Outcome of cephalosporin treatment for serious infections due to apparently susceptible organisms producing extended-spectrum beta-lactamases: implications for the clinical microbiology laboratory

Basic pharmacodynamics of antibacterials with clinical applications to the use of β-lactams, glycopeptides, and linezolid

Time above MIC for free drug concentrations is the important PK-PD parameter correlating with the efficacy of beta-lactam antibiotics. The duration of time plasma concentrations needed to exceed the MIC is relatively similar for most organisms except staphylococci. Neutrophils contribute very little to the overall activity of beta-lactams. The appearance of increasing antimicrobial resistance can challenge the efficacy of these drugs when concentrations do not exceed the MIC for 40% to 50% of the dosing interval. Time above MIC with oral amoxicillin and amoxicillin-clavulanate can be enhanced with high-dose formulations. Time above MIC with parenteral preparations can be enhanced by longer intravenous infusions or even continuous infusion. The 24-hour AUC-MIC is probably the important PK-PD parameter correlating with the efficacy of vancomycin and teicoplanin. It clearly is the important parameter for the efficacy of linezolid. Usual doses of these drugs generally provide adequate plasma concentrations to treat effectively infections in which plasma concentrations are predictive of tissue concentrations. Penetration of these drugs into respiratory secretions, such as ELF, is enhanced for linezolid and reduced for vancomycin. This may give linezolid an advantage over vancomycin in certain respiratory infections.

Detection of antimicrobial resistance by small rural hospital microbiology laboratories: comparison of survey responses with current NCCLS laboratory standards

Microbiology laboratory personnel from 77 rural hospitals in Idaho, Nevada, Utah, and eastern Washington were surveyed in July 2000 regarding their routine practices for detecting antimicrobial resistance. Their self-reported responses were compared to recommended laboratory practices. Most hospitals reported performing onsite bacterial identification and susceptibility testing. Many reported detecting targeted antimicrobial resistant organisms. While only 5/61 hospitals (8%) described using screening tests capable of detecting all 8 targeted types of resistance, most (57/61, 93%) were capable of accurately screening for at least 6 types. Conversely, most hospitals (58/61, 95%) reported confirmatory testing capable of identifying only 3 or fewer resistance types with high-level penicillin resistance among pneumococci, methicillin and vancomycin resistance among staphylococci and enterococci, and extended spectrum beta-lactamase production by Gram-negative bacilli presenting the greatest difficulties. Furthermore, only 50% of hospitals compiled annual antibiogram reports to help physicians choose initial therapy for suspected infectious illnesses. This survey suggests that the antimicrobial susceptibility testing in many rural hospitals may be unreliable.

Extended spectrum beta lactamase-producing Klebsiella pneumoniae infections: a review of the literature

Infections caused by extended-spectrum beta-lactamase (ESBL)-producing pathogens, particularly Klebsiella pneumoniae, are increasing. The epidemiology of ESBL-producing K. pneumoniae, the mechanisms of resistance, and treatment strategies for infections caused by these organisms are reviewed.

Relationships between patient- and institution-specific variables and decreased antimicrobial susceptibility of Gram-negative pathogens

The identification of patients infected with antibiotic-resistant strains of bacteria for inclusion in clinical trials remains a serious challenge for the future development of agents for use against such infections. To identify patient- and institution-specific factors predictive of reduced susceptibility of Enterobacter species, Pseudomonas aeruginosa, and Klebsiella pneumoniae to cefepime, ciprofloxacin, and piperacillin-tazobactam, 5 years (1997-2001) of North American surveillance data were analyzed. The relationship between minimum inhibitory concentration (MIC) values for each organism-agent pair and patient- and institution-specific variables was analyzed using multivariable general linear modeling. The variables most commonly associated with decreases in susceptibility were duration of hospital stay before pathogen isolation, hospital size, primary diagnosis, and medical service. Combinations of these variables were associated with increases in observed MIC90 values of as much as 16-32-fold. Our findings demonstrate a relationship between MIC and certain patient- and institution-specific variables. Such data should be considered in the design of clinical trials directed at the study of resistant pathogens.

Pediatric septic shock and multiple organ failure

In the last 5 years, the understanding of the epidemiology and pathogenesis of pediatric sepsis, septic shock, and multiple organ failure has expanded greatly. There has also been a substantial increase in the number of successful randomized trials in which success has been measured as reduction in mortality in adults, children, and newborns. This article discusses these advances, updating the 1997 article on septic shock written by the author and by Dr. Robert E. Cunnion and following the format of the 1997 article.

