Comparative study of a novel plasmid-mediated beta-lactamase, CAZ-2, and the CTX-1 and CAZ-1 enzymes conferring resistance to broad-spectrum cephalosporins

Epidemiology of β-Lactamase-Producing Pathogens

β-Lactam antibiotics have been widely used as therapeutic agents for the past 70 years, resulting in emergence of an abundance of β-lactam-inactivating β-lactamases. Although penicillinases in Staphylococcus aureus challenged the initial uses of penicillin, β-lactamases are most important in Gram-negative bacteria, particularly in enteric and nonfermentative pathogens, where collectively they confer resistance to all β-lactam-containing antibiotics. Critical β-lactamases are those enzymes whose genes are encoded on mobile elements that are transferable among species. Major β-lactamase families include plasmid-mediated extended-spectrum β-lactamases (ESBLs), AmpC cephalosporinases, and carbapenemases now appearing globally, with geographic preferences for specific variants. CTX-M enzymes include the most common ESBLs that are prevalent in all areas of the world. In contrast, KPC serine carbapenemases are present more frequently in the Americas, the Mediterranean countries, and China, whereas NDM metallo-β-lactamases are more prevalent in the Indian subcontinent and Eastern Europe. As selective pressure from β-lactam use continues, multiple β-lactamases per organism are increasingly common, including pathogens carrying three different carbapenemase genes. These organisms may be spread throughout health care facilities as well as in the community, warranting close attention to increased infection control measures and stewardship of the β-lactam-containing drugs in an effort to control selection of even more deleterious pathogens.

Antibiotic resistance in the institutionalized elderly

Geriatric patients frequently are cared for in long term care facilities (LTCFs), which are now a major component of our health care delivery system. Nearly half of the 2.2 million people who turned 65 years old in 1990 will enter an LTCF at least once before they die. Infections are one of the principal causes of morbidity and mortality in LTCFs. Because LTCFs are a less costly alternative to hospitalization, clinicians are treating many serious infections in the nursing home. As a result of antibiotic use, LTCFs will increasingly be recognized as sources of organisms resistant to multiple antibiotics. b-Lactams are a valuable class of potent antimicrobials with broad-spectrum activity against Gram-negative and Gram-positive organisms. The safety and efficacy of this class of antibiotics make them easy choices for empiric treatment of infections in the elderly. Unfortunately, excessive use of these antibiotics has created serious threats to our therapeutic armamentarium: the emergence of methicillin-resistant Staphylococcus aureus and of Gram-negative pathogens resistant to third-generation cephalosporins such as cefotaxime, ceftazidime, and ceftriaxone. Of these third-generation cephalosporins, resistance to ceftazidime is most frequently recognized. The major mechanism responsible for ceftazidime resistance in Gram-negative bacteria is the production of b-lactamases. This article summarizes the diversity of b-lactamases, highlights the important enzymes that confer ceftazidime resistance in LTCFs, and details some methods used to identify and characterize these enzymes. A clear challenge is to apply these techniques to epidemiologic and molecular studies conducted in LTCFs.

Survey of plasmid-associated genetic markers in enterobacteriaceae with reduced susceptibilities to cephalosporins

Clinical isolates of Enterobacteriaceae with reduced susceptibilities to cephalosporins were collected from 1993 to 2000. The organisms were screened for the extended-spectrum beta-lactamase (ESBL) phenotype, and plasmid extracts were screened for genetic markers by hybridization. A bla(TEM) probe was derived from pUC19; other probes were derived from pACM1, the plasmid responsible for the first known appearance of an ESBL in our institution. These probes included bla(SHV), int, aac(3)-Ia, dfrA1, IS6100, tetA, IncM markers, and Anon 13, a marker for the Klebsiella pneumoniae chromosomal sequences that flank bla(SHV-5). There were 42 hybridization patterns among 237 isolates. Patterns designated pACM1-like occurred in 44% of the isolates (eight species) and were always associated with the clavulanic acid (CA)-susceptible ESBL phenotype. The TEM marker was not predictive of the ESBL phenotype. Mapping indicated the presence of an SHV marker and up to 7.5 kb of its flanking chromosomal sequences in three non-IncM plasmids obtained in transformation experiments. We theorize that this DNA segment spread to other plasmids from pACM1-like sources. CA insensitivity became more frequent with time and was usually associated with either the TEM marker or the absence of both bla markers. One plasmid-encoded enzyme with characteristics of an AmpC beta-lactamase was observed in a transformant lacking both TEM and SHV markers. Although SHV type ESBLs were a continuing source of reduced susceptibility to cephalosporins in our institution, organisms with different resistance mechanisms were added to the hospital microflora in later years. These changes might be related, in part, to ESBL control strategies implemented in 1995.

