Potential impact of the VITEK 2 system and the Advanced Expert System on the clinical laboratory of a university-based hospital.

A study was designed to assess the impact of the VITEK 2 automated system and the Advanced Expert System (AES) on the clinical laboratory of a typical university-based hospital. A total of 259 consecutive, nonduplicate isolates of Enterobacteriaceae members, Pseudomonas aeruginosa, and Staphylococcus aureus were collected and tested by the VITEK 2 system for identification and antimicrobial susceptibility testing, and the results were analyzed by the AES. The results were also analyzed by a human expert and compared to the AES analyses. Among the 259 isolates included in this study, 245 (94.6%) were definitively identified by VITEK 2, requiring little input from laboratory staff. For 194 (74.9%) isolates, no inconsistencies between the identification of the strain and the antimicrobial susceptibility determined by VITEK 2 were detected by the AES. Thus, no input from laboratory staff was required for these strains. The AES suggested one or more corrections to results obtained with 65 strains to remove inconsistencies. The human expert thought that most of these corrections were appropriate and that some resulted from a failure of the VITEK 2 system to detect certain forms of resistance. Antimicrobial phenotypes assigned to the strains by the AES for beta-lactams, aminoglycosides, quinolones, macrolides, tetracyclines, and glycopeptides were similar to those assigned by the human expert for 95.7 to 100% of strains. These results indicate that the VITEK 2 system and AES can provide accurate information in tests for most of the clinical isolates examined and remove the need for human analysis of results for many. Certain problems were identified in the study that should be remediable with further work on the software supporting the AES.

Pharmacodynamics of moxifloxacin, levofloxacin and sparfloxacin against Streptococcus pneumoniae.

An in vitro pharmacokinetic model (IVPM) was used to simulate the human serum pharmacokinetics of moxifloxacin, levofloxacin and sparfloxacin, and to compare their pharmacodynamics against Streptococcus pneumoniae exhibiting a wide range of susceptibilities to fluoroquinolones. Logarithmic-phase cultures were exposed to peak concentrations achieved in human serum of moxifloxacin, levofloxacin or sparfloxacin with oral doses of 400, 500 and 200 mg, respectively. Human elimination pharmacokinetics were simulated, and viable bacterial counts were measured at 0, 1, 2, 4, 6, 8, 24 and 36 h. Moxifloxacin was rapidly bactericidal (>3 logs of killing) against all 10 S. pneumoniae strains, with 99.9% kills of eight strains occurring within 1-3 h after dosing. Maximum kills ranged from 5 to >6 logs. Moxifloxacin eradicated seven strains from the IVPM within 8 h of the first dose, and eradicated two other strains within 24 h. Although levofloxacin and sparfloxacin were also bactericidal against all 10 S. pneumoniae strains, the rates of killing were somewhat slower, with sparfloxacin exhibiting the slowest rate of kill. In summary, moxifloxacin's increased anti-pneumococcal potency compared with levofloxacin and its more favourable pharmacokinetics compared with sparfloxacin provided enhanced pharmacodynamic activity against some S. pneumoniae strains when maximum doses were simulated in an IVPM.

Mechanisms responsible for cross-resistance and dichotomous resistance among the quinolones.

Resistance to the quinolones almost always arises from the accumulation of mutations in chromosomal genes responsible for the drug targets, permeability, or active efflux. This resistance can be depicted as a stepwise process in which each step, represented by separate mutations, diminishes susceptibility on average 4- to 8-fold. The precise path followed in this stepwise process differs with the quinolone that selects resistance as well as the organism involved. At each step, the influence of each mutation on susceptibility to other quinolones not used in the selection process varies greatly, and a pattern of either cross-resistance or dichotomous resistance may be seen. From an understanding of the stepwise process by which resistance to the quinolones evolves, it is possible to use an 8-fold rule to predict which compounds may provide effective therapy for a given infection and be least likely to select for resistance.

Ability of the VITEK 2 advanced expert system To identify beta-lactam phenotypes in isolates of Enterobacteriaceae and Pseudomonas aeruginosa.

