Antibiotics and Resistance of Uropathogens

EAU Update Series 2 (2004) 125–135

Antibiotics and Resistance of Uropathogens F.M.E. Wagenlehner, K.G. Naber* Department of Urology, Urologic Clinic, Hospital St. Elisabeth, St. Elisabeth Straße 23, D-94315 Straubing, Germany

Abstract This article shall give an overview of the commonly used antibiotics in urological practice, their resistance rates and the resistance mechanisms of uropathogens. The bacterial spectrum of urinary tract infections and the resistance rates of uncomplicated community acquired and complicated, nosocomially acquired uropathogens can differ substantially from region to region and over time. The results from different surveillance studies are evaluated and exemplary the resistance rates of uropathogens in the last ten years of our institution are reflected. Especially the increasing resistance of enterobacteria to fluoroquinolones is notable. Antibiotics are widely used in urological practice for the need of modern medicine and therefore antibiotic resistance is of increasing importance for the urologist. Knowledge of resistance rates is paramount for empirical antimicrobial therapy. A basic understanding of the resistance mechanisms is of great help in this respect. # 2004 Elsevier B.V. All rights reserved. Keywords: Urinary tract infections; Antibiotic resistance; Resistance mechanisms 1. Introduction

2. Bacterial spectrum

Urinary tract infection (UTI) is one of the most common reasons for adults to seek medical consultation and is also one of the most frequently occurring nosocomial infections [1]. In Urology nosocomial UTI are almost exclusively complicated UTI, i.e. UTI associated with structural or functional abnormalities of the urinary tract, with a broad spectrum of etiologic pathogens. Empirical antimicrobial therapy in urology has to be instigated in occasions when urosepsis is pending or the general condition is deteriorated and is likely to improve significantly by the immediate usage of antimicrobials. For rational empiric therapy it is necessary to consider the bacterial spectrum and the antibiotic susceptibility of the uropathogens. Since the spectrum and the resistance rates may vary from time to time and hospital to hospital, each institution must be able to have its own local evaluation. Such a surveillance is also useful as indicator for hospital spread infections and for hospital antibiotic policy. In order to understand how bacteria may become resistant against antibiotics, it is important to know, how antibiotics act against bacteria.

The prevalence of uropathogens is different comparing uncomplicated and complicated UTI.

*

Corresponding author. Tel. þ49 9421 7101700; Fax: þ49 9421 7101717. E-mail address: [email protected] (K.G. Naber).

2.1. Bacterial spectrum in uncomplicated, community acquired UTI In uncomplicated UTI E. coli is the most common pathogen, typically being isolated from over 80% of outpatients with acute uncomplicated cystitis across the various regions of the world [2]. Staphylococcus saprophyticus accounts for 5% to 15% of these infections and is especially prevalent in younger women with cystitis. Causative pathogens in the remaining 5% to 10% of cases include aerobic Gram-negative rods such as Klebsiella and Proteus spp., and other enterobacteria. The range of pathogens associated with acute uncomplicated pyelonephritis is similar to that seen in acute uncomplicated cystitis [3]. 2.2. Bacterial spectrum in complicated, nosocomial acquired UTI The bacterial spectrum of complicated, nosocomial UTI is heterogenous and comprises a wide range of Gram-negative and Gram-positive species. The bacterial spectrum can vary geographically, over the time and between distinct specialities at the same institution.

1570-9124/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.euus.2004.06.003

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2.2.1. Bacterial spectrum of nosocomial UTI in North America (SENTRY study) The SENTRY antimicrobial surveillance program, initiated in 1997 and still ongoing, has chronologically examined urinary pathogens collected from hospitalized patients from different hospital departments across North America and thus provides insight into pathogen frequency and resistance rates [4]. A surveillance study based on 1998 data encompassed 31 North American institutions which examined 1510 urinary isolates from hospitalized patients from different departments. Approximately 25% of these isolates were nosocomially acquired strains, of which 20% were isolated from intensive care units [5,6]. For Gram-negative organisms, E. coli was responsible for 47% of UTI, followed by Klebsiella spp. (11%), Pseudomonas spp. (8%), Proteus spp. (5%), Enterobacter spp. (4%), and Citrobacter spp. (3%) (Table 1). The most commonly isolated Gram-positive uropathogens from this cohort of hospitalized patients were Enterococcus spp. (13%), coagulase-negative staphylococcus (3%), and S. aureus (3%) (Table 1).

