Emergence of resistance to imipenem in Pseudomonas aeruginosa

Levofloxacin-induced MexS mutation triggers imipenem-relebactam resistance in a KPC-producing Pseudomonas aeruginosa

OBJECTIVES

Imipenem-relebactam (IMR), a novel β-lactam/β-lactamase inhibitor combination, is recommended for infections caused by difficult-to-treat Pseudomonas aeruginosa. This study aimed to investigate the evolution trajectory of IMR resistance under the selection of levofloxacin in P. aeruginosa.

METHODS

Antimicrobial susceptibility testing, complete genome sequencing and gene manipulation experiments were performed. Quantitative reverse transcription PCR for specific genes and porin levels were detected. Evolution trajectory was simulated in vitro by induction assay.

RESULTS

P. aeruginosa HS347 and HS355 were isolated from abdominal drainage of two neighbouring patients (S and Z) undergoing surgery of colon carcinoma in Shanghai, China, with the latter patient having received levofloxacin. They were closely related ST16 strains, and both carried blaKPC-2 plasmids highly similar to those of P. aeruginosa endemic clones from Zhejiang province, where patient Z had received enteroscopy before this admission. Acquisition of resistance was observed for both IMR and fluoroquinolones in HS355, likely prompted by treatment with levofloxacin. The T274I substitution in MexS (putative oxidoreductase), upregulated efflux pump operon mexEF-oprN and decreased production of porin OprD leading to cross-resistance to fluoroquinolones and IMR, which was also verified by in vitro mutant selection under levofloxacin selection.

CONCLUSIONS

The emergence of a rare blaKPC-2-plasmid-bearing ST16 clone implies the horizonal spread and inter-regional dissemination of a high-risk plasmid-clone combination, representing a public health challenge. Levofloxacin exposure can select for mexS inactivating mutation, which in turn leads to IMR resistance phenotype, implicating the role of an unrelated, widely used antimicrobial agent in insidiously triggering the development of cross resistance to a latest β-lactam/β-lactamase inhibitor combination.

Upstream region of OprD mutations in imipenem-resistant and imipenem-sensitive Pseudomonas isolates

The current study was aimed at investigating the prevalence of the mutations upstream of the oprD coding region and its promoters among imipenem-resistant and sensitive Pseudomonas aeruginosa isolated from educational hospitals in Yazd City, Iran. All isolates were identified by the conventional biochemical tests. Then, the antibiotic resistance of these isolates was determined using the disk diffusion method according to the CLSI guidelines. Also, the E.test was performed to determine the minimum inhibitory concentrations (MIC) of imipenem. The mutations of this gene were recognized by the amplification of this region and subsequently sequenced. Sequencing of the genomic region upstream of oprD these regions were done in the 29 clinical strains. Statistical analysis was done by the statistical software SPSS-18. Seventy (77.7%) of isolates had MIC ≥ 16 and were resistant to imipenem. Mutations of the upstream of the oprD gene and its promoters were seen in 25 (86.2%) isolates and 4 isolates had no mutation. One isolate had a base substitution A→Cat nt 25 in the coding region and this isolate had a point mutation leading to an amino acid change at positions 9 (I→L). Our study results indicated that none of the strains had mutation in Shine-Dalgarno and the point mutations were the most common mutations upstream of the oprD coding region among P. aeruginosa isolates. Mutations were observed in imipenem-resistant isolates and it seems this mechanism is effective in resistance of isolates to imipenem and this confirmed that the indiscriminate use of antibiotic should be controlled.

Lipoxin A4 promotes reduction and antibiotic efficacy against Pseudomonas aeruginosa biofilm

Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic bacterium commonly found in wound infections and airways of cystic fibrosis patients. P. aeruginosa readily forms biofilms which can reduce the efficacy of antibiotics used to eradicate the pathogen. We have previously shown that a Specialized Pro-resolving Mediator (SPM), Lipoxin A4 (LxA4) is a quorum sensing inhibitor which can reduce P. aeruginosa virulence. In this study, we examined the direct actions of LxA4 and RvD2 on P. aeruginosa biofilm formation and virulence gene expression. The influence of LxA4 on antibiotic efficacy and the combined effects on biofilm formation were also investigated. LxA4 and RvD2 reduced P. aeruginosa biofilm formation and virulence gene expression. LxA4 increased ciprofloxacin inhibition on biofilm formation but did not affect ciprofloxacin's action on non-adherent bacteria. On the other hand, LxA4 increased bacterial killing action of imipenem but did not affect imipenem's action on biofilm. We also found that LxA4 can increase ciprofloxacin's bacterial killing ability in established biofilm. Together these results suggest that LxA4 has direct effects on P. aeruginosa biofilm formation and can increase antibiotic efficacy directly.

The 2018 Garrod Lecture: Preparing for the Black Swans of resistance

The need for governments to encourage antibiotic development is widely agreed, with 'market entry rewards' being suggested. Unless these are to be spread widely-which is unlikely given the $1 billion sums proposed-we should be wary, for this approach is likely to evolve into one of picking, or commissioning, a few 'winners' based on extrapolation of current resistance trends. The hazard to this is that whilst the evolution of resistance has predictable components, notably mutation, it also has completely unpredictable ones, contingent upon 'Black Swan' events. These include the escape of 'new' resistance genes from environmental bacteria and the recruitment of these genes by promiscuous mobile elements and epidemic strains. Such events can change the resistance landscape rapidly and unexpectedly, as with the rise of Escherichia coli ST131 with CTX-M ESBLs and the emergence of 'impossible' VRE. Given such unpredictability, we simply cannot say with any certainty, for example, which of the four current approaches to combating MBLs offers the best prospect of sustainable prizeworthy success. Only time will tell, though it is encouraging that multiple potential approaches to overcoming these problematic enzymes are being pursued. Rather than seeking to pick winners, governments should aim to reduce development barriers, as with recent relaxation of trial regulations. In particular, once β-lactamase inhibitors have been successfully trialled with one partner drug, there is scope to facilitate licensing them for partnering with other established β-lactams, thereby insuring against new emerging resistance.