The Advanced Expert System (AES) was used in conjunction with the VITEK 2 automated antimicrobial susceptibility test system to ascertain the beta-lactam phenotypes of 196 isolates of the family Enterobacteriaceae and the species Pseudomonas aeruginosa. These isolates represented a panel of strains that had been collected from laboratories worldwide and whose beta-lactam phenotypes had been characterized by biochemical and molecular techniques. The antimicrobial susceptibility of each isolate was determined with the VITEK 2 instrument, and the results were analyzed with the AES to ascertain the beta-lactam phenotype. The results were then compared to the beta-lactam resistance mechanism determined by biochemical and molecular techniques. Overall, the AES was able to ascertain a beta-lactam phenotype for 183 of the 196 (93.4%) isolates tested. For 111 of these 183 (60.7%) isolates, the correct beta-lactam phenotype was identified definitively in a single choice by the AES, while for an additional 46 isolates (25.1%), the AES identified the correct beta-lactam phenotype provisionally within two or more choices. For the remaining 26 isolates (14.2%), the beta-lactam phenotype identified by the AES was incorrect. However, for a number of these isolates, the error was due to remediable problems. These results suggest that the AES is capable of accurate identification of the beta-lactam phenotypes of gram-negative isolates and that certain modifications can improve its performance even further.

A pilot study of antibiotic cycling in a hematology-oncology unit.

OBJECTIVE: To determine the safety and treatment efficacy of cycling antibiotic regimens for prophylaxis or treatment of patients with profound neutropenia. DESIGN: A prospective, nonrandomized, observational trial. SETTING: A 20-bed adult hematology-oncology inpatient unit at a university referral hospital. PATIENTS: Hospitalized adult patients with chemotherapy- or radiation-induced neutropenia (absolute neutrophil count less than 500 cells/mm3). INTERVENTION: Between July 1994 and January 1996, 295 hospitalized patients were evaluated on an intent-to-treat basis for the cycling protocol. Of these, 271 were eligible and assigned to one of four antibiotic regimens being used at the time of enrollment: (1) ceftazidime+vancomycin; (2) imipenem; (3) aztreonam+cefazolin; (4) ciprofloxacin+clindamycin. Data on infection rates and types, and antibiotic resistance patterns, toxicity, and effectiveness were collected. RESULTS: Twenty-four patients were excluded. Of the 271 evaluable patients, 123 (42%) were able to complete treatment on the assigned regimen. Of the 148 patients (50%) unable to do so, the reasons for failure included persistent fever (79%), breakthrough bacteremia (14%), and drug toxicity (7%). The antibiotic susceptibility profiles over the study period showed no increase in resistance. However, there was a marked increase in enterococcal infections. CONCLUSIONS: Our data show no significant increase in side effects or decrease in efficacy while cycling antibiotics among neutropenic patients and thus support further study of its role.

Regulation of inducible AmpC beta-lactamase expression among Enterobacteriaceae.

AmpC ss-lactamases are active-site serine enzymes that are primarily cephalosporinases. In many gram negative organisms, including Enterobacter spp.,Citrobacter freundii, Serratia marcescens, Morganella morganii and Pseudomonas aeruginosa, the expression of chromosomal ampC genes is low but inducible in response to ss-lactams and other stimuli. The current working model for AmpC induction requires exposure of bacterial cells to ss-lactam drugs or other stimuli and is linked to the cell wall recycling pathway. Induction of ampC appears to involve several gene products associated with this pathway. These gene products include AmpR, AmpD, and AmpG. In addition, anhydro forms of cell wall precursor muropeptides are believed to act as cofactors for AmpC induction. These cofactors bind to the DNA binding protein, AmpR, and define the role of AmpR as activator. Recent debate has ensued in the literature as to the identification of the precursor muropeptide involved in the activation process. Two candidate muropeptides include 1,6-anhydro-N-acetylmuramic acid L-Ala-D-Glu-meso-diaminopimelic acid (anhydro-MurNAc-tripeptide) and anhydro-MurNAc-L-Ala-D-Glu-meso-diaminopimelic acid- D-Ala-D-Ala (pentapeptide). The intent of this review is to address the general mechanism involved in AmpC induction. In doing so, the genes and gene products required for the process of AmpC induction are described. In addition, we review the data addressing cell wall recycling as it relates to AmpC induction.

Activity of moxifloxacin against pathogens with decreased susceptibility to ciprofloxacin.

