Penicillin-binding proteins and induction of AmpC beta-lactamase

Antibiotic Treatment of Infections Caused by AmpC-Producing Enterobacterales

AmpC enzymes are a class of beta-lactamases produced by Gram-negative bacteria, including several Enterobacterales. When produced in sufficient amounts, AmpCs can hydrolyze third-generation cephalosporins (3GCs) and piperacillin/tazobactam, causing resistance. In Enterobacterales, the AmpC gene can be chromosomal- or plasmid-encoded. Some species, particularly Enterobacter cloacae complex, Klebsiella aerogenes, and Citrobacter freundii, harbor an inducible chromosomal AmpC gene. The expression of this gene can be derepressed during treatment with a beta-lactam, leading to AmpC overproduction and the consequent emergence of resistance to 3GCs and piperacillin/tazobactam during treatment. Because of this phenomenon, the use of carbapenems or cefepime is considered a safer option when treating these pathogens. However, many areas of uncertainty persist, including the risk of derepression related to each beta-lactam; the role of piperacillin/tazobactam compared to cefepime; the best option for severe or difficult-to-treat cases, such as high-inoculum infections (e.g., ventilator-associated pneumonia and undrainable abscesses); the role of de-escalation once clinical stability is obtained; and the best treatment for species with a lower risk of derepression during treatment (e.g., Serratia marcescens and Morganella morganii). The aim of this review is to collate the most relevant information about the microbiological properties of and therapeutic approach to AmpC-producing Enterobacterales in order to inform daily clinical practice.

Quantification of β-lactamase producing bacteria in German surface waters with subsequent MALDI-TOF MS-based identification and β-lactamase activity assay

Environmental oligotrophic bacteria are suspected to be highly relevant carriers of antimicrobial resistance (AMR). However, there is a lack of validated methods for monitoring in the aquatic environment. Since extended-spectrum β-lactamases (ESBLs) play a particularly important role in the clinical sector, a culturing method based on R2A-medium spiked with different combinations of β-lactams was applied to quantify β-lactamase-producing environmental bacteria from surface waters. In German surface water samples (n = 28), oligotrophic bacteria ranging from 4.0 × 103 to 1.7 × 104 CFU per 100 mL were detected on the nutrient-poor medium spiked with 3rd generation cephalosporins and carbapenems. These numbers were 3 log10 higher compared to ESBL-producing Enterobacteriales of clinical relevance from the same water samples. A MALDI-TOF MS identification of the isolates demonstrated, that the method leads to the isolation of environmentally relevant strains with Pseudomonas, Flavobacterium, and Janthinobacterium being predominant β-lactam resistant genera. Subsequent micro-dilution antibiotic susceptibility tests (Micronaut-S test) confirmed the expression of β-lactamases. The qPCR analysis of surface waters DNA extracts showed the presence of β-lactamase genes (blaTEM, blaCMY-2, blaOXA-48, blaVIM-2, blaSHV, and blaNDM-1) at concentrations of 3.7 (±1.2) to 1.0 (±1.9) log10 gene copies per 100 mL. Overall, the results demonstrate a widespread distribution of cephalosporinase and carbapenemase enzymes in oligotrophic environmental bacteria that have to be considered as a reservoir of ARGs and contribute to the spread of antibiotic resistance.

A whole-cell hypersensitive biosensor for beta-lactams based on the AmpR-AmpC regulatory circuit from the Antarctic Pseudomonas sp. IB20

Detecting antibiotic residues is vital to minimize their impact. Yet, existing methods are complex and costly. Biosensors offer an alternative. While many biosensors detect various antibiotics, specific ones for beta-lactams are lacking. To address this gap, a biosensor based on the AmpC beta-lactamase regulation system (ampR-ampC) from Pseudomonas sp. IB20, an Antarctic isolate, was developed in this study. The AmpR-AmpC system is well-conserved in the genus Pseudomonas and has been extensively studied for its involvement in peptidoglycan recycling and beta-lactam resistance. To create the biosensor, the ampC coding sequence was replaced with the mCherry fluorescent protein as a reporter, resulting in a transcriptional fusion. This construct was then inserted into Escherichia coli SN0301, a beta-lactam hypersensitive strain, generating a whole-cell biosensor. The biosensor demonstrated dose-dependent detection of penicillins, cephalosporins and carbapenems. However, the most interesting aspect of this work is the high sensitivity presented by the biosensor in the detection of carbapenems, as it was able to detect 8 pg/mL of meropenem and 40 pg/mL of imipenem and reach levels of 1-10 ng/mL for penicillins and cephalosporins. This makes the biosensor a powerful tool for the detection of beta-lactam antibiotics, specifically carbapenems, in different matrices.

Enterobacter cloacae infection characteristics and outcomes in battlefield trauma patients

Enterobacter cloacae is a Gram-negative rod with multidrug-resistant potential due to chromosomally-induced AmpC β-lactamase. We evaluated characteristics, antibiotic utilization, and outcomes associated with battlefield-related E. cloacae infections (2009-2014). Single initial and serial E. cloacae isolates (≥24 hours from initial isolate from any site) associated with a clinical infection were examined. Susceptibility profiles of initial isolates in the serial isolation group were contrasted against last isolate recovered. Characteristics of 112 patients with E. cloacae infections (63 [56%] with single initial isolation; 49 [44%] with serial isolation) were compared to 509 patients with bacterial infections not attributed to E. cloacae. E. cloacae patients sustained more blast trauma (78%) compared to non-E. cloacae infections patients (75%; p<0.001); however, injury severity scores were comparable (median of 34.5 and 33, respectively; p = 0.334). Patients with E. cloacae infections had greater shock indices (median 1.07 vs 0.92; p = 0.005) and required more initial blood products (15 vs. 14 units; p = 0.032) compared to patients with non-E. cloacae infections. Although E. cloacae patients had less intensive care unit admissions (80% vs. 90% with non-E. cloacae infection patients; p = 0.007), they did have more operating room visits (5 vs. 4; p = 0.001), longer duration of antibiotic therapy (43.5 vs. 34 days; p<0.001), and lengthier hospitalizations (57 vs. 44 days; p<0.001). Patients with serial E. cloacae had isolation of infecting isolates sooner than patients with single initial E. cloacae (median of 5 vs. 8 days post-injury; p = 0.046); however, outcomes were not significantly different between the groups. Statistically significant resistance to individual antibiotics did not develop between initial and last isolates in the serial isolation group. Despite current combat care and surgical prophylaxis guidelines recommending upfront provision of AmpC-inducing antibiotics, clinical outcomes did not differ nor did significant antibiotic resistance develop in patients who experienced serial isolation of E. cloacae versus single initial isolation.

