A new SHV-derived extended-spectrum beta-lactamase (SHV-24) that hydrolyzes ceftazidime through a single-amino-acid substitution (D179G) in the -loop

A Genotype-Phenotype Correlation Study of SHV β-Lactamases Offers New Insight into SHV Resistance Profiles

The SHV β-lactamases (BLs) have undergone strong allele diversification that has changed their substrate specificities. Based on 147 NCBI entries for SHV alleles, in silico mathematical models predicted 5 positions as relevant for the β-lactamase inhibitor (BLI)-resistant (2br) phenotype, 12 positions as relevant for the extended-spectrum BL (ESBL) (2be) phenotype, and 2 positions as related solely to the narrow-spectrum (2b) phenotype. These positions and six additional positions described in other studies (including one promoter mutation) were systematically substituted and investigated for their substrate specificities in a BL-free Escherichia coli background, representing, to our knowledge, the most comprehensive substrate and substitution analysis for SHV alleles to date. An in vitro analysis confirmed the essentiality of positions 238 and 179 for the 2be phenotype and of position 69 for the 2br phenotype. The E240K and E240R substitutions, which do not occur alone in known 2br SHV variants, led to a 2br phenotype, indicating a latent BLI resistance potential of these substitutions. The M129V, A234G, S271I, and R292Q substitutions conferred latent resistance to cefotaxime. In addition, seven positions that were found not always to be associated with the ESBL phenotype resulted in increased resistance to ceftaroline. We also observed that coupling of a strong promoter (IS26) to an A146V mutant with the 2b phenotype resulted in highly increased resistance to BLIs, cefepime, and ceftaroline but not to third-generation cephalosporins, indicating that SHV enzymes represent an underestimated risk for empirical therapies that use piperacillin-tazobactam or cefepime to treat different infectious diseases caused by Gram-negative bacteria.

Systematic research to overcome newly emerged multidrug-resistant bacteria

In the 1980s, I found that the chromosomal β-lactamase of Klebsiella pneumoniae LEN-1 showed a very high similarity to the R-plasmid-mediated penicillinase TEM-1 on the amino acid sequence level, and this strongly suggested the origination of TEM-1 from the chromosomal penicillinases of K. pneumoniae or related bacteria. Moreover, the chromosomal K1 β-lactamase (KOXY) of Klebsiella oxytoca was found to belong to the class A β-lactamases that include LEN-1 and TEM-1, although KOXY can hydrolyze cefoperazone (CPZ) like the chromosomal AmpC-type cephalosporinases of various Enterobacteriaceae that can hydrolyze several cephalosporins including CPZ. Furthermore, my collaborators and I found plural novel serine-type β-lactamases, such as MOX-1, SHV-24, TEM-91, CTX-M-64, CMY-9, CMY-19, GES-3, GES-4, and TLA-3, mediated by plasmids. Besides these serine-type β-lactamases, we also first identified exogenously acquired metallo-β-lactamases (MBLs), IMP-1 and SMB-1, in imipenem-resistant Serratia marcescens, and the IMP-1-producing S. marcescens TN9106 became the index case for carbapenemase-producing Enterobacteriaceae. I developed the sodium mercaptoacetic acid (SMA)-disk test for the simple identification of MBL-producing bacteria. We were also the first to identify a variety of plasmid-mediated 16S ribosomal RNA methyltransferases, RmtA, RmtB, RmtC, and NpmA, from various Gram-negative bacteria that showed very high levels of resistance to a wide range of aminoglycosides. Furthermore, we first found plasmid-mediated quinolone efflux pump (QepA) and fosfomycin-inactivating enzymes (FosA3 and FosK). We also first characterized penicillin reduced susceptible Streptococcus agalactiae, macrolide-resistant Mycoplasma pneumoniae, as well as Campylobacter jejuni, and Helicobacter pylori, together with carbapenem-resistant Haemophilus influenzae. We constructed a PCR-based open reading frame typing method for rapid identification of Acinetobacter baumannii international clones.