A panel of 279 clinical isolates of Gram-positive cocci and Gram-negative bacilli with varying levels of resistance to ciprofloxacin were analysed for susceptibility to moxifloxacin, ciprofloxacin, ofloxacin and nalidixic acid. Moxifloxacin was eight- to 32-fold more potent than ciprofloxacin and ofloxacin against staphylococci and Streptococcus pneumoniae, and equivalent to eight-fold more potent against enterococci. Although ciprofloxacin was intrinsically more potent than the other quinolones against highly susceptible Gram-negative isolates, the percentages of Gram-negative isolates susceptible to 1 mg/L of moxifloxacin or ciprofloxacin, or 2 mg/L of ofloxacin were 78%, 80% and 76%, indicating in-vitro equivalence of the agents against a collection that included isolates with diminished quinolone susceptibility. Staphylococci were analysed according to their ciprofloxacin susceptibility status. As ciprofloxacin resistance increased to high levels, all quinolone MICs increased, but moxifloxacin and ofloxacin MICs increased less than ciprofloxacin MICs. In mutational studies moxifloxacin inhibited more mutants (69%) at a concentration of 1 mg/L than did ciprofloxacin (63%) at 1 mg/L or ofloxacin at 2 mg/L (31%). The study indicated that moxifloxacin is more potent than ciprofloxacin and ofloxacin against Gram-positive pathogens, may be comparable in activity against less quinolone-susceptible Gram-negative isolates (other than Pseudomonas aeruginosa), and is less affected than ciprofloxacin by mechanisms responsible for increasing quinolone resistance in staphylococci.

Molecular characterization of a multiply resistant Klebsiella pneumoniae encoding ESBLs and a plasmid-mediated AmpC.

Organisms encoding multiple antibiotic resistance genes are becoming increasingly prevalent. In this report we describe a multiply resistant Klebsiella pneumoniae which possesses at least five different beta-lactamase genes. Isoelectric focusing, polymerase chain reaction and restriction fragment length polymorphism analysis identified TEM-1, multiple SHVs, OXA-9 and a plasmid-mediated ampC, beta-lactamase. Furthermore, Southern analysis and conjugation experiments established that most of the resistance genes were encoded on one large transferable plasmid. This report demonstrates the complexity of multiply resistant organisms.

In-vitro activity of levofloxacin against Streptococcus pneumoniae with various levels of penicillin resistance.

This in-vitro study was designed to compare the activity of levofloxacin with that of ciprofloxacin, ofloxacin, erythromycin, penicillin, amoxycillin, loracarbef, cefaclor, cefpodoxime, ceftriaxone, trimethoprim-sulphamethoxazole, clindamycin and vancomycin against a collection of 202 Streptococcus pneumoniae isolates (56% susceptible to penicillin, 34% intermediate, 10% resistant). The isolates (60% nasopharyngeal, 40% middle ear) were obtained from otherwise healthy children at child care centres in urban and rural Nebraska, and at a paediatric clinic in rural Kentucky. MICs were determined by NCCLS agar dilution methodology using an inoculum of 10(4) cfu/spot. Using NCCLS breakpoints, the percentage of penicillin-intermediate and -resistant strains susceptible to the evaluable agents were, respectively, as follows: levofloxacin (99%, 100%), ofloxacin (87%, 100%), erythromycin (52%, 65%), ceftriaxone (93%, 25%), trimethoprim-sulphamethoxazole (7%, 0%), clindamycin (93%, 100%) and vancomycin (100%, 100%). Without NCCLS interpretive criteria, no conclusions could be made concerning the susceptibility of penicillin-intermediate and -resistant strains to the other study drugs. All beta-lactam antibiotics, erythromycin and trimethoprim-sulphamethoxazole were less active against penicillin-resistant strains, indicating that these may be suboptimal agents for empirical therapy for suspected S. pneumoniae infections in these patient populations. However, levofloxacin, ofloxacin, clindamycin and vancomycin were equally active against penicillin-susceptible and -resistant strains. These data suggest that the efficacy of levofloxacin should be examined in both adult and paediatric S. pneumoniae infections involving body sites where levofloxacin concentrations > 2 mg/L can be achieved safely.

Pharmacodynamics of levofloxacin and ciprofloxacin against Streptococcus pneumoniae.