Evaluation of MicroScan ESBL confirmation panel for Enterobacteriaceae-producing, extended-spectrum beta-lactamases isolated in Japan

We assessed use of the MicroScan ESBL confirmation panel (Dade Behring, Tokyo, Japan) for the detection of eight Enterobacteriaceae-producing extended-spectrum beta-lactamases (ESBL) species. Of 137 bacterial strains isolated from patients in 32 hospitals in the Kinki area of Japan, 91 produced ESBL and comprised 60 bacteria (of E. coli, K. oxytoca, and K. pneumoniae) targeted by the NCCLS ESBL test and 31 non-target bacteria such as chromosomal AmpC-producing bacteria (e.g., Serratia marcescens, Enterobacter spp.). Sensitivity and specificity of the MicroScan panel for the target bacteria were 92% and 93%, respectively; sensitivity and specificity for non-target bacteria were 52% and 100%, respectively. There were 20 ESBL-positive strains that were not inhibited by clavulanic acid in the MicroScan panel (3 of 32 ESBL-producing E. coli strains, 1 of 24 K. pneumoniae, 1 of 4 K. oxytoca, 8 of 13 E. cloacae, and 7 of 14 S. marcescens), and most of them were bacteria not targeted by the NCCLS test. In 19 of the 20 strains, the synergy effect of clavulanic acid was observed in the modified-double-disk synergy test using only the cefepime-disk. Because these strains had high MICs of > or = 16 microg/ml for cephamycins such as cefoxitin and cefmetazole, these strains might produce high levels of AmpC in addition to ESBL. The MicroScan ESBL confirmation panel showed excellent performance in detecting target, but not other bacteria. Addition of cefepime and clavulanic acid to the MicroScan panel may significantly improve detection of non-target bacteria.

Serious bacterial infections in febrile infants younger than 90 days of age: the importance of ampicillin-resistant pathogens

BACKGROUND

Intrapartum antibiotic prophylaxis against group B Streptococcus (GBS) has reduced the occurrence of serious bacterial infections (SBI) in young infants caused by GBS. Recommendations for initial antibiotic therapy for the febrile infant 1 to 90 days old were developed when infections with GBS were common and antibiotic resistance was rare.

OBJECTIVE

To document the pathogens responsible for SBI in recent years in febrile infants 1 to 90 days old and the antibiotic susceptibility of these organisms.

METHODS

The results of bacterial cultures from infants 1 to 90 days old evaluated for fever at Primary Children's Medical Center in Salt Lake City, Utah, between July 1999 and April 2002 were analyzed. Antibiotic susceptibility profiles were collected and patient records were reviewed to determine if initial antibiotic therapy was changed following the identification of the organism.

RESULTS

Of 1298 febrile infants enrolled from the Primary Children's Medical Center emergency department, 105 (8%) had SBI. The mean age of the infants with SBI was 39 days (range 2-82 days) and 2 (2%) were <7 days. SBI included urinary tract infection (UTI; 67%), bacteremia (16%), bacteremia and UTI (6%), bacteremia and meningitis (5%), meningitis (2%), abscess (2%), meningitis and UTI (1%), and meningitis and gastroenteritis (1%). Eighty-three (79%) of 105 episodes of SBI were caused by Gram-negative bacteria, including 92% of UTI, 54% of bacteremia, and 44% of meningitis cases. The most common pathogen was Escherichia coli (61%). Other Gram-negative pathogens were responsible for 19% of SBI. Staphylococcus aureus was the most common Gram-positive pathogen, causing 8% of SBI. GBS accounted for 6% of SBI. Of the 105 pathogens, 56 (53%) were resistant to ampicillin. Of the pathogens causing meningitis, UTI, and bacteremia, 78%, 53%, and 50%, respectively, were resistant to ampicillin. Antibiotic therapy was changed in 54% of cases of SBI following identification of the organism.

CONCLUSIONS

In Utah, ampicillin-resistant Gram-negative bacteria are the most common cause of SBI in febrile infants <90 days old. This finding impacts antibiotic selection, especially in cases of meningitis. Local surveillance of pathogens and antibiotic susceptibility patterns is critical to determine appropriate antibiotic therapy.

Quality control for beta-lactam susceptibility testing with a well-defined collection of Enterobacteriaceae and Pseudomonas aeruginosa strains in Spain

