Cloning, nucleotide sequencing, and analysis of the gene encoding an AmpC beta-lactamase in Acinetobacter baumannii

Correlation of antimicrobial resistance with beta-lactamases, the OmpA-like porin, and efflux pumps in clinical isolates of Acinetobacter baumannii endemic to New York City

Acinetobacter baumannii strains resistant to all beta-lactams, aminoglycosides, and fluoroquinolones have emerged in many medical centers. Potential mechanisms contributing to antimicrobial resistance were investigated in 40 clinical isolates endemic to New York City. The isolates were examined for the presence of various beta-lactamases, aminoglycoside-modifying enzymes, and mutations in gyrA and parC. Expression of the genes encoding the beta-lactamase AmpC, the efflux systems AdeABC and AbeM, and the OmpA-like porin was also examined by real-time reverse transcription-PCR. No VIM, IMP, KPC, OXA-23-type, OXA-24-type, or OXA-58 beta-lactamases were detected, although several isolates had acquired bla(SHV-5). Most cephalosporin-resistant isolates had increased levels of expression of ampC and/or had acquired bla(SHV-5); however, isolates without these features still had reduced susceptibility to cefepime that was mediated by the AdeABC efflux system. Although most isolates with ISAba1 upstream of the bla(OXA-51)-like carbapenemase gene were resistant to meropenem, several remained susceptible to imipenem. The presence of aminoglycoside-modifying enzymes and gyrase mutations accounted for aminoglycoside and fluoroquinolone resistance, respectively. The increased expression of adeABC was not an important contributor to aminoglycoside or fluoroquinolone resistance but did correlate with reduced susceptibility to tigecycline. The expression of abeM and ompA and phenotypic changes in OmpA did not correlate with antimicrobial resistance. A. baumannii has become a well-equipped nosocomial pathogen; defining the relative contribution of these and other mechanisms of antimicrobial resistance will require further investigation.

Genetic relatedness of multidrug-resistant Acinetobacter baumannii endemic to New York City

Multidrug-resistant isolates of Acinetobacter baumannii from New York City generally belong to one of three ribotypes. To assess the accuracy of ribotyping, the relatedness of representative isolates was further assessed by rep-PCR, pulsed-field gel electrophoresis (PFGE), and DNA sequencing of five genes potentially associated with antimicrobial resistance (ampC, ompA, adeB, adeR, and abeM). The isolates fell into several major groups. The first group shared the same ribotype and had common mutations affecting OmpA, AdeR, and AbeM, but consisted of two subtypes with distinctive rep-PCR and PFGE patterns and ampC mutations. The second and third groups shared common alterations in OmpA, AdeR, and AbeM, but had distinct ribotype, rep-PCR, and PFGE patterns. The resistant isolates were unrelated to the beta-lactam susceptible isolates. Many of the resistant strains shared OmpA and AdeB patterns observed in several European clonal groups. Further development of a multilocus sequencing typing scheme will help determine if multidrug-resistant isolates from diverse geographic areas are indeed ancestrally related.

Emergence of carbapenem resistance in Acinetobacter baumannii in the Czech Republic is associated with the spread of multidrug-resistant strains of European clone II

OBJECTIVES

The aim of this study was to analyse the emergence of carbapenem resistance among hospital strains of Acinetobacter in the Czech Republic.

METHODS

Acinetobacter isolates were collected prospectively in 2005-06 from 19 diagnostic laboratories. They were identified to species level by AFLP, typed using AFLP, pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing, and tested for susceptibility to 14 antimicrobials and for the presence of 20 genes associated with antimicrobial resistance.

RESULTS

A total of 150 Acinetobacter isolates were obtained from 56 intensive care units of 20 hospitals in 15 cities. They were identified as Acinetobacter baumannii (n = 108) or other species. A. baumannii isolates were allocated to EU clone I (n = 5), EU clone II (n = 66) or other, mostly unique genotypes. Two-thirds of the clone II isolates had nearly identical AFLP and PFGE fingerprints. As many as 85% and 88% isolates were susceptible to meropenem and imipenem (<or=4 mg/L), respectively. Carbapenem MICs of >or=8 mg/L were found in 23 A. baumannii isolates, of which 20 belonged to clone II. Isolates with bla(OXA-58-like) (n = 3)(,) bla(OXA-24-like) (n = 1) or ISAba1 adjacent to bla(OXA-51-like) (n = 34) had carbapenem MICs of 2 to >16 mg/L, while those without these elements showed MICs of <or=0.5-4 mg/L. Clone II isolates varied in susceptibility to some antibiotics including carbapenems and carried 6-12 resistance genes in 17 combinations.

CONCLUSIONS

The emergence of Acinetobacter carbapenem resistance in the Czech Republic is associated with the spread of A. baumannii strains of EU clone II. The variation in susceptibility in these strains is likely to result from both the horizontal spread of resistance genes and differential expression of intrinsic genes.

