Potency and antimicrobial spectrum update for piperacillin/tazobactam (2000): emphasis on its activity against resistant organism populations and generally untested species causing community-acquired respiratory tract infections

Piperacillin/tazobactam-resistant, cephalosporin-susceptible Escherichia coli bloodstream infections are driven by multiple acquisition of resistance across diverse sequence types

Resistance to piperacillin/tazobactam (TZP) in Escherichia coli has predominantly been associated with mechanisms that confer resistance to third-generation cephalosporins. Recent reports have identified E. coli strains with phenotypic resistance to piperacillin/tazobactam but susceptibility to third-generation cephalosporins (TZP-R/3GC-S). In this study we sought to determine the genetic diversity of this phenotype in E. coli (n=58) isolated between 2014–2017 at a single tertiary hospital in Liverpool, UK, as well as the associated resistance mechanisms. We compare our findings to a UK-wide collection of invasive E. coli isolates (n=1509) with publicly available phenotypic and genotypic data. These data sets included the TZP-R/3GC-S phenotype (n=68), and piperacillin/tazobactam and third-generation cephalosporin-susceptible (TZP-S/3GC-S, n=1271) phenotypes. The TZP-R/3GC-S phenotype was displayed in a broad range of sequence types, which was mirrored in the same phenotype from the UK-wide collection, and the overall diversity of invasive E. coli isolates. The TZP-R/3GC-S isolates contained a diverse range of plasmids, indicating multiple acquisition events of TZP resistance mechanisms rather than clonal expansion of a particular plasmid or sequence type. The putative resistance mechanisms were equally diverse, including hyperproduction of TEM-1, either via strong promoters or gene amplification, carriage of inhibitor-resistant β-lactamases, and an S133G bla_CTX-M-15 mutation detected for the first time in clinical isolates. Several of these mechanisms were present at a lower abundance in the TZP-S/3GC-S isolates from the UK-wide collection, but without the associated phenotypic resistance to TZP. Eleven (19%) of the isolates had no putative mechanism identified from the genomic data. Our findings highlight the complexity of this cryptic phenotype and the need for continued phenotypic monitoring, as well as further investigation to improve detection and prediction of the TZP-R/3GC-S phenotype from genomic data.

Metal ions weaken the hydrophobicity and antibiotic resistance of Bacillus subtilis NCIB 3610 biofilms

Surface superhydrophobicity makes bacterial biofilms very difficult to fight, and it is a combination of their matrix composition and complex surface roughness which synergistically protects these biomaterials from wetting. Although trying to eradicate biofilms with aqueous (antibiotic) solutions is common practice, this can be a futile approach if the biofilms have superhydrophobic properties. To date, there are not many options available to reduce the liquid repellency of biofilms or to prevent this material property from developing. Here, we present a solution to this challenge. We demonstrate how the addition of metal ions such as copper and zinc during or after biofilm formation can render the surface of otherwise superhydrophobic B. subtilis NCIB 3610 biofilms completely wettable. As a result of this procedure, these smoother, hydrophilic biofilms are more susceptible to aqueous antibiotics solutions. Our strategy proposes a scalable and widely applicable step in a multi-faceted approach to eradicate biofilms.

Topographical alterations render bacterial biofilms susceptible to chemical and mechanical stress

Treatment with concentrated ethanol, saline or glucose solutions smoothens biofilm surface topography and initially superhydrophobic/omniphobic biofilms are rendered hydrophilic.

Endothelial Cell Toxicity of Vancomycin Infusion Combined with Other Antibiotics

French guidelines recommend central intravenous (i.v.) infusion for high concentrations of vancomycin, but peripheral intravenous (p.i.v.) infusion is often preferred in intensive care units. Vancomycin infusion has been implicated in cases of phlebitis, with endothelial toxicity depending on the drug concentration and the duration of the infusion. Vancomycin is frequently infused in combination with other i.v. antibiotics through the same administrative Y site, but the local toxicity of such combinations has been poorly evaluated. Such an assessment could improve vancomycin infusion procedures in hospitals. Human umbilical vein endothelial cells (HUVEC) were challenged with clinical doses of vancomycin over 24 h with or without other i.v. antibiotics. Cell death was measured with the alamarBlue test. We observed an excess cellular death rate without any synergistic effect but dependent on the numbers of combined infusions when vancomycin and erythromycin or gentamicin were infused through the same Y site. Incompatibility between vancomycin and piperacillin-tazobactam was not observed in our study, and rinsing the cells between the two antibiotic infusions did not reduce endothelial toxicity. No endothelial toxicity of imipenem-cilastatin was observed when combined with vancomycin. p.i.v. vancomycin infusion in combination with other medications requires new recommendations to prevent phlebitis, including limiting coinfusion on the same line, reducing the infusion rate, and choosing an intermittent infusion method. Further studies need to be carried out to explore other drug combinations in long-term vancomycin p.i.v. therapy so as to gain insight into the mechanisms of drug incompatibility under multidrug infusion conditions.