Structures of two aptamers with differing ligand specificity reveal ruggedness in the functional landscape of RNA

Two classes of riboswitches related to the ykkC guanidine-I riboswitch bind phosphoribosyl pyrophosphate (PRPP) and guanosine tetraphosphate (ppGpp). Here we report the co-crystal structure of the PRPP aptamer and its ligand. We also report the structure of the G96A point mutant that prefers ppGpp over PRPP with a dramatic 40,000-fold switch in specificity. The ends of the aptamer form a helix that is not present in the guanidine aptamer and is involved in the expression platform. In the mutant, the base of ppGpp replaces G96 in three-dimensional space. This disrupts the S-turn, which is a primary structural feature of the ykkC RNA motif. These dramatic differences in ligand specificity are achieved with minimal mutations. ykkC aptamers are therefore a prime example of an RNA fold with a rugged fitness landscape. The ease with which the ykkC aptamer acquires new specificity represents a striking case of evolvability in RNA.

ESBL-producing Escherichia coli and Its Rapid Rise among Healthy People

Since around the 2000s, Escherichia coli (E. coli) resistant to both oxyimino-cephalosporins and fluoroquinolones has remarkably increased worldwide in clinical settings. The kind of E. coli is also identified in patients suffering from community-onset infectious diseases such as urinary tract infections. Moreover, recoveries of multi-drug resistant E. coli from the feces of healthy people have been increasingly documented in recent years, although the actual state remains uncertain. These E. coli isolates usually produce extended-spectrum β-lactamase (ESBL), as well as acquisition of amino acid substitutions in the quinolone-resistance determining regions (QRDRs) of GyrA and/or ParC, together with plasmid-mediated quinolone resistance determinants such as Qnr, AAC(6')-Ib-cr, and QepA. The actual state of ESBL-producing E. coli in hospitalized patients has been carefully investigated in many countries, while that in healthy people still remains uncertain, although high fecal carriage rates of ESBL producers in healthy people have been reported especially in Asian and South American countries. The issues regarding the ESBL producers have become very complicated and chaotic due to rapid increase of both ESBL variants and plasmids mediating ESBL genes, together with the emergence of various "epidemic strains" or "international clones" of E. coli and Klebsiella pneumoniae harboring transferable-plasmids carrying multiple antimicrobial resistance genes. Thus, the current state of ESBL producers outside hospital settings was overviewed together with the relation among those recovered from livestock, foods, pets, environments and wildlife from the viewpoint of molecular epidemiology. This mini review may contribute to better understanding about ESBL producers among people who are not familiar with the antimicrobial resistance (AMR) threatening rising globally.

A Review of SHV Extended-Spectrum β-Lactamases: Neglected Yet Ubiquitous

β-lactamases are the primary cause of resistance to β-lactams among members of the family Enterobacteriaceae. SHV enzymes have emerged in Enterobacteriaceae causing infections in health care in the last decades of the Twentieth century, and they are now observed in isolates in different epidemiological settings both in human, animal and the environment. Likely originated from a chromosomal penicillinase of Klebsiella pneumoniae, SHV β-lactamases currently encompass a large number of allelic variants including extended-spectrum β-lactamases (ESBL), non-ESBL and several not classified variants. SHV enzymes have evolved from a narrow- to an extended-spectrum of hydrolyzing activity, including monobactams and carbapenems, as a result of amino acid changes that altered the configuration around the active site of the β -lactamases. SHV-ESBLs are usually encoded by self-transmissible plasmids that frequently carry resistance genes to other drug classes and have become widespread throughout the world in several Enterobacteriaceae, emphasizing their clinical significance.

Activity of ceftazidime/avibactam against isogenic strains of Escherichia coli containing KPC and SHV β-lactamases with single amino acid substitutions in the Ω-loop

OBJECTIVES

The objective of this study was to explore the activity of ceftazidime and ceftazidime/avibactam against a collection of isogenic strains of Escherichia coli DH10B possessing SHV and KPC β-lactamases containing single amino acid substitutions in the Ω-loop (residues 164-179).

