Detection of extended-spectrum beta-lactamases in clinical isolates of Klebsiella pneumoniae and Escherichia coli

Multicenter survey of the in vitro activity of four expanded-spectrum beta-lactams against consecutive contemporary clinical isolates

A survey of the in vitro susceptibility of consecutive clinical bacterial isolates to cefotaxime, ceftazidime, cefpodoxime, and aztreonam was conducted at 10 North American medical centers during the first quarter of 1997. All four drugs had good activity against Enterobacteriaceae and fastidious Gram-negative bacteria and poor activity against enterococci and methicillin-resistant staphylococci. No significant trends in susceptibility to cefotaxime or ceftazidime were observed when compared with a previous similar survey. The use of cefpodoxime MIC > 2.0 micrograms/mL as a marker for extended-spectrum beta-lactamase production by Escherichia coli and Klebsiella spp. had good sensitivity, but the specificity was very poor for E. coli. Thus, at least for E. coli, this screening test seems to be of questionable value unless used to indicate the need for additional tests such as clavulanic acid inhibition. Disk diffusion zone diameters correlated well with the MICs for all four drugs and support the current interpretive criteria.

Conventional and genetic laboratory tests used to guide antimicrobial therapy

Detection of antimicrobial resistance is important so that clinicians can make rational decisions about optimal antimicrobial therapy for their patients. During the past decade, new types of antimicrobial resistance have emerged, some of which present new challenges for the clinical microbiology laboratory. In most cases, conventional culture-based testing methods continue to be useful. In other situations in which the organism responsible for infection grows slowly (for example, Mycobacterium tuberculosis), culture methods are technically difficult (such as for human immunodeficiency virus), or genotypes are inconsistently expressed (for instance, methicillin resistance in staphylococci), genetic susceptibility testing methods may offer special advantages. Determining serum concentrations of antimicrobial agents may be useful both to ensure adequacy of treatment and to prevent toxicity. In this review, methods are described for conventional and genetic tests used to guide antimicrobial therapy.

Klebsiella spp. as nosocomial pathogens: epidemiology, taxonomy, typing methods, and pathogenicity factors

Bacteria belonging to the genus Klebsiella frequently cause human nosocomial infections. In particular, the medically most important Klebsiella species, Klebsiella pneumoniae, accounts for a significant proportion of hospital-acquired urinary tract infections, pneumonia, septicemias, and soft tissue infections. The principal pathogenic reservoirs for transmission of Klebsiella are the gastrointestinal tract and the hands of hospital personnel. Because of their ability to spread rapidly in the hospital environment, these bacteria tend to cause nosocomial outbreaks. Hospital outbreaks of multidrug-resistant Klebsiella spp., especially those in neonatal wards, are often caused by new types of strains, the so-called extended-spectrum-beta-lactamase (ESBL) producers. The incidence of ESBL-producing strains among clinical Klebsiella isolates has been steadily increasing over the past years. The resulting limitations on the therapeutic options demand new measures for the management of Klebsiella hospital infections. While the different typing methods are useful epidemiological tools for infection control, recent findings about Klebsiella virulence factors have provided new insights into the pathogenic strategies of these bacteria. Klebsiella pathogenicity factors such as capsules or lipopolysaccharides are presently considered to be promising candidates for vaccination efforts that may serve as immunological infection control measures.

Can results obtained with commercially available MicroScan microdilution panels serve as an indicator of beta-lactamase production among escherichia coli and Klebsiella isolates with hidden resistance to expanded-spectrum cephalosporins and aztreonam?