2.2.2. Bacterial spectrum of nosocomial UTI in Europe (ESGNI-003 study) A European multi-center one-day study on nosocomial UTI of patients from different hospital departments tested 607 uropathogens from 228 hospitals throughout Europe [7]. Patients from different departments throughout the hospital were evaluated. The bacterial spectrum is shown in Table 1. E. coli was responsible for 36% of UTI, followed by Klebsiella spp. (8%), Proteus spp. (8%), Pseudomonas spp. (7%), Enterobacter spp. (4%), and Citrobacter spp. (2%). With Grampositive uropathogens Enterococcus spp. was isolated in 13%, coagulase-negative staphylococcus in 2% and S. aureus in 2%. Candida spp. was isolated in 9%. 2.2.3. Bacterial spectrum of nosocomial UTI in urological patients in Europe (PEP study) A European multi-center one-day study on nosocomial UTI in urology tested 320 uropathogens from 232 urological departments throughout Europe [8]. The bacterial spectrum is shown in Table 1. E. coli was responsible for 35% of UTI, followed by Pseudomonas

Table 1 Bacterial spectrum of nosocomial uropathogens (2%) from distinct surveillance studies Name of study

SENTRY [6]

ESGNI-003 [7]

PEP study [8]

Straubing [9]

Regions of the world Year of surveillance Type of surveillance Origin of samples

Europe 2000 Cross-section Different departments in the hospital n ¼ 607

Europe 2003 Cross-section Urology departments

Germany 2003 Longitudinal Urology department

Number of pathogens

North America 1998 Longitudinal Different departments in the hospital n ¼ 1510

n ¼ 320

n ¼ 479

Species E. coli Klebsiella spp. Pseudomonas spp. Proteus spp. Enterobacter spp. Citrobacter spp. Enterococcus spp. Staphylococcus spp.

47% 11% 8% 5% 4% 3% 13% 6%

36% 8% 7% 8% 4% 2% 16% 4%

35% 10% 13% 7% 3% n.r. 9% 4%

41% 7% 6% 9% 3% 3% 18% 14%

Resistance rates of antibiotics Ampicillin Ampicillin þ BLI TMP/SMX Ciprofloxacin Gentamicin Ceftazidime Amikacin Piperacillin/tazobactam Imipenem Vancomycin

37%a n.r. 23%a 3%–40%b n.r. 14%c 3%c 8%c 9%c 5%d

55%a 14%a 28%a 9%a 6%a 13%c 19%c n.r. 14%c 1%d

51% 30% 45% 34% 34% 17% 14% 15% 7% n.r.

47% 30% 22% 24% 28% 28% n.r. 8% n.r. 0%d

n.r.: not reported. a E. coli. b Gram-negative bacteria. c P. aeruginosa. d Enterococcus spp.

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Table 2 Bacterial spectrum of uropathogens (>2%) isolated from the urine of hospitalzed urological patients of the department of urology in Straubing, Germany, in the years 1994 until 2003 [9] Uropathogen E. coli Klebsiella spp. Proteus spp. Enterobacter spp. Citrobacter spp. P. aeruginosa Enterococcus spp. CNS S. aureus Total (n ¼ 100%)

1994

1995

28% 6% 8% 5% 4% 13% 21% 11% 5% 448

27% 7% 6% 6% 3% 14% 21% 14% 3% 448

1996 30% 9% 8% 4% 1% 12% 22% 11% 4% 374

1997 32% 8% 8% 5% 4% 12% 17% 13% 3% 407

1998 28% 9% 6% 5% 3% 12% 20% 13% 4% 389

1999 35% 8% 6% 4% 3% 10% 20% 11% 3% 405

2000 31% 10% 8% 6% 1% 11% 23% 8% 2% 623

2001 32% 8% 9% 6% 3% 7% 22% 10% 4% 550

2002 41% 6% 7% 5% 3% 8% 19% 10% 3% 508

2003 41% 7% 9% 3% 3% 6% 18% 8% 6% 479

CNS: coagulase-negative staphylococci.

