SHV-type Enzymes Research Articles

Silver resistance genes are overrepresented among Escherichia coli isolates with CTX-M production.

Members of the Enterobacteriaceae with extended-spectrum beta-lactamases (ESBLs) of the CTX-M type have disseminated rapidly in recent years and have become a threat to public health. In parallel with the CTX-M type expansion, the consumption and widespread use of silver-containing products has increased. To determine the carriage rates of silver resistance genes in different Escherichia coli populations, the presence of three silver resistance genes (silE, silP, and silS) and genes encoding CTX-M-, TEM-, and SHV-type enzymes were explored in E. coli isolates of human (n = 105) and avian (n = 111) origin. The antibiotic profiles were also determined. Isolates harboring CTX-M genes were further characterized, and phenotypic silver resistance was examined. The silE gene was present in 13 of the isolates. All of them were of human origin. Eleven of these isolates harbored ESBLs of the CTX-M type (P = 0.007), and eight of them were typed as CTX-M-15 and three as CTX-M-14. None of the silE-positive isolates was related to the O25b-ST131 clone, but 10 out of 13 belonged to the ST10 or ST58 complexes. Phenotypic silver resistance (silver nitrate MIC > 512 mg/liter) was observed after silver exposure in 12 of them, and a concomitant reduced susceptibility to piperacillin-tazobactam developed in three. In conclusion, 12% of the human E. coli isolates but none of the avian isolates harbored silver resistance genes. This indicates another route for or level of silver exposure for humans than that caused by common environmental contamination. Since silE-positive isolates were significantly more often found in CTX-M-positive isolates, it is possible that silver may exert a selective pressure on CTX-M-producing E. coli isolates.

Prevalence of extended‐spectrum beta‐lactamase‐producing Enterobacteriaceae in healthy Swedish preschool children

The objective was to determine the prevalence of extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae in faeces from healthy Swedish preschool children and to establish whether transmission took place between children in preschools. Diapers from children attending preschools in Uppsala city were collected during September to October 2010, and the faeces was cultured. Antibiotic profiles and carriage of CTX-M, TEM, SHV and AmpC type enzymes were determined. PCR-positive isolates were further characterized by sequencing and epidemiological typing. Statistics on antibiotic use and ESBL producers in paediatric patients at Uppsala University Hospital were extracted for comparison. A total of 313 stool specimens were obtained, representing 24.5% of all preschool children in Uppsala city. The carriage rate of ESBL-producing Enterobacteriaceae was 2.9% among these healthy children. The corresponding figure for patients in the same age group was 8.4%. Escherichia coli with CTX-M type enzymes predominated, and only one E.coli isolate carried genes-encoding CMY. CTX-M-producing E.coli isolates with identical genotypes were found in children with no familial relation at two different preschools. Using diapers, the prevalence of ESBL-producing Enterobacteriaceae in children was quickly established, and, most likely, a transmission of ESBL-producing E.coli was for the first time documented between children at the same preschool.

Molecular characterization of extended-spectrum β-lactamases in clinical Escherichia coli and Klebsiella pneumoniae isolates from Surabaya, Indonesia

No detailed reports regarding extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae are currently available from Indonesia, the fourth most populous country in the world. A survey was carried out to investigate the molecular epidemiology and genetic characteristics of clinical ESBL-producing Escherichia coli and Klebsiella pneumoniae isolates originating from the Dr. Soetomo Academic Hospital in Surabaya, Indonesia, over a 4 month period (January to April 2005). ESBLs were characterized by isoelectric focusing and PCR assays. Clonality of the isolates was assessed by PFGE and repetitive-sequence-based PCR (rep-PCR). Phylogenetic grouping was performed among CTX-M-15-producing E. coli. In total, 73 consecutive non-duplicate ESBL-positive E. coli and 72 K. pneumoniae strains were isolated. The bla(CTX-M-15) gene was found to be highly prevalent (69/73 strains, 94.5%) among the 73 ESBL-positive E. coli isolates. The gene was detected in both clonal and non-clonal isolates, as defined by PFGE and rep-PCR. Sixteen CTX-M-15-positive E. coli could be assigned to a single rep-PCR type and phylogenetic group B2 and belonged to the well-known O25b-ST131 clone. Among the 72 ESBL-positive K. pneumoniae isolates, bla(CTX-M-15) was again the most prevalent ESBL (40/72, 55.6%). Several SHV-type enzymes were also frequently detected: SHV-5 (n = 28); SHV-12 (n = 13); and SHV-2 (n = 6). TEM-type ESBLs were not detected in any of the isolates. Indonesia is another developing country affected by the emergence and spread of bacterial strains harbouring ESBL genes, including the CTX-M-15-producing B2-E. coli O25b-ST131 clone.