Ceftazidime. An update of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Ceftazidime is a third generation cephalosporin antibacterial agent which, since its introduction in the early 1980s, has retained a broad spectrum of in vitro antimicrobial activity and clinical utility in serious infections. However, increasing resistance to ceftazidime and other third generation cephalosporins, particularly among Enterobacteriaceae, due to the emergence of plasmid-mediated extended spectrum beta-lactamases and the class I chromosomally mediated beta-lactamases, is of concern. There is now a wealth of information on the pharmacokinetics of the drug. enabling ceftazidime to be used predictably, and with a low potential for adverse effects, in a diversity of patient populations. Overall, ceftazidime remains an effective agent for the treatment of serious infection, particularly those due to major nosocomial pathogens, and respiratory infections in patients with cystic fibrosis. Ceftazidime-containing regimens also remain an important option for the empirical therapy of febrile episodes in neutropenic patients. The tolerability profile of ceftazidime makes the drug a useful option in seriously ill patients who are at risk of developing adverse events with other antibacterial agents. Although patterns of bacterial resistance have changed in the ensuing years since its introduction, judicious use of this important agent will help maintain its present clinical utility.

Detection of Klebsiella pneumoniae and Escherichia coli strains producing extended-spectrum beta-lactamases

Plasmids encoding extended-spectrum beta-lactamases of the TEM, SHV, and AmpC families were introduced into common Escherichia coli and Klebsiella pneumoniae hosts to create a homogeneous panel for evaluating the abilities of five test systems to detect resistance to eight beta-lactam antibiotics. Although MICs, as determined by agar dilution or E test strips, were increased and disk diffusion zone diameters were diminished, breakpoints for resistance were often not reached, and neither approach was sensitive in detecting resistance to oxyimino-beta-lactams. The MicroScan 18-h microdilution or Vitek rapid automated procedures were similarly insensitive. Ceftazidime was the best single test antibiotic for detecting extended-spectrum beta-lactamase production. beta-Lactamases TEM-7 and TEM-12 were particularly difficult to detect. Because of such difficulties, the prevalence of extended-spectrum beta-lactamases is likely to be greater than is currently appreciated.

Selective digestive decontamination by erythromycin-base in a polyvalent intensive care unit

OBJECTIVE

To study the effect of selective digestive tract decontamination by erythromycin-base on the incidence of carriage and infection with MR Enterobacteriaceae producing an extended spectrum beta-lactamase (ESB).

DESIGN

After a 10-week prospective survey to ascertain the baseline incidence in two bays (1 and 3) of the same ICU, bay 1 was compared with bay 3 during a further survey of 6 months. The patients in bay 1 received erythromycin-base.

SETTING

Two non-contiguous bays, 1 and 3, of 4 beds, in the same polyvalent ICU of a university hospital.

PATIENT

Consecutive patients with unit stay longer than 2 days; 34 patients were included during the control period, 43 in bay 1 (decontamination) and 46 in bay 3 (control) during the trial period.

INTERVENTION

Erythromycin-base, 1 g t.i.d. in powder form administered by gastric tube to patients in bay 1 from admission to discharge.