An in-vitro pharmacokinetic model was used to compare the pharmacodynamics of levofloxacin and ciprofloxacin against four penicillin-susceptible and four penicillin-resistant Streptococcus pneumoniae. Logarithmic-phase cultures were exposed to the peak concentrations of levofloxacin or ciprofloxacin observed in human serum after 500 mg and 750 mg oral doses, human elimination pharmacokinetics were simulated, and viable bacterial counts were measured at 0, 1, 2, 4, 6, 8, 12, 24 and 36 h. Levofloxacin was rapidly and significantly bactericidal against all eight strains evaluated, with eradication of six strains occurring despite area under the inhibitory curve over 24 h (AUIC24) values of only 32-64 SIT(-1) x h (serum inhibitory titre over time). The pharmacodynamics of ciprofloxacin were more variable and the rate of bacterial killing was consistently slower than observed with levofloxacin. Ciprofloxacin eradicated five strains despite having an AUIC24 of only 44 SIT(-1) x h. These data suggest that the increased potency of levofloxacin and more favourable pharmacokinetics compared with ciprofloxacin provide enhanced pharmacodynamic activity against S. pneumoniae. Furthermore, these data suggest that the minimum AUIC required for clinical efficacy against and eradication of S. pneumoniae with levofloxacin and ciprofloxacin may be well below the 125 SIT(-1) x h identified by other studies.

Pharmacodynamics of trovafloxacin, ofloxacin, and ciprofloxacin against Streptococcus pneumoniae in an in vitro pharmacokinetic model.

An in vitro pharmacokinetic model was used to simulate the pharmacokinetics of trovafloxacin, ofloxacin, and ciprofloxacin in human serum and to compare their pharmacodynamics against eight Streptococcus pneumoniae strains. The MICs of ofloxacin and ciprofloxacin ranged from 1 to 2 micrograms/ml. Trovafloxacin was 8- to 32-fold more potent, with MICs of 0.06 to 0.12 microgram/ml. Logarithmic-phase cultures were exposed to peak concentrations of trovafloxacin, ofloxacin, or ciprofloxacin achieved in human serum after 200-, 400-, and 750-mg oral doses, respectively. Trovafloxacin was dosed at 0 and 24 h, and ofloxacin and ciprofloxacin were dosed at 0, 12, and 24 h. Human elimination pharmacokinetics were simulated, and viable bacterial counts were measured at 0, 2, 4, 6, 8, 12, 24, and 36 h. Trovafloxacin was rapidly and significantly bactericidal against all eight strains evaluated, with viable bacterial counts decreasing at least 5 logs to undetectable levels. Times to 99.9% killing were only 1 to 3 h. Although the rate of killing with ofloxacin was substantially slower than that with trovafloxacin, ofloxacin was also able to eradicate all eight strains from the model, despite a simulated area under the inhibitory curve/MIC ratio (AUC/MIC) of only 49. In contrast, ciprofloxacin eradicated only five strains (AUC/MIC = 44) from the model. Against the other three strains (AUC/MIC = 22), the antibacterial activity of ciprofloxacin was substantially diminished. These data corroborate clinical data and suggest that trovafloxacin has a pharmacodynamic advantage over ciprofloxacin and ofloxacin against S. pneumoniae in relation to its enhanced antipneumococcal activity.

Use of microdilution panels with and without beta-lactamase inhibitors as a phenotypic test for beta-lactamase production among Escherichia coli, Klebsiella spp., Enterobacter spp., Citrobacter freundii, and Serratia marcescens.

Over the past decade, a number of new beta-lactamases have appeared in clinical isolates of Enterobacteriaceae that, unlike their predecessors, do not confer beta-lactam resistance that is readily detected in routine antibiotic susceptibility tests. Because optimal methodologies are needed to detect these important new beta-lactamases, a study was designed to evaluate the ability of a panel of various beta-lactam antibiotics tested alone and in combination with beta-lactamase inhibitors to discriminate between the production of extended-spectrum beta-lactamases, AmpC beta-lactamases, high levels of K1 beta-lactamase, and other beta-lactamases in 141 isolates of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter cloacae, Enterobacter aerogenes, Citrobacter freundii, and Serratia marcescens possessing well-characterized beta-lactamases. The microdilution panels studied contained aztreonam, cefpodoxime, ceftazidime, cefotaxime, and ceftriaxone, with and without 1, 2, and 4 microg of clavulanate per ml or 8 microg of sulbactam per ml and cefoxitin and cefotetan with and without 8 microg of sulbactam per ml. The results indicated that a minimum panel of five tests would provide maximum separation of extended-spectrum beta-lactamase high AmpC, high K1, and other beta-lactamase production in Enterobacteriaceae. These included cefpodoxime, cefpodoxime plus 4 microg of clavulanate per ml, ceftazidime, ceftriaxone, and ceftriaxone plus 8 microg of sulbactam per ml. Ceftriaxone plus 2 microg of clavulanate per ml could be substituted for cefpodoxime plus 4 microg of clavulanate per ml without altering the accuracy of the tests. This study indicated that tests with key beta-lactam drugs, alone and in combination with beta-lactamase inhibitors, could provide a convenient approach to the detection of a variety of beta-lactamases in members of the family Enterobacteriaceae.