Comparative Genomic Analyses Reveal Core-Genome-Wide Genes Under Positive Selection and Major Regulatory Hubs in Outlier Strains of Pseudomonas aeruginosa

Genomic information for outlier strains of Pseudomonas aeruginosa is exiguous when compared with classical strains. We sequenced and constructed the complete genome of an environmental strain CR1 of P. aeruginosa and performed the comparative genomic analysis. It clustered with the outlier group, hence we scaled up the analyses to understand the differences in environmental and clinical outlier strains. We identified eight new regions of genomic plasticity and a plasmid pCR1 with a VirB/D4 complex followed by trimeric auto-transporter that can induce virulence phenotype in the genome of strain CR1. Virulence genotype analysis revealed that strain CR1 lacked hemolytic phospholipase C and D, three genes for LPS biosynthesis and had reduced antibiotic resistance genes when compared with clinical strains. Genes belonging to proteases, bacterial exporters and DNA stabilization were found to be under strong positive selection, thus facilitating pathogenicity and survival of the outliers. The outliers had the complete operon for the production of vibrioferrin, a siderophore present in plant growth promoting bacteria. The competence to acquire multidrug resistance and new virulence factors makes these strains a potential threat. However, we identified major regulatory hubs that can be used as drug targets against both the classical and outlier groups.

Harnessing Metabolic Regulation to Increase Hfq-Dependent Antibiotic Susceptibility in Pseudomonas aeruginosa

The opportunistic human pathogen Pseudomonas aeruginosa is responsible for ~ 10% of hospital-acquired infections worldwide. It is notorious for its high level resistance toward many antibiotics, and the number of multi-drug resistant clinical isolates is steadily increasing. A better understanding of the molecular mechanisms underlying drug resistance is crucial for the development of novel antimicrobials and alternative strategies such as enhanced sensitization of bacteria to antibiotics in use. In P. aeruginosa several uptake channels for amino-acids and carbon sources can serve simultaneously as entry ports for antibiotics. The respective genes are often controlled by carbon catabolite repression (CCR). We have recently shown that Hfq in concert with Crc acts as a translational repressor during CCR. This function is counteracted by the regulatory RNA CrcZ, which functions as a decoy to abrogate Hfq-mediated translational repression of catabolic genes. Here, we report an increased susceptibility of P. aeruginosa hfq deletion strains to different classes of antibiotics. Transcriptome analyses indicated that Hfq impacts on different mechanisms known to be involved in antibiotic susceptibility, viz import and efflux, energy metabolism, cell wall and LPS composition as well as on the c-di-GMP levels. Furthermore, we show that sequestration of Hfq by CrcZ, which was over-produced or induced by non-preferred carbon-sources, enhances the sensitivity toward antibiotics. Thus, controlled synthesis of CrcZ could provide a means to (re)sensitize P. aeruginosa to different classes of antibiotics.

Characterization of a Carbapenem-Hydrolyzing Enzyme, PoxB, in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa is an opportunistic pathogen often associated with severe and life-threatening infections that are highly impervious to treatment. This microbe readily exhibits intrinsic and acquired resistance to varied antimicrobial drugs. Resistance to penicillin-like compounds is commonplace and provided by the chromosomal AmpC β-lactamase. A second, chromosomally encoded β-lactamase, PoxB, has previously been reported in P. aeruginosa. In the present work, the contribution of this class D enzyme was investigated using a series of clean in-frame ampC, poxB, and oprD deletions, as well as complementation by expression under the control of an inducible promoter. While poxB deletions failed to alter β-lactam sensitivities, expression of poxB in ampC-deficient backgrounds decreased susceptibility to both meropenem and doripenem but had no effect on imipenem, penicillin, and cephalosporin MICs. However, when expressed in an ampCpoxB-deficient background, that additionally lacked the outer membrane porin-encoding gene oprD, PoxB significantly increased the imipenem as well as the meropenem and doripenem MICs. Like other class D carbapenem-hydrolyzing β-lactamases, PoxB was only poorly inhibited by class A enzyme inhibitors, but a novel non-β-lactam compound, avibactam, was a slightly better inhibitor of PoxB activity. In vitro susceptibility testing with a clinical concentration of avibactam, however, failed to reduce PoxB activity against the carbapenems. In addition, poxB was found to be cotranscribed with an upstream open reading frame, poxA, which itself was shown to encode a 32-kDa protein of yet unknown function.

Role of the Outer Membrane Protein OprD2 in Carbapenem-Resistance Mechanisms of Pseudomonas aeruginosa

We investigated the relationship between the outer membrane protein OprD₂ and carbapenem-resistance in 141 clinical isolates of Pseudomonas aeruginosa collected between January and December 2013 from the First Affiliated Hospital of Anhui Medical University in China. Agar dilution methods were employed to determine the minimum inhibitory concentration of meropenem (MEM) and imipenem (IMP) for P. aeruginosa. The gene encoding OprD₂ was amplified from141 P. aeruginosa isolates and analyzed by PCR and DNA sequencing. Differences between the effects of IMP^R and IMP^S groups on the resistance of the P. aeruginosa were observed by SDS-poly acrylamide gel electrophoresis (SDS-PAGE). Three resistance types were classified in the 141 carbapenem-resistant P. aeruginosa (CRPA) isolates tested, namely IMP^RMEM^R (66.7%), IMP^RMEM^S (32.6%), and IMP^RMEM^S (0.7%). DNA sequencing revealed significant diverse gene mutations in the OprD₂-encoding gene in these strains. Thirty-four strains had large fragment deletions in the OprD₂gene, in 6 strains the gene contained fragment inserts, and in 96 resistant strains, the gene featured small fragment deletions or multi-site mutations. Only 4 metallo-β-lactamase strains and 1 imipenem-sensitive (meropenem-resistant) strain showed a normal OprD₂ gene. Using SDS-PAGE to detect the outer membrane protein in 16 CRPA isolates, it was found that 10 IMP^RMEM^R strains and 5 IMP^RMEM^S strains had lost the OprD₂ protein, while the IMP^SMEM^R strain contained a normal 46-kDa protein. In conclusion, mutation or loss of the OprD₂-encoding gene caused the loss of OprD₂, which further led to carbapenem-resistance of P. aeruginosa. Our findings provide insights into the mechanism of carbapenem resistance in P. aeruginosa.

The resistome of Pseudomonas aeruginosa in relationship to phenotypic susceptibility

Many clinical isolates of Pseudomonas aeruginosa cause infections that are difficult to eradicate due to their resistance to a wide variety of antibiotics. Key genetic determinants of resistance were identified through genome sequences of 390 clinical isolates of P. aeruginosa, obtained from diverse geographic locations collected between 2003 and 2012 and were related to microbiological susceptibility data for meropenem, levofloxacin, and amikacin. β-Lactamases and integron cassette arrangements were enriched in the established multidrug-resistant lineages of sequence types ST111 (predominantly O12) and ST235 (O11). This study demonstrates the utility of next-generation sequencing (NGS) in defining relevant resistance elements and highlights the diversity of resistance determinants within P. aeruginosa. This information is valuable in furthering the design of diagnostics and therapeutics for the treatment of P. aeruginosa infections.

