Different ratios of the piperacillin-tazobactam combination for treatment of experimental meningitis due to Klebsiella pneumoniae producing the TEM-3 extended-spectrum beta-lactamase

Repeated Piperacillin-Tazobactam Plasma Concentration Measurements in Severely Obese Versus Nonobese Critically Ill Septic Patients and the Risk of Under- and Overdosing

OBJECTIVE

Obesity and critical illness modify pharmacokinetics of antibiotics, but piperacillin-tazobactam continuous IV infusion pharmacokinetics has been poorly studied in obese critically ill patients. We aimed to compare pharmacokinetics of piperacillin in severely obese and nonobese patients with severe sepsis or septic shock. We hypothesized that plasma concentration variability would expose the critically ill to both piperacillin under and overdosing.

METHODS

Prospective comparative study. Consecutive critically ill severely obese (body mass index, > 35 kg/m) and nonobese patients (body mass index, < 30 kg/m) were treated with 16 g/2 g/24 hr continuous piperacillin-tazobactam infusion. Piperacillin plasma concentration was measured every 12 hours over a 7-day period by high-pressure liquid chromatography. Unbound piperacillin plasma concentration and fractional time of plasma concentration spent over 64 mg/L (4-fold the minimal inhibitory concentration for Pseudomonas aeruginosa) were compared between the two groups. We performed 5,000 Monte Carlo simulations for various dosing regimens and minimal inhibitory concentration and calculated the probability to spend 100% of the time over 64 mg/L.

RESULTS

We enrolled 11 severely obese and 12 nonobese patients and obtained 294 blood samples. We did not observe a statistically significant difference in piperacillin plasma concentrations over time between groups. The fractional time over 64 mg/L was 64% (43-82%) and 93% (85-100%) in obese and nonobese patients, respectively, p = 0.027 with intra- and intergroup variability. Five nonobese and two obese patients experienced potentially toxic piperacillin plasma concentrations. When 64 mg/L was targeted, Monte Carlo simulations showed that 12 g/1.5 g/24 hr was inadequate in both groups and 16 g/2 g/24 hr was adequate only in nonobese patients.

CONCLUSION

Using a conventional dosing of 16 g/2 g/24 hr continuous infusion, obese patients were more likely than nonobese patients to experience piperacillin underdosing when facing high minimal inhibitory concentration pathogens. The present study suggests that piperacillin drug monitoring might be necessary in the sickest patients who are at the highest risk of unpredictable plasma concentration exposing them to overdose, toxicity, underdosing, and treatment failure.

In vitro and in vivo activities of piperacillin-tazobactam and meropenem at different inoculum sizes of ESBL-producing Klebsiella pneumoniae

The inoculum effect is a laboratory phenomenon in which the minimal inhibitory concentration (MIC) of an antibiotic is increased when a large number of organisms are exposed. Due to the emergence of extended-spectrum β-lactamase-producing Klebsiella pneumoniae (ESBL-Kpn) infections, the inoculum effect of ESBL-Kpn on β-lactams was studied in vitro and in vivo using an experimental model of pneumonia. The in vitro inoculum effect of 45 clinical ESBL-Kpn isolates on β-lactams was evaluated at standard (10(5) CFU/mL) and high (10(7) CFU/mL) organism concentrations. The MIC50 of piperacillin-tazobactam, cefotaxime and cefepime was increased eight-fold or more and that of meropenem was increased two-fold. The in vivo inoculum effect was evaluated in an ESBL-Kpn pneumonia mouse model treated with bacteriostatic effect-adjusted doses of piperacillin-tazobactam (1000 mg/kg four times daily, %T>MIC; 32.60%) or meropenem (100 mg/kg twice daily, %T>MIC; 28.65%) at low/standard (10(4) CFU/mouse) and high (10(6) CFU/mouse) inocula. In mice administered a low inoculum, no mice died after treatment with piperacillin-tazobactam or meropenem, whereas all the control mice died. In contrast, in the high inoculum model, all mice in the piperacillin-tazobactam-treated group died, whereas all meropenem-treated mice survived and had a decreased bacterial load in the lungs and no invasion into the blood. In conclusion, meropenem was more resistant to the inoculum effect of ESBL-Kpn than piperacillin-tazobactam both in vitro and in vivo. In the management of severe pneumonia caused by ESBL-Kpn, carbapenems may be the drugs of choice to achieve a successful outcome.