The survey of genes encoding beta-lactamases, in Escherichia coli resistant to beta-lactam and non-beta-lactam antibiotics.

Objective(s) Resistance to the new generation of cephalosporins which is mediated by Extended-Spectrum beta-lactamases (ESBLs) has been found among Escherichia coli isolates throughout the world. These resistance genes and their producers, the micro-organisms carrying beta-lactamases, are responsible for serious clinical and therapeutic problems among inpatients and it is necessary to pay more attention to detection of ESBLs producing organisms. Materiasl and Methods Collectively 260 isolates of E. coli were obtained from 6 hospitals in Tehran (Iran) during April-2006 to April-2007. The antibiotic susceptibility patterns of isolates were determined by disk diffusion method. phenotypic confirmatory test (PCT) was carried out for screening of ESBLs. Microbroth dilution assay was used to determine the minimum inhibitory concentration (MIC) of ceftazidime. Isolates showing MIC≥2 µg/ml were subjected to polymerase chain reaction (PCR) targeting blaTEM, blaSHV, blaCTX and blaPER genes. Results The PCT showed that 48.08% of isolates are ESBL producers (125 of 260). The majority of cefotaxime resistant (90.8%) and ceftazidime resistant (92.5%) isolates were ESBL producers. The obtained results by PCR revealed that 5.77% (n=15 of 260) and 24.23 (n=63) of isolates can produce SHV and TEM type enzymes respectively. blaCTX was detected in 20.38% of isolates (n=53) and none of them could produce blaPER type beta-lactamases. Conclusion The results of our study showed that the ESBL genes have high prevalence among clinical isolates of E. coli. Such high dissemination of ESBLs is a serious problem for public health and therefore, it's necessary to seek a program for monitoring ESBLs in hospitals.

High Prevalence of CTX-M-Type β-Lactamases among Clinical Isolates of Enterobacteriaceae in Bamako, Mali