Eighteen Enterobacteriaceae and Pseudomonas aeruginosa strains, 16 of them with well-defined beta-lactam resistance mechanisms, were sent to 52 Spanish microbiology laboratories. Interpretative categories for 8 extended-spectrum beta-lactams were collected. Participating laboratories used their own routine susceptibility testing procedures (88% automatic systems, 10% disk diffusion, and 2% agar dilution). Control results were established by two independent reference laboratories by applying the NCCLS microdilution method and interpretative criteria. Interpretative discrepancies were observed in 16% of the results (4.4% for cefepime, 3.0% for aztreonam, 2.8% for piperacillin-tazobactam, 1.7% for cefotaxime [CTX] and ceftazidime, 1.1% for ceftriaxone, 0.9% for meropenem, and 0.3% for imipenem). High consistency with reference values (<5% of major plus very major errors) was observed with (i) American Type Culture Collection quality control strains; (ii) strains with low-efficiency mechanisms inactivating extended-spectrum beta-lactams, such as OXA-1-producing Escherichiacoli or SHV-1-hyperproducing Klebsiella pneumoniae; (iii) strains with highly efficient mechanisms, such as SHV-5 porin-deficient K. pneumoniae, CTX-M-10 in Enterobacter cloacae hyperproducing AmpC, and P. aeruginosa with the MexAB OprM efflux phenotype or hyperproducing AmpC. Low consistency (>30% major plus very major errors) was detected in K1-producing Klebsiella oxytoca, CTX-M-9-producing E. coli, and in OprD(-) P. aeruginosa strains. Extended-spectrum beta-lactamase (ESBL)-producing strains accounted for 86% of very major errors. Recognition of the ESBL phenotype was particularly low in Enterobacter cloacae strains (<35%), due to the lack of NCCLS-specific rules in this genus. A K1-producing K. oxytoca was misidentified by 10% of laboratories as an ESBL producer. The use of well-defined resistant strains is useful for improving proficiency in susceptibility testing in clinical laboratories.

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 and AmpC beta-lactamases in bloodstream isolates of Klebsiella pneumoniae: isolates harbor plasmid-mediated FOX-5 and ACT-1 AmpC beta-lactamases

We tested 190 Klebsiella pneumoniae bloodstream isolates recovered from 189 patients in 30 U.S. hospitals in 23 states to determine the occurrence of extended-spectrum beta-lactamase (ESBL) and AmpC beta-lactamase producers. Based on growth inhibition by clavulanic acid by disk and MIC test methods, 18 (9.5%) of the isolates produced ESBLs. Although the disk diffusion method with standard breakpoints identified 28 cefoxitin-nonsusceptible isolates, only 5 (18%) of these were confirmed as AmpC producers. Of two AmpC confirmatory tests, the three-dimensional extract test was easier to perform than was the double-disk approximation test using a novel inhibitor, Syn2190. Three of the five AmpC producers carried the bla(FOX-5) gene, while the other two isolates harbored the bla(ACT-1) gene. All AmpC genes were transferable. In vitro susceptibility testing with standard inocula showed that all five AmpC-producing strains were susceptible to cefepime, imipenem, and ertapenem but that with a high inoculum, more of these strains were susceptible to the carbapenems than to cefepime. All but 1 of 14 screen-positive AmpC nonproducers (and ESBL nonproducers) were susceptible to ceftriaxone and cefepime at the standard inoculum as were 6 of 6 isolates that were randomly selected and tested with a high inoculum. These results indicate that (i). a significant number of K. pneumoniae bloodstream isolates harbor ESBL or AmpC beta-lactamases, (ii). confirmatory tests are necessary to identify true AmpC producers, and (iii). in vitro, carbapenems are active against AmpC-producing strains of K. pneumoniae.

Emergence in Klebsiella pneumoniae of a chromosome-encoded SHV beta-lactamase that compromises the efficacy of imipenem

A Klebsiella pneumoniae isolate was identified that had reduced susceptibility to several expanded-spectrum cephalosporins and imipenem. That isolate produced a chromosome-encoded SHV-type beta-lactamase, SHV-38, that had an alanine to valine substitution in position Ambler 146 compared to beta-lactamase SHV-1. The kinetic parameters for purified beta-lactamases SHV-38 and SHV-1 showed that the hydrolytic spectrum of SHV-38 included only ceftazidime and imipenem. This report is the first example of an SHV-type beta-lactamase capable of hydrolyzing imipenem.

Occurrence and phenotypic characteristics of extended-spectrum beta-lactamases among members of the family Enterobacteriaceae at the Tel-Aviv Medical Center (Israel) and evaluation of diagnostic tests