Therapeutic options for Acinetobacter baumannii infections

Acinetobacter baumannii is an important cause of nosocomial infections, mainly in patients in intensive care units. This microorganism, although with slight differences depending on the country, presents resistance to multiple antimicrobial agents, occasionally including resistance to colistin: hence, it can be considered the paradigm of nosocomial multiresistant bacteria. This review analyzes the evolution of antimicrobial resistance and the molecular bases associated with the increase in antimicrobial resistance, as well as the current treatment of Acinetobacter infections. Although controversy remains, the pooled data suggest that infections by A. baumannii may be associated with considerable attributable mortality. Moreover, in cases of pneumonia and bacteraemia, inappropriate treatment is associated with, among other factors, mortality. Therefore, treatment should be carefully considered.

Phenotypic and molecular characterization of 5 novel CTX-M enzymes carried by Klebsiella pneumoniae and Escherichia coli

AIM

The aim of the present study was to study the phenotypic and molecular characterization of 5 novel CTX-M-beta-1actamases carried by 5 Klebsiella pneumoniae isolates and 3 Escherichia coli isolates collected from 4 hospitals in Hefei, China.

METHODS

The purified PCR products were ligated with pGEM-Teasy vectors, expressed, and sequenced. The complete genes of the CTX-M-beta-lactamases were ligated with the pHSG398 vector to express prokaryotic recombinant proteins. Plasmids were extracted by rapid alkaline lysis protocol, and the PCR method was performed to determine whether the prokaryotic expression was successful or not. Antimicrobial susceptibility was tested and the phenotypes of transformants were determined according to criteria recommended by the Clinical and Laboratory Standards Institute. The kinetic parameters of enzymes were confirmed. The isoelectric points (pI) were determined by isoelectric focusing assay. Pulsed-field gel electrophoresis and plasmid profiling were performed.

RESULTS

The PCR products had 1101 nucleotides and were determined as CTX-M-46, CTX-M-47, CTX-M-48, CTX-M-49, and CTX-M-50. All strains were resistant to cefotaxime, but most of them were susceptible or intermediate to ceftazidime. The phenotypes of novel enzymes were determined as extended-spectrum-beta-lactamases (ESBL). Penicillin G, cephalothin, cefuroxime, and cefotaxime were determined to good substrates, whereas ceftazidime hydrolysis was not detected. The pI of the 5 novel CTX-M-beta-lactamases were 8.0. CTX-M-derivatives could be the multiplex genesis in our area.

CONCLUSION

This is the first report of these 5 novel plasmid-mediated CTX-M ESBL produced from China in the world. Molecular typing reveals notably different origin in genes encoding different CTX-M variants of 8 strains.

Molecular epidemiology of a clonal outbreak of multidrug-resistant Acinetobacter baumannii in a university hospital in Italy

This study investigated the molecular epidemiology of a clonal outbreak of multidrug-resistant Acinetobacter baumannii that occurred between June 2003 and June 2004 in a tertiary-care hospital in Naples, Italy. A. baumannii was isolated from 74 patients, of whom 38 were infected and 36 were colonised. Thirty-three patients had ventilator-associated pneumonia, three had hospital-acquired pneumonia, and two had sepsis. Genotypic analysis of 45 available A. baumannii isolates revealed two distinct pulsed-field gel electrophoresis (PFGE) patterns. Of these, PFGE pattern 1 was represented by isolates from 44 patients and was identical to that of an epidemic A. baumannii clone isolated in another hospital of Naples during 2002. All A. baumannii isolates of PFGE type 1 showed identical multiresistant antibiotypes, characterised by resistance to all antimicrobial agents tested, including carbapenems, with the exception of colistin. In these isolates, inhibition of OXA enzymes by 200 mM NaCl reduced the imipenem MIC by up to four-fold. Molecular analysis of antimicrobial resistance genes showed that all A. baumannii isolates of PFGE type 1 harboured a class 1 integron containing the aacA4, orfX and bla(OXA-20) gene cassettes, an ampC gene and a bla(OXA-51)-like allele. Moreover, a bla(OXA-58)-like gene surrounded by the regulatory elements ISAba2 and ISAba3 was identified in a 30-kb plasmid from A. baumannii isolates of PFGE type 1, but not PFGE type 2. Thus, selection of a single A. baumannii clone producing an OXA-58-type carbapenem-hydrolysing oxacillinase was responsible for the increase in the number of A. baumannii infections that occurred in this hospital.

Molecular characterization of the gene encoding a new AmpC β-lactamase in Acinetobacter baylyi

OBJECTIVES

The main objective of the present study was to demonstrate the presence of a beta-lactamase ampC gene in the chromosome of the non-pathogenic bacterium Acinetobacter baylyi ADP1.