In 2004, Weil et al. reported the presence of CTX-M-15 in Salmonella enterica (10), and recently Ruppe et al. reported the carriage of CTX-M-15-producing isolates in children in a remote village in Senegal (8). However, since the report of SHV-type enzymes in Nigeria (1), no survey has been performed on extended-spectrum β-lactamase (ESBL) producers in West African hospitals (4, 7). Between May 2004 and April 2006, in Hopital du Point G, Bamako, Mali, of 1,193 nonrepetitive isolates of Enterobacteriaceae identified by the API 20E system (bioMerieux, Marcy l'Etoile, France), 256 produced an ESBL detected by the double-disk synergy test (9) (156/747 Escherichia coli, 82/217 Klebsiella pneumoniae, 15/81 Enterobacter cloacae, 2/12 Morganella morganii, and 1/36 Proteus mirabilis isolates). Enzymes were identified for 109 E. coli and 63 K. pneumoniae isolates randomly selected every 3 months and for all E. cloacae, M. morganii, and P. mirabilis isolates. These strains were isolated from patients hospitalized in the medical (n = 127 [66.3%]) and surgical (n = 27 [14.0%]) wards and intensive care units (n = 2 [1.2%]) and emergency patients (n = 4 [2.3%]) and outpatients (n = 30 [15.7%]) and from urine (n = 142 [74.7%]), blood (n = 12 [6.3%]), pus (n = 20 [10.4%]), and other sites (n = 16 [8.3%]) (stool, catheter, and pleural and peritoneal fluid). Antimicrobial susceptibility was determined by disk diffusion according to the recommendations of the Antibiogram Committee of the French Society for Microbiology (http://www.sfm.asso.fr/) (Table ​(Table1).1). Isoelectric focusing evidenced only one pI corresponding to a TEM-type enzyme, pI 5.4, in 121/190 isolates. Since this pI was considered as TEM-1 in most cases, we did not screen other TEM-type ESBLs by PCR. All 190 isolates were positive for blaSHV- or blaCTX-M-related genes, as assessed by PCR using previously described specific primers (2). After direct sequencing of PCR products (2), CTX-M-15 (n = 157), CTX-M-14 (n = 17), SHV-12 (n = 17), and SHV-27 (n = 1) were identified. Two isolates produced two ESBLs, CTX-M-15 and SHV-12 or SHV-27 (Table ​(Table1).1). Except in three isolates of E. coli, all SHV-type enzymes were produced by E. cloacae or K. pneumoniae, confirming the role of E. coli CTX-M-15 in the increase in ESBL incidence. To determine whether the ESBL frequency was due to the spread of some predominant strains, randomly amplified polymorphic DNA (RAPD) analysis was performed (2). A great diversity of RAPD types were observed (Table ​(Table1),1), and no isolate was the same as E. coli O25:H4-ST 131 (3, 5). In E. cloacae species, all SHV-12-producing isolates had similar RAPD patterns, in contrast to CTX-M-15-producing isolates, which were all different. TABLE 1. Frequency of resistance to non-β-lactam antibiotics of ESBL-producing Enterobacteriaceae isolates This study confirms that the worldwide diffusion of the CTX-M-15 enzyme has also reached West Africa. This was probably due to plasmid spread, since E. coli isolates, unlike SHV-12-producing E. cloacae isolates, were not genetically related (6). The spread of resistance in this area was more likely due to plasmids carrying antibiotic resistance genes than to bacterial transmission between patients. Maintenance of these plasmids was probably favored by antibiotic pressure. These findings emphasize the need for a policy of careful antibiotic use worldwide.

Extended-Spectrum β-Lactamase–Producing Enterobacteriaceae in Bulgarian Hospitals

The aim of the study was to describe the emergence, the spread, and the prevalence of extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae in Bulgaria. Over eight years (1996-2003), 442 ESBL-screen-positive isolates were collected in nine medical institutions in four Bulgarian towns. Class A ESBLs of the SHV, TEM, and CTX-M groups were identified in seven species. SHV-type enzymes persisted during the whole study period, TEM-ESBLs appeared first in 1999, and CTX-M-types appeared first in 2001. The rate of CTX-M enzyme producers increased rapidly between 2001 and 2003, while the rate of SHV producers decreased. Six different ESBL-types were identified, namely, SHV-2, -5, and -12, CTX-M-3 and -15, and a new TEM-3-like variant (TEM-139). The most widespread enzymes were SHV-12, CTX-M-15, and CTX-M-3 found in seven centers. TEM-139 was identified mainly in one center. A trend for strains harboring more than one ESBL gene, for example, CTX-M + SHV, was observed since 2002. Plasmid fingerprinting and random amplified polymorphic DNA analysis typing revealed wide dissemination of identical plasmids among different bacterial species and hospitals, as well as clonal spread of ESBL producers. Our data contribute to clarify the dynamics in the prevalence of ESBLs in Bulgaria and demonstrate the importance of molecular procedures for their analysis.