Among clinical isolates of Escherichia coli, Klebsiella pneumoniae, and Klebsiella oxytoca, there is an ever-increasing prevalence of beta-lactamases that may confer resistance to newer beta-lactam antibiotics that is not detectable by conventional procedures. Therefore, 75 isolates of these species producing well-characterized beta-lactamases were studied using two MicroScan conventional microdilution panels, Gram Negative Urine MIC 7 (NU7) and Gram Negative MIC Plus 2 (N+2), to determine if results could be utilized to provide an accurate indication of beta-lactamase production in the absence of frank resistance to expanded-spectrum cephalosporins and aztreonam. The enzymes studied included Bush groups 1 (AmpC), 2b (TEM-1, TEM-2, and SHV-1), 2be (extended spectrum beta-lactamases [ESBLs] and K1), and 2br, alone and in various combinations. In tests with E. coli and K. pneumoniae and the NU7 panel, cefpodoxime MICs of >/=2 microg/ml were obtained only for isolates producing ESBLs or AmpC beta-lactamases. Cefoxitin MICs of >16 microg/ml were obtained for all strains producing AmpC beta-lactamase and only 1 of 33 strains producing ESBLs. For the N+2 panel, ceftazidime MICs of >/=4 microg/ml correctly identified 90% of ESBL producers and 100% of AmpC producers among isolates of E. coli and K. pneumoniae. Cefotetan MICs of >/= 8 microg/ml were obtained for seven of eight producers of AmpC beta-lactamase and no ESBL producers. For tests performed with either panel and isolates of K. oxytoca, MICs of ceftazidime, cefotaxime, and ceftizoxime were elevated for strains producing ESBLs, while ceftriaxone and aztreonam MICs separated low-level K1 from high-level K1 producers within this species. These results suggest that microdilution panels can be used by clinical laboratories as an indicator of certain beta-lactamases that may produce hidden but clinically significant resistance among isolates of E. coli, K. pneumoniae, and K. oxytoca. Although it may not always be possible to differentiate between strains that produce ESBLs and those that produce AmpC, this differentiation is not critical since therapeutic options for patients infected with such organisms are similarly limited.

Outer membrane profiles of clonally related Klebsiella pneumoniae isolates from clinical samples and activities of cephalosporins and carbapenems

Fifteen isolates of Klebsiella pneumoniae producing extended-spectrum beta-lactamases (ESBLs) isolated during a nosocomial outbreak were studied. The strains belonged to the same clonal type, as shown by pulsed-field gel electrophoretic analysis of chromosomal DNA. All the isolates were resistant to extended-spectrum cephalosporins, aztreonam, gentamicin, and fluoroquinolones and were susceptible to carbapenems, tobramycin, netilmicin, and amikacin. None of the isolates expressed the OmpK36 porin. Eight isolates, for which the MICs of cefoxitin were > or = 64 micrograms/ml, showed a diminished level or no expression of a 35-kDa porin. The MICs of meropenem, cefotaxime, and cefpirome were three to eight times higher for porin-deficient isolates than for isolates expressing the 35-kDa porin, but the MICs of imipenem increased two times for porin-deficient isolates compared to those for isolates expressing the porin. This MIC increase reverted to a level similar to that for the parental strain when porin-deficient isolates were transformed with the gene coding for the K. pneumoniae porin OmpK36. It is concluded that the high level of resistance to cefoxitin and the increase in the MICs of meropenem, cefotaxime, and cefpirome for the ESBL-producing K. pneumoniae isolates studied are associated with porin deficiency.

Clinical strategies for serious infection: a North American perspective

In the United States, as in Europe, clinical strategies for serious infection are being increasingly driven by growing numbers of cephalosporin-resistant and multiresistant gram-negative bacilli. In a survey of nearly 400 hospital intensive care units in North America, resistance rates of Klebsiella to third-generation cephalosporins increased (from 3.6 to 14.4%) between 1990 and 1993. Resistance rates in Enterobacter are even higher, approaching 40%. Much of this resistance, which is due mainly to production of type-1 and extended spectrum beta-lactamases, appears to have arisen through overuse of third-generation cephalosporins and from poor hand-washing practices. In some American cities, a major reservoir of resistant organisms are nursing homes, where there is evidence of overuse of oral antibiotics. Currently, the most reliable agents available for the treatment of resistant gram-negative pathogens are the carbapenems, imipenem/cilastatin and meropenem, and the aminoglycoside, amikacin. A recent clinical study of meropenem monotherapy in patients with nosocomial pneumonia showed statistically significantly better clinical and microbiologic outcome compared with a standard regimen of ceftazidime plus tobramycin. The enhanced in vitro activity of meropenem against a number of key organisms may have been responsible for the superior results. Although the newer cephalosporins, cefepime and cefpirome, show greater stability to chromosomal type-1 beta-lactamases than ceftazidime, they have variable activity against extended spectrum beta-lactamase producers and can be rendered ineffective by permeability changes which occur in certain organisms. Carbapenems, on the other hand, possess good activity against virtually all of the pathogens which produce the clinically important beta-lactamases, and represent a reliable option for treatment.

