of proteins with sequence substitutions on drug susceptibility.

RESULTS

Three H. influenzae isolates were nonsusceptible to cefepime, one of which was also nonsusceptible to aztreonam. Genes encoding TEM, SHV and CTX-M extended-spectrum β-lactamases were not detected in the cefepime- and aztreonam-nonsusceptible isolates. Five genetic variations in four genes and 10 genetic variations in five genes were associated with cefepime and aztreonam nonsusceptibility, respectively. Phylogenetic analyses revealed that changes in FtsI were correlated strongly with the MIC of cefepime and moderately with aztreonam. FtsI Thr532Ser-Tyr557His cosubstitution linked to cefepime nonsusceptibility and Asn305Lys-Ser385Asn-Glu416Asp cosubstitution to aztreonam nonsusceptibility. Functional complementation assays revealed that these cosubstitutions increased MICs of cefepime and aztreonam in susceptible H. influenzae isolates, respectively.

CONCLUSIONS

Genetic variations relevant to resistant phenotypes of cefepime and aztreonam nonsusceptibility in H. influenzae were identified. Moreover, the effects of FtsI cosubstitutions on increasing MICs of cefepime and aztreonam in H. influenzae were demonstrated.

Cefepime pharmacodynamic targets against Enterobacterales employing neutropenic murine lung infection and in vitro pharmacokinetic models

BACKGROUND

Very limited studies, so far, have been conducted to identify the pharmacodynamic targets of cefepime, a well-established fourth-generation cephalosporin. As a result, conventional targets representing the cephalosporin class are used for cefepime target attainment analysis.

OBJECTIVES

We employed both a neutropenic murine lung infection model and an in vitro pharmacokinetic model (IVPM) to determine cefepime's pharmacodynamic target [percentage of the dosing interval during which unbound drug concentrations remain higher than the MIC (%fT>MIC)] for bacteriostatic and 1 log10 kill effects.

METHODS

Ten strains with cefepime MICs ranging from 0.03 to 16 mg/L were studied in the lung infection. In the IVPM, five cefepime-resistant strains with cefepime/tazobactam (fixed 8 mg/L) MICs ranging from 0.25 to 8 mg/L were included. Through 24 h dose fractionation, both in lung infection and IVPM (in the latter case, tazobactam 8 mg/L continuous infusion was used to protect cefepime), varying cefepime exposures and corresponding pharmacodynamic effect scenarios were generated to identify the pharmacodynamic targets.

RESULTS

Using a non-linear sigmoidal maximum-effect (Emax) model, the cefepime's plasma fT>MIC for 1 log10 kill in lung infection ranged from 17% to 53.7% and a combined exposure-response plot yielded 30%. In the case of IVPM, T>MIC ranged from 6.9% to 75.4% with a mean value of 34.2% for 1 log10 kill.

CONCLUSIONS

Both in vivo and in vitro studies showed that cefepime's pharmacodynamic requirements are lower than generally reported for cephalosporins (50%-70% fT>MIC). The lower requirement for cefepime could be linked with factors such as cefepime's better permeation properties and multiple PBP affinity-driven enhanced bactericidal action.

Cephalosporin translocation across enterobacterial OmpF and OmpC channels, a filter across the outer membrane

Gram-negative porins are the main entry for small hydrophilic molecules. We studied translocation of structurally related cephalosporins, ceftazidime (CAZ), cefotaxime (CTX) and cefepime (FEP). CAZ is highly active on E. coli producing OmpF (Outer membrane protein F) but less efficient on cells expressing OmpC (Outer membrane protein C), whereas FEP and CTX kill bacteria regardless of the porin expressed. This matches with the different capacity of CAZ and FEP to accumulate into bacterial cells as quantified by LC-MS/MS (Liquid Chromatography Tandem Mass Spectrometry). Furthermore, porin reconstitution into planar lipid bilayer and zero current assays suggest permeation of ≈1,000 molecules of CAZ per sec and per channel through OmpF versus ≈500 through OmpC. Here, the instant killing is directly correlated to internal drug concentration. We propose that the net negative charge of CAZ represents a key advantage for permeation through OmpF porins that are less cation-selective than OmpC. These data could explain the decreased susceptibility to some cephalosporins of enterobacteria that exclusively express OmpC porins.

The translocation of cephalosporins across enterobacterial OmpF and OmpC channels is monitored in real-time, demonstrating differential permeation of some cephalosporins through OmpF and OmpC.

Live-Cell Profiling of Penicillin-Binding Protein Inhibitors in Escherichia coli MG1655

Penicillin-binding proteins (PBPs) make up an essential class of bacterial enzymes that carry out the final steps of peptidoglycan synthesis and regulate the recycling of this polymeric structure. PBPs are an excellent drug target and have been the most clinically relevant antibacterial target since the 1940s with the introduction of β-lactams. Despite this, a large gap in knowledge remains regarding the individual function and regulation of each PBP homologue in most bacteria. This can be attributed to a lack of chemical tools and methods that enable the study of individual PBPs in an activity-dependent manner and in their native environment. The development of such methods in Gram-negative bacteria has been particularly challenging due to the presence of an outer membrane and numerous resistance mechanisms. To address this, we have developed an optimized live-cell assay for screening inhibitors of the PBPs in Escherichia coli MG1655. We utilized EDTA to permeabilize Gram-negative cells, enabling increased penetration of our readout probe, Bocillin-FL, and subsequent analysis of PBP-inhibition profiles. To identify scaffolds for future development of PBP-selective activity-based probes, we screened ten β-lactams, one diazabicyclooctane, and one monobactam for their PBP-selectivity profiles in E. coli MG1655. These results demonstrate the utility of our assay for the screening of inhibitors in live, non-hypersusceptible Gram-negative organisms.

Ceftazidime and cefepime antagonize 5-fluorouracil's effect in colon cancer cells

Background

Drug-drug interaction (DDI), which can occur at the pharmacokinetics and/or the pharmacodynamics (PD) levels, can increase or decrease the therapeutic or adverse response of a drug itself or a combination of drugs. Cancer patients often receive, along their antineoplastic agents, antibiotics such as ß-lactams to treat or prevent infection. Despite the narrow therapeutic indices of antibiotics and antineoplastic agents, data about their potential interaction are insufficient. 5-fluorouracil (5-FU), widely used against colon cancer, is known for its toxicity and large intra- and inter- individual variability. Therefore, knowledge about its interaction with antibiotics is crucial.