MEASUREMENTS AND RESULTS

Digestive tract carriage was monitored by cultures of gastric and rectal swab specimens, sampled twice a week. Enterobacteriaceae were isolated on Drigalski agar with incorporated ceftazidime (4 mg/l). In bay 1 there was a decrease in ESB producing Enterobacteriaceae (23% vs 10%, p = 0.0004) from rectal swab, especially in K. pneumoniae (15% vs 2%, p = 10(-5)), during the decontamination period in comparison to the control period. During the trial period the only differences observed between bays 1 and 3 were in the gastric samples: K. pneumoniae were less often isolated in bay 1 than in bay 3 (0% vs 3%, p = 0.03). Intestinal carriage with multiresistant Enterobacteriaceae occurred in 28% patients in bay 1 and 30% patients in bay 3 during the trial period (p = 0.79). Erythromycin-base did not delay the carriage by patients in bay 1 (log rank test p = 0.42).

CONCLUSION

Erythromycin-base was not effective in preventing digestive tract carriage due to Enterobacteriaceae resistant to third generation cephalosporin by production of chromosomal cephalosporinase. The decrease in isolates containing K. pneumoniae in bay 1 cannot be definitively attributed to erythromycin-base, since the number of this species in bay 3 was low.

Translocation of antibiotic resistance determinants including an extended-spectrum beta-lactamase between conjugative plasmids of Klebsiella pneumoniae and Escherichia coli

The extended-spectrum beta-lactamase CAZ-7, derived from TEMs, was produced by two different strains of the family Enterobacteriaceae, Klebsiella pneumoniae and Escherichia coli, isolated from the same patient. Both isolates were resistant to amikacin. In addition, the K. pneumoniae strain was TEM-1 producing and resistant to gentamicin. An E. coli HB101 transconjugant obtained from K. pneumoniae, selected on ceftazidime, showed that CAZ-7 and amikacin resistance were encoded by an 85-kb Inc7 or M plasmid, while an E. coli HB101 transconjugant obtained from E. coli under the same conditions showed that CAZ-7 and amikacin resistance were encoded by a greater than 150-kb Inc6 or C plasmid. Two other E. coli HB101 transconjugants obtained from K. pneumoniae, selected on gentamicin or chloramphenicol, showed that TEM-1 and gentamicin resistance could be encoded either by a greater than 150-kb Inc6 or C plasmid or by an 85-kb Inc7 or M plasmid. It was hypothesized that the genes for beta-lactam and aminoglycoside resistances were located on translocatable sequences. EcoRI digestion and hybridizations obtained with blatem, aacA4, and IS15 probes demonstrated that the CAZ-7 gene, amikacin resistance gene, and IS15 element were clustered on an approximately 20-kb fragment common to 85- and greater than 150-kb plasmids. E. coli HB101 transconjugants from K. pneumoniae and E. coli isolates were used to obtain translocations of CAZ-7 and amikacin resistance and of TEM-1 and gentamicin resistance between the 85- and greater than 150-kb plasmids. This study shows a typical example of in vivo gene dissemination involving transposable elements which translocate multiresistance genes, including an extended-spectrum beta-lactamase.

A case-control study of an outbreak of infections caused by Klebsiella pneumoniae strains producing CTX-1 (TEM-3) beta-lactamase

In July 1984 Klebsiella pneumoniae producing beta-lactamase CTX-1(TEM-3) (K. pneumoniae-CTX-1) spread from an Intensive Care Unit (ICU) throughout the hospitals of Clermont-Ferrand, France, and were isolated in four other hospitals of the region. A retrospective case control study was conducted in the ICU to characterize the risk factors for nosocomial infection with this organism. The cases were the 74 patients who had had K. pneumoniae-CTX-1 isolated from one or more clinical samples between July 1984 and December 1987. They were compared with 74 controls for host risk factors, underlying disease, procedures and antibiotic treatment. The monthly incidence of infection/colonization varied from 0% to 14.6%. The mortality rate attributable to this organism was 0.26% during the study period. The duration of stay of cases was longer than that of controls. More cases than controls had ventilatory assistance. However, the predominant risk factor was emergency abdominal surgery. Before K. pneumoniae-CTX-1 was isolated, cases received quinolones and trimethoprim sulphamethoxazole more often than controls. However, only 15% of cases had received third generation cephalosporins while at the onset of K. pneumoniae-CTX-1 infection colonization, 32 patients were no longer being given antibiotics. The use of antibiotic prophylaxis by, for example, selective digestive tract decontamination should be considered in patients at high risk of infection.