Investigating the link between imipenem resistance and biofilm formation by Pseudomonas aeruginosa

Pseudomonas aeruginosa, a ubiquitous environmental organism, is a difficult-to-treat opportunistic pathogen due to its broad-spectrum antibiotic resistance and its ability to form biofilms. In this study, we investigate the link between resistance to a clinically important antibiotic, imipenem, and biofilm formation. First, we observed that the laboratory strain P. aeruginosa PAO1 carrying a mutation in the oprD gene, which confers resistance to imipenem, showed a modest reduction in biofilm formation. We also observed an inverse relationship between imipenem resistance and biofilm formation for imipenem-resistant strains selected in vitro, as well as for clinical isolates. We identified two clinical isolates of P. aeruginosa from the sputum of cystic fibrosis patients that formed robust biofilms, but were sensitive to imipenem (MIC ≤ 2 μg/ml). To test the hypothesis that there is a general link between imipenem resistance and biofilm formation, we performed transposon mutagenesis of these two clinical strains to identify mutants defective in biofilm formation, and then tested these mutants for imipenem resistance. Analysis of the transposon mutants revealed a role for previously described biofilm factors in these clinical isolates of P. aeruginosa, including mutations in the pilY1, pilX, pilW, algC, and pslI genes, but none of the biofilm-deficient mutants became imipenem resistant (MIC ≥ 8 μg/ml), arguing against a general link between biofilm formation and resistance to imipenem. Thus, assessing biofilm formation capabilities of environmental isolates is unlikely to serve as a good predictor of imipenem resistance. We also discuss our findings in light of the limited literature addressing planktonic antibiotic resistance factors that impact biofilm formation.

Adaptive and mutational resistance: role of porins and efflux pumps in drug resistance

The substantial use of antibiotics in the clinic, combined with a dearth of new antibiotic classes, has led to a gradual increase in the resistance of bacterial pathogens to these compounds. Among the various mechanisms by which bacteria endure the action of antibiotics, those affecting influx and efflux are of particular importance, as they limit the interaction of the drug with its intracellular targets and, consequently, its deleterious effects on the cell. This review evaluates the impact of porins and efflux pumps on two major types of resistance, namely, mutational and adaptive types of resistance, both of which are regarded as key phenomena in the global rise of antibiotic resistance among pathogenic microorganisms. In particular, we explain how adaptive and mutational events can dramatically influence the outcome of antibiotic therapy by altering the mechanisms of influx and efflux of antibiotics. The identification of porins and pumps as major resistance markers has opened new possibilities for the development of novel therapeutic strategies directed specifically against these mechanisms.

Structure and function of OprD protein in Pseudomonas aeruginosa: from antibiotic resistance to novel therapies

Pseudomonas aeruginosa (P. aeruginosa) is a common pathogen isolated from patients with nosocomial infections. Due to its intrinsic and acquired antimicrobial resistance, limited classes of antibiotics can be used for the treatment of infection with P. aeruginosa. Of these, the carbapenems are very important; however, the occurrence of carbapenem-resistant strains is gradually increasing over time. Deficiency of the outer membrane protein OprD confers P. aeruginosa a basal level of resistance to carbapenems, especially to imipenem. Functional studies have revealed that loops 2 and 3 in the OprD protein contain the entrance and/or binding sites for imipenem. Therefore, any mutation in loop 2 and/or loop 3 that causes conformational changes could result in carbapenem resistance. OprD is also a common channel for some amino acids and peptides, and competition with carbapenems through the channel may also occur. Furthermore, OprD is a highly regulated protein at transcriptional and post-transcriptional levels by some metals, small bioactive molecules, amino acids, and efflux pump regulators. Because of its hypermutability and highly regulated properties, OprD is thought to be the most prevalent mechanism for carbapenem resistance in P. aeruginosa. Developing new strategies to combat infection with carbapenem-resistant P. aeruginosa lacking OprD is an ongoing challenge.

Biochemical and structural characterization of Mycobacterium tuberculosis beta-lactamase with the carbapenems ertapenem and doripenem

Despite the enormous success of beta-lactams as broad-spectrum antibacterials, they have never been widely used for the treatment of tuberculosis (TB) due to intrinsic resistance that is caused by the presence of a chromosomally encoded gene (blaC) in Mycobacterium tuberculosis. Our previous studies of TB BlaC revealed that this enzyme is an extremely broad-spectrum beta-lactamase hydrolyzing all beta-lactam classes. Carbapenems are slow substrates that acylate the enzyme but are only slowly deacylated and can therefore act also as potent inhibitors of BlaC. We conducted the in vitro characterization of doripenem and ertapenem with BlaC. A steady-state kinetic burst was observed with both compounds with magnitudes proportional to the concentration of BlaC used. The results provide apparent K(m) and k(cat) values of 0.18 microM and 0.016 min(-1) for doripenem and 0.18 microM and 0.017 min(-1) for ertapenem, respectively. FTICR mass spectrometry demonstrated that the doripenem and ertapenem acyl-enzyme complexes remain stable over a time period of 90 min. The BlaC-doripenem covalent complex obtained after a 90 min soak was determined to 2.2 A, while the BlaC-ertapenem complex obtained after a 90 min soak was determined to 2.0 A. The 1.3 A diffraction data from a 10 min ertapenem-soaked crystal revealed an isomerization occurring in the BlaC-ertapenem adduct in which the original Delta(2)-pyrroline ring was tautomerized to generate the Delta(1)-pyrroline ring. The isomerization leads to the flipping of the carbapenem hydroxyethyl group to hydrogen bond to carboxyl O2 of Glu166. The hydroxyethyl flip results in both the decreased basicity of Glu166 and a significant increase in the distance between carboxyl O2 of Glu166 and the catalytic water molecule, slowing hydrolysis.

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.

The use of carbapenems in the treatment of serious infections

Inadequate initial antimicrobial treatment in serious infections leads to increased mortality. Achieving adequate treatment is increasingly difficult because of the increasing prevalence of multidrug-resistant (MDR) pathogens. The carbapenems are potent, broad-spectrum antibiotics that have been shown to be safe and efficacious therapies in the treatment of serious infections. This review is intended to compare the 4 major members of the carbapenem class, which include imipenem, meropenem, ertapenem, and doripenem, with other widely used antimicrobial agents in the intensive care unit (ICU). The carbapenems are potent, broad-spectrum antibiotics that have been shown to be safe and efficacious therapies in the treatment of serious infections. They provide better gram-negative coverage than other beta-lactams and are stable against extended-spectrum beta-lactamases and AmpC beta-lactamases, making them effective in the treatment of many MDR bacteria. The newly approved carbapenem, doripenem, may help preserve the utility of the carbapenem class.