We assessed the prevalence and phenotypic characteristics of extended-spectrum beta-lactamase (ESBL) producers among cefuroxime-resistant (CXM-R) (MIC > or = 32 micro g/ml) members of the family Enterobacteriaceae in our institution. The 438 CXM-R clinical isolates obtained from nonurine sources among inpatients were screened. ESBL production was confirmed by disk diffusion assay using cefpodoxime (CPD), cefotaxime (CTX), and ceftazidime (CTZ) with and without clavulanate (CLAV). A difference of > or =5 mm in the size of the zone of inhibition in the presence of CLAV for at least one of the agents was considered representative of the ESBL phenotype: 186 isolates (42.5%) were confirmed as ESBL producers. The isolates tested and the rates of ESBL producers were as follows: Klebsiella spp. (n = 81), 79%; Proteus spp. (n = 58), 62%; Escherichia coli (n = 64), 53%; Enterobacter spp. (n = 69), 42%; Serratia spp. (n = 70), 14%; Citrobacter spp. (n = 25), 24%; Providencia spp. (n = 21), 24%; Morganella spp. (n = 41), 5%; and Kluyvera (n = 3), 0%. The overall sensitivity of isolated ESBL confirmatory tests was 79% for CPD-CLAV, 66% for CTZ-CLAV, and 91% for CTX-CLAV. Sensitivities of CTZ-CLAV confirmatory tests for Klebsiella spp., Proteus spp., E. coli, and Enterobacter spp. were 84, 22, 76, and 62%, respectively, and those for CTX-CLAV were 95, 97, 94, and 83%, respectively. They were 90% for CPD-CLAV and CTZ-CLAV, 95% for CPD-CLAV and CTX-CLAV, and 100% for CTZ-CLAV and CTX-CLAV. ESBL production was highly prevalent among Enterobacteriaceae. Using resistance to CXM as an ESBL screening criterion is a suitable option in high-incidence areas where Klebsiella spp. are not the dominant ESBL producers. This screening criterion may simplify the screening test and improve its sensitivity, although at the price of testing more isolates. The CTX-CLAV combination confirmed ESBL producers better than the CTZ-CLAV combination, with sensitivity varying between species. Combined CTZ-CLAV and CTX-CLAV testing detected all these strains; CPD-CLAV provided no additional benefit.

CANCER resistance surveillance program: initial results from hematology-oncology centers in North America. Chemotherapy Alliance for Neutropenics and the Control of Emerging Resistance

OBJECTIVE

The CANCER (Chemotherapy Alliance for Neutropenics and the Control of Emerging Resistance) surveillance program was initiated to collect culture data on antimicrobial and antifungal agents in hospitals treating neutropenic patients in North America, as a means to monitor the development of microbial resistance.

METHODS

A total of 2042 isolates from bloodstream, respiratory, urinary, and cutaneous infections in 2000-2001 were submitted by 33 oncology centers, clinics, and hospitals in North America, sent to a central laboratory, and tested by National Committee for Clinical Laboratory Standards methods against 42 different antimicrobials.

RESULTS

Staphylococcus aureus, Escherichia coli, coagulase-negative staphylococci, Enterococcus spp., and Klebsiella spp. represented the most frequently isolated pathogens during the initial benchmark year. The incidence of extended-spectrum beta-lactamase-producing phenotypes ranged from 1.6% to 4.6% among E. coli and Klebsiella spp. Amikacin, tobramycin, polymyxin B, and piperacillin/tazobactam provided the highest susceptibility rates against Pseudomonas aeruginosa isolates. Yeast bloodstream isolates demonstrated complete susceptibility to amphotericin B, but 14% of strains were considered to have high-level fluconazole resistance.

CONCLUSIONS

Elevated resistance rates when compared to general hospital strains were not observed in the CANCER program during the baseline year of this novel longitudinal, resistance surveillance program. The prevalence of gram-positive pathogens, although representing more than 50% of all bacterial isolates, was slightly lower than that reported previously by other investigators. Continued evaluation for antimicrobial resistance as well as changes in the prevalence of gram-positive pathogens requires the use of longitudinal surveillance programs such as the CANCER program. Such initiatives allow the development of therapeutic strategies for coping with changes in resistance and pathogen prevalence in this dynamic at-risk patient environment.

Comparison of BDPhoenix and VITEK2 automated antimicrobial susceptibility test systems for extended-spectrum beta-lactamase detection in Escherichia coli and Klebsiella species clinical isolates

The present study compares the ability to detect extended-spectrum beta-lactamases (ESBL) among a collection of 34 ESBL producing clinical isolates belonging to Escherichia coli and Klebsiella species with two new rapid susceptibility and identification instruments-VITEK2 (bioMérieux, Marcy l'Etoile, France) vs. BDPhoenix (BD Biosciences, Sparks, MD). ESBL content in these isolates was previously characterized on the basis of PCR amplification and sequencing results which were used as the reference method in our evaluation. BDPhoenix correctly determined the ESBL outcome for all strains tested (100% detection rate), whereas VITEK2 was not able to detect the ESBL status in 5 isolates (85% detection rate). Detailed analysis revealed that the discrepancies were mainly observed with 'difficult-to-detect' strains. Misidentification was either due to low oximino cephalosporin MIC in these strains or was associated with pronounced 'cefotaximase' or 'ceftazidimase' phenotypes. Klebsiella oxytoca chromosomal beta-lactamase (K1) is phenotypically quite similar to ESBL enzymes. In order to evaluate whether the K1 and ESBL enzymes could be discriminated, we expanded our analysis by 8 clinical K. oxytoca strains with K1 phenotypes. VITEK2 gave excellent identification of these strains whereas 7 out of 8 were falsely labeled ESBL-positive by the BDPhoenix system.