Methods

In this study, we evaluated at the PD levels, against HCT-116 colon cancer cells, DDI between 5-FU and several ß-lactams (ampicillin, benzypenicillin, piperacillin, meropenem, flucloxacillin, ceftazidime (CFT), and cefepime (CFP)), widely used in intensive care units. All drugs were tested at clinically achieved concentrations. MTT assay was used to measure the metabolic activity of the cells. Cell cycle profile and apoptosis induction were monitored, in HCT-116 and DLD-1 cells, using propidium iodide staining and Caspase-3/7 activity assay. The uptake of CFT and CFP by the cells was measured using LC-MS/MS method.

Results

Our data indicate that despite their limited uptake by the cells, CFT and CFP (two cephalosporins) antagonized significantly 5-FU-induced S-phase arrest (DLD-1 cells) and apoptosis induction (HCT-116 cells). Remarkably, while CFP did not affect the proliferation of colon cancer cells, CFT inhibited, at clinically relevant concentrations, the proliferation of DLD-1 cells via apoptosis induction, as evidenced by an increase in caspase 3/7 activation. Unexpectedly, 5-FU also antagonized CFT’s induced cell death in DLD-1 cells.

Conclusion

This study shows that CFP and CFT have adverse effects on 5-FU’s action while CFT is a potent anticancer agent that inhibits DLD-1 cells by inducing apoptotic cell death. Further studies are needed to decipher the mechanism(s) responsible for CFT’s effects against colon cancer as well as the observed antagonism between CFT, CFP, and 5-FU with the ultimate aim of translating the findings to the clinical settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-09125-4.

β-Lactam Resistance in Upper Respiratory Tract Pathogens Isolated from a Tertiary Hospital in Malaysia

The rise of antimicrobial resistance (AMR) among clinically important bacteria, including respiratory pathogens, is a growing concern for public health worldwide. Common causative bacteria for upper respiratory tract infections (URTIs) include Streptococcus pneumoniae and Haemophilus influenzae, and sometimes Staphylococcus aureus. We assessed the β-lactam resistant trends and mechanisms of 150 URTI strains isolated in a tertiary care hospital in Kuala Lumpur Malaysia. High rates of non-susceptibility to penicillin G (38%), amoxicillin-clavulanate (48%), imipenem (60%), and meropenem (56%) were observed in S. pneumoniae. Frequent mutations at STMK and SRNVP motifs in PBP1a (41%), SSNT motif in PBP2b (32%), and STMK and LKSG motifs in PBP2x (41%) were observed in S. pneumoniae. H. influenzae remained highly susceptible to most β-lactams, except for ampicillin. Approximately half of the ampicillin non-susceptible H. influenzae harboured PBP3 mutations (56%) and only blaTEM was detected in the ampicillin-resistant strains (47%). Methicillin-susceptible S. aureus (MSSA) strains were mostly resistant to penicillin G (92%), with at least two-fold higher median minimum inhibitory concentrations (MIC) for all penicillin antibiotics (except ticarcillin) compared to S. pneumoniae and H. influenzae. Almost all URTI strains (88-100%) were susceptible to cefcapene and flomoxef. Overall, β-lactam antibiotics except penicillins remained largely effective against URTI pathogens in this region.

The Next-Generation β-Lactamase Inhibitor Taniborbactam Restores the Morphological Effects of Cefepime in KPC-Producing Escherichia coli

Gram-negative bacteria producing carbapenemases are resistant to a variety of β-lactam antibiotics and pose a significant health risk. Given the dearth of new antibiotics, combinations of new broad-spectrum β-lactamase inhibitors (BLIs) with approved β-lactams have provided treatment options for resistant bacterial infections. Taniborbactam (formerly VNRX-5133) is an investigational BLI that is effective against both serine- and metallo-β-lactamases, including the serine carbapenemase KPC. In the current study, we assessed the effectiveness of taniborbactam to restore antibacterial activity of cefepime against KPC-3-producing Escherichia coli by inhibiting the KPC-3-dependent hydrolysis of cefepime. Time-lapse microscopy revealed that cells treated with greater than 1× MIC of cefepime (128 μg/ml) and cefepime-taniborbactam (4 μg/ml cefepime and 4 μg/ml taniborbactam) exhibited significant elongation, whereas cells treated with taniborbactam alone did not owing to a lack of standalone antibacterial activity of the BLI. The elongated cells also had frequent cellular voids thought to be formed by attempted cell divisions and pinching of the cytoplasmic membrane. Additionally, the effect of taniborbactam continued even after its removal from the growth medium. Pretreatment with 4 μg/ml taniborbactam helped to restore the antibacterial action of cefepime by neutralizing the effect of the KPC-3 β-lactamase. IMPORTANCE β-lactam (BL) antibiotics are the most prescribed antimicrobial class. The efficacy of β-lactams is threatened by the production of β-lactamase enzymes, the predominant resistance mechanism impacting these agents in Gram-negative bacterial pathogens. This study visualizes the effects of a combination treatment of taniborbactam, a broad spectrum β-lactamase inhibitor (BLI), and the BL antibiotic cefepime on a carbapenemase-producing E. coli strain. While this treatment has been described in the context of other cephalosporin-resistant bacteria, this is the first description of a microscopic evaluation of a KPC-3-producing strain of E. coli challenged by this BL-BLI combination. Live-cell microscopy analysis of cells treated with taniborbactam and cefepime demonstrated the antimicrobial effects on cellular morphology and highlighted the long-lasting inhibition of β-lactamases by taniborbactam even after it was removed from the medium. This research speaks to the importance of taniborbactam in fighting BL-mediated antibiotic resistance.

Potential Antibiotics for the Treatment of Neonatal Sepsis Caused by Multidrug-Resistant Bacteria

Neonatal sepsis causes up to an estimated 680,000 deaths annually worldwide, predominantly in low- and middle-income countries (LMICs). A significant and growing proportion of bacteria causing neonatal sepsis are resistant to multiple antibiotics, including the World Health Organization-recommended empiric neonatal sepsis regimen of ampicillin/gentamicin. The Global Antibiotic Research and Development Partnership is aiming to develop alternative empiric antibiotic regimens that fulfil several criteria: (1) affordable in LMIC settings; (2) activity against neonatal bacterial pathogens, including extended-spectrum β-lactamase producers, gentamicin-resistant Gram-negative bacteria, and methicillin-resistant Staphylococcus aureus (MRSA); (3) a licence for neonatal use or extensive experience of use in neonates; and (4) minimal toxicities. In this review, we identify five antibiotics that fulfil these criteria: amikacin, tobramycin, fosfomycin, flomoxef, and cefepime. We describe the available characteristics of each in terms of mechanism of action, resistance mechanisms, clinical pharmacokinetics, pharmacodynamics, and toxicity profile. We also identify some knowledge gaps: (1) the neonatal pharmacokinetics of cefepime is reliant on relatively small and limited datasets, and the pharmacokinetics of flomoxef are also reliant on data from a limited demographic range and (2) for all reviewed agents, the pharmacodynamic index and target has not been definitively established for both bactericidal effect and emergence of resistance, with many assumed to have an identical index/target to similar class molecules. These five agents have the potential to be used in novel combination empiric regimens for neonatal sepsis. However, the data gaps need addressing by pharmacokinetic trials and pharmacodynamic characterisation.