METHODS

beta-Lactam MICs were determined by Etest. The ampC gene was amplified by PCR, with specific oligonucleotides, then cloned into pBGS18 and pAT-RA plasmids and transformed into Escherichia coli TG1 and parental A. baylyi as hosts. The gene was sequenced and analysed. The AmpC protein was expressed, purified by affinity chromatography and the kinetic parameters determined.

RESULTS

An ampC gene was amplified from the ADP1 genome. Sequencing of the gene showed typical SVSK and KTG domains and the typical YXN Class C motif. The amplified gene showed significant identity (48.5% to 49.3%) with the AmpC enzymes of Acinetobacter baumannii and AG3 strains, which have recently been renamed ADC-1 to ADC-7. MIC analysis revealed a cephalosporinase profile for the E. coli TG1 clone as well as for the parental A. baylyi strain that overexpressed the ampC gene cloned under the control of an external promoter. Analysis of kinetic parameters of the purified enzyme showed higher catalytic efficiency for cefalotin than for ampicillin.

CONCLUSIONS

This study represents the first report of an AmpC beta-lactamase in A. baylyi, which was shown by biochemical and microbiological experiments to have a typical cephalosporinase profile. The presence of the respective gene in the chromosome of A. baylyi ADP1 suggested that this ampC gene is the naturally occurring cephalosporinase in this species, as previously reported for other Acinetobacter spp. We tentatively named the enzyme ADC-8.

Carbapenem resistance in Acinetobacter baumannii: mechanisms and epidemiology

The increasing trend of carbapenem resistance in Acinetobacter baumannii worldwide is a concern since it limits drastically the range of therapeutic alternatives. Metallo-beta-lactamases (VIM, IMP, SIM) have been reported worldwide, especially in Asia and western Europe, and confer resistance to all beta-lactams except aztreonam. The most widespread beta-lactamases with carbapenemase activity in A. baumannii are carbapenem-hydrolysing class D beta-lactamases (CHDLs) that are mostly specific for this species. These enzymes belong to three unrelated groups of clavulanic acid-resistant beta-lactamases, represented by OXA-23, OXA-24 and OXA-58, that can be either plasmid- or chromosomally-encoded. A. baumannii also possesses an intrinsic carbapenem-hydrolysing oxacillinase, the expression of which may vary, that may play a role in carbapenem resistance. In addition to beta-lactamases, carbapenem resistance in A. baumannii may also result from porin or penicillin-binding protein modifications. Several porins, including the 33-kDa CarO protein, that constitute a pore channel for influx of carbapenems, might be involved in carbapenem resistance.

Mechanisms of multidrug resistance in Acinetobacter species and Pseudomonas aeruginosa

Acinetobacter species and Pseudomonas aeruginosa are noted for their intrinsic resistance to antibiotics and for their ability to acquire genes encoding resistance determinants. Foremost among the mechanisms of resistance in both of these pathogens is the production of beta -lactamases and aminoglycoside-modifying enzymes. Additionally, diminished expression of outer membrane proteins, mutations in topoisomerases, and up-regulation of efflux pumps play an important part in antibiotic resistance. Unfortunately, the accumulation of multiple mechanisms of resistance leads to the development of multiply resistant or even "panresistant" strains.

Cephalosporinase over-expression resulting from insertion of ISAba1 in Acinetobacter baumannii

ISAba1-like sequences were identified immediately upstream of the bla(ampC) gene in ceftazidime-resistant Acinetobacter baumannii isolates, but were absent in ceftazidime-susceptible A. baumannii isolates. AmpC over-expression resulted from insertion of ISAba1-like sequences upstream of bla(ampC). ISAba1 provided strong promoter sequences, and it was demonstrated that the change in the ribosome binding site sequence resulting from insertion of ISAba1 did not influence expression of the bla(ampC) gene. Sequence analysis revealed that AmpC sequences of A. baumannii isolates were almost identical and that ISAba1 elements had a high percentage of identity.

Cloning and functional analysis of the gene encoding the 33- to 36-kilodalton outer membrane protein associated with carbapenem resistance in Acinetobacter baumannii

We investigated a multiresistant strain of Acinetobacter baumannii isolated in our hospital. Analysis of the N-terminal peptide sequence of the outer membrane proteins (OMPs) purified from the strain allowed us to clone and sequence the nucleotides of the gene encoding the 33- to 36-kDa OMP associated with carbapenem resistance in A. baumannii.