Detection of small RNAs in Pseudomonas aeruginosa by RNomics and structure-based bioinformatic tools

Inactivation of the Pseudomonas aeruginosa (PAO1) hfq gene, encoding the Sm-like Hfq protein, resulted in pleiotropic effects that included an attenuated virulence. As regulation by Hfq often involves the action of small regulatory RNAs (sRNAs), we have used a shotgun cloning approach (RNomics) and bioinformatic tools to identify sRNAs in strain PAO1. For cDNA library construction, total RNA was extracted from PAO1 cultures either grown to stationary phase or exposed to human serum. The cDNA libraries were generated from small-sized RNAs of PAO1 after co-immunoprecipitation with Hfq. Of 400 sequenced cDNA clones, 11 mapped to intergenic regions. Band-shift assays and Northern blot analyses performed with two selected sRNAs confirmed that Hfq binds to and affects the steady-state levels of these RNAs. A proteome study performed upon overproduction of one sRNA, PhrS, implicated it in riboregulation. PhrS contains an ORF, and evidence for its translation is presented. In addition, based on surveys with structure-based bioinformatic tools, we provide an electronic compilation of putative sRNA and non-coding RNA genes of PAO1 based on their evolutionarily conserved structure.

An important role of a "probable ATP-binding component of ABC transporter" during the process of Pseudomonas aeruginosa resistance to fluoroquinolone

In order to find new drug target to eliminate the fluoroquinolone resistance, the in vitro progress of Pseudomonas aeruginosa fluoroquinolone resistance was mimicked, and then proteomic analysis was applied to comparing different protein profiles during the resistant process. The results show that the expression of a "probable ATP-binding component of ATP binding cassette (ABC) transporter" existed in ciprofloxacin-intermediate and -resistant strains, but not in sensitive strain. In addition, the ciprofloxacin concentrations in P. aeruginosa strains, which were obtained from the progress of P. aeruginosa fluoroquinolone resistance, were determined by means of HPLC; the results show that the decrease of the intracellular concentration of drug and the expression of this new protein nearly take place simultaneously. The changes of mRNA levels of the probable ATP-binding component of ABC transporter were detected by virtue of RT-PCR and showed that this protein did not express in the sensitive strains but expressed increasingly in the intermediate and resistant strains. In order to determine the relationships between the development of antibiotic resistance and this protein further, a DNAzyme was designed to aim at the mRNA of the probable ATP-binding component of ABC transporter directly; the ciprofloxacin resistance of P. aeruginosa was partially reduced in vivo by inhibiting the expression of this protein. This DNAzyme has no effect on sensitive strain. And the comparison of drug intracellular concentrations between DNAzyme-treated strains and its control strains shows that this protein may be included in the course of active drug efflux.

Potency of Carbapenems for the Prevention of Carbapenem-Resistant Mutants of Pseudomonas aeruginosa

The potencies of the carbapenems; doripenem (DRPM), meropenem (MEPM) and imipenem (IPM) in preventing the emergence of carbapenem-resistant mutants were examined in Pseudomonas aeruginosa strains. The carbapenems predominantly selected carbapenem-resistant mutants or carbapenem mutants with reduced susceptibilities that were specifically resistant to carbapenems and had arisen as a result of the reduced level of expression of the outer membrane protein with a molecular weight of about 48,000 (OprD). The potency of carbapenems in preventing the growth of the mutants differed for DRPM, MEPM and IPM. The isolation frequency of the mutant was examined on agar plates containing each of the carbapenems at a concentration of 1/2 or 1/4 MIC of each carbapenem for that mutant. Mutants were not selected on agar containing DRPM at a frequency of greater than 10(-9) per cell per generation, whereas mutants of each strain were selected on agar containing MEPM or IPM at frequencies of 10(-7) to 10(-9) per cell per generation. The drug concentrations and the drug concentration range for the selective increase of carbapenem resistant mutants in the broth culture containing each carbapenem differed for each carbapenem. DRPM exhibited both the lowest drug concentration and the narrowest range of drug concentration for selection of the carbapenem-resistant mutants. The results shown in this report indicated that DRPM exhibited the greatest ability to prevent the emergence of the mutant.

The risk factors for acquisition of imipenem-resistant Pseudomonas aeruginosa in the burn unit

OBJECTIVE

This study was conducted to determine the risk factors for acquisition of imipenem-resistant Pseudomonas aeruginosa (IRPA) in the burn unit.

METHODS

Patients hospitalized in the burn unit from July 2003 to November 2004 were included in this study. The features of patients with isolated IRPA were compared with those of patients with isolated imipenem-susceptible P. aeruginosa (ISPA). Demographic features, total burn surface area (TBSA), burn depth, antimicrobials used in 15-day period before isolation, and presence of IRPA in the unit at the same period were included in the risk factors analysis.

RESULTS

P. aeruginosa was recovered from 133 patients in this period, 93 were IRPA and 40 were ISPA. There was no significant difference between patients with ISPA and patients with IRPA in terms of age, TBSA, and burn depth. In multivariate logistic regression analysis, hospital stay before isolation (odds ratio (OR): 3.6), carbapenem usage (OR: 7.4), broad-spectra antibiotic usage (OR: 6.5), previous presence of ISPA in the patient (OR: 1.7) and presence of IRPA in the unit at the same period (OR: 2.6) were independent risk factors for acquisition of IRPA.

CONCLUSION

Long hospitalization time, previous imipenem/meropenem use, previous broad-spectra antibiotic use, previous presence of ISPA in a patient and presence of IRPA in a unit at the same period were associated with acquisition of IRPA in the burn unit. In order to decrease the incidence of IRPA isolation, the usage of broad-spectra antibiotics, especially carbapenem, should be restricted, hospitalization time should be shortened if possible, and universal isolation precautions should be strictly applied to prevention cross-contamination.

Comparison of β-lactams in counter-selecting resistance of Pseudomonas aeruginosa

Suppression of resistance in a dense population of Pseudomonas aeruginosa has been shown with optimized quinolone exposures. However, relevance to the beta-lactams is questionable because of the unknown impact of the inoculum effect. We explored the bactericidal activity of various beta-lactams and their propensity to suppress spontaneous resistance. Minimal killing and resistance selection was seen with piperacillin, most likely due to the inoculum effect. Moderate and significant killing was seen at 24 h with ceftazidime and meropenem, respectively; however, regrowth and selective resistance amplification was apparent at 48 h. Our results suggest that different beta-lactam subclasses have a distinct killing profile and propensity to suppress resistance against a dense population of P. aeruginosa. The pharmacodynamics of each subclass of agent should be examined individually, and investigations longer than 24 h may be necessary to fully evaluate the relationship between drug exposures and the likelihood of resistance suppression.