Pulmonary penetration of piperacillin and tazobactam in critically ill patients

Pulmonary infections in critically ill patients are common and are associated with high morbidity and mortality. Piperacillin–tazobactam is a frequently used therapy in critically ill patients with pulmonary infection. Antibiotic concentrations in the lung reflect target‐site antibiotic concentrations in patients with pneumonia. The aim of this study was to assess the plasma and intrapulmonary pharmacokinetics (PK) of piperacillin–tazobactam in critically ill patients administered standard piperacillin–tazobactam regimens. A population PK model was developed to describe plasma and intrapulmonary piperacillin and tazobactam concentrations. The probability of piperacillin exposures reaching pharmacodynamic end points and the impact of pulmonary permeability on piperacillin and tazobactam pulmonary penetration was explored. The median piperacillin and tazobactam pulmonary penetration ratios were 49.3 and 121.2%, respectively. Pulmonary piperacillin and tazobactam concentrations were unpredictable and negatively correlated with pulmonary permeability. Current piperacillin–tazobactam regimens may be insufficient to treat pneumonia caused by piperacillin–tazobactam–susceptible organisms in some critically ill patients.

Clinical Pharmacology & Therapeutics (2014); 96 4, 438–448. doi:10.1038/clpt.2014.131

Antimicrobial treatment of serious gram-negative infections in newborns

The choice of antibiotics for serious Gram-negative bacterial infections in the newborn must balance delivery of effective antibiotics to the site(s) of infection with the need to minimize selection of antibiotic resistance. To reduce the risk of selective pressure from large-scale cephalosporin usage, a penicillin-aminoglycoside combination is recommended as empiric therapy for neonatal sepsis. Where Gram-negative sepsis is strongly suspected or proven, a third-generation cephalosporin should ordinarily replace penicillin. Piperacillin-tazobactam can provide better Gram-negative cover than penicillin-aminoglycoside combinations, without the risk of selecting antibiotic resistance seen with cephalosporins, but further clinical studies are required before this approach to empiric therapy can be recommended. For antibiotic-resistant infections, a carbapenem remains the mainstay of treatment. However, rapid emergence and spread of resistance to these antibiotics means that in the future, neonatologists may have to rely on antibiotics such as colistin, whose pharmacokinetics, safety, and clinical efficacy in neonates are not well-defined.

Comparative assessment of inoculum effects on the antimicrobial activity of amoxycillin-clavulanate and piperacillin-tazobactam with extended-spectrum beta-lactamase-producing and extended-spectrum beta-lactamase-non-producing Escherichia coli isolates

A significant inoculum-size effect has been observed with piperacillin-tazobactam, and has been associated with beta-lactamase production in extended-spectrum beta-lactamase (ESBL) producers. This association has not been previously studied in the case of amoxycillin-clavulanate. Piperacillin-tazobactam and amoxycillin-clavulanate were compared, using high inocula of susceptible strains either harbouring ESBLs or not. Two non-ESBL-producing and 15 amoxycillin-clavulanate-susceptible and piperacillin-tazobactam-susceptible ESBL-producing Escherichia coli isolates, and their respective transconjugants, were tested in dilution susceptibility tests using standard and 100-fold higher inocula. Three ESBL-producing strains and E. coli ATCC 25922 were selected for time-kill studies using standard and high initial inocula. At high inocula, MICs of piperacillin increased >eight-fold for non-ESBL-producing strains, and MICs of piperacillin-tazobactam (8:1 ratio or with tazobactam fixed at 4 mg/L) increased>eight-fold for all ESBL-producing strains. However, amoxycillin MICs were not affected by a high inoculum with non-ESBL-producing strains, whereas the MICs of amoxycillin-clavulanate (2:1 and 4:1) increased <or=four-fold for ESBL producers, using the broth and agar dilution methods. In kinetic studies at a high inoculum, amoxycillin and amoxycillin-clavulanate were bactericidal against E. coli ATCC 25922, whereas piperacillin and piperacillin-tazobactam yielded decreases of <1 log(10) CFU/mL. Similarly, at a high inoculum, only amoxycillin-clavulanate was able to maintain bactericidal rates of killing over 24 h against the ESBL-positive E. coli isolates. The stability of amoxycillin-clavulanate and the contrasting results obtained with piperacillin-tazobactam against high inocula of ESBL-non-producing and ESBL-producing E. coli strains appear to be related to aspects other than the amount of beta-lactamase production.