METHODS

Ceftazidime and ceftazidime/avibactam MICs were determined by the agar dilution method for a panel of isogenic E. coli strains expressing SHV-1 and KPC-2 with amino acid substitutions at positions 164, 167, 169 or 179. Two KPC-2 β-lactamase variants that possessed elevated MICs of ceftazidime/avibactam were selected for further biochemical analyses.

RESULTS

Avibactam restored susceptibility to ceftazidime for all Ω-loop variants of SHV-1 with MICs <8 mg/L. In contrast, several of the Arg164 and Asp179 variants of KPC-2 demonstrated MICs of ceftazidime/avibactam >8 mg/L. β-Lactamase kinetics showed that the Asp179Asn variant of KPC-2 demonstrated enhanced kinetic properties against ceftazidime. The Ki app, k2/K and koff of the Arg164Ala and Asp179Asn variant KPC-2 β-lactamases indicated that avibactam effectively inhibited these enzymes.

CONCLUSIONS

Several KPC-2 variants demonstrating ceftazidime resistance as a result of single amino acid substitutions in the Ω-loop were not susceptible to ceftazidime/avibactam (MICs >8 mg/L). We hypothesize that this observation is due to the stabilizing interactions (e.g. hydrogen bonds) of ceftazidime within the active site of variant β-lactamases that prevent avibactam from binding to and inhibiting the β-lactamase. As ceftazidime/avibactam is introduced into the clinic, monitoring for new KPC-2 variants that may exhibit increased ceftazidime kinetics as well as resistance to this novel antibiotic combination will be important.

[One-year surveillance study of antimicrobial resistance in major bacteria except for MRSA isolated in eight medical facilities in Kumamoto prefecture]

To investigate the current status of drug-resistant bacteria (except MRSA) in Kumamoto prefecture, a study was conducted to determine the isolation numbers and ratios of extended-spectrum beta-lactamase (ESBL)-Escherichia coli, ESBL-Klebsiella species, ESBL-Proteus mirabilis, two-drug resistant Pseudomonas aeruginosa, (resistant to two drugs either carbapenems, quinolones and aminoglycosides) multi-drug-resistant Pseudomonas aeruginosa, multi-drug-resistant Acinetobacter baumannii, and vancomyacin-resistant Enterococcii in eight general hospitals from May in 2009 to April in 2010. ESBL-E. coli was mostly isolated, and two-drug resistant P. aeruginosa came second. The isolation ratio of overall drug-resistant bacteria did not increase, while the isolation ratio of two-drugs resistant P. aeruginosa declined, suggesting that infection control was successfully conducted in these hospitals. However, the isolation numbers of ESBL-Klebsiella spp. and two-drug resistant P. aeruginosa were variable in each hospital. Furthermore, drug-resistant bacteria were occasionally spread into another medical facilities by patients transferred from these hospitals, indicating that sharing information on drug-resistant bacteria between medical facilities is required.

Ligand-dependent disorder of the Omega loop observed in extended-spectrum SHV-type beta-lactamase

Among Gram-negative bacteria, resistance to β-lactams is mediated primarily by β-lactamases (EC 3.2.6.5), periplasmic enzymes that inactivate β-lactam antibiotics. Substitutions at critical amino acid positions in the class A β-lactamase families result in enzymes that can hydrolyze extended-spectrum cephalosporins, thus demonstrating an "extended-spectrum" β-lactamase (ESBL) phenotype. Using SHV ESBLs with substitutions in the Ω loop (R164H and R164S) as target enzymes to understand this enhanced biochemical capability and to serve as a basis for novel β-lactamase inhibitor development, we determined the spectra of activity and crystal structures of these variants. We also studied the inactivation of the R164H and R164S mutants with tazobactam and SA2-13, a unique β-lactamase inhibitor that undergoes a distinctive reaction chemistry in the active site. We noted that the reduced Ki values for the R164H and R164S mutants with SA2-13 are comparable to those with tazobactam (submicromolar). The apo enzyme crystal structures of the R164H and R164S SHV variants revealed an ordered Ω loop architecture that became disordered when SA2-13 was bound. Important structural alterations that result from the binding of SA2-13 explain the enhanced susceptibility of these ESBL enzymes to this inhibitor and highlight ligand-dependent Ω loop flexibility as a mechanism for accommodating and hydrolyzing β-lactam substrates.