Risk factors for ICU-acquired imipenem-resistant Gram-negative bacterial infections

Intensive care units (ICUs) are high-risk areas for infections caused by antibiotic-resistant bacteria. This study investigated the risk factors for ICU-acquired imipenem-resistant Gram-negative infections. It was conducted prospectively in three surgical ICUs and one medical ICU from April to December 2002. ICU-acquired Gram-negative infections were found in 128 patients. Of these, 42 had imipenem-resistant and 86 had imipenem-sensitive Gram-negative bacteria as the cause of infection. According to the univariate analysis results, hospital stay before ICU admission, hospitalization period before ICU admission, length of ICU stay, surgical ICU stay, surgical operation and previous antibiotic use were significant risk factors for the acquisition of imipenem-resistant infections. In the multivariate analysis, length of ICU stay, surgical operation and previous carbapenem use were independently associated with imipenem resistance.

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.

Intrinsic resistance of Escherichia coli to mureidomycin A and C due to expression of the multidrug efflux system AcrAB-TolC: comparison with the efflux systems of mureidomycin-susceptible Pseudomonas aeruginosa

Intrinsic resistance to mureidomycin is shown in Escherichia coli. This is in contrast to Pseudomonas aeruginosa, which generally displays intrinsic resistance to a variety of antimicrobial agents, but not to mureidomycin. Isogenic efflux system mutants from both species were subjected to antibiotic susceptibility tests. These studies showed that the differences regarding the susceptibility of E. coli and P. aeruginosa to mureidomycin A and C may be explained by the expression of efflux systems that mediate resistance to mureidomycin A and C.

Function of pseudomonas porins in uptake and efflux

Porins are proteins that form water-filled channels across the outer membranes of Gram-negative bacteria and thus make this membrane semipermeable. There are four types of porins: general/nonspecific porins, substrate-specific porins, gated porins, and efflux porins (also called channel-tunnels). The recent publication of the genomic sequence of Pseudomonas aeruginosa PAO1 has dramatically increased our understanding of the porins of this organism. In particular this organism has 3 large families of porins: the OprD family of specific porins (19 members), the OprM family of efflux porins (18 members), and the TonB-interacting family of gated porins (35 members). These familial relationships underlie functional similarities such that well-studied members of these families become prototypes for other members. We summarize here the latest information on these porins.

Prevention of resistance: a goal for dose selection for antimicrobial agents

Drug-resistant microorganisms have become a major problem around the world. In nosocomial and community settings, many important pathogens have demonstrated high-grade resistance to many of our most important agents. In addition, the adverse impact of resistance has not been limited to the bacterial realm. In chemotherapy to treat human immunodeficiency virus (HIV) and other viral diseases, resistance has become a major problem. We are starting to see the beginnings of a resistance problem, even among fungi. Strangely, little attention has been focused on the impact of dosing on the probability with which emergence of resistance occurs. After delineation of the pharmacodynamically linked variable, it is possible to generate dosing regimens that can lower the probability of resistance. In addition, circumstances exist in which combination therapy may be required (e.g., therapy of HIV and tuberculosis). Here, too, it is possible to optimize therapy to prevent resistance by understanding how the drugs in the regimen interact. We can do better with our choices of dose, schedule, and combinations of agents. We will need to lower the probability of resistance and maintain the utility of the drugs currently in our therapeutic armamentarium.

Metallo-beta-lactamase VIM-2 in clinical isolates of Pseudomonas aeruginosa from Portugal

Resistance to carbapenems is emerging, and it is a great problem to therapeutics. Three isolates of Pseudomonas aeruginosa from a Portuguese hospital identified in urine and sputum, in 1995, presented a high-level resistance to imipenem (> 32 mg/L). Afterward, one isolate of P. aeruginosa recovered from urine of an ambulatory patient in 1998 showed high resistance to imipenem and meropenem. The resistance to carbapenems in these strains was associated with the production of a class B beta-lactamase, as was demonstrated by imipenem hydrolysis and inhibition by EDTA. Using primers described for bla(IMP) and bla(VIM), the amplification of the latter was observed in all isolates and a VIM-2 metallo-enzyme was identified. The pulsed-field gel electrophoresis (PFGE) patterns of these isolates were indistinguishable, suggesting dissemination to the community of this VIM-2 producer.

Analysis of the Pseudomonas aeruginosa oprD gene from clinical and environmental isolates

Genomes are constantly evolving. Our report highlights the wide mutational diversity of clinical as well as environmental isolates, compared with the laboratory strain(s), through the systematic genetic analysis of a chromosomal porin gene (oprD) in relation to a specific antibiotic resistance. Mutational inactivation of the oprD gene is associated with carbapenem resistance in Pseudomonas aeruginosa. The sequence of the oprD gene of 55 Pseudomonas aeruginosa natural isolates obtained from across the world--from sources as diverse as patients and rhizospheres--was analysed. A microscale mosaic structure for this gene--resulting from multiple intra- and possibly interspecies recombinational events--is reported. An array of independent and seemingly fast-occurring defective oprD mutations were found, none of which had been described before. A burn wound isolate demonstrated unusually high overall sequence variability typical of mutator strains. We also present evidence for the existence of OprD homologues in other fluorescent pseudomonads.

Plasma pharmacokinetics and tissue fluid concentrations of meropenem after intravenous and subcutaneous administration in dogs

OBJECTIVE

To estimate pharmacokinetic variables and measure tissue fluid concentrations of meropenem after IV and SC administration in dogs.

ANIMALS

6 healthy adult dogs.

PROCEDURE

Dogs were administered a single dose of meropenem (20 mg/kg) IV and SC in a crossover design. To characterize the distribution of meropenem in dogs and to evaluate a unique tissue fluid collection method, an in vivo ultrafiltration device was used to collect interstitial fluid. Plasma, tissue fluid, and urine samples were analyzed by use of high-performance liquid chromatography. Protein binding was determined by use of an ultrafiltration device.

RESULTS

Plasma data were analyzed by compartmental and noncompartmental pharmacokinetic methods. Mean +/- SD values for half-life, volume of distribution, and clearance after IV administration for plasma samples were 0.67 +/- 0.07 hours, 0.372 +/- 0.053 L/kg, and 6.53 +/- 1.51 mL/min/kg, respectively, and half-life for tissue fluid samples was 1.15 +/- 0.57 hours. Half-life after SC administration was 0.98 +/- 0.21 and 1.31 +/- 0.54 hours for plasma and tissue fluid, respectively. Protein binding was 11.87%, and bioavailability after SC administration was 84%.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of our data revealed that tissue fluid and plasma (unbound fraction) concentrations were similar. Because of the kinetic similarity of meropenem in the extravascular and vascular spaces, tissue fluid concentrations can be predicted from plasma concentrations. We concluded that a dosage of 8 mg/kg, SC, every 12 hours would achieve adequate tissue fluid and urine concentrations for susceptible bacteria with a minimum inhibitory concentration of 0.12 microg/mL.