An update on cefepime and its future role in combination with novel β-lactamase inhibitors for MDR Enterobacterales and Pseudomonas aeruginosa

Cefepime, a wide-spectrum β-lactam antibiotic, has been in use for the treatment of serious bacterial infections for almost 25 years. Since its clinical development, there has been a dramatic shift in its dosing, with 2 g every 8 hours being preferred for serious infections to optimize pharmacokinetic/pharmacodynamic considerations. The advent of ESBLs has become a threat to its ongoing use, although future coadministration with β-lactamase inhibitors (BLIs) under development is an area of intense study. There are currently four new cefepime/BLI combinations in clinical development. Cefepime/zidebactam is generally active against MBL-producing Enterobacterales and Pseudomonas aeruginosa, in vitro and in animal studies, and cefepime/taniborbactam has activity against KPC and OXA-48 producers. Cefepime/enmetazobactam and cefepime/tazobactam are potential carbapenem-sparing agents with activity against ESBLs. Cefepime/enmetazobactam has completed Phase III and cefepime/taniborbactam is in Phase III clinical studies, where they are being tested against carbapenems or piperacillin/tazobactam for the treatment of complicated urinary tract infections. While these combinations are promising, their role in the treatment of MDR Gram-negative infections can only be determined with further clinical studies.

Combating Multidrug-Resistant Bacteria by Integrating a Novel Target Site Penetration and Receptor Binding Assay Platform Into Translational Modeling

Multidrug-resistant bacteria are causing a serious global health crisis. A dramatic decline in antibiotic discovery and development investment by pharmaceutical industry over the last decades has slowed the adoption of new technologies. It is imperative that we create new mechanistic insights based on latest technologies, and use translational strategies to optimize patient therapy. Although drug development has relied on minimal inhibitory concentration testing and established in vitro and mouse infection models, the limited understanding of outer membrane permeability in Gram-negative bacteria presents major challenges. Our team has developed a platform using the latest technologies to characterize target site penetration and receptor binding in intact bacteria that inform translational modeling and guide new discovery. Enhanced assays can quantify the outer membrane permeability of β-lactam antibiotics and β-lactamase inhibitors using multiplex liquid chromatography tandem mass spectrometry. While β-lactam antibiotics are known to bind to multiple different penicillin-binding proteins (PBPs), their binding profiles are almost always studied in lysed bacteria. Novel assays for PBP binding in the periplasm of intact bacteria were developed and proteins identified via proteomics. To characterize bacterial morphology changes in response to PBP binding, high-throughput flow cytometry and time-lapse confocal microscopy with fluorescent probes provide unprecedented mechanistic insights. Moreover, novel assays to quantify cytosolic receptor binding and intracellular drug concentrations inform target site occupancy. These mechanistic data are integrated by quantitative and systems pharmacology modeling to maximize bacterial killing and minimize resistance in in vitro and mouse infection models. This translational approach holds promise to identify antibiotic combination dosing strategies for patients with serious infections.

Design, Synthesis, and Biological Evaluation of Light-Activated Antibiotics

The spatial and temporal control of bioactivity of small molecules by light (photopharmacology) constitutes a promising approach for study of biological processes and ultimately for the treatment of diseases. In this study, we investigated two different "caged" antibiotic classes that can undergo remote activation with UV-light at λ = 365 nm, via the conjugation of deactivating and photocleavable units through a short synthetic sequence. The two widely used antibiotics vancomycin and cephalosporin were thus enhanced in their performance by rendering them photoresponsive and thereby suppressing undesired off-site activity. The antimicrobial activity against Bacillus subtilis ATCC 6633, Staphylococcus aureus ATCC 29213, S. aureus ATCC 43300 (MRSA), Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853 could be spatiotemporally controlled with light. Both molecular series displayed a good activity window. The vancomycin derivative displayed excellent values against Gram-positive strains after uncaging, and the next-generation caged cephalosporin derivative achieved good and broad activity against both Gram-positive and Gram-negative strains after photorelease.

Clinical Biofilm Ring Test® Reveals the Potential Role of β-Lactams in the Induction of Biofilm Formation by P. aeruginosa in Cystic Fibrosis Patients

Biofilms are characterized by high tolerance to antimicrobials. However, conventional antibiograms are performed on planktonic microorganisms. Through the clinical Biofilm Ring Test^® (cBRT), initially aimed to measure the adhesion propensity of bacteria, we discerned a variable distribution of biofilm-producer strains among P. aeruginosa samples isolated from expectorations of cystic fibrosis (CF) patients. Despite a majority of spontaneous adherent isolates, few strains remained planktonic after 5 h of incubation. Their analysis by an adapted protocol of the cBRT revealed an induction of the biofilm early formation by sub-inhibitory doses of β-lactams. Microscopic observations of bacterial cultures stained with Syto 9/Propidium Iodide (PI) confirmed the ability of antimicrobials to increase either the bacterial biomass or the biovolume occupied by induced sessile cells. Finally, the cBRT and its derivatives enabled to highlight in a few hours the potential inducer property of antibiotics on bacterial adhesion. This phenomenon should be considered carefully in the context of CF since patients are constantly under fluctuating antimicrobial treatments. To conclude, assays derived from the Biofilm Ring Test^® (BRT) device, not only define efficient doses preventing biofilm formation, but could be useful for the antimicrobial selection in CF, to avoid inducer molecules of the early biofilm initiation.

Transcriptional Responses of Pseudomonas aeruginosa to Inhibition of Lipoprotein Transport by a Small Molecule Inhibitor

A key set of lipoprotein transport components, LolCDE, were inhibited by both a small molecule as well as genetic downregulation of their expression. The data show a unique signature in the Pseudomonas aeruginosa transcriptome in response to perturbation of outer membrane biogenesis. In addition, we demonstrate a transcriptional response in key genes with marked specificity compared to several antibiotic classes with different mechanisms of action. As a result of this work, we identified genes that could be of potential use as biomarkers in a cell-based screen for novel antibiotic inhibitors of lipoprotein transport in P. aeruginosa.