Extended-spectrum beta-lactamases: a clinical update

Extended-spectrum beta-lactamases (ESBLs) are a rapidly evolving group of beta-lactamases which share the ability to hydrolyze third-generation cephalosporins and aztreonam yet are inhibited by clavulanic acid. Typically, they derive from genes for TEM-1, TEM-2, or SHV-1 by mutations that alter the amino acid configuration around the active site of these beta-lactamases. This extends the spectrum of beta-lactam antibiotics susceptible to hydrolysis by these enzymes. An increasing number of ESBLs not of TEM or SHV lineage have recently been described. The presence of ESBLs carries tremendous clinical significance. The ESBLs are frequently plasmid encoded. Plasmids responsible for ESBL production frequently carry genes encoding resistance to other drug classes (for example, aminoglycosides). Therefore, antibiotic options in the treatment of ESBL-producing organisms are extremely limited. Carbapenems are the treatment of choice for serious infections due to ESBL-producing organisms, yet carbapenem-resistant isolates have recently been reported. ESBL-producing organisms may appear susceptible to some extended-spectrum cephalosporins. However, treatment with such antibiotics has been associated with high failure rates. There is substantial debate as to the optimal method to prevent this occurrence. It has been proposed that cephalosporin breakpoints for the Enterobacteriaceae should be altered so that the need for ESBL detection would be obviated. At present, however, organizations such as the Clinical and Laboratory Standards Institute (formerly the National Committee for Clinical Laboratory Standards) provide guidelines for the detection of ESBLs in klebsiellae and Escherichia coli. In common to all ESBL detection methods is the general principle that the activity of extended-spectrum cephalosporins against ESBL-producing organisms will be enhanced by the presence of clavulanic acid. ESBLs represent an impressive example of the ability of gram-negative bacteria to develop new antibiotic resistance mechanisms in the face of the introduction of new antimicrobial agents.

Practical methods using boronic acid compounds for identification of class C beta-lactamase-producing Klebsiella pneumoniae and Escherichia coli

Detection of the resistance mediated by class C beta-lactamases remains a challenging issue, considering that transferable plasmid-mediated class C beta-lactamases are of worldwide concern. Methods for the identification of strains that produce extended-spectrum beta-lactamases (ESBLs) or metallo-beta-lactamases (MBLs) have been developed and applied for routine use in clinical microbiology laboratories, but no practical methods for identification of plasmid-mediated class C producers have been established to date. We therefore developed three simple methods for clinical microbiology laboratories that allow identification of plasmid-mediated class C beta-lactamase-producing bacteria using a boronic acid derivative, 3-aminophenylboronic acid (APB), one of the specific inhibitors of class C beta-lactamases. Detection by the disk potentiation test was based on the enlargement of the growth-inhibitory zone diameter (by greater than or equal to 5 mm) around a Kirby-Bauer disk containing a ceftazidime (CAZ) or a cefotaxime (CTX) disk in combination with APB. In a double-disk synergy test, the discernible expansion of the growth-inhibitory zone around the CAZ or the CTX disk toward a disk containing APB was indicative of class C beta-lactamase production. A greater than or equal to eightfold decrease in the MIC of CAZ or CTX in the presence of APB was the criterion for detection in the microdilution test. By using these methods, Escherichia coli and Klebsiella pneumoniae isolates producing plasmid-mediated class C beta-lactamases, ACT-1, CMY-2, CMY-9, FOX-5, LAT-1, and MOX-1, were successfully distinguished from those producing other classes of beta-lactamases, such as ESBLs and MBLs. These methods will provide useful information needed for targeted antimicrobial therapy and better infection control.