Mechanism of β-lactam action in Streptococcus pneumoniae: the piperacillin paradox

The human pathogen Streptococcus pneumoniae has been treated for decades with β-lactam antibiotics. Its resistance is now widespread, mediated by the expression of mosaic variants of the target enzymes, the penicillin-binding proteins (PBPs). Understanding the mode of action of β-lactams, not only in molecular detail but also in their physiological consequences, will be crucial to improving these drugs and any counterresistances. In this work, we investigate the piperacillin paradox, by which this β-lactam selects primarily variants of PBP2b, whereas its most reactive target is PBP2x. These PBPs are both essential monofunctional transpeptidases involved in peptidoglycan assembly. PBP2x participates in septal synthesis, while PBP2b functions in peripheral elongation. The formation of the "lemon"-shaped cells induced by piperacillin treatment is consistent with the inhibition of PBP2x. Following the examination of treated and untreated cells by electron microscopy, the localization of the PBPs by epifluorescence microscopy, and the determination of the inhibition time course of the different PBPs, we propose a model of peptidoglycan assembly that accounts for the piperacillin paradox.

APPROPRIATE AND TIMELY EMPIRICAL ANTIMICROBIAL TREATMENT OF ICU INFECTIONS - A ROLE FOR CARBAPENEMS

Treatment of serious nosocomial infections in the intensive care unit requires swift, effective, well-tolerated and appropriate therapy from the outset. The consequences of inappropriate treatment, i.e. the use of antibiotics that are ineffective against the causative pathogen(s) or delayed therapy, are numerous and impact negatively upon both the patient and the ever-dwindling healthcare resources in many hospitals. Although antibiotics have revolutionised the treatment of infections, their inappropriate and untimely use within the intensive care setting has led to the emergence and spread of antibiotic-resistant bacteria worldwide. Consequently, to ensure successful patient outcomes (reduce morbidity and mortality), it is important that any antibiotic treatment employed is right first time. Treatment of serious infections in the intensive care unit requires an empirical stratagem providing broad-spectrum coverage to a wide range of suspected or difficult-to-treat pathogens such as Pseudomonas aeruginosa. However, to prevent the errors of the past, this needs to be tailored as soon as the pathogen has been identified and resistance patterns are known. The carbapenems are potent parenteral antibiotics, with an ultra-broad spectrum of activity that encompasses multi-drug resistant and difficult-to-treat Gram-negative bacteria. Clinical trial data supports the clinical effectiveness of these agents in patients with difficult to treat pathogens.

Activityin vitro of twelve antibiotics against clinical isolates of extended-spectrum beta-lactamase producing Escherichia coli

Twelve beta-lactam and non-beta-lactam antibiotics were evaluated against 115 clinical isolates of extended-spectrum beta-lactamase-producing (ESBLs) Escherichia coli using a broth microdilution test in accordance with the CLSI guidelines. Susceptibility was 100% with imipenem, ertapenem and amikacin, 95.7% with piperacillin-tazobactam, 91.3% with cefoxitin, 87% with tobramycin, 81.7% with amoxicillin-clavulanate, 80% with cefepime, 67.8% with ceftazidime, 27.8% with ciprofloxacin, 27% with levofloxacin and 13% with ceftriaxone. Ertapenem was the antibiotic with the lowest minimum inhibitory concentrations (MICs) for all isolates. There were no clinically relevant differences in the activity of the antibiotics in the presence of CTX-M-9 or SHV enzymes.