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.

Ceftibuten and bactericidal kinetics. Comparative in vitro activity against Enterobacteriaceae producing extended spectrum beta-lactamases

Ceftibuten, compared to cefixime, cefetamet, cefpodoxime, loracarbef, cefprozil, cefuroxime, cefaclor, and cefadroxil, was the most active oral cephalosporin derivative against Enterobacteriaceae producing plasmid-encoded broad spectrum beta-lactamases. In a pharmacodynamic model, ceftibuten was bactericidal for Haemophilus influenzae and Streptococcus pneumoniae at concentrations simulating human serum levels following 200 mg, p.o., b.i.d.

Outbreak of ceftazidime resistance caused by extended-spectrum beta-lactamases at a Massachusetts chronic-care facility

During a 4-month period in late 1988, we isolated ceftazidime-resistant strains of Klebsiella pneumoniae and other members of the family Enterobacteriaceae from 29 patients at a chronic-care facility in Massachusetts. Ceftazidime resistance resulted from two distinct extended-spectrum beta-lactamases of the TEM type which efficiently hydrolyzed the cephalosporin: YOU-1 with a pI of 5.57 and YOU-2 with a pI of 5.2. Genes encoding these enzymes were present on different but closely related high-molecular-weight, multiple antibiotic resistance plasmids of the H12 incompatibility group and were transferable by conjugation in vitro. Agarose gel electrophoresis of extracts from clinical isolates indicated that this outbreak arose from plasmid transmission among different strains of the family Enterobacteriaceae rather than from dissemination of a single resistant isolate. Isolation rates of ceftazidime-resistant organisms transiently decreased after use of this drug was restricted, but resistant isolates continued to be recovered 7 months after empiric use of ceftazidime ceased.

Discrimination of extended-spectrum beta-lactamases by a novel nitrocefin competition assay

We describe a nitrocefin competition assay for determining inhibition profiles as a useful adjunct to existing biochemical methods for the discrimination of beta-lactamases. The hydrolysis rate of nitrocefin was measured with a plate photometer as the change in A480 over 45 min in the presence of 17 inhibitors. Fourteen well-established beta-lactamases and 13 extended-spectrum beta-lactamases were tested. Correlations with data from isoelectric focusing and amino acid sequencing suggested that the inhibition profile reflects alterations in the active-site configuration of beta-lactamases. The method was especially useful in measuring the relative affinities of beta-lactamases against poorly hydrolyzed substrates and in screening large numbers of isolates for the detection of new beta-lactamase types.

In vitro activities of 15 oral beta-lactams against Klebsiella pneumoniae harboring new extended-spectrum beta-lactamases

The activities of 15 oral beta-lactams against Klebsiella pneumoniae harboring new extended-spectrum beta-lactamases were studied. All compounds were affected by these enzymes, especially by the SHV derivatives. Except for ceftibuten, the compounds with the greatest intrinsic activity were more affected by the presence of these enzymes than were older compounds with moderate intrinsic activity.

Failure of ceftazidime-amikacin therapy for bacteremia and meningitis due to Klebsiella pneumoniae producing an extended-spectrum beta-lactamase

A multiple trauma patient failed treatment with ceftazidime and amikacin for bacteremia and meningitis due to a Klebsiella pneumoniae strain that produced a novel, plasmid-mediated beta-lactamase. Both pre- and posttreatment isolates were resistant to ceftazidime (MIC, greater than or equal to 64 micrograms/ml) and various penicillins but not to other expanded-spectrum cephalosporins. The beta-lactamase had a pI of 5.25 and was encoded on a conjugal plasmid of approximately 150 kilobases. DNA hybridization studies indicated that the enzyme was a TEM derivative.

