the rapid spread of resistance among common pathogenic microorganisms is a serious challenge around the word . this phenomenon affects antibiotics ' effectiveness and limits available options for the treatment of common infections in human . serious infections due to beta - lactamase producing microorganisms , especially in hospitalized patients are increasing now . both enzymatic and nonenzymatic pathways cause resistance to third - generation cephalosporins , aminoglycosides , fluoroquinolones , and carbapenems . antibiotic resistance occurs following mutation in chromosomal genes or by horizontal transfer of genes between different microorganisms . the main mechanism of antimicrobial resistance in enterobacteriaceae family is transferring of plasmid encoding extended - spectrum beta - lactamase ( esbl ) . esbl producers ( esbl - p ) are gram - negative microorganisms which almost always belong to the enterobacteriaceae species . these gram - negative bacteria secret esbl enzyme in periplasmic space and hydrolyze the beta - lactam ring in penicillins , cephalosporins , and aztreonam . in general , esbl - p pathogens can cause severe and life - threatening infections such as bacteremia , sepsis , pneumonia , and meningitis . in the united states , 26,000 infections and 17,000 deaths per 2012 were due to esbl - p species . according to the centers for disease control and prevention ( cdc ) report , more than 19% of healthcare - associated infections are resistant to extended - spectrum cephalosporins . in the united states , the mortality rate was 57% more common in patients with bloodstream infection caused by esbl - p than nonproducers . prolong hospitalization , presence of invasive medical devices , receiving total parenteral nutrition , age < 12 weeks , prior treatment with cephalosporins and aminoglycosides , recent surgery , and hemodialysis are defined as risk factors for colonization with esbl - p species . the incidence of infections due to resistant microorganisms is increasing in the recent years in iran . in this study , frequency and antimicrobial sensitivity pattern of multidrug resistant ( mdr ) gram - negative microorganisms were evaluated in a referral teaching hospital in tehran , iran . this cross - sectional study was performed between december 2014 and january 2016 in imam khomeini hospital , a referral teaching hospital affiliated to the tehran university of medical sciences , tehran , iran . biologic clinical samples including urine , cerebrospinal fluid ( csf ) , blood , and tracheal secretions that were referred to the central laboratory department from different wards of the hospital were analyzed according to the clinical and laboratory standard institute instructions . antimicrobial sensitivity patterns of all isolates were recognized by using standard antibiotic disks on mueller - hinton agar . following antibiotic disks from himedia , bioscience company , india , was used for the primary antibiogram and esbl screening ; ciprofloxacin ( 5 g ) , ceftriaxone ( 30 g ) , cefotaxime ( 30 g ) , ceftazidime ( 30 g ) , amikacin ( 30 g ) , ampicillin - sulbactam ( 10/10 g ) , imipenem ( 10 g ) , and meropenem ( 10 g ) . after 24 h of incubation , if an inhibitory concentration zone was < 25 mm for ceftriaxone , 27 mm for cefotaxime or 22 mm for ceftazidime , phenotypic confirmatory test was performed with double disk synergy test . for this test , cefotaxime / clavulanic acid ( 30/10 g ) and ceftazidime / clavulanic acid ( 30/10 g ) discs were used . increasing of 5 mm in the inhibition zone diameter in double synergy test versus the antibiotic tested alone was considered in favor of esbl - p isolates . non - esbl isolates that were resistant to imipenem or meropenem was categorized as probable carbapenem - resistant gram - negative microorganisms . patients , in whom mdr gram - negative pathogens were confirmed phenotypically , were detected and followed by the clinical pharmacists during the course of hospitalization . mdr was defined as resistance to at least three classes of antibiotics ( aminoglycosides , anti - mrsa cephalosporins , antipseudomonal penicillins + beta - lactamase inhibitors , carbapenems , and nonextended spectrum cephalosporins ; first and second generation cephalosporins , extended - spectrum cephalosporins ; and third and fourth generation cephalosporins , cephamycins , fluoroquinolones , folate pathway inhibitors , glycylcyclines , monobactams , penicillins , penicillins + beta - lactamase inhibitors , polymyxins , phosphonic acids , phenicols , and tetracyclines ) . demographic data , baseline diseases , type of biological sample , isolated microorganism , type of infection , antibiotic regimen before availability of the culture result , and change in the antibiotic regimen following receiving the antibiogram results , response to the treatment regimen , duration of hospitalization , and patient 's outcome were considered variables for each recruited patient . cultures compatible with patient clinical status were measured as true infection according to the cdc definitions for health - care associated infections . chi - square or fisher exact test was used for comparing the categorical variables between the groups . during the study period , fifty patients with mdr gram - negative infections including confirmed esbl or probably carbapenem - resistant enterobacteriaceae ( cre ) were detected . the mean  sd of patients ' age was 59.02  17.96 years old and thirty ( 60% ) of them were males . tracheal secretions ( 17 [ 34% ] ) , urine ( 15 [ 30% ] ) , blood ( 8 [ 16% ] ) , soft tissue ( 3 [ 6% ] ) , peritoneal fluid ( 2 [ 4% ] ) , csf ( 1 [ 2% ] ) , and pleural fluid ( 1 [ 2% ] ) were positive in the patients . most patients were hospitalized in intensive care unit ( 35 [ 70% ] ) and general ward ( 8 [ 16% ] ) , followed by emergency , neurosurgery , and coronary care unit wards . klebsiella species ( 78% ) , escherichia coli ( 20% ) , and enterobacter cloacae ( 2% ) were isolated microorganisms from the patients ' biological samples . all of isolated e. coli and e. cloacae but only 56% of isolated klebsiellas species were sensitive to carbapenems . the result of antimicrobial susceptibility tests revealed that 30% of the isolated microorganisms were resistant to carbapenems that may be cre . however , most of these species ( 86.7% ) were esbl negative . according to the double disk synergy test , 17 ( 34% ) of all isolates were esbl - p and others