Lipoprotein transport from the inner to the outer membrane, carried out by the Lol machinery, is essential for the biogenesis of the Gram-negative cell envelope and, consequently, for bacterial viability. Recently, small molecule inhibitors of the Lol system in Escherichia coli have been identified and shown to inhibit the growth of this organism by interfering with the function of the LolCDE complex. Analysis of the transcriptome of E. coli treated with one such molecule (compound 2) revealed that a number of envelope stress response pathways were induced in response to LolCDE inhibition. However, Pseudomonas aeruginosa is refractory to inhibition by the same small molecule, but we could demonstrate that E. colilolCDE could be substituted for the P. aeruginosa orthologues, where it functions in the correct transport of Pseudomonas lipoproteins, and the cells are inhibited by the more potent compound 2A. In the present study, we took advantage of the functionality of E. coli LolCDE in P. aeruginosa and determined the P. aeruginosa transcriptional response to LolCDE inhibition by compound 2A. We identified key genes that responded to LolCDE inhibition and also demonstrated that the same genes appeared to be affected by genetic depletion of the native P. aeruginosa LolCDE proteins. Several of the major changes were in an upregulated cluster of genes that encode determinants of alginate biosynthesis and transport, and the levels of alginate were found to be increased either by treatment with the small molecule inhibitor or upon depletion of native LolCDE. Finally, we tested several antibiotics with differing mechanisms of action to identify potential specific reporter genes for the further development of compounds that would inhibit the native P. aeruginosa Lol system.

IMPORTANCE A key set of lipoprotein transport components, LolCDE, were inhibited by both a small molecule as well as genetic downregulation of their expression. The data show a unique signature in the Pseudomonas aeruginosa transcriptome in response to perturbation of outer membrane biogenesis. In addition, we demonstrate a transcriptional response in key genes with marked specificity compared to several antibiotic classes with different mechanisms of action. As a result of this work, we identified genes that could be of potential use as biomarkers in a cell-based screen for novel antibiotic inhibitors of lipoprotein transport in P. aeruginosa.

Preserving the Dwindling β-lactams-Based Empiric Therapy Options for Gram-Negative Infections in Challenging Resistance Scenario: Lessons Learned and Way Forward

Appropriate empiric therapy reduces mortality and morbidity associated with serious Gram-negative infections. β-lactams (BLs) owing to their safety, efficacy, and coverage spectrum are the most preferred agents for empiric use. Inappropriate use of older penicillins and cephalosporins led to selection and spread of resistant clones. As a result, these valuable agents have lost their reliability compelling clinicians to often use erstwhile last-line therapies such as carbapenems. Excessive carbapenems use imposed collateral damage by selecting difficult-to-treat carbapenem-resistant organisms. Lack of empiric therapeutic options amenable for use in infections caused by contemporary pathogens was realized by the pharmaceutical industry leading to intensive efforts in discovering novel antibiotics. These efforts led to the approval of newer β-lactams and β-lactamase inhibitor (BL-BLI) combination. This review elaborates the past trends in empirical use of BLs and ensuing patterns of resistance emergence in Gram-negatives. Furthermore, a critical appraisal of newer BL-BLIs has been presented to identify the appropriate clinical situations for their use to ensure clinical efficacy coupled with minimal resistance selection. These learning have been derived from past trends of clinical usage of older empiric therapies so that the therapeutic utility of newer agents is preserved for long in light of dwindling global antibiotics pipeline.

Social Behavior of Antibiotic Resistant Mutants Within Pseudomonas aeruginosa Biofilm Communities

The complex spatial structure and the heterogeneity within biofilms lead to the emergence of specific social behaviors. However, the impact of resistant mutants within bacterial communities is still mostly unknown. Thus, we determined whether antibiotic resistant mutants display selfish or altruistic behaviors in mixed Pseudomonas aeruginosa biofilms exposed to antibiotics. ECFP-tagged P. aeruginosa strain PAO1 and its EYFP-tagged derivatives hyperproducing the β-lactamase AmpC or the efflux pump MexAB-OprM were used to develop single or mixed biofilms. Mature biofilms were challenged with different concentrations of β-lactams to monitor biofilm structural dynamics, using confocal laser scanning microscopy (CLSM), and population dynamics, through enumeration of viable cells. While exposure of single wild-type PAO1 biofilms to β-lactams lead to a major reduction in bacterial load, it had little effect on biofilms formed by the resistant mutants. However, the most reveling finding was that bacterial load of wild-type PAO1 was significantly increased when growing in mixed biofilms compared to single biofilms. In agreement with CFU enumeration data, CLSM images revealed the amplification of the resistant mutants and their protection of susceptible populations. These findings show that mutants expressing diverse resistance mechanisms, including β-lactamases, but also, as evidenced for the first time, efflux pumps, protect the whole biofilm community, preserving susceptible populations from the effect of antibiotics. Thus, these results are a step forward to understanding antibiotic resistance dynamics in biofilms, as well as the population biology of bacterial pathogens in chronic infections, where the coexistence of susceptible and resistant variants is a hallmark.

Penicillin-binding protein 3 is a common adaptive target among Pseudomonas aeruginosa isolates from adult cystic fibrosis patients treated with β-lactams

OBJECTIVE

Determining the mechanisms that modulate β-lactam resistance in clinical Pseudomonas aeruginosa (P. aeruginosa) isolates can be challenging, as the molecular profiles identified in mutation-based or expression-based resistance determinant screens may not correlate with in vitro phenotypes. One of the lesser studied resistance mechanisms in P. aeruginosa is the modification of penicillin-binding protein 3 (pbpB/ftsI). This study reported that nonsynonymous polymorphisms within pbpB frequently occur among β-lactam resistant sputum isolates, and are associated with unique antibiotic susceptibility patterns.

METHODS

Longitudinally collected isolates (n = 126) from cystic fibrosis (CF) patients with or without recent β-lactam therapy or of non-clinical origin were tested for susceptibility to six β-lactams (aztreonam, ceftazidime, cefsulodin, cefepime, meropenem, and piperacillin). Known β-lactam resistance mechanisms were characterised by polymerase chain reaction (PCR)-based methods, and polymorphisms in the transpeptidase-encoding domain of pbpB identified by sequencing.