Molecular characterization of a cephamycin-hydrolyzing and inhibitor-resistant class A beta-lactamase, GES-4, possessing a single G170S substitution in the omega-loop

The nosocomial spread of six genetically related Klebsiella pneumoniae strains producing GES-type beta-lactamases was found in a neonatal intensive care unit, and we previously reported that one of the six strains, strain KG525, produced a new beta-lactamase, GES-3. In the present study, the molecular mechanism of cephamycin resistance observed in strain KG502, one of the six strains described above, was investigated. This strain was found to produce a variant of GES-3, namely, GES-4, which was responsible for resistance to both cephamycins (cefoxitin MIC, >128 microg/ml) and beta-lactamase inhibitors (50% inhibitory concentration of clavulanic acid, 15.2 +/- 1.7 microM). The GES-4 enzyme had a single G170S substitution in the Omega-loop region compared with the GES-3 sequence. This single amino acid substitution was closely involved with the augmented hydrolysis of cephamycins and carbapenems and the decreased affinities of beta-lactamase inhibitors to GES-4. A cloning experiment and sequencing analysis revealed that strain KG502 possesses duplicate bla(GES-4) genes mediated by two distinct class 1 integrons with similar gene cassette configurations. Moreover, the genetic environments of the bla(GES-4) genes found in strain KG502 were almost identical to that of bla(GES-3) in strain KG525. From these findings, these two phenotypically different strains were suggested to belong to a clonal lineage. The bla(GES-4) gene found in strain KG502 might well emerge from a point mutation in the bla(GES-3) gene harbored by its ancestor strains, such as strain KG525, under heavy antibiotic stress in order to acquire extended properties of resistance to cephamycins and carbapenems.

Escherichia coli producing CTX-M-2 beta-lactamase in cattle, Japan

From November 2000 to June 2001, Escherichia coli strains producing CTX-M-2 β-lactamase were isolated from 6 (1.5%) of 396 cattle fecal samples and 2 (0.7%) of 270 surface swabs of cattle carcasses in Japan. The bla_CTX-M-2 gene responsible for CTX-M-2 production was encoded on transferable plasmids, and the gene was transferred to E. coli CSH2 with a very high frequency (2 x 10^-4 to 6 x 10^-1 per donor cells) by conjugation. Random amplified polymorphic DNA analysis of nine isolates showed at least five different patterns. These findings suggest that CTX-M-2 producers might have originated from cattle through the use of cephalosporins such as ceftiofur and that cattle could be a reservoir of CTX-M-2–producing E. coli. Continuous and strategic surveillance of antimicrobial-resistant bacteria in livestock is essential to suppress further dissemination of these bacteria into society at large.

A new TEM-derived extended-spectrum beta-lactamase (TEM-91) with an R164C substitution at the omega-loop confers ceftazidime resistance

A new plasmid-mediated TEM-derived extended-spectrum beta-lactamase, TEM-91, was identified in a ceftazidime-resistant (MIC, >128 microg per ml) Escherichia coli strain isolated in 1996 in Japan. TEM-91 has three amino acid substitutions, R164C, M184T, and E240K, compared with TEM-1 penicillinase. The isoelectric point (pI), K(m), and k(cat) of TEM-91 for ceftazidime were 5.7, 179 microM, and 29.0 s(-1), respectively. The K(i) of clavulanic acid for ceftazidime hydrolysis was 30.3 nM.

Prevalence of extended-spectrum beta-lactamase-producing Gram-negative bacteria in septicaemic neonates in a tertiary care hospital

The present study was undertaken to investigate the high incidence of multiresistant Gram-negative bacilli causing neonatal septicaemia. Samples of neonatal blood from 728 suspected cases were obtained in brain heart infusion broth with sodium polyanethol sulfonate. All Gram-negative rods isolated were subsequently subjected to routine antimicrobial susceptibility testing and tests for extended-spectrum beta-lactamase (ESBL) production, as per NCCLS recommendations. ESBL was detected in 86.6% of Klebsiella spp., 73.4% of Enterobacter spp. and 63.6% of Escherichia coli strains. It was also observed that 74.4-80.9% of these ESBL producers were resistant to cefotaxime and 47.6-59.5% were resistant to ceftazidime in routine susceptibility testing. Some ESBL producers (36.3-61.5%) were found to be susceptible to either or both cephalosporins used in this study. It is concluded that indiscriminate use of third-generation cephalosporins may be responsible for the selection of ESBL-producing multiresistant strains in the neonatal intensive-care unit (NICU).