Quinolone resistance from a transferable plasmid

BACKGROUND

Bacteria can mutate to acquire quinolone resistance by target alterations or diminished drug accumulation. Plasmid-mediated resistance to quinolones in clinical isolates has been claimed but not confirmed. We investigated whether a multiresistance plasmid could transfer resistance to quinolones between bacteria.

METHODS

We transferred resistance between strains by conjugation. The resistance plasmid was visualised in different hosts by agarose-gel electrophoresis. We determined the frequency of spontaneous mutations to ciprofloxacin or nalidixic-acid resistance in Escherichia coli strains, with or without the quinolone resistance plasmid.

FINDINGS

A multiresistance plasmid (pMG252) from a clinical isolate of Klebsiella pneumoniae was found to increase quinolone resistance to minimum inhibitory concentrations (MICs) as high as 32 microg/mL for ciprofloxacin when transferred to strains of K pneumoniae deficient in outer-membrane porins. Much lower resistance was seen when pMG252 was introduced into K pneumoniae or E coli strains with normal porins. The plasmid had a wide host range and expressed quinolone resistance in other enterobacteriaceae and in Pseudomonas aeruginosa. From a plasmid-containing E coli strain with ciprofloxacin MIC of 0.25 microg/mL and nalidixic-acid MIC of 32 microg/mL, quinolone-resistant mutants could be obtained at more than 100 times the frequency of a plasmid-free strain, reaching MICs for ciprofloxacin of 4 microg/mL and for nalidixic acid of 256 microg/mL.

INTERPRETATION

Transferable resistance to fluoroquinines and nalidixic acid has been found in a clinical isolate of K pneumoniae on a broad host range plasmid. Although resistance was low in wild-type strains, higher levels of quinolone resistance arose readily by mutation. Such a plasmid can speed the development and spread of resistance to these valuable antimicrobial agents.

Antimicrobial activity and spectrum investigation of eight broad-spectrum beta-lactam drugs: a 1997 surveillance trial in 102 medical centers in the United States. Cefepime Study Group

Because antimicrobial agents become less effective after the emergence of resistance mechanisms in clinically prevalent pathogens, physicians must utilize local, regional, and national antimicrobial susceptibility surveillance data to assist in choices of appropriate agents. An investigation of the spectrum and potency of eight broad-spectrum beta-lactam drugs (cefepime, cefotaxime, ceftazidime, ceftriaxone, imipenem, piperacillin with or without tazobactam, and ticarcillin/clavulanic acid) was performed using a common protocol and method (Etest; AB BIODISK, Solna, Sweden) in 102 clinical microbiology laboratories in the United States. A total of 9777 strains of Gram-negative bacilli were tested from late 1996 through April 1997. Quality assurance measures using three control strains observed quality control failures in 13 laboratories (usually ticarcillin/clavulanic acid or piperacillin), but only 2% of results required deletion. A total of 33.4% of Enterobacter spp. (1977 strains) were either resistant or intermediately susceptible to ceftazidime. Only imipenem (99.6% susceptible) and cefepime (99.1%) remained highly active against strains of Enterobacter, as well as Citrobacter freundii, indole-positive Proteae, and Serratia spp. Ceftazidime-resistant Escherichia coli and Klebsiella pneumoniae were detected at rates of 10.3% and 23.8%, respectively. Although these were participant-selected strains, only imipenem and cefepime had broad-spectrum coverage (> or = 97.1%) against these extended-spectrum beta-lactamase phenotypes. A dominant number of these extended-spectrum beta-lactamase phenotypes were reported from medical centers in the Northeast, but a nationwide distribution was observed. Among the nonenteric Gram-negative bacilli (4057 strains), the rank order of susceptibility (percent inhibited at published breakpoint concentrations) was: imipenem (86.1%) > piperacillin/tazobactam (80.1%) > cefepime (77.1%) > ceftazidime = piperacillin (74.9%) > ticarcillin/clavulanic acid (61.6%) > cefotaxime (18.2%) > ceftriaxone (12.9%). The cephalosporins, cefepime and ceftazidime, had rates of resistance for the 3005 Pseudomonas aeruginosa isolates of 10.1% and 14.4%, respectively. For all Gram-negative strains tested, only two contemporary beta-lactam antimicrobials exhibited > 90% inhibition of strains, imipenem at 93.6% and cefepime at 90.2%. These drugs were superior to the other tested compounds (48.8-84.3%). Ticarcillin/clavulanic acid had the narrowest spectrum of activity (48.8% of isolates susceptible). These results indicate that carbapenems and a new fourth-generation cephalosporin, cefepime, possess usable in vitro potencies against current clinical strains of Gram-negative bacilli, many of which harbored resistance to other antimicrobial agents.