RESULTS

Twelve nonsynonymous polymorphisms were detected among 86 isolates (67%) from five CF patients with a history of β-lactam therapy, compared with one polymorphism in 30 (3.3%) from three patients who had not received β-lactam treatments. No nonsynonymous polymorphisms were found in ten environmental isolates. Multiple pbpB alleles, often with different combinations of polymorphisms, were detected within the population of strains from each CF patient for up to 2.6 years. Traditional patterns of ampC or mexA de-repression reduced expression of oprD or the presence of extended-spectrum β-lactamases were not observed in resistant isolates with nonsynonymous polymorphisms in pbpB.

CONCLUSION

This study's findings suggest that pbpB is a common adaptive target, and may contribute to the development of β-lactam resistance in P. aeruginosa.

First Penicillin-Binding Protein Occupancy Patterns of β-Lactams and β-Lactamase Inhibitors in Klebsiella pneumoniae

Penicillin-binding proteins (PBPs) are the high-affinity target sites of all β-lactam antibiotics in bacteria. It is well known that each β-lactam covalently binds to and thereby inactivates different PBPs with various affinities. Despite β-lactams serving as the cornerstone of our therapeutic armamentarium against Klebsiella pneumoniae, PBP binding data are missing for this pathogen. We aimed to generate the first PBP binding data on 13 chemically diverse and clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae PBP binding was determined using isolated membrane fractions from K. pneumoniae strains ATCC 43816 and ATCC 13883. Binding reactions were conducted using β-lactam concentrations from 0.0075 to 256 mg/liter (or 128 mg/liter). After β-lactam exposure, unbound PBPs were labeled by Bocillin FL. Binding affinities (50% inhibitory concentrations [IC50]) were reported as the β-lactam concentrations that half-maximally inhibited Bocillin FL binding. PBP occupancy patterns by β-lactams were consistent across both strains. Carbapenems bound to all PBPs, with PBP2 and PBP4 as the highest-affinity targets (IC50, <0.0075 mg/liter). Preferential PBP2 binding was observed by mecillinam (amdinocillin; IC50, <0.0075 mg/liter) and avibactam (IC50, 2 mg/liter). Aztreonam showed high affinity for PBP3 (IC50, 0.06 to 0.12 mg/liter). Ceftazidime bound PBP3 at low concentrations (IC50, 0.06 to 0.25 mg/liter) and PBP1a/b at higher concentrations (4 mg/liter), whereas cefepime bound PBPs 1 to 4 at more even concentrations (IC50, 0.015 to 2 mg/liter). These PBP binding data on a comprehensive set of 13 clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae enable, for the first time, the rational design and optimization of double β-lactam and β-lactam-β-lactamase inhibitor combinations.

Development of MAST: A Microscopy-Based Antimicrobial Susceptibility Testing Platform

Antibiotic resistance is compromising our ability to treat bacterial infections. Clinical microbiology laboratories guide appropriate treatment through antimicrobial susceptibility testing (AST) of patient bacterial isolates. However, increasingly, pathogens are developing resistance to a broad range of antimicrobials, requiring AST of alternative agents for which no commercially available testing methods are available. Therefore, there exists a significant AST testing gap in which current methodologies cannot adequately address the need for rapid results in the face of unpredictable susceptibility profiles. To address this gap, we developed a multicomponent, microscopy-based AST (MAST) platform capable of AST determinations after only a 2 h incubation. MAST consists of a solid-phase microwell growth surface in a 384-well plate format, inkjet printing-based application of both antimicrobials and bacteria at any desired concentrations, automated microscopic imaging of bacterial replication, and a deep learning approach for automated image classification and determination of antimicrobial minimal inhibitory concentrations (MICs). In evaluating a susceptible strain set, 95.8% were within ±1 and 99.4% were within ±2, twofold dilutions, respectively, of reference broth microdilution MIC values. Most (98.3%) of the results were in categorical agreement. We conclude that MAST offers promise for rapid, accurate, and flexible AST to help address the antimicrobial testing gap.

Penicillin-Binding Protein 3 Is Essential for Growth of Pseudomonas aeruginosa

Penicillin-binding proteins (PBPs) function as transpeptidases, carboxypeptidases, or endopeptidases during peptidoglycan synthesis in bacteria. As the well-known drug targets for β-lactam antibiotics, the physiological functions of PBPs and whether they are essential for growth are of significant interest. The pathogen Pseudomonas aeruginosa poses a particular risk to immunocompromised and cystic fibrosis patients, and infections caused by this pathogen are difficult to treat due to antibiotic resistance. To identify potential drug targets among the PBPs in P. aeruginosa, we performed gene knockouts of all the high-molecular-mass (HMM) PBPs and determined the impacts on cell growth and morphology, susceptibility to β-lactams, peptidoglycan structure, virulence, and pathogenicity. Disruptions of the transpeptidase domains of most HMM PBPs, including double disruptions, had only minimal effects on cell growth. The exception was PBP3, where cell growth occurred only when the protein was conditionally expressed on an integrated plasmid. Conditional deletion of PBP3 also caused a defect in cell division and increased susceptibility to β-lactams. Knockout of PBP1a led to impaired motility, and this observation, together with its localization at the cell poles, suggests its involvement in flagellar function. Overall, these findings reveal that PBP3 represents the most promising target for drug discovery against P. aeruginosa, whereas other HMM PBPs have less potential.

Characterization of Escherichia coli NDM isolates with decreased susceptibility to aztreonam/avibactam: role of a novel insertion in PBP3

OBJECTIVES

The spread of NDM-1 amongst Enterobacteriaceae has highlighted a significant threat to the clinical management of serious infections. The combination of aztreonam and avibactam, a non-β-lactam β-lactamase inhibitor, may provide a much-needed therapeutic alternative. This combination was potent against most NDM-containing Enterobacteriaceae, although activity was diminished against many Escherichia coli isolates. These E. coli isolates were characterized to elucidate the mechanism of decreased susceptibility to aztreonam/avibactam.

METHODS

MIC determinations were performed using broth microdilution, and whole-genome sequencing was performed to enable sequence-based analyses.

RESULTS

The decreased susceptibility was not due to avibactam being unable to inhibit the serine β-lactamases found in the E. coli isolates. Rather, it was manifested by a four-amino-acid insertion in PBP3. This same insertion was also found in non-NDM-containing E. coli that had reduced susceptibility to aztreonam/avibactam. Construction of an isogenic mutant confirmed that this insertion resulted in decreased susceptibility to aztreonam and several cephalosporins, but had no impact on carbapenem potency. Structural analysis suggests that this insertion will impact the accessibility of the β-lactam drugs to the transpeptidase pocket of PBP3.