Treatment options for infections caused by multidrug-resistant Gram-negative bacteria: a guide to good clinical practice

The rapid emergence of multidrug-resistant Gram-negative bacterial infections necessitates the development of new treatments or the repurposing of available antibiotics. Here, treatment options for treatment of these infections, recent guidelines and evidence are reviewed. Studies that included treatment options for infections caused by multidrug-resistant Gram-negative bacteria (Enterobacterales and nonfermenters), as well as extended-spectrum β-lactamase-producing and carbapenem-resistant bacteria, were considered. Potential agents for the treatment of these infections, considering type of microorganism, mechanism of resistant, source and severity of infection as well as pharmacotherapy considerations, are summarized.

Ceftazidime/Avibactam in Ventilator-Associated Pneumonia Due to Difficult-to-Treat Non-Fermenter Gram-Negative Bacteria in COVID-19 Patients: A Case Series and Review of the Literature

Ventilator-associated pneumonia (VAP) in critically ill patients with COVID-19 represents a very huge global threat due to a higher incidence rate compared to non-COVID-19 patients and almost 50% of the 30-day mortality rate. Pseudomonas aeruginosa was the first pathogen involved but uncommon non-fermenter gram-negative organisms such as Burkholderia cepacea and Stenotrophomonas maltophilia have emerged as other potential etiological causes. Against carbapenem-resistant gram-negative microorganisms, Ceftazidime/avibactam (CZA) is considered a first-line option, even more so in case of a ceftolozane/tazobactam resistance or shortage. The aim of this report was to describe our experience with CZA in a case series of COVID-19 patients hospitalized in the ICU with VAP due to difficult-to-treat (DTT) P. aeruginosa, Burkholderia cepacea, and Stenotrophomonas maltophilia and to compare it with data published in the literature. A total of 23 patients were treated from February 2020 to March 2022: 19/23 (82%) VAPs were caused by Pseudomonas spp. (16/19 DTT), 2 by Burkholderia cepacea, and 6 by Stenotrophomonas maltophilia; 12/23 (52.1%) were polymicrobial. Septic shock was diagnosed in 65.2% of the patients and VAP occurred after a median of 29 days from ICU admission. CZA was prescribed as a combination regimen in 86% of the cases, with either fosfomycin or inhaled amikacin or cotrimoxazole. Microbiological eradication was achieved in 52.3% of the cases and the 30-day overall mortality rate was 14/23 (60.8%). Despite the high mortality of critically ill COVID-19 patients, CZA, especially in combination therapy, could represent a valid treatment option for VAP due to DTT non-fermenter gram-negative bacteria, including uncommon pathogens such as Burkholderia cepacea and Stenotrophomonas maltophilia.

Detection and Characterization of Carbapenemases in Enterobacterales With a New Rapid and Simplified Carbapenemase Detection Method Called rsCDM

Objective

This study aimed to develop a new rapid and simplified carbapenemase detection method (rsCDM) for detection and characterization of carbapenemase with 3-aminophenylboronic acid (APBA), ethylenediaminetetraacetic acid (EDTA), and cloxacillin (CLO) β-lactamase inhibitors.

Methods

A panel of 182 carbapenem-resistant Enterobacterales (CRE) strains with blaKPC (88), blaNDM (60), blaIMP (10), blaVIM (3), blaOXA-181 (5), blaKPC, and blaNDM (7), porin changes in combination with an extended-spectrum β-lactamase (ESBL) (3) or AmpC hyper-production (6) and 43 carbapenem-susceptible Enterobacterales isolates were used to evaluate the performance of rsCDM and EDTA-carbapenem inactivation method (eCIM). Carbapenemase class was determined with specific inhibitors at 4, 6, and 18 h by rsCDM, and the difference between imipenem (IMI) and meropenem (MEM) disks was simultaneously compared.

Results

The sensitivity of rsCDM using IMI was 97.1% at the three time points, with a specificity of 100%, independent of the culture duration. Similar to IPM, MEM disk also showed high sensitivity (97.1%) and specificity (100%) at 6 h. And the sensitivity of eCIM was 95.4% and the specificity was 100%. Based on a decision algorithm, the characterization number of IMI and MEM in KPC-producing isolates was 88 vs. 87, metallo-β-lactamases (MBLs) was 73 vs. 72, KPC and NDM carbapenemase was 7 vs. 7 at 4 h, respectively. After 6 h, the category number changed insignificantly except for isolates with combined AmpC overproduction and porin changes, showing an increase in IMI (6) and MEM (2), and there was no difference in the results between 6 and 18 h for the two tablets. OXA-181-producing strains can’t be distinguished by rsCDM. For eCIM, the characterization number in KPC-, OXA- 181-, and MBLs-producing strains was 88, 5, and 72, but it failed to detect multi-enzyme-producing isolates (KPC and NDM).

Conclusion

rsCDM accurately discriminated carbapenemase within 4 h and could differentiate multi-enzyme (KPC and NDM) and AmpC in conjunction with porin changes strains. Hence, rsCDM represents a rapid, simple, easy readout, and accurate tool that can be used without any specialized equipment.

Infectious Diseases Society of America Guidance on the Treatment of AmpC β-lactamase-Producing Enterobacterales, Carbapenem-Resistant Acinetobacter baumannii, and Stenotrophomonas maltophilia Infections

BACKGROUND

The Infectious Diseases Society of America (IDSA) is committed to providing up-to-date guidance on the treatment of antimicrobial-resistant infections. A previous guidance document focused on infections caused by extended-spectrum β-lactamase-producing Enterobacterales (ESBL-E), carbapenem-resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with difficult-to-treat resistance (DTR-P. aeruginosa). Here, guidance is provided for treating AmpC β-lactamase-producing Enterobacterales (AmpC-E), carbapenem-resistant Acinetobacter baumannii (CRAB), and Stenotrophomonas maltophilia infections.