Channel formation by CarO, the carbapenem resistance-associated outer membrane protein of Acinetobacter baumannii

It has been recently shown that resistance to both imipenem and meropenem in multidrug-resistant clinical strains of Acinetobacter baumannii is associated with the loss of a heat-modifiable 25/29-kDa outer membrane protein, called CarO. This study aimed to investigate the channel-forming properties of CarO. Mass spectrometry analyses of this protein band detected another 25-kDa protein (called Omp25), together with CarO. Both proteins presented similar physicochemical parameters (M(w) and pI). We overproduced and purified the two polypeptides as His-tagged recombinant proteins. Circular dichroism analyses demonstrated that the secondary structure of these proteins was mainly a beta-strand conformation with spectra typical of porins. We studied the channel-forming properties of proteins by reconstitution into artificial lipid bilayers. In these conditions, CarO induced ion channels with a conductance value of 110 pS in 1 M KCl, whereas the Omp25 protein did not form any channels, despite its suggested porin function. The pores formed by CarO showed a slight cationic selectivity and no voltage closure. No specific imipenem binding site was found in CarO, and this protein would rather form unspecific monomeric channels.

Contribution of acquired carbapenem-hydrolyzing oxacillinases to carbapenem resistance in Acinetobacter baumannii

Carbapenem-hydrolyzing oxacillinases are reported increasingly in Acinetobacter baumannii. Since they hydrolyze carbapenems at low levels, the roles of carbapenem-hydrolyzing oxacillinases OXA-23, OXA-40, and OXA-58 in A. baumannii were determined. The bla(OXA-23), bla(OXA-40), and bla(OXA-58) genes were inserted in broad-host-range plasmid pAT801 and transformed in Escherichia coli DH10B and in A. baumannii CIP 70.10 and its point mutant derivative A. baumannii BM4547, which overexpresses the efflux pump AdeABC. Natural plasmids harboring the bla(OXA-23) and bla(OXA-58) genes were also transformed in A. baumannii CIP 70.10. In addition, the bla(OXA-40) gene was inactivated at its chromosome location in A. baumannii CLA-1. Intermediate levels of resistance or reduced susceptibilities to carbapenems were observed for A. baumannii transformants expressing OXA-23, OXA-40, and OXA-58. The inactivation of bla(OXA-40) in A. baumannii CLA-1 yielded reduced susceptibilities to carbapenems. Carbapenem-hydrolyzing oxacillinases OXA-23, OXA-40, and to a lesser extent OXA-58 play a role in carbapenem resistance in A. baumannii, and overexpression of efflux pump AdeABC may also contribute to higher levels of resistance to beta-lactams, including carbapenems.

Identification of a new allelic variant of the Acinetobacter baumannii cephalosporinase, ADC-7 beta-lactamase: defining a unique family of class C enzymes

Acinetobacter spp. are emerging as opportunistic hospital pathogens that demonstrate resistance to many classes of antibiotics. In a metropolitan hospital in Cleveland, a clinical isolate of Acinetobacter baumannii that tested resistant to cefepime and ceftazidime (MIC = 32 microg/ml) was identified. Herein, we sought to determine the molecular basis for the extended-spectrum-cephalosporin resistance. Using analytical isoelectric focusing, a beta-lactamase with a pI of > or = 9.2 was detected. PCR amplification with specific A. baumannii cephalosporinase primers yielded a 1,152-bp product which, when sequenced, identified a novel 383-amino-acid class C enzyme. Expressed in Escherichia coli DH10B, this beta-lactamase demonstrated greater resistance against ceftazidime and cefotaxime than cefepime (4.0 microg/ml versus 0.06 microg/ml). The kinetic characteristics of this beta-lactamase were similar to other cephalosporinases found in Acinetobacter spp. In addition, this cephalosporinase was inhibited by meropenem, imipenem, ertapenem, and sulopenem (K(i) < 40 microM). The amino acid compositions of this novel enzyme and other class C beta-lactamases thus far described for A. baumannii, Acinetobacter genomic species 3, and Oligella urethralis in Europe and South Africa suggest that this cephalosporinase defines a unique family of class C enzymes. We propose a uniform designation for this family of cephalosporinases (Acinetobacter-derived cephalosporinases [ADC]) found in Acinetobacter spp. and identify this enzyme as ADC-7 beta-lactamase. The coalescence of Acinetobacter ampC beta-lactamases into a single common ancestor and the substantial phylogenetic distance separating them from other ampC genes support the logical value of developing a system of nomenclature for these Acinetobacter cephalosporinase genes.