Piperacillin-tazobactam: a beta-lactam/beta-lactamase inhibitor combination

Piperacillin-tazobactam is a beta-lactam/beta-lactamase inhibitor combination with a broad spectrum of antibacterial activity that includes Gram-positive and -negative aerobic and anaerobic bacteria. Piperacillin-tazobactam retains its in vitro activity against broad-spectrum beta-lactamase-producing and some extended-spectrum beta-lactamase-producing Enterobacteriaceae, but not against isolates of Gram-negative bacilli harboring AmpC beta-lactamases. Piperacillin-tazobactam has recently been reformulated to include ethylenediaminetetraacetic acid and sodium citrate; this new formulation has been shown to be compatible in vitro with the two aminoglycosides, gentamicin and amikacin, allowing for simultaneous Y-site infusion, but not with tobramycin. Multicenter, randomized, double-blinded clinical trials have demonstrated piperacillin-tazobactam to be as clinically effective as relevant comparator antibiotics. Clinical trials have demonstrated piperacillin-tazobactam to be effective for the treatment of patients with intra-abdominal infections, skin and soft tissue infections, lower respiratory tract infections, complicated urinary tract infections, gynecological infections and more recently, febrile neutropenia. Piperacillin-tazobactam has an excellent safety and tolerability profile and continues to be a reliable option for the empiric treatment of moderate-to-severe infections in hospitalized patients.

[Activity of fosfomycin against extended-spectrum beta-lactamase producing Escherichia coli and Klebsiella pneumoniae]

INTRODUCTION

Infection due to extended-spectrum beta-lactamase (ESBL)-producing microorganisms is an emerging problem in the community; a high proportion of these microorganisms have been isolated from urine samples of women with uncomplicated urinary tract infections (UTI). The options for oral treatment of uncomplicated UTI are limited because of the multiple drug resistance typical of ESBL-producing strains.

METHODS

The in vitro activity of fosfomycin (FOS) was determined against 428 ESBL-producing strains, including 290 (68%) E. coli and 138 (32%) K. pneumoniae. Activity of fosfomycin was compared with that of amoxicillin-clavulanate (AMC), ciprofloxacin (CIP) and cotrimoxazole (SxT). MICs of AMC, CIP, and SxT, and detection of ESBL production were tested by the broth microdilution method, whereas FOS MICs were determined by the agar dilution method. ESBLs were characterized by isoelectric focusing, polymerase chain reaction (PCR) and direct sequencing of encoding genes. The genetic relationship among the isolates was determined by REP-PCR.

RESULTS

Among the 428 ESBL-producing isolates studied, 417 (97.4%) were susceptible to FOS (MIC < or = 64 microg/mL). The resistance rate of E. coli to FOS was 0.3%, and was lower than resistance to AMC (11.7%), whereas the resistance rate of K. pneumoniae was 7.2% and was equal to resistance to AMC. SxT and CIP were the least active antibiotic agents against ESBL-producing isolates (sensitivity < 50%). There were no differences in fosfomycin activity against strains expressing different types of ESBLs.

CONCLUSION

Fosfomycin showed maintained activity against ESBL-producing strains and did not present co-resistance with other antimicrobial groups.

The Meropenem Yearly Susceptibility Test Information Collection antimicrobial susceptibility program in Spain: a 5-year analysis

The Meropenem Yearly Susceptibility Test Information Collection program is a global study providing in vitro surveillance data on antimicrobial susceptibility in centers prescribing meropenem. This study summarizes data on the activity of meropenem and 5 comparators against 4022 clinical isolates from 7 centers in Spain (1999-2003). Those bacteria intrinsically resistant to meropenem were excluded. Among Enterobacteriaceae, 100% of Enterobacter spp., Citrobacter spp., and Serratia spp. were susceptible to meropenem. Escherichia coli and Klebsiella pneumoniae susceptibilities to carbapenems were 100% and > or =98%, respectively. Extended-spectrum beta-lactamase-producing Enterobacteriaceae were 3.8% of isolates, and all of them were susceptible to meropenem. Ciprofloxacin resistance in E. coli was around 20%. Meropenem and piperacillin/tazobactam were the most active agents against Pseudomonas aeruginosa. Acinetobacter baumannii were 61-90% susceptible to carbapenems, but only 6-21% susceptible to ciprofloxacin. In this period, around 100% of oxacillin-susceptible staphylococci were susceptible to meropenem. There was no significant decrease in susceptibility to the carbapenems throughout the 5-year period. The clinical use of meropenem in 7 Spanish centers did not increase bacterial resistance to this agent in the microorganisms evaluated.