spp. (13%), Klebsiella spp. (10%), Proteus spp. (7%) and Enterobacter spp. (3%). With Gram-positive uropathogens Enterococcus spp. was isolated in 9% and Staphylococcus spp. in 4%. Candida spp. was isolated in 3%. 2.2.4. Bacterial spectrum of UTI in hospitalized urological patients in Straubing, Germany, within a ten-year period Since 1994 an ongoing longitudinal surveillance on nosocomial uropathogens is performed at our institution [9]. The isolated species are shown in Table 2. Per year 374 (in the year 1996) to 623 (in the year 2000) isolates were evaluated. Only one isolate of one species with the same susceptibility pattern from each patient was included into the study to avoid duplicates. E. coli was the most frequently isolated Gram-negative pathogen with a percentage of 27% (in the year 1995) to 41% (in the year 2003). Gram-positive pathogens were isolated with a percentage of 32% (in the year 2003) to 38% (in the year 1995).

less based upon knowledge of national or international surveillance studies. 3.1.1. Antibiotic resistance in uncomplicated, community acquired UTI in North America In a surveillance study of urinary E. coli isolates from outpatient women in the United States, collected during the year 2000, the rates of ampicillin and TMP/ SMX susceptibility (60% and 76%, respectively) were far lower than the incidence of susceptibility to ciprofloxacin (96%) [10]. Similar findings were reported in the analysis of 16,745 E. coli isolates from female outpatients with UTI collected in the Pacific region of the United States in 2001 (ampicillin resistance: 38%; TMP/SMX resistance: 20%; ciprofloxacin resistance: 2%) [11].

Since antibiotics have been introduced into clinical medicine, antibiotic resistant bacteria have evolved. The epidemiology of antibiotic resistant bacteria, however, varies from region to region, from speciality to speciality, from infection type to infection type and from year to year.

3.1.2. Antibiotic resistance in uncomplicated, community acquired UTI in Europe In a 1999–2000 European survey, over 93% of 1927 uropathogenic E. coli isolates from outpatients with UTI were susceptible to ciprofloxacin, with little variability across individual countries [12]. Similar findings were also reported in a separate study of 2478 uropathogenic E.coli isolates from female outpatients with UTI in European countries and Canada. The rate of TMP/SMX-resistant E. coli averaged about 14% but varied widely with the lowest rates in the Nordic countries and Austria and the highest rates in Portugal and Spain, whereas ciprofloxacin resistance was 2% on average [13].

3.1. Antibiotic resistance in uncomplicated, community acquired UTI In clinical practice urine culturing is routinely not performed in the setting of uncomplicated UTI. Antibiotic therapy therefore is mostly empiric and more or

3.2. Antibiotic resistance in complicated, nosocomial acquired UTI Nosocomial uropathogens are frequently subject to antibiotic pressure and cross-infection. The influence of these parameters can vary between regions and