ESBL Genotypes in Fluoroquinolone‐Resistant and Fluoroquinolone‐Susceptible ESBL‐Producing Escherichia coli Urinary Isolates in Manitoba

Extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli are increasingly common in nosocomial and community settings. Furthermore, fluoroquinolone (FQ) and even multidrug resistance (MDR) appear to be associated with certain ESBL genotypes. The purpose of the present study was to determine which ESBL genotypes are associated with FQ and MDR in E coli urinary isolates in Manitoba. The authors determined the antimicrobial susceptibility, genetic similarity and ESBL genotype of 27 FQ-resistant and seven FQ-susceptible, ESBL-producing urinary isolates submitted to the clinical microbiology laboratories of two teaching hospitals between October 2000 and April 2005. Susceptibilities to beta-lactams, FQs, trimethoprim-sulfamethoxazole (SXT), doxycycline (DOX), gentamicin (GM) and tigecycline were determined by microbroth dilution; pulsed-field gel electrophoresis (PFGE) was used to determine genetic relatedness, and ESBL genotype was determined by polymerase chain reaction and sequencing. Of 34 ESBL-producing organisms, 27 (79.4%) were found to be ciprofloxacin (CIP) resistant, 27 (79.4%) were SXT resistant, eight (23.5%) were GM resistant and 29 (85.3%) were DOX resistant. Twenty-three (67.6%) had MDR, with concomitant resistance to CIP and SXT; 16 had concomitant resistance to CIP, SXT and DOX; and seven (20.6%) had MDR, with concomitant resistance to CIP, SXT, DOX and GM. All isolates were susceptible to tigecycline. Of 27 FQ-resistant ESBL-producing organisms, seven (25.9%) were genotype CTX-M-14, 19 (70.4%) were genotype CTX-M-15 and one (3.7%) was genotype CTX-M-24. Among the seven FQ-susceptible strains, three (42.8%) expressed SHV-type enzymes, three (42.8%) expressed TEM-type enzymes and one (14.3%) expressed CTX-M-9. CTX-M-15 was the most common MDR-associated genotype. Of a total of 19 strains, 18 (94.7%) were resistant to FQs and SXT; 15 (78.9%) were resistant to FQs, SXT and DOX; and five (26.3%) were resistant to FQs, SXT, DOX and GM. PFGE analysis revealed genetic similarity within CTX-M-15-producing isolates only. CTX-M-15 in E coli is strongly associated with an MDR phenotype compared with other genotypes. CTX-M-14 is associated with FQ resistance only. PFGE suggests clonality of CTX-M-15-producing isolates within and among hospitals.

Extended-Spectrum β-Lactamases and Plasmid-Mediated AmpC Enzymes among Clinical Isolates of Escherichia coli and Klebsiella pneumoniae from Seven Medical Centers in Taiwan

Production of extended-spectrum beta-lactamases and plasmid-mediated AmpC enzymes was investigated among 291 Escherichia coli and 282 Klebsiella pneumoniae isolates that showed decreased susceptibilities to extended-spectrum cephalosporins from seven Taiwanese medical centers. CTX-M-type and SHV-type enzymes were the most prevalent extended-spectrum beta-lactamases. CMY-2-like and DHA-1-like beta-lactamases were the most prevalent AmpC-type enzymes.

Extended spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae: considerations for diagnosis, prevention and drug treatment.

Extended spectrum beta-lactamase (ESBL)-producing organisms pose unique challenges to clinical microbiologists, clinicians, infection control professionals and antibacterial-discovery scientists. ESBLs are enzymes capable of hydrolysing penicillins, broad-spectrum cephalosporins and monobactams, and are generally derived from TEM and SHV-type enzymes. ESBLs are often located on plasmids that are transferable from strain to strain and between bacterial species. Although the prevalence of ESBLs is not known, it is clearly increasing, and in many parts of the world 10-40% of strains of Escherichia coli and Klebsiella pneumoniae express ESBLs. ESBL-producing Enterobacteriaceae have been responsible for numerous outbreaks of infection throughout the world and pose challenging infection control issues. Clinical outcomes data indicate that ESBLs are clinically significant and, when detected, indicate the need for the use of appropriate antibacterial agents. Unfortunately, the laboratory detection of ESBLs can be complex and, at times, misleading. Antibacterial choice is often complicated by multi-resistance. Many ESBL-producing organisms also express AmpC beta-lactamases and may be co-transferred with plasmids mediating aminoglycoside resistance. In addition, there is an increasing association between ESBL production and fluoroquinolone resistance. Although in in vitro tests ESBLs are inhibited by beta-lactamase inhibitors such as clavulanic acid, the activity of beta-lactam/beta-lactamase inhibitor combination agents is influenced by the bacterial inoculum, dose administration regimen and specific type of ESBL present. Currently, carbapenems are regarded as the drugs of choice for treatment of infections caused by ESBL-producing organisms. Unfortunately, use of carbapenems has been associated with the emergence of carbapenem-resistant bacterial species such as Stenotrophomonas sp. or Pseudomonas sp.