Detection and characterization of beta-lactamase genes in subgingival bacteria from patients with refractory periodontitis

Fifty-three beta-lactamase-producing strains of oral bacteria isolated from patients with refractory periodontitis in Norway and USA were screened for the presence of the bla(TEM), bla(SHV), bla(OXA), bla(ampC), bla(cfxA), and bla(cepA/cblA) genes by the polymerase chain reaction (PCR). The PCR products were characterized by direct sequencing of the amplified DNA. Thirty-four of the 53 enzyme-producing strains (64%) were positive in one of the PCR assays. All beta-lactamase-producing Prevotella and Capnocytophaga spp. were CfxA positive. TEM-type beta-lactamases were identified in one strain each of Escherichia coli and Neisseria sp., and one strain of Citrobacter freundii possessed an AmpC-type beta-lactamase. Screening for gene cassettes and genes known to be associated with integrons did not reveal the presence of integrons in these oral bacteria. Sequence analyses showed that most CfxA positive Prevotella and Capnocytophaga isolates from patients with refractory periodontitis harboured variants of the CfxA2 and CfxA3 enzyme. The present study also showed that many different genetic determinants of beta-lactamase production are found in bacteria isolated from refractory periodontitis, many of which remain to be characterized.

OXA-58, a novel class D {beta}-lactamase involved in resistance to carbapenems in Acinetobacter baumannii

A carbapenem-resistant Acinetobacter baumannii strain was isolated in Toulouse, France, in 2003. Cloning and expression in Escherichia coli identified the carbapenem-hydrolyzing beta-lactamase OXA-58, which is weakly related (less than 50% amino acid identity) to other oxacillinases. It hydrolyzed penicillins, oxacillin, and imipenem but not expanded-spectrum cephalosporins. The bla(OXA-58) gene was located on a ca. 30-kb non-self-transferable plasmid. After electrotransformation in the A. baumannii CIP7010(T) reference strain, it conferred reduced susceptibility to carbapenems. The bla(OXA-58) gene was bracketed by two novel ISAba3-like insertion elements. This study describes a newly characterized beta-lactamase that may contribute to carbapenem resistance in A. baumannii.

Nosocomial transmission of CTX-M-2 beta-lactamase-producing Acinetobacter baumannii in a neurosurgery ward

Three strains of cefotaxime (CTX)-resistant Acinetobacter baumannii, FM0209680, FM0300106, and FM0301433, were isolated from transtracheal aspirate cultures of three patients with probable nosocomial infections in a neurosurgery ward in Japan. The CTX MICs for these isolates were greater than 128 microg/ml but were drastically reduced in the presence of 4 microg of clavulanic acid per ml. These strains were also resistant to ceftriaxone, cefpodoxime, and aztreonam but were susceptible to ceftazidime and imipenem. The profile of resistance to various broad-spectrum beta-lactams was transferred by conjugation. Strain FM0209680 was not eradicated from case patient 1 by administration of imipenem, ceftazidime, and levofloxacin, even after a 6-month hospitalization period. Strains FM0300106 and FM0301433 were isolated from case patients 2 and 3 during the sixth week following admission, respectively, and then each patient was colonized for 3 weeks. Eradication of FM0300106 was successfully obtained from case patient 2 by imipenem treatment, while administration of imipenem was continued to prevent pneumonia. Prophylactic antimicrobial therapy was discontinued in case patient 3 because of the lack of pneumonic symptoms, and FM0301433 disappeared after the discontinuation of antimicrobial chemotherapy. All three strains carried the bla(CTX-M-2) gene, and the appearance of colonies in the growth-inhibitory zones around disks of CTX and aztreonam in double-disk synergy tests suggested inducible beta-lactamase production in these A. baumannii strains. The ribotyping investigation suggested that all these strains belong to the same clonal lineage. The plasmids harbored by A. baumannii had the same restriction profile as those harbored by Proteus mirabilis strains previously isolated in a urology ward of the Funabashi Medical Center.

Molecular characterization of the gene encoding a new AmpC beta-lactamase in a clinical strain of acinetobacter genomic species 3

A presumptive chromosomal cephalosporinase (pI, 9.0) from a clinical strain of Acinetobacter genomic species 3 (AG3) is reported. The nucleotide sequence of this beta-lactamase shows for the first time the gene encoding an AmpC enzyme in AG3. In addition, the biochemical properties of the novel AG3 AmpC beta-lactamase are reported

Antimicrobial susceptibility and mechanisms of resistance to quinolones and beta-lactams in Acinetobacter genospecies 3

Antimicrobial susceptibility was determined in 15 epidemiologically unrelated clinical isolates of Acinetobacter genospecies 3. Moreover, the mechanisms of resistance to some beta-lactam antibiotics may be associated with the presence of a chromosomal cephalosporinase, AmpC, and the resistance to quinolones related to mutations in the gyrA and parC genes.

Genetic environment and transcription of ampC in an Acinetobacter baumannii clinical isolate

An ampC gene was cloned from a clinical isolate of Acinetobacter baumannii (strain RAN). DNA sequencing and primer extension studies showed that ampC is transcribed from a promoter contained within a putative insertion sequence element which has been found to abut several different genes in Acinetobacter spp.