In vitro killing of parenteral beta-lactams against standard and high inocula of extended-spectrum beta-lactamase and non-ESBL producing Klebsiella pneumoniae

Minimum inhibitory concentrations and time-kill curves were performed against 8 Klebsiella pneumoniae (4 non-extended-spectrum beta-lactamase[ESBL] and 4 ESBL) for piperacillin/tazobactam (40/5 microg/mL), cefepime (20 microg/mL), and meropenem (4 microg/mL) by using a standard and high inocula. Imipenem was evaluated only at the standard inoculum for the non-ESBL and ESBL isolates. Samples were withdrawn at 7 predetermined time-points over 24 hours and plated on trypticase soy agar plates. Minimum inhibitory concentrations were: piperacillin/tazobactam 4 to 8 microg/mL (ESBL and non-ESBL), cefepime 1 to 2 microg/mL (ESBL) and 0.06 to 0.125 microg/mL (non-ESBL), imipenem 0.125 to 0.25 microg/mL (ESBL and non-ESBL), and meropenem 0.03 to 0.06 microg/mL (ESBL and non-ESBL). Each antibiotic reached and maintained bactericidal killing (> or =3 log killing) for 24 hours against all non-ESBL isolates for both the standard and high inoculum. Cefepime, imipenem, and meropenem showed the same bactericidal activity against each ESBL isolate at the standard inoculum, whereas piperacillin/tazobactam showed bactericidal killing against only 1 ESBL isolate. At the high inoculum, cefepime and piperacillin/tazobactam were unable to maintain bactericidal activity against any of the ESBL isolates. Only meropenem was able to maintain bactericidal killing over 24 hours against the ESBL isolates at the high inoculum. In summary, meropenem and imipenem maintained bactericidal activity against non-ESBL and ESBL K. pneumoniae irrespective of the inoculum size. While piperacillin/tazobactam and cefepime are bactericidal against non-ESBL K. pneumoniae, their activity against ESBL K. pneumoniae is limited and based on the size of the inoculum. Until more data are available, piperacillin/tazobactam and cefepime should not be used for the treatment of ESBL K. pneumoniae.

Clinical and microbiologic analysis of a hospital's extended-spectrum beta-lactamase-producing isolates over a 2-year period

STUDY OBJECTIVE

To evaluate the microbiologic and clinical outcomes of patients with extended-spectrum beta-lactamase (ESBL)-producing isolates over a 2-year period.

DESIGN

Retrospective analysis.

SETTING

Tertiary care teaching hospital.

PATIENTS

Twenty-one patients with cultures of confirmed ESBL-producing Escherichia coli, Klebsiella pneumoniae, or Klebsiella oxytoca.

MEASUREMENTS AND MAIN RESULTS

Antimicrobial susceptibilities of piperacillin-tazobactam, cefotetan, carbapenems, aminoglycosides, fluoroquinolones, trimethoprim-sulfamethoxazole, and nitrofurantoin (nitrofurantoin for urinary isolates only) of confirmed ESBL producers at our institution were determined, as well as clinical outcomes of patients with ESBL-producing isolates. Microbiologic and medical records were reviewed for patient sex and age, antimicrobial susceptibilities, antimicrobial therapy, and clinical and microbiologic outcomes. From January 2000-December 2001, 31 isolates were confirmed as ESBL producers (6 E. coli, 11 K. pneumoniae, and 14 K. oxytoca). A statistically significant increase occurred over the 2-year period from 9 (0.6%) of 1414 isolates in 2000 to 22 (1.8%) of 1218 isolates in 2001 (p = 0.0055). All isolates were susceptible to carbapenems, and more than 88% were susceptible to amikacin, cefotetan, or nitrofurantoin. Less than 70% of isolates were susceptible to gentamicin, fluoroquinolones, piperacillin-tazobactam, or trimethoprim-sulfamethoxazole. All patients treated with a carbapenem experienced clinical cure. Piperacillin-tazobactam alone and in combination resulted in an overall clinical cure rate of 55%, with a 50% cure rate for isolates susceptible to piperacillin-tazobactam. All patients in whom antibiotic therapy failed had been treated with piperacillin-tazobactam or cefepime, either alone or in combination with a fluoroquinolone.