Activities of beta-lactam antibiotics against Escherichia coli strains producing extended-spectrum beta-lactamases

Seven extended-spectrum beta-lactamases related to TEM and four enzymes derived from SHV-1 were transferred to a common Escherichia coli host so that the activity of a variety of beta-lactams could be tested in a uniform genetic environment. For most derivatives, penicillinase activity was 10% or less than that of strains making TEM-1, TEM-2, or SHV-1 beta-lactamase, suggesting that reduced catalytic efficiency accompanied the broader substrate spectrum. Despite this deficit, resistance to aztreonam, carumonam, cefdinir, cefepime, cefixime, cefmenoxime, cefotaxime, cefotiam, cefpirome, cefpodoxime, ceftazidime, ceftibuten, ceftizoxime, ceftriaxone, cefuroxime, and E1040 was enhanced. For strains producing TEM-type enzymes, however, MICs of carumonam, cefepime, cefmenoxime, cefotiam, cefpirome, and ceftibuten were 8 micrograms/ml or less. Susceptibilities of cefmetazole, cefotetan, cefoxitin, flomoxef, imipenem, meropenem, moxalactam, temocillin, FCE 22101, and Sch 34343 were unaffected. FCE 22101, imipenem, meropenem, and Sch 34343 were inhibitory for all strains at 1 microgram/ml or less. In E. coli an OmpF- porin mutation in combination with an extended-spectrum beta-lactamase enhanced resistance to many of these agents, but generally by only fourfold. Hyperproduction of chromosomal AmpC beta-lactamase increased resistance to 7-alpha-methoxy beta-lactams but not that to temocillin. When tested at 8 micrograms/ml, clavulanate was more potent than sulbactam or tazobactam in overcoming resistance to ampicillin, while cefoperazone-sulbactam was more active than ticarcillin-clavulanate or piperacillin-tazobactam, especially against TEM-type extended-spectrum beta-lactamases.

Structural features related to hydrolytic activity against ceftazidime of plasmid-mediated SHV-type CAZ-5 beta-lactamase

Tryptic peptides of the novel ceftazidimase CAZ-5 were sequenced by manual Edman degradation and aligned according to strong homology (more than 98%) with SHV-1 and SHV-2 beta-lactamase sequences. CAZ-5 differed from SHV-1 by five amino acid substitutions. Unusually high activity of CAZ-5 towards ceftazidime was imputed to substitution of a Lys for a Glu at position 214 of the mature protein.

Prospective survey of colonization and infection caused by expanded-spectrum-beta-lactamase-producing members of the family Enterobacteriaceae in an intensive care unit

The occurrence of expanded-spectrum-beta-lactamase-producing strains was prospectively studied in 56 patients in an intensive care unit. Ten patients were infected or colonized; some of these patients had asymptomatic intestinal colonization. Four different expanded-spectrum beta-lactamases were identified and used as epidemiological markers to survey nosocomial infections.

Multiplicity of TEM-derived beta-lactamases from Klebsiella pneumoniae strains isolated at the same hospital and relationships between the responsible plasmids