METHODS

A panel of six infectious diseases specialists with expertise in managing antimicrobial-resistant infections formulated questions about the treatment of AmpC-E, CRAB, and S. maltophilia infections. Answers are presented as suggestions and corresponding rationales. In contrast to guidance in the previous document, published data on optimal treatment of AmpC-E, CRAB, and S. maltophilia infections are limited. As such, guidance in this document is provided as "suggested approaches" based on clinical experience, expert opinion, and a review of the available literature. Because of differences in the epidemiology of resistance and availability of specific anti-infectives internationally, this document focuses on the treatment of infections in the United States.

RESULTS

Preferred and alternative treatment suggestions are provided, assuming the causative organism has been identified and antibiotic susceptibility results are known. Approaches to empiric treatment, duration of therapy, and other management considerations are also discussed briefly. Suggestions apply for both adult and pediatric populations.

CONCLUSIONS

The field of antimicrobial resistance is highly dynamic. Consultation with an infectious diseases specialist is recommended for the treatment of antimicrobial-resistant infections. This document is current as of September 17, 2021 and will be updated annually. The most current versions of IDSA documents, including dates of publication, are available at www.idsociety.org/practice-guideline/amr-guidance-2.0/.

Molecular Basis of AmpC β-Lactamase Induction by Avibactam in Pseudomonas aeruginosa: PBP Occupancy, Live Cell Binding Dynamics and Impact on Resistant Clinical Isolates Harboring PDC-X Variants

Avibactam belongs to the new class of diazabicyclooctane β-lactamase inhibitors. Its inhibitory spectrum includes class A, C and D enzymes, including P. aeruginosa AmpC. Nonetheless, recent reports have revealed strain-dependent avibactam AmpC induction. In the present work, we wanted to assess the mechanistic basis underlying AmpC induction and determine if derepressed PDC-X mutated enzymes from ceftazidime/avibactam-resistant clinical isolates were further inducible. We determined avibactam concentrations that half-maximally inhibited (IC₅₀) bocillin FL binding. Inducer β-lactams were also studied as comparators. Live cells’ time-course penicillin-binding proteins (PBPs) occupancy of avibactam was studied. To assess the ampC induction capacity of avibactam and comparators, qRT-PCR was performed in wild-type PAO1, PBP4, triple PBP4, 5/6 and 7 knockout derivatives and two ceftazidime/avibactam-susceptible/resistant XDR clinical isolates belonging to the epidemic high-risk clone ST175. PBP4 inhibition was observed for avibactam and β-lactam comparators. Induction capacity was consistently correlated with PBP4 binding affinity. Outer membrane permeability-limited PBP4 binding was observed in the live cells’ assay. As expected, imipenem and cefoxitin showed strong induction in PAO1, especially for carbapenem; avibactam induction was conversely weaker. Overall, the inducer effect was less remarkable in ampC-derepressed mutants and nonetheless absent upon avibactam exposure in the clinical isolates harboring mutated AmpC variants and their parental strains.

A Primer on AmpC β-Lactamases: Necessary Knowledge for an Increasingly Multidrug-resistant World

Understanding the nuances of AmpC β-lactamase-mediated resistance can be challenging, even for the infectious diseases specialist. AmpC resistance can be classified into 3 categories: (1) inducible chromosomal resistance that emerges in the setting of a β-lactam compound, (2) stable derepression due to mutations in ampC regulatory genes, or (3) the presence of plasmid-mediated ampC genes. This review will mainly focus on inducible AmpC resistance in Enterobacteriaceae. Although several observational studies have explored optimal treatment for AmpC producers, few provide reliable insights into effective management approaches. Heterogeneity within the data and inherent selection bias make inferences on effective β-lactam choices problematic. Most experts agree it is prudent to avoid expanded-spectrum (ie, third-generation) cephalosporins for the treatment of organisms posing the greatest risk of ampC induction, which has best been described in the context of Enterobacter cloacae infections. The role of other broad-spectrum β-lactams and the likelihood of ampC induction by other Enterobacteriaceae are less clear. We will review the mechanisms of resistance and triggers resulting in AmpC expression, the species-specific epidemiology of AmpC production, approaches to the detection of AmpC production, and treatment options for AmpC-producing infections.

Complex Response of the CpxAR Two-Component System to β-Lactams on Antibiotic Resistance and Envelope Homeostasis in Enterobacteriaceae

The Cpx stress response is widespread among Enterobacteriaceae We previously reported a mutation in cpxA in a multidrug-resistant strain of Klebsiella aerogenes isolated from a patient treated with imipenem. This mutation yields a single-amino-acid substitution (Y144N) located in the periplasmic sensor domain of CpxA. In this work, we sought to characterize this mutation in Escherichia coli by using genetic and biochemical approaches. Here, we show that cpxA^Y144N is an activated allele that confers resistance to β-lactams and aminoglycosides in a CpxR-dependent manner, by regulating the expression of the OmpF porin and the AcrD efflux pump, respectively. We also demonstrate the effect of the intimate interconnection between the Cpx system and peptidoglycan integrity on the expression of an exogenous AmpC β-lactamase by using imipenem as a cell wall-active antibiotic or by inactivating penicillin-binding proteins. Moreover, our data indicate that the Y144N substitution abrogates the interaction between CpxA and CpxP and increases phosphotransfer activity on CpxR. Because the addition of a strong AmpC inducer such as imipenem is known to cause abnormal accumulation of muropeptides (disaccharide-pentapeptide and N-acetylglucosamyl-1,6-anhydro-N-acetylmuramyl-l-alanyl-d-glutamy-meso-diaminopimelic-acid-d-alanyl-d-alanine) in the periplasmic space, we propose these molecules activate the Cpx system by displacing CpxP from the sensor domain of CpxA. Altogether, these data could explain why large perturbations to peptidoglycans caused by imipenem lead to mutational activation of the Cpx system and bacterial adaptation through multidrug resistance. These results also validate the Cpx system, in particular, the interaction between CpxA and CpxP, as a promising therapeutic target.