Occurrence of extended-spectrum beta-lactamases in members of the family Enterobacteriaceae in Italy: implications for resistance to beta-lactams and other antimicrobial drugs.

An Italian nationwide survey was carried out to assess the prevalences and the antimicrobial susceptibilities of members of the family Enterobacteriaceae producing extended-spectrum beta-lactamases (ESBLs). Over a 6-month period, 8,015 isolates were obtained from hospitalized patients and screened for resistance to extended-spectrum cephalosporins and monobactams. On the basis of a synergistic effect between clavulanate and selected beta-lactams (ceftazidime, aztreonam, cefotaxime, cefepime, and ceftriaxone), 509 isolates were found to be ESBL positive (6.3%). Colony blot hybridization with bla(TEM) and bla(SHV) DNA probes allowed one to distinguish four different genotypes: TEM-positive, SHV-positive, TEM- and SHV-positive, and non-TEM, non-SHV ESBL types. MICs for each isolate (E-test) were obtained for widely used beta-lactams, combinations of beta-lactams with beta-lactamase inhibitors, aminoglycosides, and fluoroquinolones. Among ESBL-positive strains, Klebsiella pneumoniae, Proteus mirabilis, and Escherichia coli accounted for 73.6% of isolates. Overall, TEM-type ESBLs were more prevalent than SHV-type enzymes (234 versus 173), whereas the prevalence of strains producing both TEM- and SHV-type ESBLs was similar to that of isolates producing non-TEM, non-SHV enzymes (55 and 38, respectively). In vitro, all but one of the ESBL-producing isolates remained susceptible to imipenem. Susceptibility to other drugs varied: piperacillin-tazobactam, 91%; amoxicillin-clavulanic acid, 85%; cefoxitin, 78%; amikacin, 76%; ampicillin-sulbactam, 61%; ciprofloxacin, 58%; and gentamicin, 56%. Associated resistance to aminoglycosides and ciprofloxacin was observed most frequently among TEM-positive strains. Since therapeutic options for multiresistant Enterobacteriaceae are limited, combinations of beta-lactams and beta-lactamase inhibitors appear to represent an important alternative for treating infections caused by ESBL-producing ENTEROBACTERIACEAE:

Detection of extended-spectrum beta-lactamases in clinical isolates of Enterobacter cloacae and Enterobacter aerogenes.

The aim of the present study was to investigate the frequency of extended-spectrum beta-lactamases (ESBLs) in a consecutive collection of clinical isolates of Enterobacter spp. The abilities of various screening methods to detect ESBLs in enterobacters were simultaneously tested. Among the 68 consecutive isolates (56 Enterobacter cloacae and 12 Enterobacter aerogenes isolates) that were analyzed for beta-lactamase content, 21 (25 and 58%, respectively) possessed transferable ESBLs with pIs of 8.2 and phenotypic characteristics of SHV-type enzymes, 8 (14.3%) of the E. cloacae isolates produced a previously nondescribed, clavulanate-susceptible ESBL that exhibited a pI of 6.9 and that conferred a ceftazidime resistance phenotype on Escherichia coli transconjugants, and 2 E. cloacae isolates produced both of these enzymes. Among the total of 31 isolates that were considered ESBL producers, the Vitek ESBL detection test was positive for 2 (6.5%) strains, and the conventional double-disk synergy test (DDST) with amoxicillin-clavulanate and with expanded-spectrum cephalosporins and aztreonam was positive for 5 (16%) strains. Modifications of the DDST consisting of closer application of the disks (at 20 instead of 30 mm), the use of cefepime, and the use of both modifications increased the sensitivity of this test to 71, 61, and 90%, respectively. Of the 37 isolates for which isoelectric focusing failed to determine ESBLs, the Vitek test was false positive for 1 isolate and the various forms of DDSTs were false-positive for 3 isolates.

