with the increasing utility of invasive procedures in inpatient settings , nosocomial infections are encountered with more prevalence among the hospitalized population . hospital-/noncommunity acquired infections ( hai ) not only lengthen the duration of hospital stay with increased demand for additional health care resources but also are potential sources for acquisition of microorganisms by the health care providers , burdens health care system itself , and sets up the transmission into shorter cycle [ 1 , 2 ] . in essence , the most common pathogens responsible for hospital - acquired infections differ from those of community - acquired infections . on the other hand , antimicrobial susceptibility of each organism is influenced by the origin of infection , periodic alterations , and epidemiological factors of that region [ 1 , 35 ] . urinary and respiratory tract infections are the leading cause of in - hospital infections consuming huge amounts of health resources . in addition , these infections impose the risk of high morbidity and mortality on admitted patients . empirical treatment for hai should be instituted based on the knowledge of susceptibility of responsible pathogens to available antibiotics [ 57 ] . although this approach optimizes the effects of antimicrobial agents , inappropriate extended - spectrum antibiotics should strongly be discouraged . the importance is more sensible if increased resistance to many antibiotics is taken into account and action programs have developed to restrict overuse of antibiotics [ 3 , 5 , 79 ] . for this reason , specific nationwide surveillance studies should be conducted to provide for microbiological resistance pattern and antibacterial activity of antibiotics . the aim of our study was to provide objective evidence in terms of antibiotic susceptibility and resistance of pathogens isolated from patients afflicted by either community- or noncommunity acquired uti and rti in a tertiary care center in iran . the study population included patients attending to the laboratory or admitted to cardiac wards in tehran heart center ( thc ) , tehran , iran . the inclusion criteria comprised clinical evidence of urinary or respiratory tract infection , according to the definition of center for disease control and prevention ( cdc ) , in patients admitted to the hospital before 48 hours of admission and in patients presenting to the outpatient clinic having symptoms for less than 72 hours . the inclusion criteria comprised community - acquired infections diagnosed in patients attending to the laboratory without prolonged duration ( > 72  hrs ) and previous antibiotics reception and hospital - acquired infections considered in patients developing u / r tract infections more than 48 hours after hospital admission . admitted patients with documented uti or rti on admission or outpatient subjects who had received antibiotic therapy within 48 hours , immune compromised patients , and those with documented complicated infections were excluded from the study . the study design and protocol was approved by the research and ethics committee of thc . patients with suggestive symptoms of upper respiratory tract infection or clinical evidence of urinary tract infection underwent sampling for sputum and urine analysis before antibiotic therapy , respectively . fifty milliliter ( ml ) of clean - catch midstream urine from patients with a probable diagnosis of uti was collected in a sterile container for uti as soon as possible after admission . in those with evidence of an rti , 20 to 30  ml of early morning sputum was obtained in a sterile plastic container . bacteriuria was determined with quantitative technique using 0.01  ml calibrated wire loop to inoculate the blood agar , which provides for growth of most organisms . the same technique was accomplished with the macconkey culture medium specialized for isolation of gram - negative bacteria . culture was considered positive if at least 10  cfu / ml was isolated for a single organism . identification of isolates was obtained by their characteristic appearance on their respective media and gram staining . the isolated colony was confirmed by the pattern of biochemical reactions using the standard method   and also by pcr for the type of microorganism . antibiotic susceptibility was evaluated by kirby - bauer disc diffusion method on mueller - hinton agar according to the guidelines of clinical and laboratory standards institute ( clsi )   and antibiogram was obtained . the followings are the concentrations of drugs used for disc diffusion testing : ampicillin ( 10  g ) , amikacin ( 30  g ) , coamoxiclav ( 20/10  g ) , cefepime ( 30  g ) , cefoxitin ( 10  g ) , ceftazidime ( 30  g ) , ceftriaxone ( 30  g ) , cephalotin ( 30  