Effects of selective decontamination of digestive tract on mortality and acquisition of resistant bacteria in intensive care: a randomised controlled trial

BACKGROUND

Selective decontamination of the digestive tract (SDD) is an infection-prevention regimen used in critically ill patients. We assessed the effects of SDD on intensive-care-unit (ICU) and hospital mortality, and on the acquisition of resistant bacteria in adult patients admitted to intensive care.

METHODS

We did a prospective, controlled, randomised, unblinded clinical trial. 934 patients admitted to a surgical and medical ICU were randomly assigned oral and enteral polymyxin E, tobramycin, and amphotericin B combined with an initial 4-day course of intravenous cefotaxime (SDD group n=466), or standard treatment (controls n=468). Primary endpoints were ICU and hospital mortality and the acquisition of resistant bacteria.

FINDINGS

In the SDD group 69 (15%) patients died in the ICU compared with 107 (23%) in the control group (p=0.002). Hospital mortality was lower in the SDD groups than in the control group (113 [24%] vs 146 [31%], p=0.02). During their stay in intensive care, colonisation with gram-negative bacteria resistant to ceftazidime, ciprofloxacin, imipenem, polymyxin E, or tobramycin occurred in 61 (16%) of 378 SDD patients and in 104 (26%) of 395 patients in the control group (p=0.001). Colonisation with vancomycin-resistant enterococcus occurred in five (1%) SDD patients and in four (1%) controls (p=1.0). No patient in either group was colonised with meticillin-resistant Staphylococcus aureus.

INTERPRETATION

In a setting with low prevalence of vancomycin-resistant enterococcus and meticillin-resistant S aureus, SDD can decrease ICU and hospital mortality and colonisation with resistant gram-negative aerobic bacteria.

Molecular epidemiology and mechanisms of carbapenem resistance in Acinetobacter baumannii endemic in New York City

Multidrug-resistant Acinetobacter baumannii has emerged as a serious nosocomial pathogen in certain areas. In Brooklyn, New York, citywide surveillance revealed that approximately 2 of every 3 isolates were resistant to carbapenem antibiotics. Genetic fingerprinting revealed that 2 strains accounted for 82% of these resistant isolates. Compared with carbapenem-susceptible isolates, carbapenem-resistant isolates had reduced expression of 47-, 44-, and 37-kDa outer-membrane proteins. No specific carbapenemase was found; however, carbapenem-resistant isolates expressed greater levels of a class C cephalosporinase. Although expression of penicillin-binding proteins varied among strains, no consistent pattern appeared to account for carbapenem resistance. An efflux pump, present in several strains, did not appear to contribute to carbapenem resistance. Clonal spread of carbapenem-resistant A. baumannii has occurred in hospitals in Brooklyn. The preliminary findings for a small number of strains suggest that diminished production of outer-membrane porins, together with increased expression of a class C cephalosporinase, appear to be important factors leading to carbapenem resistance in this region.

Chromosomal integration of a cephalosporinase gene from Acinetobacter baumannii into Oligella urethralis as a source of acquired resistance to beta-lactams

Clinical Oligella urethralis isolate COH-1, which was uncommonly resistant to penicillins and narrow-spectrum cephalosporins, was recovered from a 55-year-old patient with a urinary tract infection. Shotgun cloning into Escherichia coli and expression experiments gave recombinant clones expressing either an AmpC beta-lactamase-type phenotype of resistance or a carbenicillin-hydrolyzing beta-lactamase-type phenotype of resistance. The AmpC beta-lactamase identified (ABA-1), which had a pI value of 8.2, had 98% amino acid identity with a chromosomally encoded cephalosporinase of Acinetobacter baumannii. A 820-bp insertion sequence element, ISOur1, belonging to the IS6 family of insertion sequence elements, was identified immediately upstream of bla(ABA-1), providing a -35 promoter sequence and likely giving rise to a hybrid promoter region. The carbenicillin-hydrolyzing beta-lactamase identified (CARB-8), which had a pI value of 6.4, differed from CARB-5 by two amino acid substitutions. Hybridization of CeuI fragment I-restricted DNA fragments of O. urethralis COH-1 with bla(ABA-1)-, bla(CARB-8)-, and 16S rRNA-specific probes indicated the chromosomal integration of the beta-lactamase genes. PCR and hybridization experiments failed to detect bla(CARB-8)- and bla(ABA-1)-like genes in three O. urethralis reference strains, indicating that the beta-lactamase genes identified were the source of acquired resistance in O. urethralis COH-1. This is one of the few examples of the interspecies transfer and the chromosomal integration of a gene encoding a naturally occurring beta-lactamase.