Multicenter evaluation of the in vitro activity of six broad-spectrum beta-lactam antimicrobial agents in Puerto Rico. The Puerto Rico Antimicrobial Resistance Study Group

The minimum inhibitory concentrations of 6 broad-spectrum beta-lactam antimicrobial agents were determined by use of the Etest versus a total of 569 bacteria in 7 Puerto Rican hospital laboratories. These included 342 recent clinical isolates of Enterobacteriaceae, 63 Pseudomonas aeruginosa, 54 Acinetobacter species, and 110 oxacillin-susceptible staphylococci. Extended spectrum beta-lactamase production was noted among 11% of Klebsiella pneumoniae isolates. Hyperproduction of Amp C cephalosporinase was observed with > 20% of isolates of Enterobacter spp., Serratia spp., and Citrobacter freundii. The overall rank order of activity of the six beta-lactams examined in this study versus all clinical isolates was imipenem (95.8% susceptible) > cefepime (91.1%) > piperacillin/ tazobactam (82.3%) > cefotaxime (77.6%) > piperacillin (72.5%) > ceftazidime (67.0%).

Extended-spectrum beta-lactamases and other enzymes providing resistance to oxyimino-beta-lactams

Bacteria have once again demonstrated their remarkably versatility in meeting the introduction of new classes of beta-lactam antibiotics by modifying available plasmid mediated beta-lactamases to expand their spectrum of action and by incorporating chromosomal beta-lactamase genes onto plasmids that permit their spread to new hosts. Such resistance is more common than presently is appreciated because current NCCLS breakpoints for resistance underestimate its prevalence. A number of risk factors for acquisition of ESBL-producing K. pneumoniae have been defined, but most will be no easier to control than those for infection by MRSA or VRE. More clinical and animal model studies are needed to evaluate options for treatment. Most strains remain susceptible to imipenem and other carbapenems, but carbapenem resistance has appeared either by spread of metallo-beta-lactamase or by production of an AmpC enzyme combined with loss of an outer membrane porin channel. Attack on our adversaries' latest biological weapons is likely to require enhanced versatility on our part as well.

Occurrence and detection of extended-spectrum beta-lactamases in members of the family Enterobacteriaceae at a veterans medical center: seek and you may find

A total of 907 consecutive isolates of members of the family Enterobacteriaceae recovered during a 20-week period were tested for production of extended-spectrum beta-lactamases (ESBLs) by the double-disk (DD) potentiation method. Of 84 DD-positive isolates, 83 (9.2%) produced ESBLs based on isoelectric focusing. SHV-derived ESBLs and several TEM-derived ESBLs were present in nine species, including the first isolate of Citrobacter koserii and Morganella morganii known to harbor an SHV-derived ESBL. Results of testing 58 nonrepeat isolates for ESBL production by several recommended methods were as follows (percent detected in parentheses): DD method with aztreonam (95), ceftazidime (79), ceftriaxone (88), or cefpodoxime (90); broth microdilution method with ceftazidime (86) or cefotaxime (91) alone or in combination with clavulanate; and the standard disk diffusion method with new breakpoints and standard concentrations of aztreonam (78), ceftazidime (79), ceftriaxone (83), or cefpodoxime (98) or a novel concentration (5 microg) of ceftazidime (88). In three instances during an extended part of the study, an ESBL-producing isolate and a non-ESBL-producing isolate of the same species were recovered from a single blood culture bottle. These data indicate that ESBLs occur in several species of Enterobacteriaceae and at a relatively high incidence at our institution and that the standard disk diffusion method with cefpodoxime and the DD method with several beta-lactams are practical and cost-effective methods for detecting ESBL-producing isolates of Enterobacteriaceae.