Activity of tigecycline against clinical isolates of Staphylococcus aureus and extended-spectrum β-lactamase-producing Escherichia coli in Granada, Spain

We evaluated the in vitro activity of tigecycline using the Etest and disk diffusion method according to Clinical and Laboratory Standards Institute guidelines against clinical isolates of methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA) as well as for CTX-M-9 extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli and SHV ESBL-producing E. coli. All isolates were susceptible to tigecycline according to US Food and Drug Administration cut-off points. There were no differences in the activity of tigecycline between MSSA and MRSA isolates or between the presence of either type of ESBL. For each type of microorganism studied, we established the equation relating the minimum inhibitory concentration to the diameter of the zone of inhibition.

An overview of the Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Program: 1997-2004

This overview provides a summary of the Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Program over an 8-year period from 1997 to 2004. The evolution of the MYSTIC Program is described, as well as its design compared with other surveillance programs. In addition, the global MYSTIC Program data, published to date, are summarized, and the empiric use of carbapenems, their current indications, and meropenem usage versus resistance was discussed. From 1997 to 2004, 120 medical centers that were actively prescribing meropenem in 32 countries worldwide participated in the program. The MYSTIC Program results demonstrate the sustained potency and continued effectiveness of meropenem globally against clinically relevant Gram-negative and Gram-positive pathogens including extended spectrum beta-lactamase- and AmpC beta-lactamase-producing organisms, which may also display resistance to the fluoroquinolones and/or aminoglycosides. Furthermore, in centers actively prescribing meropenem, resistance to meropenem is not increasing despite greater resistance among the comparator antimicrobial agents. Thus, antipseudomonal carbapenems such as meropenem and imipenem remain an effective treatment option.

Comparison of piperacillin/tazobactam and imipenem/cilastatin, both in combination with tobramycin, administered every 6 h for treatment of nosocomial pneumonia

This randomized, double-blind, multicenter study compared the efficacy and safety of piperacillin/tazobactam (P/T) and imipenem/cilastatin (IMP), both in combination with an aminoglycoside, in hospitalized patients with acute nosocomial pneumonia (NP). Patients with acute NP, defined as pneumonia with symptoms > or = 48 h after admission or < or =7 days after hospital discharge, received infusions of 4 g/500 mg P/T or 500 mg/500 mg IMP every 6 h. Endpoints were clinical cure and microbiological response rates; pathogen eradication rates; length of hospital stay; hospital readmissions; and adverse events (AEs). Of 437 patients in the intent-to-treat population, 197 were efficacy evaluable. At test-of-cure, response rates were similar between groups. Within the efficacy evaluable population, 68% of P/T patients and 61% of IMP patients were clinically cured (P = 0.256). Microbiological responses for P/T and IMP patients were: eradication, 64% versus 59%; persistence, 29% versus 21%; relapse, 0% versus 5%; and superinfection, 7% versus 15%, respectively. Gram-positive isolates were eradicated in 83% of P/T patients and 75% of IMP patients; Gram-negative pathogens were eradicated in 72% of P/T patients and 77% of IMP patients. Treatment groups had similar number of mean hospital days, readmission rates, and frequency of AEs. This study showed that P/T administered four times per day was as safe and efficacious as IMP in treating hospitalized patients with NP.

In vitro activity of β-lactam and non–β-lactam antibiotics in extended-spectrum β-lactamase–producing clinical isolates of Escherichia coli

The activity of different beta-lactam and non-beta-lactam antibiotics was assessed against extended-spectrum beta-lactamase (ESBL)-producing and non-ESBL-producing clinical isolates of Escherichia coli. A phenotypic study to discover the presence of ESBLs in 399 clinical isolates of E. coli was made by the disk diffusion method following the Clinical and Laboratory Standards Institute (formely NCCLS, 2004) guidelines. The activity of different antibiotics was subsequently studied using the automated VITEK 2 system (bioMérieux, Marcy l'Etoile, France). One hundred fifteen isolates proved to be ESBL-producing and 284 non-ESBL-producing. Among the former, percentage susceptibilities to the antibiotics assayed were meropenem and amikacin, 100%; piperacillin/tazobactam, 97.4%; cefepime, 94.8%; amoxicillin/clavulanic acid, 84.3%; tobramycin, 84.3%; gentamicin, 83.5%; cefoxitin, 83.5%; nitrofurantoin, 71.3%; cotrimoxazole, 46.1%; norfloxacin, 29.6%; ciprofloxacin, 27%; and ofloxacin, 26.1%.