An analysis of why highly similar enzymes evolve differently

The TEM-1 and SHV-1 beta-lactamases are important contributors to resistance to beta-lactam antibiotics in gram-negative bacteria. These enzymes share 68% amino acid sequence identity and their atomic structures are nearly superimposable. Extended-spectrum cephalosporins were introduced to avoid the action of these beta-lactamases. The widespread use of antibiotics has led to the evolution of variant TEM and SHV enzymes that can hydrolyze extended-spectrum antibiotics. Despite being highly similar in structure, the TEM and SHV enzymes have evolved differently in response to the selective pressure of antibiotic therapy. Examples of this are at residues Arg164 and Asp179. Among TEM variants, substitutions are found only at position 164, while among SHV variants, substitutions are found only at position 179. To explain this observation, the effects of substitutions at position 164 in both TEM-1 and SHV-1 on antibiotic resistance and on enzyme catalytic efficiency were examined. Competition experiments were performed between mutants to understand why certain substitutions preferentially evolve in response to the selective pressure of antibiotic therapy. The data presented here indicate that substitutions at position Asp179 in SHV-1 and Arg164 in TEM-1 are more beneficial to bacteria because they provide increased fitness relative to either wild type or other mutants.

Three cefotaximases, CTX-M-9, CTX-M-13, and CTX-M-14, among Enterobacteriaceae in the People's Republic of China

Of 15 extended-spectrum beta-lactamase (ESBL)-producing isolates of the family Enterobacteriaceae collected from the First Municipal People's Hospital of Guangzhou, in the southern part of the People's Republic of China, 9 were found to produce CTX-M ESBLs, 3 produced SHV-12, and 3 produced both CTX-M and SHV-12. Eleven isolates produced either TEM-1B or SHV-11, in addition to an ESBL. Nucleotide sequence analysis of the 12 isolates carrying bla(CTX-M) genes revealed that they harbored three different bla(CTX-M) genes, bla(CTX-M-9) (5 isolates), bla(CTX-M-13) (1 isolate), and bla(CTX-M-14) (6 isolates). These genes have 98% nucleotide homology with bla(Toho-2). The bla(CTX-M) genes were carried on plasmids that ranged in size from 35 to 150 kb. Plasmid fingerprints and pulsed-field gel electrophoresis showed the dissemination of the bla(CTX-M) genes through transfer of different antibiotic resistance plasmids to different bacteria, suggesting that these resistance determinants are highly mobile. Insertion sequence ISEcp1, found on the upstream region of these genes, may be involved in the translocation of the bla(CTX-M) genes. This is the first report of the occurrence of SHV-12 and CTX-M ESBLs in China. The presence of strains with these ESBLs shows both the evolution of bla(CTX-M) genes and their dissemination among at least three species of the family Enterobacteriaceae, Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae, isolated within a single hospital. The predominance of CTX-M type enzymes seen in this area of China appears to be similar to that seen in South America but is different from those seen in Europe and North America, suggesting different evolutionary routes and selective pressures. A more comprehensive survey of the ESBL types from China is urgently needed.

CTX-M-type extended-spectrum beta-lactamase that hydrolyzes ceftazidime through a single amino acid substitution in the omega loop