CONCLUSIONS

The acquisition of β-lactamases is the predominant mechanism of β-lactam resistance in Enterobacteriaceae. We have demonstrated that small PBP3 changes will affect the susceptibility to a broad range of β-lactams. These changes were identified in multiple MLST lineages of E. coli, and were enriched in NDM-containing isolates. However, they were not present in other key species of Enterobacteriaceae despite significant conservation among the PBP3 proteins.

Dual beta-lactam therapy for serious Gram-negative infections: is it time to revisit?

We are rapidly approaching a crisis in antibiotic resistance, particularly among Gram-negative pathogens. This, coupled with the slow development of novel antimicrobial agents, underscores the exigency of redeploying existing antimicrobial agents in innovative ways. One therapeutic approach that was heavily studied in the 1980s but abandoned over time is dual beta-lactam therapy. This article reviews the evidence for combination beta-lactam therapy. Overall, in vitro, animal and clinical data are positive and suggest that beta-lactam combinations produce a synergistic effect against Gram-negative pathogens that rivals that of beta-lactam-aminoglycoside or beta-lactam-fluoroquinolone combination therapy. Although the precise mechanism of improved activity is not completely understood, it is likely attributable to an enhanced affinity to the diverse penicillin-binding proteins found among Gram negatives. The collective data indicate that dual beta-lactam therapy should be revisited for serious Gram-negative infections, especially in light of the near availability of potent beta-lactamase inhibitors, which neutralize the effect of problematic beta-lactamases.

Comparative activity of several antimicrobial agents against nosocomial Gram-negative rods isolated across Canada

In 1992, a surveillance study was performed in Canada to determine the susceptibility of nosocomial Gram-negative rods to several wide spectrum antimicrobials. Consecutive isolates from 10 institutions, as well as additional strains of selected species of Enterobacteriaceae that are known to possess the Bush group 1 beta-lactamase, were tested for susceptibility to 12 antimicrobials. Third-generation cephalosporin resistance was found to be as high as 29% in Enterobacter cloacae that possesses the Bush group 1 beta-lactamase and less than 4% in those isolates not possessing this enzyme. Cefepime equalled or exceeded the activity of the third-generation cephalosporins against the species of Enterobacteriaceae that demonstrated resistance to the third-generation cephalosporins.

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.

Resistance to the third-generation cephalosporin ceftazidime by a deacylation-deficient mutant of the TEM β-lactamase by the uncommon covalent-trapping mechanism

The Glu166Arg/Met182Thr mutant of Escherichia coli TEM(pTZ19-3) β-lactamase produces a 128-fold increase in the level of resistance to the antibiotic ceftazidime in comparison to that of the parental wild-type enzyme. The single Glu166Arg mutation resulted in a dramatic decrease in both the level of enzyme expression in bacteria and the resistance to penicillins, with a concomitant 4-fold increase in the resistance to ceftazidime, a third-generation cephalosporin. Introduction of the second amino acid substitution, Met182Thr, restored enzyme expression to a level comparable to that of the wild-type enzyme and resulted in an additional 32-fold increase in the minimal inhibitory concentration of ceftazidime to 64 μg/mL. The double mutant formed a stable covalent complex with ceftazidime that remained intact for the entire duration of the monitoring, which exceeded a time period of 40 bacterial generations. Compared to those of the wild-type enzyme, the affinity of the TEM(pTZ19-3) Glu166Arg/Met182Thr mutant for ceftazidime increased by at least 110-fold and the acylation rate constant was augmented by at least 16-fold. The collective experimental data and computer modeling indicate that the deacylation-deficient Glu166Arg/Met182Thr mutant of TEM(pTZ19-3) produces resistance to the third-generation cephalosporin ceftazidime by an uncommon covalent-trapping mechanism. This is the first documentation of such a mechanism by a class A β-lactamase in a manifestation of resistance.

Affinity of the new cephalosporin CXA-101 to penicillin-binding proteins of Pseudomonas aeruginosa

CXA-101, previously designated FR264205, is a new antipseudomonal cephalosporin. The objective of this study was to determine the penicillin-binding protein (PBP) inhibition profile of CXA-101 compared to that of ceftazidime (PBP3 inhibitor) and imipenem (PBP2 inhibitor). Killing kinetics, the induction of AmpC expression, and associated changes on cell morphology were also investigated. The MICs for CXA-101, ceftazidime, and imipenem were 0.5, 1, and 1 microg/ml, respectively. Killing curves revealed that CXA-101 shows a concentration-independent bactericidal activity, with concentrations of 1x the MIC (0.5 microg/ml) producing a > 3-log reduction in bacterial load after 8 h of incubation. Live-dead staining showed that concentrations of CXA-101 as low as 0.5x the MIC stopped bacterial septation and induced an intense filamentation, which is consistent with the documented high affinity of PBP3. CXA-101 was found to be a potent PBP3 inhibitor and showed affinities > or = 2-fold higher than those of ceftazidime for all of the essential PBPs (1b, 1c, 2, and 3). Compared to imipenem, in addition to the obvious inverse PBP2/PBP3 affinities, CXA-101 showed a significantly higher affinity for PBP1b but a lower affinity for PBP1c. Furthermore, CXA-101, like ceftazidime and in contrast to imipenem, was found to be a very weak inducer of AmpC expression, consistent with the low PBP4 affinity documented.

The safety of cefepime in the treatment of infection

BACKGROUND

Cefepime is a fourth-generation cephalosporin usually reserved for treating severe nosocomial pneumonia, as well as empirical treatment of febrile neutropenia, uncomplicated and complicated urinary tract infections, uncomplicated skin and skin structure infections, and complicated intra-abdominal infections.

OBJECTIVE

Since reports of neurotoxic effects and of an all-cause mortality higher with cefepime than with comparators have created some concerns regarding its safety, this paper reviews data available in the PubMed database up to December 2007 on cefepime safety.

METHODS

Literature data from PubMed obtained by combining cefepime and safety, or cefepime and clinical trials, were examined.

RESULTS/CONCLUSIONS

Caution in the use of cefepime should be adopted until new evidence on cefepime safety is available.