Clavulanate induces expression of the Pseudomonas aeruginosa AmpC cephalosporinase at physiologically relevant concentrations and antagonizes the antibacterial activity of ticarcillin.

Although previous studies have indicated that clavulanate may induce AmpC expression in isolates of Pseudomonas aeruginosa, the impact of this inducer activity on the antibacterial activity of ticarcillin at clinically relevant concentrations has not been investigated. Therefore, a study was designed to determine if the inducer activity of clavulanate was associated with in vitro antagonism of ticarcillin at pharmacokinetically relevant concentrations. By the disk approximation methodology, clavulanate induction of AmpC expression was observed with 8 of 10 clinical isolates of P. aeruginosa. Quantitative studies demonstrated a significant induction of AmpC when clavulanate-inducible strains were exposed to the peak concentrations of clavulanate achieved in human serum with the 3.2- and 3.1-g doses of ticarcillin-clavulanate. In studies with three clavulanate-inducible strains in an in vitro pharmacodynamic model, antagonism of the bactericidal effect of ticarcillin was observed in some tests with regimens simulating a 3.1-g dose of ticarcillin-clavulanate and in all tests with regimens simulating a 3.2-g dose of ticarcillin-clavulanate. No antagonism was observed in studies with two clavulanate-noninducible strains. In contrast to clavulanate. No antagonism was observed in studies with two clavulanate-noninducible strains. In contrast to clavulanate, tazobactam failed to induce AmpC expression in any strains, and the pharmacodynamics of piperacillin-tazobactam were somewhat enhanced over those of piperacillin alone against all strains studied. Overall, the data collected from the pharmacodynamic model suggested that induction per se was not always associated with reduced killing but that a certain minimal level of induction by clavulanate was required before antagonism of the antibacterial activity of its companion drug occurred. Nevertheless, since clinically relevant concentrations of clavulanate can antagonize the bactericidal activity of ticarcillin, the combination of ticarcillin-clavulanate should be avoided when selecting an antipseudomonal beta-lactam for the treatment of P. aeruginosa infections, particularly in immunocompromised patients. For piperacillin-tazobactam, induction is not an issue in the context of treating this pathogen.

Use of an isogenic Escherichia coli panel to design tests for discrimination of beta-lactamase functional groups of Enterobacteriaceae.

A study was designed to determine if an isogenic panel of Escherichia coli strains containing many different beta-lactamases could be used for the preliminary screening of a large number of beta-lactam agents to identify which might be most useful in the development of a definitive test for specific beta-lactamases found among the members of family Enterobacteriaceae. The susceptibilities of 46 strains, comprising the isogenic panel, to expanded-spectrum cephalosporins, cephamycins, and aztreonam were determined in the presence and absence of beta-lactamase inhibitors in broth microdilution tests. The results indicated that strains producing extended-spectrum beta-lactamases (ESBLs) could be distinguished from strains producing other Bush-Jacoby-Medeiros functional group 2 or group 1 beta-lactamases. For strains producing group 1 beta-lactamases, cefpodoxime and ceftazidime MICs were > or = 4 micrograms/ml and addition of clavulanate did not reduce the MICs more than fourfold. For strains producing group 2 enzymes other than ESBLs, cefpodoxime and ceftazidime MICs were < or = 2 micrograms/ml. With a single exception (ceftazidime for the strain producing SHV-3), among strains producing ESBLs, cefpodoxime and ceftazidime MICs were > or = 4 micrograms/ml and addition of clavulanate reduced the MICs by more than eightfold. Cephamycins could also be used to discriminate between strains producing group 1 beta-lactamases and ESBLs, since only the former required cefotetan concentrations as high as 8 micrograms/ml or cefoxitin concentrations of > 16 micrograms/ml for inhibition. Other cephalosporins provided some discrimination between the various beta-lactamase producers, although they were not as reliable as either cefpodoxime or ceftazidime. These results indicate the utility of an isogenic panel for identification of candidate drugs among many for further testing with clinical isolates of the family Enterobacteriaceae to determine the best agents for detection of specific beta-lactamases in this family.