Escherichia coli ILT-1, Klebsiella pneumoniae ILT-2, and K. pneumoniae ILT-3 were isolated in May 1999 in Paris, France, from a rectal swab of a hospitalized 5-month-old girl. These isolates had a clavulanic acid-inhibited substrate profile that included expanded-spectrum cephalosporins. The MICs of cefotaxime were higher for E. coli ILT-1 and K. pneumoniae ILT-2 than for K. pneumoniae ILT-3, while the opposite was found for the MICs of ceftazidime. Genetic and biochemical analyses revealed that E. coli ILT-1 and K. pneumoniae ILT-2 produced the CTX-M-18 beta-lactamase, while K. pneumoniae ILT-3 produced the CTX-M-19 beta-lactamase. The amino acid sequence of the CTX-M-18 beta-lactamase differed from that of the CTX-M-9 beta-lactamase by an Ala-to-Val change at position 231, while CTX-M-19 possessed an additional Pro-to-Ser change at position 167 in the omega loop of Ambler class A enzymes. The latter amino acid substitution may explain the CTX-M-19-mediated hydrolysis of ceftazidime, which has not been reported for other CTX-M-type enzymes. The bla(CTX-M-18) and bla(CTX-M-19) genes were located on transferable plasmids that varied in size (ca. 60 and 50 kb, respectively) but that showed similar restriction patterns.

Evolution and epidemiology of extended-spectrum beta-lactamases (ESBLs) and ESBL-producing microorganisms

The rapid and irrepressible increase in antimicrobial resistance of pathogenic bacteria that has been observed over the last two decades is widely accepted to be one of the major problems of human medicine today. Several aspects of this situation are especially worrying. There are resistance mechanisms that eliminate the use of last-choice antibiotics in the treatment of various kinds of infection. Many resistance mechanisms that emerge and spread in bacterial populations are those of wide activity spectra, which compromise all or a majority of drugs belonging to a given therapeutic group. Some mechanisms of great clinical importance require specific detection procedures, as they may not confer clear resistance in vitro on the basis of the interpretive criteria used in standard susceptibility testing. Finally, multiple mechanisms affecting the same and/or different groups of antimicrobials coexist and are even co-selected in more and more strains of pathogenic bacteria. The variety of beta-lactamases with wide spectra of substrate specificity illustrates very well all the phenomena mentioned above. Being able to hydrolyze the majority of beta-lactams that are currently in use, together they constitute the most important resistance mechanism of Gram-negative rods. Three major groups of these enzymes are usually distinguished, class C cephalosporinases (AmpC), extended-spectrum beta-lactamases (ESBLs) and different types of beta-lactamases with carbapenemase activity, of which the so-called class B metallo-beta-lactamases (MBLs) are of the greatest concern. This review is focused on various aspects of the evolution and epidemiology of ESBLs; it does not cover the problems of ESBL detection and clinical relevance of infections caused by ESBL-producing organisms.

SHV-16, a beta-lactamase with a pentapeptide duplication in the omega loop

A clinical isolate of Klebsiella pneumoniae was found to be resistant to ampicillin (MIC of 128 microg/ml), ticarcillin (MIC of 512 microg/ml), and ceftazidime (MIC of 128 microg/ml) and susceptible to all other beta-lactams; a synergistic effect between clavulanate and ceftazidime suggested the presence of an extended-spectrum beta-lactamase (ESBL). Transconjugants in Escherichia coli were obtained at low levels (10(-7) per donor cell) and exhibited a similar beta-lactam resistance pattern (resistant to ampicillin, ticarcillin, and ceftazidime at 64 microg/ml). The ESBL, pI 7.6, was encoded by a large plasmid (>100 kb) which did not carry any other resistance determinant. The ESBL-encoding gene was amplified by PCR using bla(SHV)-specific primers and was sequenced. The deduced amino acid sequence of the SHV-16 ESBL showed that it differed from SHV-1 by only a pentapeptide insertion (163DRWET167) corresponding to a tandem duplication in the omega loop. The implication of the 163a-DRWET163b-DRWET sequence in ceftazidime resistance was confirmed by cloning either bla(SHV-1) or bla(SHV-16) in the same vector, subsequently introduced in the same E. coli strain. Under these isogenic conditions, SHV-16 conferred a 32-fold increase in ceftazidime MIC compared to that with SHV-1. Furthermore, site-directed mutagenesis experiments modifying either E166aA or E166bA revealed that the functional glutamic residue was that located in the first copy of the duplicated sequence. But surprisingly, the second E166b also conferred a low-level resistance to ceftazidime. This work is the first description of a class A enzyme exhibiting an extended substrate specificity due to an insertion instead of a nucleotide substitution(s) in a clinical isolate.