CONCLUSION

Carbapenems remain the treatment of choice for ESBL-producing pathogens. Piperacillin-tazobactam and cefepime should not be routinely administered for the treatment of these organisms.

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.

Hospital-acquired infections in the neonatal intensive care unit--Klebsiella pneumoniae

Klebsiella pneumoniae is medically the most important organism of the Klebsiella species. It is responsible for a significant proportion of hospital-acquired infections including septicemias, urinary tract infections, pneumonia, and soft tissue infections especially in the immunocompromised hosts such as the neonate. The hands of healthcare workers and the gastrointestinal tract of hospitalized infants serve as reservoirs for the transmission of the organism and are responsible for multiple hospital outbreaks. In recent years, there has been an increase in the incidence of outbreaks caused by multidrug resistance K. pneumoniae organisms or the extended spectrum beta-lactamase (ESBL)-producing K. pneumoniae. The problems associated with extended spectrum beta-lactamase-producing organisms include difficulties in accurate antimicrobial susceptibility testing, limited treatment options and increased morbidity and perhaps mortality. Hence, prevention through implementation of strict infection control guidelines, effective hand washing and judicious use of antimicrobials such as third generation cephalosporins is important to effectively reduce the morbidity associated with this infection.

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.

Therapeutic challenges associated with extended-spectrum, beta-lactamase-producing Escherichia coli and Klebsiella pneumoniae

The emergence of extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae, particularly Escherichia coli and Klebsiella pneumoniae, presents significant diagnostic and therapeutic challenges to the management of infections due to these organisms. Detection of resistant isolates is difficult based on routine susceptibility testing performed by a clinical microbiology laboratory. In addition, the utility of penicillins, cephalosporins, and aztreonam in treating serious infections due to these organisms is uncertain due to reports of treatment failure despite apparent in vitro susceptibility. A critical evaluation of the English literature was performed on treatment outcomes associated with ESBL-producing Enterobacteriaceae. Imipenem and extended-spectrum cephalosporins were commonly administered. Discordant outcomes in relation to in vitro susceptibility of the agent did not occur exclusively with cephalosporins but with all drugs including imipenem. Until more outcome data are available, drug selection must take into consideration whether or not an outbreak is occurring and whether therapy is empirical or definitive.

Polymicrobial meningitis revealing an anterior sacral meningocele in a 23-year-old woman

Polymicrobial meningitis has become increasingly rare during recent decades. Historically, it has mainly been reported as being associated with disorders of the ENT-sphere. The treatment of these infections being optimized, polymicrobial meningitis nowadays is essentially a complication of gastrointestinal or gynaecological disorders and trauma. We present a case of polymicrobial meningitis following puncture of a unrecognized pre-sacral meningocele in a patient with Currarino syndrome and review of the relevant literature.

Efficacy of beta-lactam and inhibitor combinations in a diffusion chamber model in rabbits