Role of AmpD, OprF and penicillin-binding proteins in beta-lactam resistance in clinical isolates of Pseudomonas aeruginosa

In this study, the mechanisms leading to increased chromosomal AmpC beta-lactamase expression and the contributory roles of the outer-membrane protein OprF and penicillin-binding proteins were analysed in 33 characterized clinical isolates of Pseudomonas aeruginosa. The genes ampD and ampE were analysed by PCR and DNA sequencing. Expression of the gene oprF was assessed using real-time RT-PCR, and penicillin-binding proteins were analysed using a chemiluminescence assay. Several of the isolates with increased ampC expression had major deletions affecting ampD, although in some isolates the mechanism of increased ampC expression could not be ascertained. Occasional isolates had increased expression of both ampC and oprF but remained susceptible to cephalosporins, suggesting that increased beta-lactamase activity could not offset increased outer-membrane permeability. There were no discernible changes in penicillin-binding proteins. Genomic deletions in ampD were observed in selected clinical isolates of P. aeruginosa with increased expression of the AmpC beta-lactamase. For some isolates, cephalosporin resistance was dependent upon the interplay of ampC and oprF expression.

Binding of ceftobiprole and comparators to the penicillin-binding proteins of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus pneumoniae

Ceftobiprole exhibited tight binding to PBP2a in methicillin-resistant Staphylococcus aureus, PBP2x in penicillin-resistant Streptococcus pneumoniae, and PBP3 and other essential penicillin-binding proteins in methicillin-susceptible S. aureus, Escherichia coli, and Pseudomonas aeruginosa. Ceftobiprole also bound well to PBP2 in the latter organisms, contributing to the broad-spectrum antibacterial activity against gram-negative and gram-positive bacteria.

Using microarray gene signatures to elucidate mechanisms of antibiotic action and resistance

Microarray analyses reveal global changes in gene expression in response to environmental changes and, thus, are well suited to providing a detailed picture of bacterial responses to antibiotic treatment. These responses are represented by patterns of gene expression, termed expression signatures, which provide insight into the mechanism of action of antibiotics as well as the general physiological responses of bacteria to antibiotic-related stresses. The complexity of such signatures is challenging the notion that antibiotics act on single targets and this is consistent with the concept that there are multiple targets coupled with common stress responses. A more detailed knowledge of how known antibiotics act should reveal new strategies for antimicrobial drug discovery.

Empiric use of cefepime in the treatment of serious urinary tract infections in children

BACKGROUND

Urinary tract infections (UTIs) are common in childhood. They represent a significant proportion (10%) of hospital-acquired infections in children. Bacteria causing UTIs in children vary, depending on the setting (community-acquired vs. nosocomial), underlying anatomic anomalies and concurrent medical conditions.

OBJECTIVE

To review published and unpublished clinical studies that have used cefepime for the treatment of UTIs in children.

METHODS AND RESULTS

In two recent multicenter, randomized trials, cefepime (50 mg/kg/dose every 8 h and every 12 h) was compared with ceftazidime (50 mg/kg/dose every 8 h) for the treatment of serious urinary tract infections including pyelonephritis in children less than 12 years of age. In these studies a favorable clinical and microbiologic response was observed in >95% of cefepime-treated and ceftazidime-treated children assessed at the end of treatment.

CONCLUSIONS

These results indicate that cefepime represents an important therapeutic option for the treatment of serious UTIs in children.

New criteria for selecting the proper antimicrobial chemotherapy for severe sepsis and septic shock

The mortality rate resulting from severe bacterial sepsis, particularly that associated with shock, still approaches 50% in spite of appropriate antimicrobial therapy and optimum supportive care. Bacterial endotoxins that are part of the cell wall are one of the cofactors in the pathogenesis of sepsis and septic shock and are often induced by antimicrobial chemotherapy even if it is administered rationally. Not all antimicrobial agents are equally capable of inducing septic shock; this is dependant on their mechanism of action rather than on the causative pathogen species. The quantity of endotoxin released depends on the drug dose and whether filaments or spheroplast formation predominates. Some antibiotics such as carbapenems, ceftriaxone, cefepime, glycopeptides, aminoglycosides and quinolones do not have the propensity to provoke septic shock because their rapid bactericidal activity induces mainly spheroplast or fragile spheroplast-like bacterial forms.

Use of microdilution panels with and without beta-lactamase inhibitors as a phenotypic test for beta-lactamase production among Escherichia coli, Klebsiella spp., Enterobacter spp., Citrobacter freundii, and Serratia marcescens

Over the past decade, a number of new beta-lactamases have appeared in clinical isolates of Enterobacteriaceae that, unlike their predecessors, do not confer beta-lactam resistance that is readily detected in routine antibiotic susceptibility tests. Because optimal methodologies are needed to detect these important new beta-lactamases, a study was designed to evaluate the ability of a panel of various beta-lactam antibiotics tested alone and in combination with beta-lactamase inhibitors to discriminate between the production of extended-spectrum beta-lactamases, AmpC beta-lactamases, high levels of K1 beta-lactamase, and other beta-lactamases in 141 isolates of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter cloacae, Enterobacter aerogenes, Citrobacter freundii, and Serratia marcescens possessing well-characterized beta-lactamases. The microdilution panels studied contained aztreonam, cefpodoxime, ceftazidime, cefotaxime, and ceftriaxone, with and without 1, 2, and 4 microg of clavulanate per ml or 8 microg of sulbactam per ml and cefoxitin and cefotetan with and without 8 microg of sulbactam per ml. The results indicated that a minimum panel of five tests would provide maximum separation of extended-spectrum beta-lactamase high AmpC, high K1, and other beta-lactamase production in Enterobacteriaceae. These included cefpodoxime, cefpodoxime plus 4 microg of clavulanate per ml, ceftazidime, ceftriaxone, and ceftriaxone plus 8 microg of sulbactam per ml. Ceftriaxone plus 2 microg of clavulanate per ml could be substituted for cefpodoxime plus 4 microg of clavulanate per ml without altering the accuracy of the tests. This study indicated that tests with key beta-lactam drugs, alone and in combination with beta-lactamase inhibitors, could provide a convenient approach to the detection of a variety of beta-lactamases in members of the family Enterobacteriaceae.

Antibiotic-induced release of bacterial cell wall components in the pathogenesis of sepsis and septic shock: a review

This article reviews the new criteria for selecting the proper antimicrobial agent and dosage regimen for standard treatment of severe sepsis, with the intention of preventing septic shock. After introducing new concepts on the pathogenesis of sepsis and septic shock, the authors analyze the parameters of betalactam antibacterial activity, the antibiotic-induced release of bacterial endotoxin and the interrelationships between pharmacokinetics and pharmacodynamics of antibiotics in the search for an optimum dosage regimen of antimicrobial mono- or polytherapy for severely ill septic patients admitted to the intensive care unit.