Evaluation of the Etest ESBL and the BD Phoenix, VITEK 1, and VITEK 2 automated instruments for detection of extended-spectrum beta-lactamases in multiresistant Escherichia coli and Klebsiella spp

Seventy-four isolates of multiresistant Escherichia coli and Klebsiella spp. recovered during a 3-year period and 17 control strains with genotypically identified beta-lactamases were tested for the production of extended-spectrum beta-lactamases (ESBLs) by using the Etest and the VITEK 1, VITEK 2, and Phoenix automated instruments. The use of the Etest was evaluated by investigating its accuracy in detecting the ESBLs of the control strains and by comparing interpretation results of laboratory technicians and experts. The accuracy of the Etest was 94%. With the Etest as the reference for the clinical strains and the genotype as the reference for the control strains, the automated instruments detected the ESBLs with accuracies of 78% (VITEK 2), 83% (VITEK 1), and 89% (Phoenix). No significant difference between the systems with regard to the control strains was detected. The VITEK 2 did, however, perform less well than the Phoenix (P = 0.03) on the collection of clinical isolates, mainly because of its high percentage of indeterminate test results (11%). No significant difference between the performances of the VITEK 1 and either the VITEK 2 or the Phoenix was found. However, because of its associated BDXpert system the Phoenix showed the best performance. The Etest was found to be an accurate test but was limited by its indeterminate results (4%), its inability to differentiate between K1 hyperproduction and ESBLs, questionable guidelines concerning mutants inside the inhibition zones, and the inability of the technicians to recognize subtle zone deformations.

Clinical implications of extended-spectrum beta-lactamase-producing Klebsiella pneumoniae bacteraemia

To identify the clinical implications of extended-spectrum beta-lactamase (ESBL) production, 162 cases of Klebsiella pneumoniae bacteraemia in 154 adults were analysed. Of these cases, 44 (27.2%) were ESBL-producing (ESBLKP). Common sources of ESBLKP bacteraemia included primary bacteraemia (34.1%) and biliary infection (29.5%). The placement of a biliary drainage catheter, nosocomial acquisition, and prior antibiotic therapy were independently associated with ESBL production in multivariate analysis. More cases of ESBLKP than non-ESBLKP received inappropriate antibiotic therapy before culture results were reported (54.5 vs. 3.4%; P = 0.001). In 19 cases of ESBLKP, no significant difference in mortality was observed between patients who received appropriate empiric antibiotic therapy and those who did not (26.3 vs. 20.8%; P = 0.67). The mean length of hospital stay after the onset of bacteraemia was longer in the cases of ESBLKP than in the cases of non-ESBLKP (39.6 vs. 23.9 days; P = 0.008). Directly related mortality was not significantly different between the cases of ESBLKP and the cases of non-ESBLKP (23.3 vs. 20.0%; P = 0.65). None of the patients with biliary infection due to ESBLKP died (0/12; P = 0.03). In conclusion, ESBL production was not significantly associated with death but it had a considerable impact on patients with K. pneumoniae bacteraemia.

Antibiotic susceptibility of bacterial strains isolated from patients with various infections

AIMS

Isolates from various samples obtained during 1998 and 1999 were identified and their susceptibility to third-generation cephalosporins, monobactams and/or cephamycins studied along with any production of ESBLs.

METHODS AND RESULTS

Of these samples, bacteria most frequently isolated by the conventional techniques and Vitek GNI card were Escherichia coli (37%), Klebsiella pneumoniae (27%) and Enterobacter cloacae (16%). Using disk diffusion and double-disk synergy tests, we found that 71% strains produced ESBLs and 18% strains produced ESBLs and cephamycinases. Banding patterns of PCR amplification with the designed primers showed that 57% strains were capable of harbouring bla(SHV) genes. The bla(TEM), bla(CMY) and bla(AmpC) genes were harboured by 55%, 31% and 12% strains, respectively. Forty-five percent of strains contained more than two types of beta-lactamase genes. In particular, one strain contained bla(TEM), bla(SHV), bla(CMY) and bla(AmpC) genes.

CONCLUSIONS

The percentage of ESBL-producing strains was high. The most prevalent beta-lactamase gene was bla(SHV) gene. The bla(CMY) genes have been prevalent in cephamycin-resistant strains. The multidrug-resistant strains resistant to third-generation cephalosporins and cephamycins were detected in high percentage.