Comparison of screening methods for detection of extended-spectrum beta-lactamases and their prevalence among blood isolates of Escherichia coli and Klebsiella spp. in a Belgian teaching hospital

Using a set of 33 well-defined extended-spectrum beta-lactamase (ESBL)-producing strains of Escherichia coli and Klebsiella pneumoniae, we compared three screening methods for ESBL detection: (i) a double-disk synergy test, (ii) a three-dimensional test (both the double-disk synergy test and the three-dimensional test were performed with ceftriaxone, ceftazidime, aztreonam, and cefepime), and (iii) the Etest ESBL screen (AB Biodisk, Solna, Sweden), based on the recognition of a reduction in the ceftazidime MIC in the presence of clavulanic acid. In the double-disk test, all four indicator antibiotics scored equally and 31 of the 33 reference strains were recognized. In the three-dimensional test, ceftriaxone was the only satisfactory indicator and 30 ESBL-positive strains were detected by this antibiotic. Both systems produced two false-positive results with cefepime. With the Etest ESBL screen, 15 of 16 TEM-related and 11 of 16 SHV-related ESBL-producing strains scored positive. In 10 cases the clavulanic acid on one end of the strip interfered with the MIC determination for ceftazidime, which was read on the opposite end. This MIC had to be determined with an extra ceftazidime-only strip. No false-positive results were noted. Eighty-six blood isolates of E. coli and Klebsiella species were screened for ESBL expression by the double-disk and three-dimensional tests, both with ceftriaxone. Six strains with suspicious antibiogram phenotypes also gave positive results by the double-disk test. One E. coli strain remained undetected by the three-dimensional test. Identification of the enzymes suspected of being ESBLs by isoelectric focusing (all strains) and DNA sequencing (1 strain) confirmed the screening test results except for one Klebsiella oxytoca strain, which proved to be a hyperproducer of its chromosomal enzyme and which also had a negative Etest score. The five true ESBL producers were all confirmed by the Etest ESBL screen. Pulsed-field gel electrophoresis proved that the E. coli strains were unrelated, but that two of the three K. pneumoniae strains were closely related.

Comparative in vitro activities of carbapenem L-749,345 and other antimicrobials against multiresistant gram-negative clinical pathogens

Carbapenems L-749,345 and imipenem had the lowest MICs at which 90% of isolates were inhibited (0.5 microg/ml) of 14 antimicrobial agents tested against 76 multiresistant gram-negative clinical isolates with TEM- or SHV-type extended-spectrum beta-lactamases and chromosomal or plasmid-determined AmpC beta-lactamases, but the MIC of L-749,345 for one isolate of Klebsiella pneumoniae was 16 microg/ml.

Detection and clinical significance of extended-spectrum beta-lactamases in a tertiary-care medical center

The prevalence of extended-spectrum beta-lactamase (ESBL)-mediated resistance remains unknown for most hospitals, and national guidelines for testing and reporting ESBL-mediated resistance have not yet been developed. We undertook a study to determine the prevalence of ESBLs and the clinical need for testing in our tertiary-care medical center. Members of the family Enterobacteriaceae isolated over a 6-month period for which ceftazidime or ceftriaxone MICs were greater than 1 microg/ml were tested for production of ESBLs by the double-disk synergy method. Approximately 1.5% of isolates of the family Enterobacteriaceae (50 of 3,273), which were isolated from 1.2% of patients (23 of 1,844), were found to express ESBLs. ESBL-producing strains included eight different species and were isolated from patients located throughout the hospital, including outpatient clinics. By using the interpretive guidelines of the National Committee for Clinical Laboratory Standards, 26 to 39% of the isolates would have been reported to be susceptible to ceftazidime, depending upon the routine susceptibility method used. However, tests with cefpodoxime found all of the ESBL-producing strains to be resistant or intermediate. Nine patients infected with ESBL-producing isolates were treated with therapy which included an expanded-spectrum cephalosporin. Seven were cured. The deaths of the other two patients were not attributed to bacterial resistance missed by routine susceptibility testing. These observations suggest that in our tertiary-care medical center, it may not be clinically necessary or cost-effective at this time to institute additional testing on a routine basis to detect ESBL production in all clinical isolates of the family Enterobacteriaceae.