Five plasmid-mediated beta-lactamases conferring high-level resistance to ceftazidime were isolated from Klebsiella pneumoniae strains in the same hospital. These enzymes had isoelectric points ranging from 5.3 to 6.5 (CAZ-1, 5.55; CAZ-2, 6.0; CAZ-3, 5.3; CAZ-6, 6.5; and CAZ-7, 6.3). All isolates and their Escherichia coli transconjugants were highly resistant to amoxicillin (MICs, greater than 4,096 micrograms/ml), piperacillin (64 to 256 micrograms/ml), cephalothin (32 to 256 micrograms/ml), and ceftazidime (32 to 512 micrograms/ml) but remained moderately susceptible to cefotaxime (0.5 to 8 micrograms/ml). Only CAZ-6- and CAZ-7-producing strains were highly resistant to aztreonam (64 to 128 micrograms/ml). All the isolates remained susceptible to moxalactam and imipenem. The reduced activity of piperacillin, cefotaxime, ceftazidime, or aztreonam was restored by 2 micrograms of clavulanate, sulbactam, tazobactam, or brobactam per ml for E. coli producing CAZ-2, CAZ-3, and CAZ-7. Sulbactam had a lower protective effect than other inhibitors for E. coli harboring CAZ-1 and especially CAZ-6. Except for CAZ-1, which was mediated by a 150-kilobase (kb) plasmid (pCFF14), the other ceftazidimases were mediated by plasmids of 85 kb with EcoRI digestion patterns similar to that of pCFF04 encoding CTX-1 beta-lactamase. A TEM probe hybridized with a 19-kb EcoRI fragment of all these closely related plasmids.

TEM-4, a new plasmid-mediated beta-lactamase that hydrolyzes broad-spectrum cephalosporins in a clinical isolate of Escherichia coli

A clinical isolate of Escherichia coli, strain CB-134, recovered in 1986 from an abdominal abscess, exhibited resistance to penams, oxyimino-beta-lactams including broad-spectrum cephalosporins (cefotaxime, ceftriaxone, ceftazidime), and aztreonam but remained susceptible to cephamycins (cefoxitin, cefotetan) and to moxalactam and imipenem. Clavulanate (2 micrograms/ml) restored the susceptibility of the strain to broad-spectrum cephalosporins and aztreonam. A beta-lactamase with an isoelectric point (pI) of 5.9 was detected in strain CB-134, and the corresponding gene was transferred by conjugation to E. coli together with the associated aminoglycoside resistance determinant [AAC(3)-II] and tetracycline, trimethoprim, and sulfonamide resistance. The beta-lactamase efficiently hydrolyzed cefotaxime and ceftriaxone but only moderately hydrolyzed ceftazidime and was inhibited by clavulanate and sulbactam (1 microM) and by anti-TEM-1 and anti-TEM-2 sera. This extended-spectrum beta-lactamase, conferring resistance to cefotaxime, ceftriaxone, ceftazidime, and aztreonam, was comparable to CTX-1 (TEM-3) but differed from it by pI. Agarose gel electrophoresis of the plasmid DNA indicated that this new enzyme was coded by pUD16, a plasmid of 220 kilobases which belongs to the Inc6 incompatibility group. Hybridization with an intragenic probe for TEM-1 revealed that this beta-lactamase derives from TEM-type beta-lactamases and hence it was named TEM-4.

Novel plasmid-mediated beta-lactamase (TEM-10) conferring selective resistance to ceftazidime and aztreonam in clinical isolates of Klebsiella pneumoniae

Two clinical isolates of Klebsiella pneumoniae from seriously ill patients in Chicago, Ill., have been identified as resistant to ceftazidime and aztreonam but susceptible to other cephalosporins. This unusual antibiogram was shown to be due to a novel plasmid-mediated beta-lactamase which readily hydrolyzed ceftazidime and aztreonam in addition to penicillins such as piperacillin and carbenicillin. This enzyme and its attendant resistance were transferred to Escherichia coli by conjugation on a 50-kilobase plasmid. Isoelectric focusing revealed a single beta-lactamase band with a molecular weight of 29,000 and an isoelectric point of 5.57 in the resistant isolates and transconjugants. The beta-lactamase inhibitors clavulanic acid and sulbactam restored beta-lactam susceptibility in the resistant isolates. Fifty percent inhibitory concentrations of clavulanic acid and sulbactam were 4.4 and 940 nM, respectively. DNA hybridization studies indicated that this enzyme, designated TEM-10, is related to well-established TEM-type beta-lactamases. However, the TEM-10 enzyme was inhibited by p-chloromercuribenzoate, in contrast to TEM-2 beta-lactamase. On the basis of substrate and inhibition profiles, the TEM-10 enzyme could be easily discriminated from TEM-5 and RHH-I beta-lactamases.

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)