Using a diffusion chamber in rabbits, we evaluated therapy with the combination of ceftriaxone plus the beta-lactamase inhibitor tazobactam in comparison with ceftriaxone alone. One sensitive and one resistant strain of Escherichia coli, Enterobacter cloacae and Klebsiella pneumoniae were inoculated into one of the six diffusion chambers, implanted in the same animal. In order to simulate pharmacokinetics in humans, both substances were administered in decreasing doses. Ceftriaxone was given 0, 2, 4 and 6 h after infection in dosages of 45, 35, 25 and 15 mg/kg of body weight, while tazobactam was administered either in one dose at 0 h, or divided into two doses at 0 and 1 h or 0 and 4 h, or divided into three doses at 0, 1 and 4 h after infection. The ratio of ceftriaxone:tazobactam was fixed at 8:1. Ceftriaxone, in combination with tazobactam, given in one dose immediately after infection showed a significant reduction in bacterial count. All other combinations of ceftriaxone and tazobactam did not differ from ceftriaxone in monotherapy. Co-administration of the beta-lactamase inhibitor tazobactam significantly enhanced the activity of ceftriaxone against all three tested species.

Assessment of biliary excretion of piperacillin-tazobactam in humans

Piperacillin-tazobactam concentrations in serum and bile were measured intraoperatively in 10 patients undergoing cholecystectomy (group 1) and 5 cholecystectomized patients provided with external bile duct drainage (group 2). Each patient received a single intravenous dose of piperacillin at 4 g plus tazobactam at 0.5 g over 30 min. Drug concentrations in both serum and bile were measured by high-performance liquid chromatography. In group 1 patients, serum and bile specimens and gallbladder wall fragments were collected at mean times of 70 and 83 min postinfusion, respectively. The mean concentrations of piperacillin and tazobactam were, respectively, 69.1 +/- 41.5 (standard deviation) and 9.9 +/- 5.1 microg/ml in serum, 630.4 microg/ml (range, 24.8 to 1,194 microg/ml) and 11.8 microg/ml (range, 3.6 to 22 microg/ml) in choledochal bile, 342.3 microg/ml (range, 1.1 to 1,149 microg/ml) and 7.7 microg/ml, (range, 0.2 to 23.1 microg/ml) in gallbladder bile, and 49.3 microg/g (range, 9.7 to 223 microg/g) and 2.9 microg/g (range, 0.1 to 5.9 microg/g) in the gallbladder wall. In group 2 patients, the amounts of drugs recovered in bile drainage obtained over 12 h were 28.4 +/- 18.0 and 1.0 +/- 0.5 mg for piperacillin and tazobactam, respectively. Peak piperacillin and tazobactam concentrations in bile reached 358 +/- 242 and 10.8 +/- 4.2 microg/ml, respectively. Comparison of drug levels in serum and bile suggests an underlying active secretion process for piperacillin elimination into the bile, unlike that of tazobactam. From a therapeutic viewpoint, given the concentrations of tazobactam recorded in bile fluid and tissue, the addition of this beta-lactamase inhibitor to piperacillin therapy might be of interest in the management of biliary tract infections, mostly in patients at risk of mixed aerobic-anaerobic infections due to beta-lactamase-producing organisms.

Efficacies of piperacillin-tazobactam and cefepime in rats with experimental intra-abdominal abscesses due to an extended-spectrum beta-lactamase-producing strain of Klebsiella pneumoniae

The in vivo activities of piperacillin-tazobactam and cefepime were compared with those of ticarcillin-clavulanate, ceftazidime, cefotaxime, and imipenem in a rat model of intra-abdominal abscess with a strain of Klebsiella pneumoniae elaborating an extended-spectrum beta-lactamase (TEM-26). With the exception of ceftazidime, all of the antimicrobial agents significantly reduced bacterial counts within abscesses at the end of therapy compared with those in untreated controls. Residual viable cell counts (mean +/- standard deviation in log10 CFU/gram) were as follows: control, 8.76 +/- 0.97; ceftazidime, 8.00 +/- 0.76; piperacillin-tazobactam, 3.87 +/- 1.72; ticarcillin-clavulanate, 3.74 +/- 1.34; cefepime, 3.15 +/- 1.19; cefotaxime, 2.61 +/- 0.77; imipenem, 2.41 +/- 0.93. Imipenem was more effective than either of the inhibitor combinations (P < 0.05). Cefotaxime was unexpectedly effective given its poor in vivo activity against this organism in our earlier studies, which used a different dose and total duration of therapy (L. B. Rice, J. D. C. Yao, K. Klimm, G. M. Eliopoulos, and R. C. Moellering, Jr., Antimicrob. Agents Chemother. 35:1243-1244, 1991). These observations suggest that the effectiveness of cephalosporins in the treatment of experimental infections caused by extended-spectrum beta-lactamase-producing K. pneumoniae may be highly dependent on dosing regimens, even for a specific organism and site of infection.