The effects of increasing levels of quinolone resistance on in-vitro activity of four quinolones.

A panel of 266 clinically isolated Gram-positive cocci and Gram-negative bacilli with varying levels of resistance to ciprofloxacin were analysed for susceptibility to Du-6859a, ciprofloxacin, ofloxacin, temafloxacin and nalidixic acid. Staphylococci were divided into ciprofloxacin-susceptible, moderately resistant and highly resistant subgroups. Du-6859a was the most potent quinolone against all taxa. As ciprofloxacin resistance increased to high levels, MICs of all quinolones increased but Du-6859a MICs increased least, and ciprofloxacin MICs increased most. Less susceptible single-step mutants were selected from 80% of 15 representative clinical isolates exposed to ciprofloxacin, 71% of isolates exposed to temafloxacin, 67% of isolates exposed to Du-6859a and 53% of isolates exposed to ofloxacin. Du-6859a inhibited more mutants (67%) at a concentration of 1 mg/L than did the other quinolones (26-43%) at their susceptible breakpoints. Du-6859a was the most rapidly bactericidal quinolone in time-kill studies with Enterococcus faecalis and Enterococcus faecium. This study indicated that Du-6859a is more potent than the comparator quinolones, is less affected by the mechanisms responsible for high-level quinolone resistance and may be less likely to select resistant mutants if it has a susceptible breakpoint of 1 mg/L.

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.

Cefepime-aztreonam: a unique double beta-lactam combination for Pseudomonas aeruginosa.

An in vitro pharmacokinetic model was used to determine if aztreonam could enhance the pharmacodynamics of cefepime or ceftazidime against an isogenic panel of Pseudomonas aeruginosa 164, including wild-type (WT), partially derepressed (PD), and fully derepressed (FD) phenotypes. Logarithmic-phase cultures were exposed to peak concentrations achieved in serum with 1- or 2-g intravenous doses, elimination pharmacokinetics were simulated, and viable bacterial counts were measured over three 8-h dosing intervals. In studies with cefepime and cefepime-aztreonam against the PD strain, samples were also filter sterilized, assayed for active cefepime, and assayed for nitrocefin hydrolysis activity before and after overnight dialysis. Against WT strains, the cefepime-aztreonam combination was the most active regimen, but viable counts at 24 h were only 1 log below those in cefepime-treated cultures. Against PD and FD strains, the antibacterial activity of cefepime-aztreonam was significantly enhanced over that of each drug alone, with 3.5 logs of killing by 24 h. Hydrolysis and bioassay studies demonstrated that aztreonam was inhibiting the extracellular cephalosporinase that had accumulated and was thus protecting cefepime in the extracellular environment. In contrast to cefepime-aztreonam, the pharmacodynamics of ceftazidime-aztreonam were not enhanced over those of aztreonam alone. Further pharmacodynamic studies with five other P. aeruginosa strains producing increased levels of cephalosporinase demonstrated that the enhanced pharmacodynamics of cefepime-aztreonam were not unique to the isogenic panel. The results of these studies demonstrate that aztreonam can enhance the antibacterial activity of cefepime against derepressed mutants of P. aeruginosa producing increased levels of cephalosporinase. This positive interaction appears to be due in part to the ability of aztreonam to protect cefepime from extracellular cephalosporinase inactivation. Clinical evaluation of this combination is warranted.

beta-Lactamases responsible for resistance to expanded-spectrum cephalosporins in Klebsiella pneumoniae, Escherichia coli, and Proteus mirabilis isolates recovered in South Africa.