Trends in antimicrobial susceptibility in UK centres: the MYSTIC Programme (1997–2002)

Trends in antimicrobial susceptibilities in three UK centres participating in the MYSTIC Programme were examined from 1997 to 2002. Isolates were tested using standard methodology to determine the susceptibility breakpoints of meropenem and several other antimicrobial agents including imipenem, ceftazidime, piperacillin/tazobactam, ciprofloxacin and gentamicin. Data are grouped in 2-year blocks. The carbapenems were the most active agents tested against the Enterobacteriaceae (99-100% and 98-100% susceptibility to meropenem and imipenem, respectively) and non-fermenters, including Pseudomonas spp. and Acinetobacter spp. With the exception of susceptibility to ciprofloxacin, which decreased among Enterobacteriaceae at the end of the 6-year period, all antibiotics tested retained their levels of activity. The proportion of extended-spectrum beta-lactamase (ESBL)- and AmpC-producing Enterobacteriaceae increased during the study (4.8% and 11.3% in 1997-1998; 7.4% and 16.7% in 2001-2002, respectively). Both meropenem and imipenem retained their potency against these ESBL- and AmpC-producing isolates (100% for all time periods). All the other antimicrobial agents tested had much lower susceptibility against these resistant isolates and this decreased further over the 6-year period, with the exception of tazobactam, which maintained its low levels. Although all antibiotics tested retained acceptable activity, the carbapenems remained the most active antimicrobial agents against Gram-negative bacteria, including ESBL- and AmpC-producing isolates.

Frequency of isolation and antimicrobial susceptibility of bacteria isolated from bloodstream infections at Children's Medical Center, Tehran, Iran, 1996–2000

Antimicrobial susceptibility patterns of major bloodstream pathogens from Iran provide essential information regarding the selection of antibiotic therapy for patients with bloodstream infections (BSIs) living in Iran. Unfortunately, data regarding the isolation frequency and antimicrobial susceptibility patterns of endemic BSI pathogens are scarce in Iran. To shed some light on the susceptibility patterns of BSI pathogens endemic to Tehran, Iran, we investigated the antimicrobial susceptibility patterns of 2248 bloodstream isolates from patients in Children's Medical Center (CMC) Hospital in Tehran between January 1996 and December 2000. Microbiology reports of 24600 blood specimens collected from inpatients in CMC Hospital were retrospectively reviewed. Specimen culture, bacterial identification and disk diffusion susceptibility testing were performed according to National Committee for Clinical Laboratory Standards guidelines. Overall, Gram-positive bacteria comprised 72% (1627/2248) of recovered isolates and Gram-negative bacteria comprised 28% (621/2248). Coagulase-negative staphylococci (CoNS) comprised 48.4% of all isolates, followed by Staphylococcus aureus (16.7%) and Klebsiella spp. (8.5%). Among the 621 Gram-negative organisms, Klebsiella spp. (31%) were the most frequently isolated, followed by Escherichia coli (21%) and Pseudomonas aeruginosa (17%). The rates of oxacillin resistance for S. aureus and CoNS isolates were similar (60% versus 61%); however, the rate of S. aureus vancomycin resistance was almost twice that of CoNS resistance (21% versus 11%). Over 55% of S. pneumoniae were resistant to penicillin and co-trimoxazole. Although all isolates of enterococci were susceptible to vancomycin, only 21% were susceptible to gentamicin. Among Gram-negative isolates, amikacin was shown to be very effective, with susceptibility rates of 84%. The susceptibility of Klebsiella spp. to ampicillin and co-trimoxazole was 1% and 39%, respectively. The susceptibility of Klebsiella spp., E. coli and Enterobacter spp. to ceftriaxone was 47%, 86% and 67%, respectively. There were notable differences in the order of the five most common organisms isolated from blood cultures, which can help set priorities for focused control efforts. Our findings highlight the importance of a nationwide surveillance programme to monitor the trends in isolation frequency of bacteria and their antimicrobial resistance patterns throughout Iran.