Origin and impact of plasmid-mediated extended-spectrum beta-lactamases

Resistance to oxyimino cephalosporins was originally highlighted by the emergence of plasmid-encoded extended-spectrum beta-lactamases deriving by mutation from TEM-1, TEM-2 and SHV type enzymes (class A). The broader spectrum of resistance produced by these enzymes is related to more amino acid substitutions, but susceptibility to seven alpha-methoxyimino cephalosporins and carbapenems was preserved until recently. Clavulanate-sensitive extended-spectrum beta-lactamases are distributed worldwide, mainly among Klebsiella pneumoniae isolates. Novel clavulanate-sensitive extended-spectrum beta-lactamases deriving from other class A enzymes (e.g. MEN-1 from beta la OXY, OXA-11 in Pseudomonas aeruginosa from PSE-2) have been reported. Recently, clavulanate-resistant extended-spectrum beta-lactamases (class C) were encountered amongst single isolates, mostly Klebsiella pneumoniae. These cephalosporinases or cefamycinases (usually chromosomally mediated) have expanded the spectrum of plasmid-encoded resistance to include seven alpha-methoxyimino cephalosporins. Thus far, only two isolates (1 Pseudomonas aeruginosa, 1 Bacteroides fragilis), both recovered in Japan, with plasmid-mediated resistance to carbapenems have been found.

Characterization of chromosomally encoded penicillinases in clinical isolates of Klebsiella pneumoniae.

Four Klebsiella pneumoniae strains, isolated in two geographically distant French hospitals, were found to produce constitutive beta-lactamases with an unusual isoelectric point for this species (8.1). The four enzymes were chromosomally encoded and related to the Ambler's class A plasmid-mediated SHV-type enzymes. The four enzymes were predominantly penicillinases, with good activity against amino- and ureidopenicillins. They poorly hydrolysed cephalothin, cephaloridine and cefoperazone and did not hydrolyse third-generation cephalosporins and aztreonam. The four enzymes were susceptible to the inhibitory effect of clavulanic acid. Finally, the strains were all found to produce an acetyl-esterase. The acetyl-esterases catalysized hydrolysis of the acetoxy group of cephalothin and cefotaxime although this did not affect their antibacterial activities. These esterases were not susceptible to the inhibitory effect of clavulanic acid and EDTA.

New Aspects of Antimicrobial Resistance and the Resulting Therapeutic Dilemmas

Emergence of resistance to antimicrobial agents among previously susceptible organisms continues to be an important obstacle to the successful treatment of bacterial infections. In hospitals, plasmid-mediated resistance to third-generation cephalosporins and monobactams has recently appeared in gram-negative bacilli, due primarily to mutations in TEM- and SHV-type enzymes. Among nosocomial enterococci, vancomycin resistance, beta-lactamase production, and high-level resistance to all aminoglycosides have recently been added to this organism's already formidable armamentarium of resistance properties. Also, resistance to fluoroquinolones and rifampin has been emerging in methicillin-resistant Staphylococcus aureus. In the community, organisms in which resistance plays a particularly important role are shigellae, Haemophilus influenzae, gonococci, and pneumococci, particularly in developing countries. beta-lactamase-producing meningococci have been reported for the first time. The selective pressure generated by the use of antimicrobial agents, together with the ability of bacteria to acquire and spread resistance and the capacity of humans to transmit bacteria, suggest that antimicrobial resistance will continue to be a problem for the foreseeable future.