Kinetics of piperacillin and tazobactam in ventricular cerebrospinal fluid of hydrocephalic patients

Its broad antibacterial spectrum qualifies the combination of piperacillin and tazobactam for therapy of nosocomial bacterial central nervous system (CNS) infections. Since these infections sometimes are accompanied by only minor dysfunction of the blood-cerebrospinal fluid (CSF) barrier, patients with noninflammatory occlusive hydrocephalus who had undergone external ventriculostomy were studied (n = 9; age range, 48 to 75 years). After administration of the first dose of piperacillin (6 g)-tazobactam (0.5 g) over 30 min intravenously, serum and CSF were drawn repeatedly and analyzed by high-performance liquid chromatography. Pharmacokinetics were determined by noncompartmental analysis. Maximum concentrations of piperacillin in CSF ranged from 8.67 to <0.37 mg/liter (median, 3.42 mg/liter), and those of tazobactam ranged from 1.37 to 0.11 mg/liter (median, 0.45 mg/liter). CSF maxima were observed, in median, 1.5 and 2 h after the end of the infusion. Elimination in CSF was considerably slower than in serum (median half-life at beta phase for piperacillin, 5.9 h in CSF versus 1.47 h in serum; for tazobactam, 6.1 h versus 1.34 h). For tazobactam, the ratio of the area under the concentration-time curve (AUC) in CSF to the AUC in serum was approximately three times as high as that for piperacillin (medians, 0.106 versus 0.034). In view of the tazobactam concentrations in CSF observed in this study, the practice of using a constant concentration of 4 mg of tazobactam per liter for MIC determination is inadequate for intracranial infections. Larger amounts of tazobactam than the standard dose of 0.5 g three times daily may be necessary for CNS infections.

Importance of beta-lactamase inhibitor pharmacokinetics in the pharmacodynamics of inhibitor-drug combinations: studies with piperacillin-tazobactam and piperacillin-sulbactam

An in vitro pharmacokinetic model was used to study the pharmacodynamics of piperacillin-tazobactam and piperacillin-sulbactam against gram-negative bacilli producing plasmid-encoded beta-lactamases. Logarithmic-phase cultures were exposed to peak antibiotic concentrations observed in human serum after the administration of intravenous doses of 3 g of piperacillin and 0.375 g of tazobactam or 0.5 g of sulbactam. Piperacillin and inhibitor were either dosed simultaneously or piperacillin was dosed sequentially 0.5 h after dosing with the inhibitor. In studies with all four test strains, the pharmacodynamics observed after simultaneous dosing were similar to those observed with the sequential regimen. Since the ratio between piperacillin and tazobactam was in constant fluctuation after sequential dosing, these data suggest that the pharmacodynamics of the piperacillin-inhibitor combinations were not dependent upon maintenance of a critical ratio between the components. Furthermore, when regrowth was observed, the time at which bacterial counts began to increase was similar between the simultaneous and sequential dosing regimens. Since the pharmacokinetics of the inhibitors were the same for all regimens, these data suggest that the length of time that the antibacterial activity was maintained over the dosing interval with these combinations was dictated by the pharmacokinetics of the beta-lactamase inhibitor in the combination. The antibacterial activity of the combination appeared to be lost when the amount of inhibitor available fell below some critical concentration. This critical concentration varied depending upon the type and amount of enzyme produced, as well as the specific inhibitor used. These results indicate that the antibacterial activity of drug-inhibitor combinations, when dosed at their currently recommended ratios, is more dependent on the pharmacokinetics of the inhibitor than on those of the beta-lactam drug.