g ) , ciprofloxacin ( 5  g ) , clindamycin ( 2  g ) , cotrimoxazole ( 23.75  g sulfamethoxazole/1.25  g trimethoprim ) , erythromycin ( 15  g ) , gentamicin ( 10  g ) , imipenem ( 10  g ) , meropenem ( 10  g ) , nitrofurantoin ( 100  g ) , penicillin ( 10 units ) , tobramycin ( 10  g ) , and vancomycin ( 30  g ) . data were assessed using the statistical package for social sciences ( ibm spss statistics , version 19 , ibm corporation ) and presented as number ( percent ) and mean  sd when appropriate . urinary and respiratory samples from admitted patients in 4 intensive care units ( icu ) , 5 coronary care units ( ccu ) , 12 inpatient wards , and those attended to the laboratory were analyzed . of 1099 positive cultures , 835 ( 75.9% ) and 264 ( 24.1% ) samples were related to uti and rti , respectively . of 835 positive urine cultures , 664 samples ( 79.5% ) were obtained from hospitalized patients compared with 171 samples ( 20.5% ) from outpatient . e. coli was the most prevalent isolated organism in both the hospital - acquired and the community - acquired utis followed by klebsiella and enterococcus ( table 2 ) . in outpatients , e. coli was the leading cause followed by klebsiella , staphylococcus epidermidis , and inactive e. coli . e. coli was highly resistant to cephalotin in the majority of samples ( 56.6% ) followed by cotrimoxazole ( 49.4% ) and ceftriaxone ( 38.5% ) as presented in table 3 . antibiotic susceptibility and resistance pattern of klebsiella and enterococcus species have been also presented in table 3 . it is perceived from the table that the best antibiotics for klebsiella and enterococcus were imipenem and nitrofurantoin , respectively . on the other hand , we found a high susceptibility of e. coli to imipenem , amikacin , and nitrofurantoin . table 4 summarizes the prevalence of microorganisms isolated from urine samples according to the source of infection and time of sampling . two hundred sixty - four positive cultures were obtained from respiratory secretions . of these , 135 samples ( 51.1% ) were related to icu wards compared with 129 samples ( 48.9% ) obtained from hospital wards . regarding the admission wards , acinetobacter was the most prevalent organism in the icu , while pseudomonas aeruginosa was the most prevalent organism in non - icu wards . acinetobacter demonstrated the greatest resistance to ceftriaxone with the greatest susceptibility and lowest resistance to imipenem . staphylococcus aureus was highly susceptible to vancomycin and highly resistant to erythromycin and cefoxitin , respectively . susceptibility and resistance to antibiotics of the isolated respiratory organisms are summarized in table 5 . the importance of regional specification of rti and uti etiology and their antibiotic susceptibility has been convincingly proved by several studies [ 5 , 1318 ] . however , the knowledge needs to be regularly revised with changing trends in microbiological patterns of the responsible organisms . this accelerates the initiation of adequate antibiotic treatment in nosocomial infection with better results in terms of outcome and improved survival . our study confirmed that e. coli is still the most frequent pathogen in both outpatient and inpatient settings . although this is in accord with a number of previous reports [ 13 , 16 , 17 ] , muvunyi and colleagues ( 2011 ) proposed that the prevalence of urinary infections with e. coli is decreasing probably due to other superseding members of the enterobacteriaceae . the second and the third most prevalent uropathogens in the hospitalized population of our study were klebsiella and enterococcus , respectively . similar to other investigations , e. coli constituted a lower percentage of isolates in inpatients than outpatients . however , the pattern of isolates in an ambulatory setting may be quite variable , with e. coli being the most common organism , followed by staphylococcus saprophyticus , proteus mirabilis , and enteric gram - negative rods in one study . another observational study on microbiologic patterns of uti in general practice also revealed e. coli as the most common culprit followed by typical uropathogens and enterococcus faecalis . according to current evidence , irrespective of the chronological pattern of the infection , e. coli still accounts for the most common organism isolated from urinary tract infections in either of the inpatient or outpatient populations which should be investigated in terms of antimicrobial susceptibility . with respect to rti , acinetobacter constituted the most common pathogen in 2008 followed by pseudomonas aeruginosa and staphylococcus aureus . nevertheless , the trend has been different in 2007 with pseudomonas aeruginosa being the most prevalent and acinetobacter