SIGNIFICANCE AND IMPACT OF THE STUDY

Resistance mechanisms to beta-lactams, comprising mostly extended-spectrum beta-lactamase (ESBL) production, lead to the resistance against even recently developed beta-lactams in enterobacteria, which is now a serious threat to antibiotic therapy. The high prevalence of bla(CMY) genes and multidrug-resistant genes may also cause therapeutic failure and lack of eradication of these strains by third-generation cephalosporins or cephamycins.

Detection of extended spectrum beta-lactamases in the routine clinical microbiology laboratory

AIMS

To compare three methods of confirming the presence of an extended spectrum beta-lactamase (ESBL) enzyme with the initial detection (i.e., screening) by the Vitek AMS.

METHODS

Gram-negative bacteria which flagged as ESBL-positive in the Vitek GNS card, or were suspected of harbouring an enzyme, were further tested by each of the following methods: (a) combination disc test using cefpodoxime, ceftazidime and cefotaxime with and without clavulanate; (b) cefotaxime ESBL Etest; and (c) Jarlier keyhole method with cefpodoxime (10 microg), cefotaxime (5 microg) and aztreonam (30 microg) placed 15mm away from an augmentin (30 microg) disc.

RESULTS

A total of 52 isolates were investigated, representing an 18-month time period. Fifty of these were positive by Vitek. Twenty-eight (56%) were confirmed by other methods (true positives). Of the 44% Vitek-positive/confirmatory test-negative (false positives), eight were Escherichia coli which was 53% of all E. coli tested. The majority of other false-positive isolates were Klebsiella oxytoca (24% overall) which were all Vitek- and Etest-positive but negative by the combination disc test.

CONCLUSIONS

All ESBL-positive strains by Vitek should be confirmed by the combination disc test using all three antibiotics. This will enable differentiation of 'true' ESBLs from false-positive organisms, including K1 hyperbetalactamase-producing Klebsiella oxytoca and AmpC-producing organisms. The cefpodoxime combination discs gave the best differentiation in this study with only one ESBL organism being missed.

Bloodstream infections by extended-spectrum beta-lactamase-producing Escherichia coli and Klebsiella pneumoniae in children: epidemiology and clinical outcome

To determine the epidemiologic features and clinical outcomes of bloodstream infections caused by extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae isolates, cases of bacteremia caused by these organisms in children were analyzed retrospectively. Among the 157 blood isolates recovered from 1993 to 1998 at the Seoul National University Children's Hospital, the prevalence of ESBL production was 17.9% among the E. coli isolates and 52.9% among the K. pneumoniae isolates. The commonest ESBLs were SHV-2a and TEM-52. A novel ESBL, TEM-88, was identified. Pulsed-field gel electrophoresis analysis of the ESBL-producing organisms showed extensive diversity in clonality. The medical records of 142 episodes were reviewed. The risk factors for bloodstream infection with ESBL-producing organisms were prior hospitalization, prior use of oxyimino-cephalosporins, and admission to an intensive care unit within the previous month. There was no difference in clinical severity between patients infected with ESBL-producing strains (the ESBL group) and those infected with ESBL-nonproducing strains (the non-ESBL group) at the time of presentation. However, the overall fatality rate for the ESBL group was significantly higher than that for the non-ESBL group: 12 of 45 (26.7%) versus 5 of 87 (5.7%) (P = 0.001). In a subset analysis of patients treated with extended-spectrum cephalosporins with or without an aminoglycoside, favorable response rates were significantly higher in the non-ESBL group at the 3rd day (6 of 17 versus 33 of 51; P = 0.035), the 5th day (6 of 17 versus 36 of 50; P < 0.05), and the end of therapy (9 of 17 versus 47 of 50; P < 0.001). In conclusion, the ESBL production of the infecting organisms has a significant impact on the clinical course and survival of pediatric patients with bacteremia caused by E. coli and K. pneumoniae.

Ceftriaxone activity against Gram-positive and Gram-negative pathogens isolated in US clinical microbiology laboratories from 1996 to 2000: results from The Surveillance Network (TSN) Database-USA