Regulation of AmpC-Driven β-Lactam Resistance in Pseudomonas aeruginosa: Different Pathways, Different Signaling

The hyperproduction of the chromosomal AmpC β-lactamase is the main mechanism driving β-lactam resistance in Pseudomonas aeruginosa, one of the leading opportunistic pathogens causing nosocomial acute and chronic infections in patients with underlying respiratory diseases. In the current scenario of the shortage of effective antipseudomonal drugs, understanding the molecular mechanisms mediating AmpC hyperproduction in order to develop new therapeutics against this fearsome pathogen is of great importance. It has been accepted for decades that certain cell wall-derived soluble fragments (muropeptides) modulate AmpC production by complexing with the transcriptional regulator AmpR and acquiring different conformations that activate/repress ampC expression. However, these peptidoglycan-derived signals have never been characterized in the highly prevalent P. aeruginosa stable AmpC hyperproducer mutants. Here, we demonstrate that the previously described fragments enabling the transient ampC hyperexpression during cefoxitin induction (1,6-anhydro-N-acetylmuramyl-pentapeptides) also underlie the dacB (penicillin binding protein 4 [PBP4]) mutation-driven stable hyperproduction but differ from the 1,6-anhydro-N-acetylmuramyl-tripeptides notably overaccumulated in the ampD knockout mutant. In addition, a simultaneous greater accumulation of both activators appears linked to higher levels of AmpC hyperproduction, although our results suggest a much stronger AmpC-activating potency for the 1,6-anhydro-N-acetylmuramyl-pentapeptide. Collectively, our results propose a model of AmpC control where the activator fragments, with qualitative and quantitative particularities depending on the pathways and levels of β-lactamase production, dominate over the repressor (UDP-N-acetylmuramyl-pentapeptide). This study represents a major step in understanding the foundations of AmpC-dependent β-lactam resistance in P. aeruginosa, potentially useful to open new therapeutic conceptions intended to interfere with the abovementioned cell wall-derived signaling.IMPORTANCE The extensive use of β-lactam antibiotics and the bacterial adaptive capacity have led to the apparently unstoppable increase of antimicrobial resistance, one of the current major global health challenges. In the leading nosocomial pathogen Pseudomonas aeruginosa, the mutation-driven AmpC β-lactamase hyperproduction stands out as the main resistance mechanism, but the molecular cues enabling this system have remained elusive until now. Here, we provide for the first time direct and quantitative information about the soluble cell wall-derived fragments accounting for the different levels and pathways of AmpC hyperproduction. Based on these results, we propose a hierarchical model of signals which ultimately govern ampC hyperexpression and resistance.

Diversity and regulation of intrinsic β-lactamases from non-fermenting and other Gram-negative opportunistic pathogens

This review deeply addresses for the first time the diversity, regulation and mechanisms leading to mutational overexpression of intrinsic β-lactamases from non-fermenting and other non-Enterobacteriaceae Gram-negative opportunistic pathogens. After a general overview of the intrinsic β-lactamases described so far in these microorganisms, including circa. 60 species and 100 different enzymes, we review the wide array of regulatory pathways of these β-lactamases. They include diverse LysR-type regulators, which control the expression of β-lactamases from relevant nosocomial pathogens such as Pseudomonas aeruginosa or Stenothrophomonas maltophilia or two-component regulators, with special relevance in Aeromonas spp., along with other pathways. Likewise, the multiple mutational mechanisms leading to β-lactamase overexpression and β-lactam resistance development, including AmpD (N-acetyl-muramyl-L-alanine amidase), DacB (PBP4), MrcA (PPBP1A) and other PBPs, BlrAB (two-component regulator) or several lytic transglycosylases among others, are also described. Moreover, we address the growing evidence of a major interplay between β-lactamase regulation, peptidoglycan metabolism and virulence. Finally, we analyse recent works showing that blocking of peptidoglycan recycling (such as inhibition of NagZ or AmpG) might be useful to prevent and revert β-lactam resistance. Altogether, the provided information and the identified gaps should be valuable for guiding future strategies for combating multidrug-resistant Gram-negative pathogens.

Role of Low-Molecular-Mass Penicillin-Binding Proteins, NagZ and AmpR in AmpC β-lactamase Regulation of Yersinia enterocolitica

Yersinia enterocolitica encodes a chromosomal AmpC β-lactamase under the regulation of the classical ampR-ampC system. To obtain a further understanding to the role of low-molecular-mass penicillin-binding proteins (LMM PBPs) including PBP4, PBP5, PBP6, and PBP7, as well as NagZ and AmpR in ampC regulation of Y. enterocolitica, series of single/multiple mutant strains were systematically constructed and the ampC expression levels were determined by luxCDABE reporter system, reverse transcription-PCR (RT-PCR) and β-lactamase activity test. Sequential deletion of PBP5 and other LMM PBPs result in a continuously growing of ampC expression level, the β-lactamse activity of quadruple deletion strain YEΔ4Δ5Δ6Δ7 (pbp4, pbp5, pbp6, and pbp7 inactivated) is approached to the YEΔD123 (ampD1, ampD2, and ampD3 inactivated). Deletion of nagZ gene caused two completely different results in YEΔD123 and YEΔ4Δ5Δ6Δ7, NagZ is indispensable for YEΔ4Δ5Δ6Δ7 ampC derepression phenotype but dispensable for YEΔD123. AmpR is essential for ampC hyperproduction in these two types of strains, inactivation of AmpR notable reduced the ampC expression level in both YEΔD123 and YEΔ4Δ5Δ6Δ7.

Cell Wall Recycling-Linked Coregulation of AmpC and PenB β-Lactamases through ampD Mutations in Burkholderia cenocepacia

In many Gram-negative pathogens, mutations in the key cell wall-recycling enzyme AmpD (N-acetyl-anhydromuramyl-L-alanine amidase) affect the activity of the regulator AmpR, which leads to the expression of AmpC β-lactamase, conferring resistance to expanded-spectrum cephalosporin antibiotics. Burkholderia cepacia complex (Bcc) species also have these Amp homologs; however, the regulatory circuitry and the nature of causal ampD mutations remain to be explored. A total of 92 ampD mutants were obtained, representing four types of mutations: single nucleotide substitution (causing an amino acid substitution or antitermination of the enzyme), duplication, deletion, and IS element insertion. Duplication, which can go through reversion, was the most frequent type. Intriguingly, mutations in ampD led to the induction of two β-lactamases, AmpC and PenB. Coregulation of AmpC and PenB in B. cenocepacia, and likely also in many Bcc species with the same gene organization, poses a serious threat to human health. This resistance mechanism is of evolutionary optimization in that ampD is highly prone to mutations allowing rapid response to antibiotic challenge, and many of the mutations are reversible in order to resume cell wall recycling when the antibiotic challenge is relieved.