C-terminal region of Pseudomonas aeruginosa outer membrane porin OprD modulates susceptibility to meropenem

We investigated the unusual susceptibility to meropenem observed for seven imipenem-resistant clinical isolates of Pseudomonas aeruginosa. These strains were genetically closely related, expressed OprD, as determined by Western blot analyses, and were resistant to imipenem (>5 microg/ml) but susceptible to meropenem (<1 microg/ml). The oprD genes from two isolates were entirely sequenced, and their deduced protein sequences showed 93% identity with that of OprD of strain PAO1. The major alteration consisted of the replacement of a stretch of 12 amino acids, located in putative external loop L7 of OprD, by a divergent sequence of 10 amino acid residues. The oprD gene variants and the wild-type oprD gene were cloned and expressed in a defined oprD mutant. The meropenem MICs for strains carrying the oprD genes from clinical isolates were four times lower than that for the strain carrying the wild-type oprD gene. Imipenem activities, however, were comparable for all strains. Furthermore, meropenem hypersusceptibility was obtained with a hybrid OprD porin that consisted of the PAO1 oprD gene containing loop L7 from a clinical isolate. These results show that the C-terminal portion of OprD, in particular, loop L7, was responsible for the unusual meropenem hypersusceptibility. Competition experiments suggested that the observed OprD modifications in the clinical isolates did not affect antagonism between imipenem and the basic amino acid L-lysine. We further propose that shortening of putative loop L7 of the OprD porin by 2 amino acid residues sufficiently opens the porin channel to allow optimal penetration of meropenem and increase its activity. In contrast, this alteration would not affect susceptibility to a smaller carbapenem molecule, such as imipenem.

Role of putative loops 2 and 3 in imipenem passage through the specific porin OprD of Pseudomonas aeruginosa

Mutant proteins with eight amino acid deletions in putative surface loops 2 and 3 of the imipenem-specific porin OprD of Pseudomonas aeruginosa failed to reconstitute imipenem susceptibility in an oprD-deficient background. The loop 3 deletion prevented the ability of imipenem to inhibit KCl conductance through the OprD channel, as previously shown for a loop 2 deletion. This suggests that both loops 2 and 3 have a role in imipenem binding to the OprD channel.

Negative regulation of the Pseudomonas aeruginosa outer membrane porin OprD selective for imipenem and basic amino acids

Pseudomonas aeruginosa OprD is a specific porin which facilitates the uptake of basic amino acids and imipenem, a carbapenem antibiotic. Resistance to imipenem due to the loss of OprD is an important mechanism for the loss of clinical effectiveness. To investigate the negative regulatory mechanisms influencing oprD expression, a gene upstream of the coregulated mexEF-oprN efflux operon, designated mexT, was cloned. The predicted 304-amino-acid mature MexT protein showed strong homology to LysR-type regulators. When overexpressed it induced the expression of the mexEF-oprN efflux operon while decreasing the level of expression of OprD. The use of an oprD::xylE transcriptional fusion indicated that it acted by repressing the transcription of oprD. Salicylate, a weak aromatic acid known to reduce porin expression and induce low levels of multiple antibiotic resistance in Escherichia coli, was able to induce imipenem resistance and reduce the expression of OprD but not multiple antibiotic resistance or OprN expression in P. aeruginosa. This was also demonstrated to occur at the level of transcription. Acetyl salicylate and benzoate, but not catechol, were also able to reduce the levels of OprD in the P. aeruginosa outer membranes. These OprD-suppressing compounds increased imipenem resistance even in a mexT-overexpressing and nfxC mutant backgrounds, suggesting that such resistance is independent of the MexT repressor and that oprD is influenced by more than a single mechanism of repression.

Protein D2 porin of the Pseudomonas aeruginosa outer membrane bears the protease activity

We report here our discovery that protein D2 of the outer membrane of Pseudomonas aeruginosa is a novel porin bearing protease activity. Homogeneously purified protein D2 hydrolyzed several synthetic peptides according to the Michaelis-Menten kinetics. A specific serine protease inhibitor, diisopropyl fluorophosphate (DFP), inactivated the protease activity and [3H]DFP covalently labeled protein D2. We tested the effect of two monoclonal antibodies raised against protein D2 on the protease activity. One antibody lowered the protease activity to about 20%, while the other enhanced it to about 300% of that without antibody. In addition, the fractions derived from the outer membrane of the protein D2-deficient mutants showed negligible protease activity, whereas similarly fractionated outer membrane proteins of the protein D2-positive parent strain showed strong protease activity.

Role of mutations in DNA gyrase genes in ciprofloxacin resistance of Pseudomonas aeruginosa susceptible or resistant to imipenem

In Pseudomonas aeruginosa, resistance to imipenem is mainly related to a lack of protein OprD and resistance to fluoroquinolones is mainly related to alterations in DNA gyrase. However, strains cross resistant to fluoroquinolones and imipenem have been selected in vitro and in vivo with fluoroquinolones. We investigated the mechanisms of resistance to fluoroquinolones in 30 clinical strains of P. aeruginosa resistant to ciprofloxacin (mean MIC, >8 micrograms/ml), 20 of which were also resistant to imipenem (mean MIC, >16 micrograms/ml). By immunoblotting, OprD levels were markedly decreased in all of the imipenem-resistant strains. Plasmids carrying the wild-type gyrA gene (pPAW207) or gyrB gene (pPBW801) of Escherichia coli were introduced into each strain by transformation. MICs of imipenem did not change after transformation, whereas those of ciprofloxacin and sparfloxacin dramatically decreased (25- to 70-fold) for all of the strains. For 28 of them (8 susceptible and 20 resistant to imipenem), complementation was obtained with pPAW207 but not with pPBW801. After complementation, the geometric mean MICs of ciprofloxacin and sparfloxacin (MICs of 0.3 microgram/ml and 0.5 microgram/ml, respectively) were as low as those for wild-type strains. Complementation was obtained only with pPBW801 for one strain and with pPAW207 and pPBW801 for one strain highly resistant to fluoroquinolones. These results demonstrate that in clinical practice, gyrA mutations are the major mechanism of resistance to fluoroquinolones even in the strains of P. aeruginosa resistant to imipenem and lacking OprD, concomitant resistance to these drugs being the result of the addition of at least two independent mechanisms.