Although resistance to the expanded-spectrum cephalosporins among members of the family Enterobacteriaceae lacking inducible beta-lactamases occurs virtually worldwide, little is known about this problem among isolates recovered in South Africa. Isolates of Klebsiella pneumoniae, Escherichia coli, and Proteus mirabilis resistant to expanded-spectrum cephalosporins recovered from patients in various parts of South Africa over a 3-month period were investigated for extended-spectrum beta-lactamase production. Antibiotic susceptibility was determined by standard disk diffusion and agar dilution procedures. Production of extended-spectrum beta-lactamases was evaluated by using the double-disk test, and the beta-lactamases were characterized by spectrophotometric hydrolysis assays and an isoelectric focusing overlay technique which simultaneously determined isoelectric points and general substrate or inhibitor characteristics. DNA amplification and sequencing were performed to confirm the identities of these enzymes. The P. mirabilis and E. coli isolates were found to produce TEM-26-type, SHV-2, and SHV-5 extended-spectrum beta-lactamases. An AmpC-related enzyme which had a pI of 8.0 and which conferred resistance to cefoxitin as well as the expanded-spectrum cephalosporins was found in a strain of K. pneumoniae. This is the first study which has identified organisms producing different extended-spectrum beta-lactamases from South Africa and the first report describing strains of P. mirabilis producing a TEM-26-type enzyme. The variety of extended-spectrum beta-lactamases found among members of the family Enterobacteriaceae isolated from major medical centers in South Africa is troubling and adds to the growing list of countries where these enzymes pose a serious problem for antimicrobial therapy.

Development of quinolone-resistant Campylobacter fetus bacteremia in human immunodeficiency virus-infected patients.

Campylobacter fetus subspecies fetus has been recognized as a cause of systemic illness in immunocompromised hosts, including relapsing bacteremia in human immunodeficiency virus (HIV)-infected patients. Acquired resistance to quinolone therapy, while reported for a variety of bacteria, including Campylobacter jejuni, has not been previously documented for C. fetus. Two cases of quinolone-resistant C. fetus bacteremia were detected in HIV-infected patients. Cloning and nucleotide sequencing of the C. fetus gyrA gene in the 2 resistant isolates demonstrated a G-to-T change that led to an Asp-to-Tyr amino acid substitution at a critical residue frequently associated with quinolone resistance. In addition, comparison of the pre- and posttreatment isolates from 1 patient documented outer membrane protein changes temporally linked with the development of resistance. Relapsing C. fetus infections in quinolone-treated HIV-infected patients may be associated with the acquisition of resistance to these agents, and this resistance may be multifactorial.

Plasmid-mediated resistance to expanded-spectrum cephalosporins among Enterobacter aerogenes strains.

Resistance to expanded-spectrum cephalosporins commonly develops in Enterobacter aerogenes during therapy due to selection of mutants producing high levels of the chromosomal Bush group 1 beta-lactamase. Recently, resistant strains producing plasmid-mediated extended-spectrum beta-lactamases (ESBLs) have been isolated as well. A study was designed to investigate ESBL production among 31 clinical isolates of E. aerogenes from Richmond, Va., with decreased susceptibility to expanded-spectrum cephalosporins and a positive double-disk potentiation test. Antibiotic susceptibility was determined by standard disk diffusion and agar dilution procedures. Beta-lactamases were investigated by an isoelectric focusing overlay technique which simultaneously determined isoelectric points (pIs) and substrate or inhibitor profiles. Decreased susceptibility to cefotaxime, ceftazidime, and aztreonam (MIC range, 1 to 64 microg/ml) was detected and associated with resistance to gentamicin and trimethoprim-sulfamethoxazole. All strains produced an inducible Bush group 1 beta-lactamase (pI 83). Twenty-nine of the 31 isolates also produced an enzyme similar to SHV-4 (pI 7.8), while 1 isolate each produced an enzyme similar to SHV-3 (pI 6.9) and to SHV-5 (pI 8.2). The three different SHV-derived ESBLs were transferred by transconjugation to Escherichia coli C600N and amplified by PCR. Plasmid profiles of the clinical isolates showed a variety of different large plasmids. Because of the linkage of resistance to aminoglycosides and trimethoprim-sulfamethoxazole with ESBL production, it is possible that the usage of these drugs was responsible for selecting plasmid-mediated resistance to extended-spectrum cephalosporins in E. aerogenes. Furthermore, it is important that strains such as these be recognized, because they can be responsible for institutional spread of resistance genes.