Ceftriaxone was introduced into clinical practice in the USA in 1985 and was the first extended-spectrum (third-generation) cephalosporin approved for once-daily treatment of patients with Gram-positive or Gram-negative infections. Review of ceftriaxone activity is important given its continued use since the mid-1980s and reports of emerging resistance among all antimicrobial agent classes. We reviewed the activity of ceftriaxone and relevant comparative agents against five Gram-positive and 11 Gram-negative species for a 5-year period, 1996-2000, using data from The Surveillance Network (TSN) Database-USA. All MIC results were interpreted using NCCLS breakpoint criteria. Ceftriaxone resistance among isolates of Streptococcus pneumoniae (n=17219) remained essentially unchanged over the 5 years studied and in fact was lower from 1998 to 2000 (5.0-5.1%) than in 1996 (6.3%) and 1997 (6.6%). Ceftriaxone resistance (range, 5.1-6.9%) among viridans group streptococci (n=6621) varied by <2% from 1997 to 2000. Beta-lactam-resistant Streptococcus pyogenes (n=935) and group B beta-haemolytic streptococci (n=2267) were not identified in any year. Among methicillin-susceptible Staphylococcus aureus (n=39 284) ceftriaxone resistance was 0.1-0.3% per year from 1996 to 2000. Ceftriaxone resistance among Escherichia coli (n=472407; range, 0.2-0.4%), Klebsiella oxytoca (n=16231; range, 3.5-4.8%), Klebsiella pneumoniae (n=117754; range, 1.9-2.6%), Proteus mirabilis (n=67692; range, 0.2-0.3%), Morganella morganii (n=11251; range, 0.3-2.1%) and Serratia marcescens (n=26519; range, 1.6-3.8%) was low and consistent from 1996 to 2000. Resistance to ceftriaxone among Enterobacter cloacae (n=48114; range, 21.7-23.9%) was relatively high, compared with other Enterobacteriaceae, but unchanged from 1996 to 2000. Rates of resistance to ceftriaxone among Acinetobacter spp. (n=20813) increased from 24.8% in 1996 to 45.1% in 2000. All Haemophilus influenzae (n=7911) and Neisseria gonorrhoeae (n=218) were susceptible to ceftriaxone, as were 99.7% of Moraxella catarrhalis (n=312) tested in 1996 and 1997. In summary, ceftriaxone has retained its potent activity against the most commonly encountered Gram-positive and Gram-negative human pathogens despite widespread and ongoing clinical use for more than 15 years.

A decade of antimicrobial susceptibilities at the University of Kentucky Hospital

OBJECTIVE

To determine the antimicrobial susceptibility rates for key antimicrobial agents and selective bacterial pathogens in the decade of the 1990s.

METHODS

Data from 1990 to 2000 from the University of Kentucky Clinical Microbiology Laboratory were analyzed by linear regression analysis to identify agents and pathogens that show a decline in susceptibility. For selected pathogens and antimicrobial agents, predictions were made for further declines in susceptibility for 2005 and 2010.

RESULTS

Significant declines in susceptibility to selected antimicrobial agents were found for Pseudomonas aeruginosa, Klebsiella pneumoniae, Enterobacter cloacae, Escherichia coli, Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus pneumoniae. Further declines were predicted for 2005 and 2010.

CONCLUSIONS

Examination of susceptibility rates over time in a university hospital medical center provides useful data for future planning. In our institution, antimicrobial susceptibility rates have significantly declined during the 1990s for certain antimicrobial agents and bacterial pathogens. We are attempting to change our antimicrobial use patterns through formulary manipulation and clinician education, which may retard or prevent such declines in the future.

How predictive is PK/PD for antibacterial agents?

The pharmacodynamic (PD) parameters most often used in studies of antibiotic effect include the following relationships between the antibiotic concentration curve in serum as a surrogate marker for the antibiotic concentration at the infection site, the peak/minimal inhibitory concentration (MIC) ratio, the area under the curve (AUC)/MIC ratio and the duration of time the concentration exceeds the MIC (T(>MIC)). The MIC plays an important role also as a PD marker, and its precision in this respect is discussed. The predictive role of T(>MIC) is important for drugs showing minimal concentration dependent effect such as the beta-lactam antibiotics, the macrolides and others. The time can be calculated as the chronological time measured or as the (cumulative) per cent of the dosing interval covered by the dose. Several clinical studies have confirmed this relationship. It can be deduced from experimental as well as clinical studies that there is a minimal effective time (MET), which needs to be covered by the antibiotic concentration at the site of infection in order to achieve cure. Dosing according to this MET will result in the least antibiotic needed for the shortest duration. In several cases a single dose will suffice to cover the MET. If this is not possible the antibiotic should be dosed in a way, that each dose will surpass the MIC for at least 40-50% of the dosing interval. For antibiotics with a clear concentration-dependent bacterial killing effect the most important pharmacokinetic/pharmacodynamic (PK/PD) index is the peak/MIC ratio (or the AUC/MIC ratio). This is the case for aminoglycosides and fluoroquinolones, and for both classes a peak/MIC ratio of at least 10 within the first 24 h of treatment has been shown to result in around 90% bacteriological as well as clinical cure. One consequence of clinical dosing has been the once-a-day (OD) dosing for aminoglycosides, which is the standard mode of therapy in many countries. Clinical studies in the field of antibacterial PD are still relatively scarce, and much information is needed to enable relevant dosing strategies for all types of antibiotics against all common infections and micro-organisms.