being the least frequent organisms . in a study by wang et al . from northern china , pseudomonas aeruginosa was the most common isolated organism in hospitalized patients with lower respiratory tract infection . however , a sentry antimicrobial surveillance program showed that staphylococcus aureus comprised 28 percent of isolates followed by pseudomonas aeruginosa in 20.0 percent , streptococcus pneumonia in 9.1 percent , and klebsiella saprophyticus in 7.5 percent among hospitalized patients in northern america and canada . xia and colleagues investigated the changing pattern of isolated pathogens in lower respiratory tract infection among a chinese population and found that acinetobacter was more frequently isolated in 2006 than in 2010   which is also confirmed by our study ( 15% in 2007 versus 36.8% in 2008 ) . moreover , it seems that acinetobacter is commonly isolated more from icu patients than from other wards perhaps due to the compromised immunity of the critically ill patients [ 5 , 18 ] . the urinary isolated e. coli was susceptible to nitrofurantoin in 90 percent and to gentamicin in 71.8 percent of cases . although susceptibility to imipenem and meropenem remained high ( 100 percent and 80.4 percent , resp . ) , cotrimoxazole represented a susceptible proportion of less than 50 percent . additionally , klebsiella demonstrated 80.7 percent susceptibility to gentamicin , 83.9 percent to ciprofloxacin , 58.5 percent to nitrofurantoin , and more than 60 percent to cephalosporin . however , it raised concern because of resistance to coamoxiclav in 61.8 percent and to ampicillin in 92.6 percent of the isolates . similarly , enterococcus showed susceptibility to nitrofurantoin ( 74.1% ) and gentamicin ( 48% ) with 55.2 percent resistance to vancomycin , hence , categorized as vancomycin resistant enterococcus ( vre ) . however , the susceptibility and resistance patterns were relatively comparable so as both features are close to 50 percent . in a study by cullen and colleagues , e. coli , as the most common uropathogen , showed significant resistance to penicillin , trimethoprim , and ciprofloxacin . their results also showed a decreasing susceptibility to gentamicin during the period of study for 11 years . in contrast , resistance against cefoxitin and erythromycin exceeded 80 percent implying that more than 80 percent of the organisms were methicillin resistant s. aureus ( mrsa ) . acinetobacter , as the most common isolate in rti , represented the best susceptibility to imipenem ( 50% ) . nevertheless , its resistance to imipenem ( 47.9% ) was relatively too high to be selected as the first line antibiotic . on the other hand , other antibiotics did not reveal high activity against this organism with the best susceptibility varying from 10 to 15 percent ( see table 5 ) . interestingly , pseudomonas aeruginosa had high susceptibility to available antibiotics such as meropenem ( 90% ) , ciprofloxacin ( 87.5% ) , imipenem ( 84.4% ) , and amikacin ( 73.4% ) . in their study , . rated antimicrobial resistance of pathogens in pediatric infections and reported that ceftriaxone and cefepime were generally more effective against e. coli and klebsiella pneumonia . however , imipenem was the most effective antibiotic with 100 percent susceptibility . these findings confirm that despite current trends in pediatric population , where administration of imipenem is not widely established yet , the community - acquired pathogens are still amenable with high susceptibility to this antimicrobial agent . the present study was employed as surveillance of current microbiologic pattern and antibiotic susceptibility in uti and rti among a large hospital and outpatient population in order to revise the existing knowledge for the establishment of proper and adequate empirical antibiotic therapy . however , a multicenter investigation would have complied with this objective more sufficiently . furthermore , the history of infection and previous administration of antibiotics are influential factors for striding towards understanding elements of antimicrobial resistance . on the other hand , the role of other nonbacterial microorganisms in community- and in - hospital acquired infections should be taken into account . periodic domestic antimicrobial surveillances are needed to regularly update the guidelines on proper empiric and organism - specific antibiotic treatment . our study showed the changing trends in antibiotic susceptibility of isolates from uti and rti with respect to the source of infection during the time . while e. coli and acinetobacter remain the most common urinary and respiratory organisms , respectively , their increasing resistance to wide - spectrum imipenem and meropenem is a major concern .