Characterization of class 1 integron resistance gene cassettes and the identification of a novel IS-like element in Acinetobacter baumannii

Based on hybridization studies, 21/32 multi-resistant clinical isolates of Acinetobacter baumannii contain class 1 integrons. Amplification products were obtained from 20 of the hybridization-positive strains. A single dfrA7 cassette was identified in 18 of the isolates and an integron with two cassettes (aadB-aadA4) was found in only one strain. Amplicons were not obtained from one of the hybridization positive strains. DNA sequence analysis of a 6.080-kb fragment, cloned from this strain, identified the remnant of an integron, following insertion of IS26 into the 5(')-end of intI1. The 6.080-kb sequence carries an aminoglycoside resistance gene, linked to a portion of IS1133, which in turn is linked to a sequence that has properties of IS elements, including sequences that could stimulate transcription, and ORFs encoding amino acid sequences with similarity to a transposase from Deinococcus radiodurans.

Genetic and functional analysis of the chromosome-encoded carbapenem-hydrolyzing oxacillinase OXA-40 of Acinetobacter baumannii

Clinical isolate Acinetobacter baumannii CLA-1 was resistant to a series of antibiotic molecules, including carbapenems. Cloning and expression of the beta-lactamase gene content of this isolate in Escherichia coli DH10B identified a chromosome-encoded oxacillinase, OXA-40, that differed by one or two amino acid changes from OXA-24, -25, and -26 and an AmpC-type cephalosporinase. The OXA-40 beta-lactamase had a mainly narrow-spectrum hydrolytic profile, but it included ceftazidime and imipenem. Its activity was resistant to inhibition by clavulanic acid, tazobactam, sulbactam, and, like most of the other carbapenem-hydrolyzing oxacillinases, NaCl. OXA-40 had an FGN triad replacing a YGN motif at class D beta-lactamase (DBL) positions 144 to 146. Site-directed DNA mutagenesis leading to a Phe-to-Tyr change at DBL position 144 in OXA-40 gave a mutant enzyme with increased hydrolytic activity against most beta-lactams, including imipenem. Conversely, with a gene encoding the narrow-spectrum oxacillinase OXA-1 as the template, a nucleotide substitution leading to a Tyr-to-Phe change in the YGN motif of OXA-1 gave a mutant enzyme with decreased hydrolytic activity without an increase in carbapenem-hydrolyzing activity. Thus, the Phe residue in the FGN motif was not associated with carbapenem-hydrolyzing activity by itself but instead was associated with weak overall hydrolytic activity. Finally, this Phe residue in OXA-40 explained resistance to inhibition by NaCl whereas a Tyr residue in motif YGN was related to susceptibility to NaCl.

[Interpretative reading of the non-fermenting gram-negative bacilli antibiogram]

Among non-fermenting Gram-negative rods, the most clinically important species are Pseudomonas aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia, which are frequently multiresistant. P. aeruginosa resistance to beta-lactams depends on the production of chromosomal and plasmid-mediated beta-lactamases, altered permeability (loss of OprD porin is related to carbapenem-resistance) and active efflux pumps, particularly MexAB-OprM. In aminoglycoside-resistant strains the main mechanism of resistance is the production of inactivating enzymes; the efflux pump MexXY-OprM is also involved. Quinolone-resistance in P. aeruginosa is related to changes in topoisomerases, altered permeability and efflux pumps. The mechanisms of resistance of A. baumannii have not been well characterized, which makes interpretative reading of the antibiogram in this organism difficult. Resistance to beta-lactams is associated with the production of beta-lactamases and altered penicillin-binding proteins. Resistance to aminoglycosides has been related to modifying enzymes and resistance to quinolones to altered targets. S. maltophilia is resistant to carbapenems and other beta-lactams because of the production of two beta-lactamases (L-1 and L-2). Aminoglycoside-modifying enzymes have also been described in this species. In contrast to what is observed in other organisms, S. maltophilia resistance to quinolones has been mainly related to active efflux, rather than to target alterations.

Amino acid sequence determinants of extended spectrum cephalosporin hydrolysis by the class C P99 beta-lactamase

Class C beta-lactamases are commonly encoded on the chromosome of Gram-negative bacterial species. Mutations leading to increased expression of these enzymes are a common cause of resistance to many cephalosporins including extended spectrum cephalosporins. Recent reports of plasmid- and integrin-encoded class C beta-lactamases are a cause for concern because these enzymes are likely to spread horizontally to susceptible strains. Because of their increasing clinical significance, it is critical to identify the determinants of catalysis and substrate specificity of these enzymes. For this purpose, the codons of a set of 21 amino acid residues that encompass the active site region of the P99 beta-lactamase were individually randomized to create libraries containing all possible amino acid substitutions. The amino acid sequence requirements for the hydrolysis of ceftazidime, an extended spectrum cephalosporin commonly used to treat serious infections, were determined by selecting resistant mutants from each of the 21 libraries. DNA sequencing identified the residue positions that are critical for ceftazidime hydrolysis. In addition, it was found that certain amino acid substitutions in the omega-loop region of the P99 enzyme result in increased ceftazidime hydrolysis suggesting the loop is an important determinant of substrate specificity.