Role of Pseudomonas aeruginosa low-molecular-mass penicillin-binding proteins in AmpC expression, β-lactam resistance, and peptidoglycan structure

This study aimed to characterize the role of Pseudomonas aeruginosa low-molecular-mass penicillin-binding proteins (LMM PBPs), namely, PBP4 (DacB), PBP5 (DacC), and PBP7 (PbpG), in peptidoglycan composition, β-lactam resistance, and ampC regulation. For this purpose, we constructed all single and multiple mutants of dacB, dacC, pbpG, and ampC from the wild-type P. aeruginosa PAO1 strain. Peptidoglycan composition was determined by high-performance liquid chromatography (HPLC), ampC expression by reverse transcription-PCR (RT-PCR), PBP patterns by a Bocillin FL-binding test, and antimicrobial susceptibility by MIC testing for a panel of β-lactams. Microscopy and growth rate analyses revealed no apparent major morphological changes for any of the mutants compared to the wild-type PAO1 strain. Of the single mutants, only dacC mutation led to significantly increased pentapeptide levels, showing that PBP5 is the major dd-carboxypeptidase in P. aeruginosa. Moreover, our results indicate that PBP4 and PBP7 play a significant role as dd-carboxypeptidase only if PBP5 is absent, and their dd-endopeptidase activity is also inferred. As expected, the inactivation of PBP4 led to a significant increase in ampC expression (around 50-fold), but, remarkably, the sequential inactivation of the three LMM PBPs produced a much greater increase (1,000-fold), which correlated with peptidoglycan pentapeptide levels. Finally, the β-lactam susceptibility profiles of the LMM PBP mutants correlated well with the ampC expression data. However, the inactivation of ampC in these mutants also evidenced a role of LMM PBPs, especially PBP5, in intrinsic β-lactam resistance. In summary, in addition to assessing the effect of P. aeruginosa LMM PBPs on peptidoglycan structure for the first time, we obtained results that represent a step forward in understanding the impact of these PBPs on β-lactam resistance, apparently driven by the interplay between their roles in AmpC induction, β-lactam trapping, and dd-carboxypeptidase/β-lactamase activity.

Involvement of mutation in ampD I, mrcA, and at least one additional gene in β-lactamase hyperproduction in Stenotrophomonas maltophilia

It has been reported that targeted disruption of ampD I or mrcA causes β-lactamase hyperproduction in Stenotrophomonas maltophilia. We show here that β-lactamase-hyperproducing laboratory selected mutants and clinical isolates can have wild-type ampD I and mrcA genes, implicating mutation of at least one additional gene in this phenotype. The involvement of mutations at multiple loci in the activation of β-lactamase production in S. maltophilia reveals that there are significant deviations from the enterobacterial paradigm of AmpR-mediated control of β-lactamase induction. We do show, however, that S. maltophilia ampD I can complement a mutation in Escherichia coli ampD. This suggests that an anhydromuropeptide degradation product of peptidoglycan is used to activate AmpR in S. maltophilia, as is also the case in enteric bacteria.

Inactivation of mrcA gene derepresses the basal-level expression of L1 and L2 β-lactamases in Stenotrophomonas maltophilia

OBJECTIVES

To characterize the relationship between inactivation of the mrcA gene and β-lactamase expression and β-lactams resistance in Stenotrophomonas maltophilia KJ and to investigate the involvement of ampR, ampN-ampG, ampD(I) and creBC in this.

METHODS

The mrcA deletion mutant KJΔmrcA was constructed to investigate the role of this putative penicillin-binding protein 1a (PBP1a) in β-lactamase expression and β-lactam resistance. The ΔampR, ΔampNG, ΔampDI and ΔcreBC alleles were introduced into KJΔmrcA, and KJΔDIΔBC and KJΔDIΔmrcAΔBC were also constructed for comparison. All the mutants and their corresponding parent strains were assayed for β-lactamase activities and MICs of β-lactams.

RESULTS

Inactivation of mrcA caused basal L1/L2 β-lactamase production to increase by ∼100-fold, but made little difference to cefuroxime-induced β-lactamase activity and the MICs of β-lactams. The ΔmrcA-derived basal β-lactamase hyperproduction was ampR and ampN-ampG dependent. Simultaneous inactivation of ampD(I) and mrcA did not augment β-lactamase production over and above that seen in an ampD(I) mutant alone. Furthermore, we could find no evidence for a role of the creBC two-component regulatory system in β-lactamase hyperproduction in a ΔampD(I) or ΔmrcA background.

CONCLUSIONS

Inactivation of mrcA, predicted to encode PBP1a, causes basal L1/L2 β-lactamase hyperproduction in S. maltophilia.

Beyond Susceptible and Resistant, Part I: Treatment of Infections Due to Gram-Negative Organisms With Inducible β-Lactamases

Inactivation of β-lactams by the action of β-lactamase enzymes is the most common mode of resistance to these drugs among Gram-negative organisms. The genomes of some key clinical pathogens such as Enterobacter and Pseudomonas encode AmpC, an inducible chromosomal β-lactamase. The potent activity of AmpC against broad-spectrum β-lactams complicates treatment of organisms with this gene. Antibiotic exposure can select for mutants expressing high levels of this enzyme, leading to the emergence of resistant isolates and failure of therapy, even when the initial isolate is fully susceptible. The risk of selecting for resistant organisms varies according to the particular β-lactam used for treatment. This article reviews the microbiology of these enzymes, summarizes clinical data on the frequency emergence of resistance, and discusses considerations for antimicrobial treatment of these organisms.

Beyond Susceptible and Resistant, Part I: Treatment of Infections Due to Gram-Negative Organisms With Inducible β-Lactamases

Inactivation of β-lactams by the action of β-lactamase enzymes is the most common mode of resistance to these drugs among Gram-negative organisms. The genomes of some key clinical pathogens such as Enterobacter and Pseudomonas encode AmpC, an inducible chromosomal β-lactamase. The potent activity of AmpC against broad-spectrum β-lactams complicates treatment of organisms with this gene. Antibiotic exposure can select for mutants expressing high levels of this enzyme, leading to the emergence of resistant isolates and failure of therapy, even when the initial isolate is fully susceptible. The risk of selecting for resistant organisms varies according to the particular β-lactam used for treatment. This article reviews the microbiology of these enzymes, summarizes clinical data on the frequency emergence of resistance, and discusses considerations for antimicrobial treatment of these organisms.