Empiric Treatment of Nosocomial Intra-Abdominal Infections: A Focus on the Carbapenems

BACKGROUND

Serious nosocomial intra-abdominal infections are associated with high morbidity and mortality and represent a substantial drain on healthcare resources. Effective management of this type of infection requires the early use of appropriate, broad-spectrum empiric antimicrobial therapy. The consequences of delayed or inappropriate antimicrobial treatment can be severe-leading to an increased risk of death, re-operation, or prolonged hospitalization. Therefore, it is necessary to begin treatment as soon as possible with the most appropriate regimen, in terms of spectrum, timing, and duration.

METHODS

Review of pertinent English-language literature.

RESULTS

Serious nosocomial intra-abdominal infections require broad-spectrum coverage because of the wide range of possible pathogens, which include difficult-to-treat organisms such as Pseudomonas aeruginosa and Bacteroides spp., and resistant strains of Klebsiella spp., Escherichia coli, and methicillin-resistant Staphylococcus aureus acquired from the hospital flora. The early use of appropriate, broad-spectrum empiric antimicrobial therapy for treating high-risk patients with intra-abdominal infections is considered, and appropriate use of the carbapenems, meropenem, and imipenem/cilastatin, is described.

CONCLUSION

The carbapenems meropenem and imipenem/cilastatin have a spectrum of antimicrobial activity that covers the majority of expected pathogens, including anaerobes, as well as difficult-to-treat and resistant gram-negative strains. Early and appropriate use can reduce mortality and morbidity. Data from published clinical trials support the clinical effectiveness of these two carbapenems in intra-abdominal infections.

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.

Argentinean collaborative multicenter study on the in vitro comparative activity of piperacillin-tazobactam against selected bacterial isolates recovered from hospitalized patients

The in vitro activity of piperacillin-tazobactam and several antibacterial drugs commonly used in Argentinean hospitals for the treatment of severe infections was determined against selected but consecutively isolated strains from clinical specimens recovered from hospitalized patients at 17 different hospitals from 9 Argentinean cities from different geographic areas during the period November 2001-March 2002. Out of 418 Enterobacteriaceae included in the Study 84% were susceptible to piperacillin-tazobactam. ESBLs putative producers were isolated at an extremely high rate since among those isolates obtained from patients with hospital acquired infections 56% of Klebsiella pneumoniae, 32% of Proteus mirabilis and 25% Escherichia coli were phenotypically considered as ESBLs producers Notably P.mirabilis is not considered by for screening for ESBL producers. ESBLs producers were 100% susceptible to imipenem and 70% were susceptible to piperacillin-tazobactam whereas more than 50% were resistant to levofloxacin. The isolates considered as amp C beta lactamase putative producers showed 99% susceptibility to carbapenems while 26.7% were resistant to piperacillin-tazobactam and 38.4% to levofloxacin. Noteworthy only 4% of the Enterobacteriaceae isolates were resistant to amikacin. Piperacillin-tazobactam was the most active agent against Pseudomonas aeruginosa isolates (MIC(90): 128 microg/ml; 78% susceptibility) but showed poor activity against Acinetobacter spp (MIC(90):>256 microg/ml; 21.7% susceptibility). Only 41.7% Acinetobacter spp isolates were susceptible to ampicillin-sulbactam. Piperacillin-tazobactam inhibited 100% of Haemophilus influenzae isolates (MIC(90) < 0.25 microg/ml) but only 16.6% of them were ampicillin resistant. The activity of piperacillin-tazobactam against oxacillin susceptible Staphylococcus aureus or coagulase negative staphylococci was excellent (MIC(90) 2 microg/ml; 100% susceptibility). Out of 150 enterococci 12 isolates (8%) were identified as E.faecium and only three isolates (2%), 2 E.faecium and 1 E.faecalis were vancomycin resistant. All the enterococci isolates were susceptible to linezolid. Piperacillin-tazobactam showed excellent activity (MIC(90) 2 microg/ml; 92% susceptibility). Regarding pneumococci all the isolates showed MICs of 16 microg/ml for piperacillin-tazobactam. Among 34 viridans group streptococci only 67% were penicillin susceptible and 85.2% ceftriaxone susceptible whereas piperacillin-tazobactam was very active (MIC(90) 4 microg/ml).Piperacillin-tazobactam is therefore a very interesting antibacterial drug to be used, preferably in combination (IE: amikacin-vancomycin) for the empiric treatment of severe infections occurring in hospitalized patients in Argentina. Caution must be taken for infections due to ESBL producers considering that the inoculum effect MICs can affect MIC values.