Characterization of a nosocomial outbreak caused by a multiresistant Acinetobacter baumannii strain with a carbapenem-hydrolyzing enzyme: high-level carbapenem resistance in A. baumannii is not due solely to the presence of beta-lactamases

From February to November 1997, 29 inpatients at Ramón y Cajal Hospital, Madrid, Spain, were determined to be either colonized or infected with imipenem- and meropenem-resistant Acinetobacter baumannii (IMRAB) strains (MICs, 128 to 256 microg/ml). A wide antibiotic multiresistance profile was observed with IMRAB strains. For typing IMRAB isolates, pulsed-field gel electrophoresis was used. For comparative purposes, 30 imipenem- and meropenem-susceptible A. baumannii (IMSAB) strains isolated before, during, and after the outbreak were included in this study. The molecular-typing results showed that the outbreak was caused by a single IMRAB strain (genotype A). By cloning experiments we identified a class D beta-lactamase (OXA-24) encoded in the chromosomal DNA of this IMRAB strain which showed carbapenem hydrolysis. Moreover, the outer membrane profile of the IMRAB strain showed a reduction in the expression of two porins at 22 and 33 kDa when compared with genetically related IMSAB isolates. In addition no efflux mechanisms were identified in the IMRAB strains. In summary, we report here the molecular characterization of a nosocomial outbreak caused by one multiresistant A. baumannii epidemic strain that harbors a carbapenem-hydrolyzing enzyme. Although alterations in the penicillin-binding proteins cannot be ruled out, the reduction in the expression of two porins and the presence of this OXA-derived beta-lactamase are involved in the carbapenem resistance of the epidemic nosocomial IMRAB strain.

Molecular characterization of FOX-4, a new AmpC-type plasmid-mediated beta-lactamase from an Escherichia coli strain isolated in Spain

A clinical strain of Escherichia coli (Ec GCE) displayed resistance to cefoxitin, cefotetan, cefotaxime, and ceftazidime. Susceptibility was not restored by the addition of clavulanic acid. Two beta-lactamases with apparent pIs of 5.4 and 6.4 were identified; the beta-lactamase with a pI of 6.4 was transferred by conjugation and associated with a 40-kb plasmid. Analysis of the nucleotide sequence showed a new ampC beta-lactamase gene that is closely related to those encoding the FOX-3, FOX-2, and FOX-1 beta-lactamases but whose product has four novel amino acid mutations, at positions 11 (M-->T), 43 (A-->E), 233 (V-->A), and 280 (Y-->H). This first cephamycinase from Spain was named FOX-4.

OXA-24, a novel class D beta-lactamase with carbapenemase activity in an Acinetobacter baumannii clinical strain

Acinetobacter baumannii RYC 52763/97, a clinical isolate involved in a prolonged nosocomial outbreak at our hospital, was resistant to all beta-lactams tested, including imipenem and meropenem, which had MICs of 128 and 256 microg/ml, respectively. This strain synthesized three beta-lactamases: a plasmid-mediated TEM-1 beta-lactamase (pI 5.4), an AmpC-type chromosomal cephalosporinase (pI 9.4), and a novel, presumptively chromosomally mediated OXA-related enzyme (pI 9.0) named OXA-24. After cloning and sequencing, the deduced amino acid sequence of the OXA-24 beta-lactamase showed 40% homology with the OXA-10 (PSE-2) and OXA-7 beta-lactamases, 39% homology with the OXA-11 and OXA-5 enzymes, and 33% homology with the LCR-1 beta-lactamase. The amino acid sequence of the OXA-24 beta-lactamase contained the STFK motif found in serine beta-lactamases, but the typical class D triad KTG was replaced by KSG and the motif YGN was replaced by FGN. The OXA-24 beta-lactamase hydrolyzed benzylpenicillin and cephaloridine but lacked activity against oxacillin, cloxacillin, and methicillin. The enzymatic activity was inhibited by chloride ions and by tazobactam (50% inhibitory concentration [IC(50)], 0.5 microM), sulbactam (IC(50), 40 microM), and clavulanic acid (IC(50), 50 microM). Carbapenem MICs for an Escherichia coli transformant (pBMB-1) expressing the cloned OXA-24 enzyme had a fourfold increase. Relative V(max)/K(m) values of 13 and 6 were obtained with imipenem and meropenem, respectively, and a positive microbiological assay result with imipenem was obtained with a purified enzymatic extract of this transformant strain. Therefore, we consider this new beta-lactamase to be involved in the carbapenem resistance of A. baumannii RYC 52763/97.