Structure-based design of a potent transition state analogue for TEM-1 beta-lactamase

The structure of the plasmid-mediated beta-lactamase TEM-1 has been solved in complex with a designed boronic acid inhibitor (1R)-1-acetamido-2-(3-carboxyphenyl)ethane boronic acid at 1.7 A resolution. The boronate inhibitor was designed based on the crystallographic coordinates of the acyl-enzyme intermediate of TEM-1 bound to the substrate penicillin G. The boronate-TEM-1 complex is highly ordered and defines a novel transition state analogue of the deacylation step in the beta-lactamase reaction pathway. The design principles of this highly effective inhibitor (Ki = 110 nM) and the resulting structural and mechanistic implications are presented.

Hospital outbreak of Klebsiella pneumoniae resistant to broad-spectrum cephalosporins and beta-lactam-beta-lactamase inhibitor combinations by hyperproduction of SHV-5 beta-lactamase

An aminoglycoside- and ceftazidime-resistant strain of Klebsiella pneumoniae K2 producing the extended-spectrum beta-lactamase SHV-5 infected or colonized 14 pediatric patients at Guy's Hospital. The patients were mostly neonates recovering from cardiac surgery for congenital defects. The organism was also isolated from a nurse and from the father of one of the children. Four patients had septicemia, and two septicemic neonates with postoperative renal failure died. Aminoglycoside and cephalosporin resistance transferred to Escherichia coli in vitro on a 160-kb plasmid, and a similar resistant E. coli strain was isolated from the stools of one of the affected children. The epidemic organism colonized the bowel and skin and was probably transmitted via staff hands. Five wards were involved because of extensive patient movements. The outbreak was controlled by patient isolation and attention to handwashing. All of the isolates of the outbreak strain were identical by phage typing, ribotyping, plasmid profiling, and biochemical and serological testing, but they varied in their production of SHV-5. Some isolates produced normal amounts of SHV-5 and were susceptible to beta-lactam-beta-lactamase inhibitor combinations. Others, including the single isolate of multiresistant E. coli, produced up to five times as much enzyme as "normal" isolates. This hyperproduction resulted in increased resistance to several penicillins and cephalosporins and to the beta-lactam-beta-lactamase inhibitor combinations amoxicillin-clavulanic acid, ampicillin-sulbactam, piperacillin-tazobactam, and ceftazidime-clavulanic acid. The hyperproduction of SHV-5 by K. pneumoniae and E. coli seen in this outbreak suggests that beta-lactam-beta-lactamase inhibitor combinations may be unreliable for the treatment of organisms producing extended-spectrum beta-lactamases.

beta-Lactamases in laboratory and clinical resistance

beta-Lactamases are the commonest single cause of bacterial resistance to beta-lactam antibiotics. Numerous chromosomal and plasmid-mediated types are known and may be classified by their sequences or phenotypic properties. The ability of a beta-lactamase to cause resistance varies with its activity, quantity, and cellular location and, for gram-negative organisms, the permeability of the producer strain. beta-Lactamases sometimes cause obvious resistance to substrate drugs in routine tests; often, however, these enzymes reduce susceptibility without causing resistance at current, pharmacologically chosen breakpoints. This review considers the ability of the prevalent beta-lactamases to cause resistance to widely used beta-lactams, whether resistance is accurately reflected in routine tests, and the extent to which the antibiogram for an organism can be used to predict the type of beta-lactamase that it produces.

In vitro activities of various beta-lactam antimicrobial agents against clinical isolates of Escherichia coli and Klebsiella spp. resistant to oxyimino cephalosporins

Broth microdilution testing was used to study the activity of several beta-lactam antimicrobial agents, including piperacillin-tazobactam and cefepime, against 108 clinically derived Escherichia coli and Klebsiella sp. strains resistant to oxyimino cephalosporins (i.e., putative extended-spectrum beta-lactamase producers). On the basis of the percentage of susceptible strains, imipenem (100%), cefotetan (> or = 92%), and piperacillin-tazobactam (> or = 86%) were the most active agents. Cefepime activity (52 to 64% susceptible) was comparable to that of cefotaxime (40 to 63% susceptible) and aztreonam (20 to 63% susceptible). Among all beta-lactams tested, imipenem and cefotetan demonstrated the highest and most consistent level of activity and were the least affected by challenges with increased sizes of inocula of these resistant organisms.