Induction of beta-lactamase production in Aeromonas hydrophila is responsive to beta-lactam-mediated changes in peptidoglycan composition

We have studied the mechanism by which beta-lactam challenge leads to beta-lactamase induction in Aeromonas hydrophila through transposon-insertion mutagenesis. Disruption of the dd-carboxypeptidases/endopeptidases, penicillin-binding protein 4 or BlrY leads to elevated monomer-disaccharide-pentapeptide levels in A. hydrophila peptidoglycan and concomitant overproduction of beta-lactamase through activation of the BlrAB two-component regulatory system. During beta-lactam challenge, monomer-disaccharide-pentapeptide levels increase proportionately with beta-lactamase production and beta-lactamase induction is inhibited by vancomycin, which binds muro-pentapeptides. Taken together, these data strongly suggest that the Aeromonas spp. beta-lactamase regulatory sensor kinase, BlrB, responds to the concentration of monomer-disaccharide-pentapeptide in peptidoglycan.

Antibacterial-resistant Pseudomonas aeruginosa: clinical impact and complex regulation of chromosomally encoded resistance mechanisms

Treatment of infectious diseases becomes more challenging with each passing year. This is especially true for infections caused by the opportunistic pathogen Pseudomonas aeruginosa, with its ability to rapidly develop resistance to multiple classes of antibiotics. Although the import of resistance mechanisms on mobile genetic elements is always a concern, the most difficult challenge we face with P. aeruginosa is its ability to rapidly develop resistance during the course of treating an infection. The chromosomally encoded AmpC cephalosporinase, the outer membrane porin OprD, and the multidrug efflux pumps are particularly relevant to this therapeutic challenge. The discussion presented in this review highlights the clinical significance of these chromosomally encoded resistance mechanisms, as well as the complex mechanisms/pathways by which P. aeruginosa regulates their expression. Although a great deal of knowledge has been gained toward understanding the regulation of AmpC, OprD, and efflux pumps in P. aeruginosa, it is clear that we have much to learn about how this resourceful pathogen coregulates different resistance mechanisms to overcome the antibacterial challenges it faces.

Antibacterial-resistant Pseudomonas aeruginosa: clinical impact and complex regulation of chromosomally encoded resistance mechanisms

Treatment of infectious diseases becomes more challenging with each passing year. This is especially true for infections caused by the opportunistic pathogen Pseudomonas aeruginosa, with its ability to rapidly develop resistance to multiple classes of antibiotics. Although the import of resistance mechanisms on mobile genetic elements is always a concern, the most difficult challenge we face with P. aeruginosa is its ability to rapidly develop resistance during the course of treating an infection. The chromosomally encoded AmpC cephalosporinase, the outer membrane porin OprD, and the multidrug efflux pumps are particularly relevant to this therapeutic challenge. The discussion presented in this review highlights the clinical significance of these chromosomally encoded resistance mechanisms, as well as the complex mechanisms/pathways by which P. aeruginosa regulates their expression. Although a great deal of knowledge has been gained toward understanding the regulation of AmpC, OprD, and efflux pumps in P. aeruginosa, it is clear that we have much to learn about how this resourceful pathogen coregulates different resistance mechanisms to overcome the antibacterial challenges it faces.

Resistance to β-lactams in bacteria isolated from different types of Portuguese cheese

The purpose of this study was to investigate the presence of β-lactam-resistant bacteria in six different types of Portuguese cheese. The numbers of ampicillin resistant (AMP^r) bacteria varied from 4.7 × 10² to 1.5 × 10⁷ CFU/g. Within 172 randomly selected β-lactam-resistant bacteria, 44 resistant phenotypes were found and 31.4% were multidrug resistant. The majority (85%) of the isolates identified belonged to the Enterobacteriaceae family. The presence of the bla_TEM gene was detected in 80.9% of the tested isolates. The results suggest that without thermal processing of the milk and good hygienic practices, cheese may act as a vehicle of transfer of β-lactam-resistant bacteria to the gastrointestinal tract of consumers.

Beta-lactam resistance response triggered by inactivation of a nonessential penicillin-binding protein

It has long been recognized that the modification of penicillin-binding proteins (PBPs) to reduce their affinity for β-lactams is an important mechanism (target modification) by which Gram-positive cocci acquire antibiotic resistance. Among Gram-negative rods (GNR), however, this mechanism has been considered unusual, and restricted to clinically irrelevant laboratory mutants for most species. Using as a model Pseudomonas aeruginosa, high up on the list of pathogens causing life-threatening infections in hospitalized patients worldwide, we show that PBPs may also play a major role in β-lactam resistance in GNR, but through a totally distinct mechanism. Through a detailed genetic investigation, including whole-genome analysis approaches, we demonstrate that high-level (clinical) β-lactam resistance in vitro, in vivo, and in the clinical setting is driven by the inactivation of the dacB-encoded nonessential PBP4, which behaves as a trap target for β-lactams. The inactivation of this PBP is shown to determine a highly efficient and complex β-lactam resistance response, triggering overproduction of the chromosomal β-lactamase AmpC and the specific activation of the CreBC (BlrAB) two-component regulator, which in turn plays a major role in resistance. These findings are a major step forward in our understanding of β-lactam resistance biology, and, more importantly, they open up new perspectives on potential antibiotic targets for the treatment of infectious diseases.