Nosocomial antibiotic resistance in multiple gram-negative species: experience at one hospital with squeezing the resistance balloon at multiple sites

Increased use of antibiotics has led to the isolation of multidrug-resistant bacteria, especially in intensive care units and long-term care facilities. Resistance in specific gram-negative bacteria, including Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa, is of great concern, because a growing number of reports have documented mechanisms whereby these microorganisms have become resistant to all available antibacterial agents used in therapy. Reduction in the selection of these multidrug-resistant bacteria can be accomplished by a combination of several strategies. These include having an understanding of the genetics of both innate and acquired characteristics of bacteria; knowing resistance potentials for specific antibacterials; monitoring resistance trends in bacteria designated as problematic organisms within a particular institution on a routine basis; modifying antibiotic formularies when and where needed; creating institutional education programs; and enforcing strict infection-control practices. Strategies appropriate for primary prevention of nosocomial resistance may differ from those required for control of existing epidemic or endemic resistance.

Molecular correlation for the treatment outcomes in bloodstream infections caused by Escherichia coli and Klebsiella pneumoniae with reduced susceptibility to ceftazidime

Data are limited on outcomes of treatment with extended-spectrum cephalosporins (ESCs) for infections caused by Enterobacteriaceae that produce extended-spectrum beta-lactamases (ESBLs). This study describes the largest treatment experience of a nonoutbreak series of bloodstream infections caused by strains of Escherichia coli (23 episodes) and Klebsiella pneumoniae (13 episodes) with a ceftazidime minimal inhibitory concentration of > or =2 microg/mL. E. coli isolates produced a greater variety of beta-lactamase types than did K. pneumoniae isolates, among which ESBL production was predominant. Five ESBL types were identified: TEM-12, TEM-71, TEM-6, SHV-12, and SHV-5. Most patients were treated empirically with an ESC-based regimen. A favorable response to treatment with a nonceftazidime ESC was observed when the causative pathogen produced either TEM-6 or TEM-12; ceftazidime treatment was associated with failure of therapy in all patients. Despite the limited clinical success, ESCs are currently not recommended for the treatment of serious infections caused by ESBL-producing strains.

Cefepime, piperacillin-tazobactam, and the inoculum effect in tests with extended-spectrum beta-lactamase-producing Enterobacteriaceae

There is little information about the clinical effectiveness of cefepime and piperacillin-tazobactam in the treatment of infections caused by extended-spectrum beta-lactamase (ESBL)-producing pathogens. Some inferences have been drawn from laboratory studies, which have usually involved only one or a few strains of ESBL-producing Klebsiella pneumoniae or Escherichia coli that produced only a limited range of ESBLs. Such studies are indirect, sometimes conflicting, indicators of efficacy. To extend previous laboratory findings, a study was designed to investigate organism-drug interactions by determining the in vitro activities of eight parenteral beta-lactam agents against 82 clinical and laboratory strains of Klebsiella, Escherichia, Enterobacter, Citrobacter, Serratia, Morganella, and Proteus species that produced 22 different ESBLs, alone or in combination with other beta-lactamases. Activities were determined in broth microdilution MIC tests using standard and 100-fold-higher inocula. An inoculum effect, defined as an eightfold or greater MIC increase on testing with the higher inoculum, was most consistently detected with cefepime, cefotaxime, and ceftriaxone and least frequently detected with meropenem and cefoteten. Piperacillin-tazobactam was intermediate between these two groups of agents. Although the inoculum effect is an in vitro laboratory phenomenon, if it has any predictive value in identifying increased risk of therapeutic failure in serious infections, these results support suggestions that cefepime may be a less-than-reliable agent for therapy of infections caused by ESBL-producing strains.

Extended-spectrum beta-lactamases: the European experience

Extended-spectrum beta-lactamases mediate resistance to cephalosporin antibiotics and were first discovered in Europe in the early 1980s. They have become a widespread problem, particularly in Klebsiella pneumoniae, but increasingly in non-typhoid Salmonella species. Traditionally, extended-spectrum beta-lactamases have been derivatives of TEM and SHV parent enzymes. The last year, however, has seen an explosion of developments in extended-spectrum beta-lactamases of non-TEM, non-SHV lineage in Europe. The CTX-M type extended-spectrum beta-lactamases have become particularly widespread. At the same time, European clinical microbiology laboratories have become more aware of the pressing need for detection methods given increasing awareness of the lack of reliability of cephalosporins in the treatment of extended-spectrum beta-lactamase producers.