Evaluation of the in vitro activity of cefepime compared to other broad-spectrum cephalosporins against clinical isolates from eighteen Brazilian hospitals by using the Etest

The in vitro activity of cefepime was compared to that of ceftazidime, ceftriaxone, and cefotaxime in a multicenter study involving 10 clinical microbiology laboratories and clinical isolates from 18 Brazilian hospitals from 7 cities (4 states). A total of 982 isolates consecutively collected between December 1995 and March 1996 were susceptibility tested by using Etest and following the NCCLS procedures for agar diffusion tests. The cefepime spectrum was broader than that of the other broad-spectrum cephalosporins against both Gram-negative rods and Gram-positive cocci. Cefepime was particularly more active against Enterobacter sp. (MIC90, 2 micrograms/ml), Serratia sp. (MIC90, 2 micrograms/ml) and oxacillin-susceptible Staphylococcus aureus (MIC90, 3 micrograms/ml). Against Pseudomonas aeruginosa, cefepime (MIC90, 16 micrograms/ml) was slightly more active than ceftazidime (MIC90, 32 micrograms/ml) and 8- to 16-fold more active than ceftriaxone of cefotaxime (MIC90, > 256 micrograms/ml). Our results show that nosocomial bacteria, especially Gram-negative rods, have a high rate of cephalosporin resistance in Brazil. However, part of these resistant bacteria remains susceptible to cefepime. The Etest was shown to be an excellent method for multicenter studies of the in vitro evaluation of new antimicrobial agents.

Survey and molecular genetics of SHV beta-lactamases in Enterobacteriaceae in Switzerland: two novel enzymes, SHV-11 and SHV-12

Sixty isolates of Enterobacteriaceae resistant to beta-lactam antibiotics were collected over a period of 2 years in Switzerland and screened by hybridization for the carriage of SHV genes. Thirty-four positive strains were found, and their SHV genes were amplified and sequenced. SHV extended-spectrum beta-lactamases (ESBLs) were found: 13 strains contained SHV-2a, 12 harbored SHV-2, and SHV-5 was found twice. Four strains were shown to contain SHV-1. In addition, we report two new SHV variants, termed SHV-11 (non-ESBL) and SHV-12 (ESBL). In spite of the carriage of SHV ESBLs, many strains showed only low resistance to one or more third-generation cephalosporins. In addition, 26 did not transfer the blaSHV gene in mating experiments.

Comparative susceptibility of clinical isolates producing extended spectrum beta-lactamases to ceftibuten: effect of large inocula

BACKGROUND

Infections caused by Enterobacteriaceae producing extended-spectrum beta-lactamases (ESBLs) are a growing clinical problem. However, there is wide variation in the level of resistance to third generation beta-lactams conferred by these enzymes.

METHODS

We studied 33 Klebsiella pneumoniae and 4 Escherichia coli isolates producing ESBLs obtained from outbreaks in 14 different hospitals and a nursing home in the United States. Microdilution testing with standard (10(4-5) colony-forming units/ml) and large (10(6-7) colony-forming units/ml) inocula, was used to compare the minimum inhibitory concentrations (MICs) of ceftibuten, a novel oral oxyimino beta-lactam, with those of other third generation beta-lactams (cefotaxime, ceftazidime, aztreonam, cefixime, cefpodoxime and cefoxitin).

RESULTS

Twenty-seven of the clinical isolates had well-characterized ESBLs of 10 different types, 7 of which produced TEM-1; 1 isolate also produced LXA-1. Two strains produced more than 1 ESBL. The remaining 10 strains produced 8 as yet uncharacterized types of ESBL. With large inocula 73% tested susceptible to ceftibuten, whereas 8 to 22% tested susceptible to the other third generation beta-lactam antibiotics. Ceftibuten MICs increased with higher inocula when tested against strains producing SHV-4 or SHV-5 and, to a lesser extent, strains producing multiple beta-lactamases. Only cefoxitin showed a smaller inoculum effect.

CONCLUSION

Ceftibuten merits clinical evaluation in infections caused by bacteria that produce ESBLs.