Properties of plasmids responsible for production of extended-spectrum beta-lactamases

The extended-spectrum beta-lactamases are believed to arise by mutations which alter the configuration around the active site of TEM- and SHV-type enzymes so as to increase their efficiency with otherwise nonhydrolyzable cephalosporins and monobactams. This hypothesis predicts that the genes for these new enzymes should be found on the same wide variety of plasmids that encode TEM-1, TEM-2, and SHV-1 beta-lactamases and that at least some of them should be mediated by transposons. Fifteen plasmids, each encoding an extended-spectrum beta-lactamase, were examined. Unlike the average TEM plasmid, all were large, ranging in size from 80 to 300 kb. All determined resistance to multiple antimicrobial agents, ranging from 5 to 11, and some conferred resistance to heavy metals and UV radiation as well. The plasmids belonged to a limited number of incompatibility (Inc) groups, including IncC, IncFI, IncHI2, and IncM. Because most of the mutations giving rise to extended-spectrum activity are G.C----A.T transitions and some of the mutant genes have as many as four base substitutions, a plasmid-determined mutator gene was searched for, but no such property was found. Several techniques were used to detect transposition of the extended-spectrum beta-lactamase genes, but a mobile genetic element could not be demonstrated even though eight of the plasmids hybridized with a DNA probe derived from the tnpR gene of Tn3. The genesis of extended-spectrum beta-lactamases may not be as simple as has been supposed.

Molecular evolution of ubiquitous β‐lactamases towards extended‐spectrum enzymes active against newer β‐lactam antibiotics

Production of beta-lactamases, and of the plasmid-encoded TEM- and SHV-type enzymes in particular, is the most common mechanism of resistance against beta-lactam antibiotics in Gram-negative bacteria. The two ubiquitous types of enzyme have a large spectrum of activity and preferentially hydrolyse the penicillins as well as some first- and second-generation cephalosporins. Recently, point mutations in the corresponding genes have been observed, apparently selected for, in the clinical setting, by originally 'beta-lactamase-stable' third-generation cephalosporins or by monobactams, which fall into the substrate range of the mutant or 'extended-spectrum' beta-lactamases. The point mutations are clustered in three areas, each adjacent to one of the seven evolutionarily conserved boxes described by Joris et al. (1988). The substituted amino acids at positions 102 (adjacent to the alpha-3 helix), 162 (adjacent to the alpha-7 helix) and 235, 236 and 237 (on the beta-3 strand) are located in close proximity to the active-site cavity and are thought to open up novel enzyme-substrate interactions, involving, in particular, the oxyimino moieties of the newer beta-lactam compounds.

Epidemiology of extended spectrum beta-lactamases.

Beta-lactamases play a major part in resistance, as recently redemonstrated by the emergence of extended spectrum beta-lactamases. Since its discovery in FR Germany, SHV-2 has been reported from four continents and CTX-1 (TEM-3) was established in at least 26 French hospitals. More than 12 other enzymes have been individualized. The newest aspect of resistance was probably underestimated because most strains of enterobacteria (mainly Klebsiella pneumoniae) appeared susceptible to oxyimino-beta-lactams as suggested by MICs or diameters of inhibition zone sizes. The double-disk synergy test between amoxicillin/clavulanic acid and oxyimino-beta-lactams was useful to easily detect two susceptibility patterns (CTX, CAZ). Extended spectrum beta-lactamases isolated among nosocomial isolates of enterobacteria (urines, blood, wound, sputum cultures) mostly from intensive care units have spread through hospitals. If outbreaks were described, numerous serotypes were identified in Klebsiella pneumoniae. In France the distribution of extended spectrum beta-lactamases showed that CTX-1 (TEM-3) was well distributed among ten species unlike SHV-type enzymes (SHV-2, SHV-3, SHV-4) preferentially detected in Klebsiella pneumoniae. A majority of strains produced CAZ-type enzymes in Escherichia coli. Some isolates produced two extended spectrum beta-lactamases. In Tunisia extended spectrum beta-lactamase producing strains were mainly identified among pediatric isolates of Klebsiella pneumoniae, Salmonella and Escherichia coli; SHV-2 was predominant but recently CTX-1 and two other types with an isoelectric point of 6.35 and 5.4 (phenotype CTX) were individualized. Because plasmid-encoded, this mechanism was spreading in France among enterobacteria with other resistance markers (e.g. netilmicin, amikacin) for CTX-1 unlike SHV-2.(ABSTRACT TRUNCATED AT 250 WORDS)