were esbl - negative . esbl was positive in 58.8% and 41.2% of isolated klebsiella species and e. coli , respectively . based on the cdc definition , the clinical condition was compatible with the isolates in 62% of all patients and 25.8% of patients with esbl - p infections . in 71% of the cases , a carbapenem alone or plus amikacin or ciprofloxacin were selected regimens for patients with esbl infections . for patients with probable cre infections , clinical response was detected in 54.5% of the patients who were treated based on the antibiogram results . clinical response was higher in the esbl - p than the non - esbl - p infections ( 75% vs. 52% ) . however , this difference was not significant ( p = 0.09 ) . most nonresponders ( 80% ) had sepsis due to klebsiella species . finally , 41.9% of the patients were discharged from the hospital and 58.2% died . characteristics of patients with esbl - p and probably cre infections were summarized in tables 2 and 3 , respectively . resistance pattern of isolated gram - negative pathogens characteristics of patients with extended - spectrum -lactamase - producing infections characteristics of patients with probable carbapenem resistant enterobacteriaceae infections inappropriate antibiotic administration and consequent increasing number of mdr pathogens including esbl - p and cre are a serious worldwide concern in recent years . rapid growing of esbl - p and cre among community and hospitalized patients is a global threat , especially in critically ill patients . considering that only limited new antimicrobial agents have been introduced in recent years ; in some situations , we do not have an effective weapon against these pathogens . following extensive use of cephalosporins in last years , resistance rate of enterobacteriaceae family to these agents is increasing around the world . cephalsporins - resistant rate of these microorganisms was 30% among 11 countries of asia in 2010 . this rate received to 87% in 2014 at latin america . only in limited studies , the prevalence of esbl - p pathogens was evaluated in iran and ranged from 43.6% in ilam to 74% in milad hospital . however , the average rate of esbl - p microorganisms was 42.2% in iran . in a recent study , more than 50% of isolated microorganisms from bile specimens were esbl - p . in european hospitals , more than 80% of isolated e. coli and klebsiella pneumonia were belonged to the esbl - p category . in the present study , the frequency of esbl - p pathogens was lower than the previous reports from our country . in a report from three hospitals of iran , all isolated esbl - p microorganisms were sensitive to carbapenems . however , in our study , some of esbl - p species and most of esbl negative strains were cre . all cre were klebsiella species . to interpret the result of clinical responses , limitations of the study esbl - p pathogens were identified phenotypically but were not confirmed by the genotypic assay method . genotypic assay is not easily available method in our hospitals and only is applied for research purpose . although double disk synergy test is a common and practical method for esbl confirmation but some isolates may be missed by this test . the sensitivity of this method could be reduced by microorganisms that show low - esbl activity . it has been shown that 13.63% of esbl positive strains were not recognized by double disk method . therefore , some of non - esbl strains in our study may be false negatives of the test . in this study , cre isolates were detected based on the results of the disk diffusion method and were not confirmed based on the phenotypic and genotypic assays . specific methods such as bromic acid in combination with clavulanate are recommended to unmask the underlying esbls among enterobacteriaceae family with carbapenemase enzyme . pharmacokinetic parameters such as inadequate tissue penetration of antimicrobial agents can influence the clinical responses in in vivo settings . most of the recruited patients had at least one of the following severe comorbidities including malignancies , respiratory disorders , ischemic heart disease , heart failure , diabetes mellitus , renal failure , cerebrovascular accident , hepatitis , immunodeficiency , and sepsis . unfortunately like other countries , cre prevalence in our country is increasing in the recent years . empiric administration of carbapenems should be restricted to patients with risk factors of infections with esbl - p bacteria and in specific clinical situations . to limit the use of last - line antibiotics such as carbapenems , availability of accurate phenotypic , and genotypic methods for detection of esbl - p and carbapenemase strains is essential in clinical practice .
objective : in this study , frequency and antimicrobial sensitivity pattern of multidrug resistant ( mdr ) microorganisms were evaluated in a referral teaching hospital in iran.methods:patients with mdr gram - negative pathogens were followed during the course of hospitalization . 
 demographic data , baseline diseases , type of biological sample , isolated microorganism , type of infection , antibiotic regimen before the availability of the culture result and change in the antibiotic regimen following receiving the antibiogram results , response to the treatment regimen , and duration of hospitalization and patient 's outcome were considered variables for each recruited patient.findings:in 71% of the patients , antibiotic regimens were changed according to the antibiogram results . 
 a carbapenem alone or plus amikacin or ciprofloxacin were selected regimens for patients with extended - spectrum beta - lactamase ( esbl ) infections . for patients with probable carbapenem - resistant enterobacteriaceae infections , 
 a carbapenem plus colistin was the most common antibiotic regimen . 
 clinical response was detected in 54.5% of the patients who were treated based on the antibiogram results . 
 clinical response was higher in the esbl producers ( esbl - p ) than the non - esbl - p infections ( 75% vs. 52% ) . however , this difference was not significant ( p = 0.09 ) . 
 most nonresponders ( 80% ) had sepsis due to klebsiella species . finally , 41.9% of the patients were discharged from the hospital and 58.2% died.conclusion:same as other countries , infections due mdr microorganisms is increasing in the recent years . 
 this type of resistance caused poor clinical response and high rate mortality in the patients .