Activity of carbapenem BMS-181139 against Pseudomonas aeruginosa is not dependent on porin protein D2

The broad antipseudomonal spectrum of the carbapenem BMS-181139 includes clinical strains and laboratory mutants of Pseudomonas aeruginosa that are resistant to imipenem. Unlike other known carbapenems (meropenem, panipenem, biapenem, and BO-2727), which have reduced activity against imipenem-resistant strains of P. aeruginosa, BMS-181139 was equally active against imipenem-susceptible (D2-sufficient) and imipenem-resistant (D2-deficient) strains. Conversely, imipenem and meropenem activities were the same against the susceptible parental strains and their BMS-181139-resistant mutants. Whereas basic amino acids antagonized the antipseudomonal activities of imipenem and meropenem, they had no effect on the activity of BMS-181139. These results suggest that the uptake of BMS-181139 into pseudomonal cells occurs by a non-D2 pathway. Compared with imipenem and meropenem, BMS-181139 may have a slightly higher affinity for penicillin-binding protein 2 (PBP-2) of P. aeruginosa. The rates of resistance development to imipenem, meropenem, and BMS-181139 in P. aeruginosa strains were similar; resistance occurred at frequencies of approximately 10(-7) to 10(-8). Resistance to BMS-181139 in P. aeruginosa is presumed to be caused by its diminished permeability since no change in their penicillin-binding protein affinities or beta-lactamase levels could be detected. In summary, BMS-181139 is a new carbapenem which differs from other known carbapenems in its lack of cross-resistance with imipenem. This difference could be explained by the permeation of BMS-181139 through a non-D2 channel, compared to the preferential uptake of other carbapenems by the D2 porin.

Cloning and expression in Enterobacteriaceae of the extended-spectrum beta-lactamase gene from a Pseudomonas aeruginosa plasmid

An extended-spectrum beta-lactamase, the gene for which is located on plasmid pMS350 in Pseudomonas aeruginosa strains, hydrolyzes carbapenems and other extended-spectrum beta-lactam antibiotics. We cloned the pMS350 beta-lactamase gene in an Escherichia coli K-12 strain using the vector plasmid pHSG398, and subcloned it into pMS360, a plasmid with a wide host-range. This resulted in the formation of the recombinant plasmid, pMS363, containing a 4.1-kb DNA insert that includes the extended-spectrum beta-lactamase gene. Plasmid pMS363 was introduced into the P. aeruginosa PAO strain or into six species of Enterobacteriaceae, and the specific activities of the beta-lactamase and MICs of various beta-lactam antibiotics were estimated. The cloned gene was capable of expression in these strains and caused resistance to carbapenem, penem and other beta-lactam antibiotics, with the exception of aztreonam.

Transient carbapenem resistance induced by salicylate in Pseudomonas aeruginosa associated with suppression of outer membrane protein D2 synthesis

Pseudomonas aeruginosa PAO1 showed increased phenotypic resistance to imipenem, panipenem, and biapenem specifically in the presence of salicylate. The antipseudomonal activity of carbapenems was reduced in proportion to the concentration of salicylate. This resistance was transient and nonheritable. The synthesis of the outer membrane protein D2 (OprD or OprD2) in P. aeruginosa PAO1 was inhibited by 4 to 32 mM salicylate in the bacterial growth medium, whereas no changes in any other outer membrane proteins were observed. These results indicate that salicylate suppresses the synthesis of OprD and therefore reduces the antipseudomonal activity of carbapenems. Under these conditions, one carbapenem--meropenem--is still active against P. aeruginosa, which indicates that meropenem can pass through the outer membrane via both the D2 channel and another undefined route(s).

Genetic definition of the substrate selectivity of outer membrane porin protein OprD of Pseudomonas aeruginosa

Earlier studies proved that Pseudomonas aeruginosa OprD is a specific porin for basic amino acids and imipenem. It was also considered to function as a nonspecific porin that allowed the size-dependent uptake of monosaccharides and facilitation of the uptake of quinolone and other antibiotics. In the present study, we utilized P. aeruginosa strains with genetically defined levels of OprD to characterize the in vivo substrate selectivity of this porin. An oprD::omega interposon mutant was constructed by gene replacement utilizing an in vitro mutagenized cloned oprD gene. In addition, OprD was overexpressed from the lac promoter by cloning the oprD gene into the broad-host-range plasmid pUCP19. To test the substrate selectivity, strains were grown in minimal medium with limiting concentrations of the carbon sources glucose, gluconate, or pyruvate. In minimal medium with 0.5 mM gluconate, the growth rates of the parent strain H103 and its oprD::omega mutant H729 were only 60 and 20%, respectively, of that of the OprD-overexpressing strain H103(pXH2). In contrast, no significant differences were observed in the growth rates of these three strains on glucose or pyruvate, indicating that OprD selectively facilitated the transport of gluconate. To determine the role of OprD in antibiotic uptake, nine strains representing different levels of OprD and OprF were used to determine the MICs of different antibiotics. The results clearly demonstrated that OprD could be utilized by imipenem and meropenem but that, even when substantially overexpressed, it could not be significantly utilized by other beta-lactams, quinolones, or aminoglycosides. In addition, competition experiments confirmed that imipenem had common binding sites with basic amino acids in the OprD channel, but not with gluconate or glucose.

Pharmacodynamic effects of extended dosing intervals of imipenem alone and in combination with amikacin against Pseudomonas aeruginosa in an in vitro model

The pharmacodynamic effects of extended imipenem dosing intervals were studied against two strains of Pseudomonas aeruginosa (ATCC 27853 and an imipenem-resistant mutant, 27853R) in an in vitro model of infection. Imipenem was administered as monotherapy (simulated 1-g bolus every 8 or every 12 h) and in combination with amikacin (7.5-mg/kg bolus every 12 h or a 15-mg/kg bolus once). Monotherapy with imipenem administered every 8 h was equally bactericidal at 24 h compared with regimens combined with amikacin for ATCC 27853. Imipenem administered every 12 h against the sensitive strain and both imipenem monotherapy regimens against the resistant strain demonstrated regrowth at 24 h. Although both amikacin regimens administered as monotherapy resulted in rapid bacterial killing activity with respect to time to a 99.9% reduction in log10 CFU/milliliter, regrowth at 24 h was observed at levels reaching or exceeding the initial inoculum. All combination regimens resulted in no detectable growth by 24 h regardless of dosing interval for either drug or initial susceptibility to imipenem. Results from this study indicate the potential for several novel dosing regimens against P. aeruginosa. Monotherapy with imipenem, 1 g every 8 h, was effective against a sensitive strain of P. aeruginosa. Combination therapy with imipenem and once-daily or twice-daily amikacin resulted in increased killing activity against imipenem-resistant P. aeruginosa. Once-daily or twice-daily amikacin in combination therapy, regardless of P. aeruginosa susceptibility, allowed for extension of imipenem dosing intervals.