Decades after their discovery, β-lactams remain key components of our antimicrobial armamentarium for the treatment of infectious diseases. Nevertheless, resistance to these antibiotics is increasing alarmingly. There are two major bacterial strategies to develop resistance to β-lactam antibiotics: the production of enzymes that inactivate them (β-lactamases), or the modification of their targets in the cell wall (the essential penicillin-binding proteins, PBPs). Using the pathogen Pseudomonas aeruginosa as a model microorganism, we show that high-level (clinical) β-lactam resistance in vitro and in vivo frequently occurs through a previously unrecognized, totally distinct resistance pathway, driven by the mutational inactivation of a nonessential PBP (PBP4) that behaves as a trap target for β-lactams. We show that mutation of this PBP determines a highly efficient and complex β-lactam resistance response, triggering overproduction of the chromosomal β-lactamase AmpC and the specific activation of a two-component regulator, which in turn plays a key role in resistance. These findings are a major step forward in our understanding of β-lactam resistance biology, and, more importantly, they open up new perspectives on potential antibiotic targets for the treatment of infectious diseases.

beta-Lactamase induction and cell wall recycling in gram-negative bacteria

beta-Lactams with the ability to induce beta-lactamase in gram-negative bacteria bind to essential penicillin-binding proteins (PBPs) after entering the periplasmic space. This leads to inactivation of transpeptidase activities and thereby a decrease in the number of peptide cross-links, allowing further degradation of murein by soluble lytic transglycosylases. If all DD-carboxypeptidases (PBP 4, 5, 6a and 6b) are inhibited as well, the degradation product aD-pentapeptide (N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic-acid-D-alanyl-D- alanine) accumulates, which is the case with inducing beta-lactams such as imipenem. These molecules in addition to tri- and tetrapeptides (N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic-acid-[D-alanine]) which are the usual degradation products of peptidoglycan, are released into the cytoplasm and displace the UDP-pentapeptide (UDP-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic-acid-D-alanyl-D-alanine) from the DNA-binding protein AmpR, converting it into an activator of AmpC beta-lactamase expression.

Structure-based optimization of a non-beta-lactam lead results in inhibitors that do not up-regulate beta-lactamase expression in cell culture

Bacterial expression of beta-lactamases is the most widespread resistance mechanism to beta-lactam antibiotics, such as penicillins and cephalosporins. There is a pressing need for novel, non-beta-lactam inhibitors of these enzymes. One previously discovered novel inhibitor of the beta-lactamase AmpC, compound 1, has several favorable properties: it is chemically dissimilar to beta-lactams and is a noncovalent, competitive inhibitor of the enzyme. However, at 26 microM its activity is modest. Using the X-ray structure of the AmpC/1 complex as a template, 14 analogues were designed and synthesized. The most active of these, compound 10, had a K(i) of 1 microM, 26-fold better than the lead. To understand the origins of this improved activity, the structures of AmpC in complex with compound 10 and an analogue, compound 11, were determined by X-ray crystallography to 1.97 and 1.96 A, respectively. Compound 10 was active in cell culture, reversing resistance to the third generation cephalosporin ceftazidime in bacterial pathogens expressing AmpC. In contrast to beta-lactam-based inhibitors clavulanate and cefoxitin, compound 10 did not up-regulate beta-lactamase expression in cell culture but simply inhibited the enzyme expressed by the resistant bacteria. Its escape from this resistance mechanism derives from its dissimilarity to beta-lactam antibiotics.

Chromosomally-encoded resistance mechanisms of Pseudomonas aeruginosa: therapeutic implications

Pseudomonas aeruginosa is an important nosocomial pathogen that presents a difficult therapeutic challenge. Although P. aeruginosa has been shown to acquire resistance mechanisms encoded on plasmids, this pathogen comes armed with multiple chromosomally-encoded mechanisms of resistance that can provide impressive intrinsic resistance, as well as the potential to mutate to high-level multi-drug resistance. Recent analysis of the sequenced genome of P. aeruginosa PAO1 suggested that we have just started to unlock the resistance potential of this pathogen. One of the most serious threats to the usefulness of beta-lactams against P. aeruginosa is the chromosomal AmpC cephalosporinase. When AmpC production increases through mutational events, overproduction of this cephalosporinase provides high-level resistance to all beta-lactams except the carbapenems. Carbapenem resistance typically requires down-regulation of the outer membrane protein (OprD), which serves as the primary route of entry for carbapenems. Perhaps the most threatening of the resistance mechanisms encoded on the P. aeruginosa chromosome are the multi-drug efflux pumps. These pumps have the ability to extrude multiple classes of antibiotics from the periplasmic space, as well as the cytoplasm. Natural expression of efflux pumps in 'wild-type' cells plays an important role in the relatively decreased susceptibility of P. aeruginosa to antibiotics. However, the greatest therapeutic problems occur when these pumps are overproduced in mutants and high-level, multi-drug resistance develops. Although the development of infections with highly resistant strains of P. aeruginosa can present serious therapeutic challenges, the most troublesome threat associated with the chromosomally-encoded resistance mechanisms is the potential for high-level resistance to emerge during the course of therapy. When resistance emerges during therapy, clinical failure can occur and the therapeutic options for second-line therapy can become severely limited. Unfortunately, the emergence of resistance during therapy is not a rare event with P. aeruginosa and these three resistance mechanisms. Therefore, clinicians must be mindful of this threat when choosing an appropriate therapy, and usually appropriate therapy includes a combination of drugs. Since the standard combination of an aminoglycoside and a beta-lactam has been shown to be ineffective in preventing the emergence of some resistance problems, the search for more effective combinations must be a priority.

Role of the murein precursor UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-meso-diaminopimelic acid-D-Ala-D-Ala in repression of beta-lactamase induction in cell division mutants

Certain beta-lactam antibiotics induce the chromosomal ampC beta-lactamase of many gram-negative bacteria. The natural inducer, though not yet unequivocally identified, is a cell wall breakdown product which enters the cell via the AmpG permease component of the murein recycling pathway. Surprisingly, it has been reported that beta-lactamase is not induced by cefoxitin in the absence of FtsZ, which is required for cell division, or in the absence of penicillin-binding protein 2 (PBP2), which is required for cell elongation. Since these results remain unexplained, we examined an ftsZ mutant and other cell division mutants (ftsA, ftsQ, and ftsI) and a PBP2 mutant for induction of beta-lactamase. In all mutants, beta-lactamase was not induced by cefoxitin, which confirms the initial reports. The murein precursor, UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-meso-diaminopimelic acid-D-Ala-D-Ala (UDP-MurNAc-pentapeptide), has been shown to serve as a corepressor with AmpR to repress beta-lactamase expression in vitro. Our results suggest that beta-lactamase is not induced because the fts mutants contain a greatly increased amount of corepressor which the inducer cannot displace. In the PBP2(Ts) mutant, in addition to accumulation of corepressor, cell wall turnover and recycling were greatly reduced so that little or no inducer was available. Hence, in both cases, a high ratio of repressor to inducer presumably prevents induction.