Cefepime-aztreonam: a unique double beta-lactam combination for Pseudomonas aeruginosa

An in vitro pharmacokinetic model was used to determine if aztreonam could enhance the pharmacodynamics of cefepime or ceftazidime against an isogenic panel of Pseudomonas aeruginosa 164, including wild-type (WT), partially derepressed (PD), and fully derepressed (FD) phenotypes. Logarithmic-phase cultures were exposed to peak concentrations achieved in serum with 1- or 2-g intravenous doses, elimination pharmacokinetics were simulated, and viable bacterial counts were measured over three 8-h dosing intervals. In studies with cefepime and cefepime-aztreonam against the PD strain, samples were also filter sterilized, assayed for active cefepime, and assayed for nitrocefin hydrolysis activity before and after overnight dialysis. Against WT strains, the cefepime-aztreonam combination was the most active regimen, but viable counts at 24 h were only 1 log below those in cefepime-treated cultures. Against PD and FD strains, the antibacterial activity of cefepime-aztreonam was significantly enhanced over that of each drug alone, with 3.5 logs of killing by 24 h. Hydrolysis and bioassay studies demonstrated that aztreonam was inhibiting the extracellular cephalosporinase that had accumulated and was thus protecting cefepime in the extracellular environment. In contrast to cefepime-aztreonam, the pharmacodynamics of ceftazidime-aztreonam were not enhanced over those of aztreonam alone. Further pharmacodynamic studies with five other P. aeruginosa strains producing increased levels of cephalosporinase demonstrated that the enhanced pharmacodynamics of cefepime-aztreonam were not unique to the isogenic panel. The results of these studies demonstrate that aztreonam can enhance the antibacterial activity of cefepime against derepressed mutants of P. aeruginosa producing increased levels of cephalosporinase. This positive interaction appears to be due in part to the ability of aztreonam to protect cefepime from extracellular cephalosporinase inactivation. Clinical evaluation of this combination is warranted.

Antibiotic-induced lipopolysaccharide (LPS) release from Salmonella typhi: delay between killing by ceftazidime and imipenem and release of LPS

It has been suggested that the antibiotic-induced release of lipopolysaccharide (LPS) is an important cause of the development of septic shock in patients treated for severe infections caused by gram-negative bacteria. Beta-lactam antibiotics change the integrity of the bacterial cell envelope by binding to penicillin-binding proteins (PBP) in the membrane and thus may affect the amount of LPS that is released and the kinetics of that release. In this respect, ceftazidime at intermediate concentrations binds with a high affinity to PBP 3 and PBP 1a and thus can induce filament formation in addition to killing, whereas imipenem preferentially binds to PBP 2 and PBP 1b, leading to spheroplast formation and rapid cell lysis. We investigated the effects of these antibiotics on the killing and the release of the radioactively labelled LPS of Salmonella typhi Ty 21A. A mathematical model was developed to calculate the delay between bacterial killing and LPS release, designated the lag time. At antibiotic concentrations inducing equal killing, the amount of LPS released was the same for both antibiotics. Only after 6 h of incubation at antibiotic concentrations above 0.5 microg/ml, the amount of 3H-LPS released was slightly higher (approximately 1.2-fold) in incubations with ceftazidime than in those with imipenem, and the maximum releases of the total label were 33.2% +/- 0.89% and 27.1% +/- 0.45%, respectively. Despite the clear concentration-dependent effect on the bacterial killing and subsequent LPS release, the lag time was independent of the antibiotic concentration. For ceftazidime as well as imipenem the lag time amounted to approximately 60 min. In conclusion, our findings imply that the mechanism of antibiotic-induced LPS release is independent of the PBP affinities for these beta-lactam antibiotics. Furthermore, once the organism is killed by either imipenem or ceftazidime, the rate of LPS release from S. typhi does not differ according to the antibiotic with which the organism is killed, and there is little difference in the relative amount of LPS released.

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.

Susceptibility to beta-lactam antibiotics of Pseudomonas aeruginosa overproducing penicillin-binding protein 3

By using a broad-host-range vector, pUCP27, the Pseudomonas aeruginosa and Escherichia coli pbpB genes, which encode penicillin-binding protein 3 (PBP3), were separately overexpressed in a P. aeruginosa strain, PAO4089, that is deficient in producing chromosomal beta-lactamase. Susceptibility studies indicated that overproduction of the P. aeruginosa PBP3 in PAO4089 resulted in twofold-increased resistance to aztreonam, fourfold-increased resistance to cefepime and cefsulodin, and eightfold-increased resistance to ceftazidime, whereas overproduction of the P. aeruginosa PBP3 in PAO4089 did not affect susceptibility to PBP1-targeted cephaloridine or PBP2-targeted imipenem. Similar results were obtained with PAO4089 overproducing E. coli PBP3, with the exception that there was no influence on the MICs or minimal bactericidal concentrations of cefsulodin and cefepime, which have very low affinities for E. coli PBP3. These data are consistent with the conclusion that PBP3 has to achieve a certain level of saturation, with beta-lactams targeted to this protein, to result in cell inhibition or death.

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.

Susceptibility of bacterial isolates to beta-lactam antibiotics from U.S. clinical trials over a 5-year period

Results are reported for agar dilution susceptibility testing of 3,075 isolates of aerobic bacteria collected from >200 U.S. institutions, located in 30 different states. These isolates were collected from 1987 through 1991 from patients who participated in cefepime clinical trials. Cefepime susceptibility was compared with ceftazidime, cefotaxime, ceftriaxone, cefoperazone, and imipenem. To avoid duplication of strains, only initial isolates were included. Cefepime minimum inhibitory concentration (MIC90) values for Enterobacteriaceae were < or = 0.5 microg/mL, except for two species, Citrobacter freundii and Providencia stuartii, with MIC90 values of 2 and 1, respectively. The MIC90 values of the other cephalosporins were higher, especially for Enterobacter aerogenes and C. freundii. The MIC90 values of cefepime for methicillin-susceptible Staphylococcus aureus (4 microg/mL) and Pseudomonas aeruginosa (8 microg/mL) were similar to those of cefotaxime for S. aureus (4 microg/mL), and to ceftazidime for P. aeruginosa (8 microg/mL). Streptococcus pneumoniae was similar in susceptibility to cefotaxime at 0.06 microg/mL. The activity of cefepime against a diverse group of gram-positive and gram-negative (1987-1991) bacteria isolates demonstrates the excellent activity of cefepime compared to third-generation cephalosporins and imipenem, particularly among C. freundii and E. aerogenes isolates, which were often resistant to other cephalosporins.