Susceptibility of Pseudomonas species to the novel antibiotics mureidomycins

Strains of Pseudomonas aeruginosa, including imipenem- or ofloxacin-resistant clinical isolates, and some other species in the genus Pseudomonas were inhibited by novel antibiotics of the mureidomycin (MRD) group. On the other hand, almost all other gram-positive and gram-negative bacteria were resistant to MRDs, though the antibiotics potently inhibited the in vitro peptidoglycan synthesis of Escherichia coli and P. aeruginosa. All of the strains in the genus Pseudomonas that were inhibited by less than or equal to 200 micrograms of MRDs per ml were classified into the rRNA groups I and III, and none of the tested strains of rRNA group I were resistant to MRDs, suggesting that these two groups are closely related to each other evolutionary. Among group I strains, P. aeruginosa, P. mendocina, P. stutzeri, and P. alcaligenes were more susceptible than the others, suggesting a closer relationship among these species.

Addition of rifampin to combination antibiotic therapy for Pseudomonas aeruginosa bacteremia: prospective trial using the Zelen protocol

A multicenter, prospective randomized trial was conducted to determine if the addition of rifampin to a combination therapy of an antipseudomonal beta-lactam agent and aminoglycoside improves the outcome of patients with Pseudomonas aeruginosa bacteremia. The Zelen protocol for randomized-consent design was used. Consent was sought only from patients randomized to the experimental therapy (rifampin+). If the experimental therapy was refused, the patient would then receive the standard combination therapy (control); however, when outcome was evaluated, all patients randomized to the rifampin+ group, including those that declined rifampin, were compared with the control group. One hundred twenty-one consecutive hospitalized patients with positive blood cultures for P. aeruginosa were enrolled. Entry was stratified for prior use of empiric antipseudomonal antibiotics, neutropenia, severity of illness, and presence of pneumonia. Fifty-eight patients were randomized to receive rifampin (600 mg orally every 8 h for the first 72 h and then every 12 h for a total of 10 days) plus a beta-lactam agent plus an aminoglycoside. Sixty-three received the standard therapy of a beta-lactam plus an aminoglycoside agent (control). Bacteriologic cure occurred significantly more frequently in patients randomized to the rifampin+ regimen. Breakthrough or relapsing bacteremias occurred in 2% of the three-drug (rifampin+) group, compared with 14% for the two-drug (standard therapy) group. Despite this favorable trend in bacteriological response, no significant differences in survival were seen for the two treatment groups. Rifamycin derivatives warrant further clinical study as antipseudomonal agents. The Zelen protocol appears well suited for comparative trials of antimicrobial agents.

In vitro activity of E1040 against imipenem-resistant Pseudomonas aeruginosa strains

E1040 showed the most potent activity (MIC for 90% of strains, 6.25 micrograms/ml) among beta-lactams tested against 70 strains of imipenem-resistant Pseudomonas aeruginosa. Two strains showed high-level resistance to E1040; one strain produced a type II oxyiminocephalosporin-hydrolyzing beta-lactamase (group 3), and the other produced an enzyme similar to a type II penicillinase (OXA-1). Both beta-lactamases contributed to resistance to E1040.

Recurrent Pseudomonas aeruginosa pneumonia in an intensive care unit

We reviewed the records of all patients in the intensive care unit (ICU) who had Pseudomonas aeruginosa pneumonia over a 2.5-year period. Of patients with P aeruginosa pneumonia, 20 of 34 survived the initial episode of pneumonia. Ten of these 20 developed recurrence. In the nonrecurrent group, nine of ten survived hospitalization, compared to only four of ten in the recurrent group. Comparing the recurrent to the nonrecurrent group, factors associated with recurrence were the APACHE 2 score (12.3 +/- 2.7 vs 8.6 +/- 4.2 [p less than 0.03]), APS score (7.0 +/- 3.5 vs 2.7 +/- 2.1 [p less than 0.01]), and chronic pulmonary disease (8/10 vs 2/10 [p less than 0.05]). The recurrent P aeruginosa group was younger (63 +/- 10 vs 74 +/- 11 years old [p less than 0.03]) and spent more time receiving mechanical ventilation (95 +/- 64 vs 26 +/- 36 days [p less than 0.01]), in the ICU (101 +/- 61 vs 33 +/- 35 days [p less than 0.01]), and in the hospital (144 +/- 77 vs 84 +/- 32 days [p less than 0.03]). Although not statistically significant, in the recurrent group, eight of ten patients had tracheostomy and seven of ten had COPD, vs three of ten and two of ten, respectively, in the nonrecurrent group. Recurrent P aeruginosa pneumonia in the ICU is associated with increased morbidity and mortality and does not appear to be related to the adequacy of antibiotic treatment. Chronic lung disease appears to predispose patients to recurrent P aeruginosa pneumonia.

Imipenem- and meropenem-resistant mutants of Enterobacter cloacae and Proteus rettgeri lack porins

Carbapenems such as imipenem and meropenem are not rapidly hydrolyzed by commonly occurring beta-lactamases. Nevertheless, it was possible, by mutagenesis and selection, to isolate mutant strains of Enterobacter cloacae and Proteus rettgeri that are highly resistant to meropenem and imipenem. Two alterations were noted in the E. cloacae mutants. First, the mutant strains appeared to be strongly derepressed in the production of beta-lactamases, which reached a very high level when the strains were grown in the presence of imipenem. Second, these mutants were deficient in the production of nonspecific porins, as judged by the pattern of outer membrane proteins as well as by reconstitution assays of permeability. As with most porin-deficient mutants, their cultures were unstable, and their cultivation in the absence of carbapenems rapidly led to an overgrowth of porin-producing revertants. Analysis of the data suggests that the synergism between the lowered outer membrane permeability and the slow but significant hydrolysis of carbapenems by the overproduced enzymes can explain the resistance phenotypes quantitatively, although the possibility of alteration of the target cannot be excluded at present. With P. rettgeri mutants, there was no indication of further derepression of beta-lactamase, but the enzyme hydrolyzed imipenem much more efficiently than the E. cloacae enzyme did. In addition, the major porin was absent in one mutant strain. These results suggest that a major factor for the carbapenem resistance of these enteric bacteria is the porin deficiency, and this conclusion forms a contrast to the situation in Pseudomonas aeruginosa, in which the most prevalent class of imipenem-resistant mutants appears to lack the specific channel protein D2 yet retains the major nonspecific porin F.