Risk factors for piperacillin-tazobactam-resistant Pseudomonas aeruginosa among hospitalized patients

Antimicrobial resistance is an emerging problem with Pseudomonas aeruginosa. This study determined risk factors for the recovery of piperacillin-tazobactam-resistant P. aeruginosa from clinical cultures from hospitalized patients. A case-control study design was used to compare two groups of case patients with control patients. The first group of case patients was defined by nosocomial isolation of piperacillin-tazobactam-resistant P. aeruginosa, and the second group of cases yielded piperacillin-tazobactam-susceptible P. aeruginosa. Controls were selected in a 6:1 ratio from the same medical or surgical services among which piperacillin-tazobactam-resistant P. aeruginosa arose in patients. Risk factors analyzed included antimicrobial drug exposure, comorbid conditions, and demographics. Bivariate and multivariable analyses were performed. Piperacillin-tazobactam-resistant P. aeruginosa was isolated from 179 patients, and piperacillin-tazobactam-susceptible P. aeruginosa was isolated from 624 patients over a 2.5-year period. Piperacillin-tazobactam (odds ratio [OR] = 6.82; 95% confidence interval [CI], 4.56 to 10.21), imipenem (OR = 2.42; 95% CI, 1.19 to 4.94), aminoglycosides (OR = 2.18; 95% CI, 1.44 to 3.28), vancomycin (OR = 1.87; 95% CI, 1.21 to 2.89), and broad-spectrum cephalosporins (OR = 2.38; 95% CI, 1.45 to 3.88) were the antibiotics associated with the isolation of piperacillin-tazobactam-resistant P. aeruginosa. Exposure to vancomycin (OR = 1.53; 95% CI, 1.13 to 2.06) or ampicillin-sulbactam (OR = 2.28; 95% CI, 1.62 to 3.21) was associated with recovery of piperacillin-tazobactam-susceptible P. aeruginosa. In this study, antibiotics associated with piperacillin-tazobactam-susceptible P. aeruginosa were different from antibiotics associated with piperacillin-tazobactam-resistant P. aeruginosa. Piperacillin-tazobactam was a strong risk factor for piperacillin-tazobactam-resistant P. aeruginosa. Our results suggest that the nosocomial isolation of piperacillin-tazobactam-resistant P. aeruginosa may be affected by multiple antibiotics.

Role of penicillin-binding proteins in the initiation of the AmpC beta-lactamase expression in Enterobacter cloacae

Penicillin-binding proteins (PBPs) are involved in the regulation of beta-lactamase expression by determining the level of anhydromuramylpeptides in the periplasmatic space. It was hypothesized that one or more PBPs act as a sensor in the beta-lactamase induction pathway. We have performed induction studies with Escherichia coli mutants lacking one to four PBPs with DD-carboxypeptidase activity. Therefore, we conclude that a strong beta-lactamase inducer must inhibit all DD-carboxypeptidases as well as the essential PBPs 1a, 1b, and/or 2.

The complexed structure and antimicrobial activity of a non-beta-lactam inhibitor of AmpC beta-lactamase

Beta-lactamases are the major resistance mechanism to beta-lactam antibiotics and pose a growing threat to public health. Recently, bacteria have become resistant to beta-lactamase inhibitors, making this problem pressing. In an effort to overcome this resistance, non-beta-lactam inhibitors of beta-lactamases were investigated for complementarity to the structure of AmpC beta-lactamase from Escherichia coli. This led to the discovery of an inhibitor, benzo(b)thiophene-2-boronic acid (BZBTH2B), which inhibited AmpC with a Ki of 27 nM. This inhibitor is chemically dissimilar to beta-lactams, raising the question of what specific interactions are responsible for its activity. To answer this question, the X-ray crystallographic structure of BZBTH2B in complex with AmpC was determined to 2.25 A resolution. The structure reveals several unexpected interactions. The inhibitor appears to complement the conserved, R1-amide binding region of AmpC, despite lacking an amide group. Interactions between one of the boronic acid oxygen atoms, Tyr150, and an ordered water molecule suggest a mechanism for acid/base catalysis and a direction for hydrolytic attack in the enzyme catalyzed reaction. To investigate how a non-beta-lactam inhibitor would perform against resistant bacteria, BZBTH2B was tested in antimicrobial assays. BZBTH2B significantly potentiated the activity of a third-generation cephalosporin against AmpC-producing resistant bacteria. This inhibitor was unaffected by two common resistance mechanisms that often arise against beta-lactams in conjunction with beta-lactamases. Porin channel mutations did not decrease the efficacy of BZBTH2B against cells expressing AmpC. Also, this inhibitor did not induce expression of AmpC, a problem with many beta-lactams. The structure of the BZBTH2B/AmpC complex provides a starting point for the structure-based elaboration of this class of non-beta-lactam inhibitors.

Bacterial enzymatic resistance: beta-lactamases and aminoglycoside-modifying enzymes

Numerous novel beta-lactamases and aminoglycoside-modifying enzymes with altered substrate profiles continue to be identified. Plasmid-mediated transmission of many of these enzymes readily occurs due to inclusion of the encoding genes in mobile gene cassettes. Recent crystallographic determinations of the structures of metallo-beta-lactamases and aminoglycoside-modifying enzymes provide the opportunity for the rational design of inhibitors.

Important and emerging beta-lactamase-mediated resistances in hospital-based pathogens: the Amp C enzymes

Resistance to third-generation cephalosporins mediated by beta-lactamases is an increasing problem for clinical therapeutics. A wide range of Enterobacteriaceae produce these AmpC enzymes (Bush-Jacoby-Medeiros group 1), including Enterobacter spp., Citrobacter freundii, Morganella morganii, Providencia spp., and Serratia marcescens. Resistance via this mechanism has been shown to be statistically correlated with the use of some third-generation cephalosporins, and the infections caused by these stably derepressed enzyme-producing species seem to occur most frequently in the seriously ill. More recently the genes encoding this enzyme have been documented on plasmids capable of transfer into other species such as Klebsiella pneumoniae. Fourth-generation cephalosporins, with stability and low affinity for the Amp C beta-lactamases and the ability to penetrate rapidly into the periplasmic space of Gram-negative organisms, offer a viable alternative in the treatment of these infections or as empiric regimens. Furthermore, these compounds (example: cefpirome) possess greater potency against the frequently occurring Gram-positive cocci such as oxacillin-susceptible staphylococci and the streptococci (including some penicillin-resistant strains) as compared to previously used anti-pseudomonal cephalosporias, ceftazidime.