3. Antibiotic resistance

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specialities. Different species of uropathogens show distinct abilities to elaborate antibiotic resistance. 3.2.1. Antibiotic resistance in nosocomial UTI in North America (SENTRY study) In the SENTRY study [5] E. coli strains showed high resistance to ampicillin (37%) and TMP/SMX (23%), while being very susceptible to fluoroquinolones, nitrofurantoin, imipenem, and aminoglycosides (i.e. >96%) (Table 1). Against strains of Klebsiella spp., third and fourth generation cephalosporins were very active (>95% susceptible), as well as imipenem (100%), piperacillin/tazobactam (95%), aminoglycosides (>95%), and fluoroquinolones (>92%). In this series, antimicrobial activity against P. aeruginosa was more variable. Amikacin (97% susceptibility), piperacillin/tazobactam (92% to 96%), cefepime (91%), and imipenem (91%) were the most active agents in vitro against pseudomonal strains. Ciprofloxacin was the most active of the fluoroquinolones tested against P. aeruginosa with 75% of strains susceptible, followed by levofloxacin (72%) and gatifloxacin (66%). Another surveillance from the SENTRY group found similar rates of fluoroquinolone resistance in hospital acquired P. aeruginosa clinical urinary isolates [14]. One hundred six P. aeruginosa strains were collected from 31 North American medical centers in 2000; resistance rates were 29% to ciprofloxacin and levofloxacin, and 28% to gatifloxacin. However, it should be noted that 31% of isolates were also resistant to gentamicin and 17% to amikacin. The most active agents tested were cefepime, imipenem, meropenem, and piperacillin/tazobactam, with resistance rates lower than 11%. Many of the tested antimicrobials had borderline activity against Enterococcus spp., although 5% were found to be vancomycin-resistant. Nitrofurantoin, rifampin, quinupristin/dalfopristin, and vancomycin had the greatest activity against coagulase-negative staphylococci strains. The fluoroquinolones had only modest in vitro activity against enterococci, coagulasenegative staphylococci, and S. aureus, the most common Gram-positive pathogens (ranging from 38% to 81% susceptible) in UTI. 3.2.2. Antibiotic resistance in nosocomial UTI in Europe (ESGNI-003 study) The European Study Group on Nosocomial Infections (29 countries) also evaluated antimicrobial susceptibility against hospital-acquired urinary isolates [7]. During 1999, 607 organisms from 522 patients with nosocomial UTI were tested. Resistance rates were for E. coli similar to those observed from the

North American SENTRY experience (Table 1): TMP/ SMX (28%), ampicillin (55%), ciprofloxacin (9%), and gentamicin (6%). However, it is worth noting that non-European Union countries tended to have higher rates of E. coli resistance than European Union countries. In particular, amikacin, ceftazidime, and cefepime were the most active agents (>90% susceptible), imipenem and tobramycin were moderately active (>85% susceptible), and ciprofloxacin and gentamicin were the least active (75% susceptible). In contrast, P. aeruginosa isolates from non-European Union countries (e.g. Estonia, Serbia) showed resistance rates of over 50% for fluoroquinolones and non-amikacin aminoglycosides. The authors speculated that this high rate of resistance to pseudomonal strains might be explained by the lack of strict antimicrobial policies in hospitals within non-European Union countries. 3.2.3. Antibiotic resistance in nosocomial UTI in urological patients in Europe (PEP study) The PEP study [8] also evaluated resistance rates of uropathogens causing nosocomial UTI in urological patients. However, there was no reference laboratory and different standards were employed for testing of the strains (178 hospitals employed NCCLS criteria, 34 DIN citeria, and 20 other criteria) and not all hospitals have tested all antibiotics. Global resistance rates were as follows (Table 1): ampicillin 51%, ampicillinþ betalactamase-inhibitor 30%, piperacillin 21%, piperacillin/tazobactam 15%, cefazolin 44%, cefuroxime 25%, ceftazidime 17%, cefepime 21%, imipenem 7%, gentamicin 34%, amikacin 14%, ciprofloxacin 34% and TMP/SMX 45%. 3.2.4. Antibiotic resistance of uropathogens of hospitalized urological patients in Straubing, Germany, within a ten-year period In our own institution the resistance rates of 12 antibacterial substances were evaluated in the time course of the last 10 years (1994 until 2003) [8]. Table 3 shows exemplary the global resistance rates of the year 2001 according to grouping of bacteria. The lowest overall rates of resistance were found with piperacillin/tazobactam (carbapenems were not tested). Ciprofloxacin and TMP/SMX showed the next favourable overall activity with resistance rates of about 20% to 25%. The development of resistance rates in the last ten years showed on the one hand trends of increasing resistance in some species like E. coli and steady resistance rates in other species: TMP/SMX showed an increase of resistance in E. coli from 15% in 1994 to 26% in 2003. Resistance to

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Table 3 Resistance rates in % of uropathogens and stratified into bacterial groups, isolated from the urine of hospitalized urological patients of the department of urology in Straubing, Germany, in the year 2001 [8] Group of pathogens

Antibiotics Cip

TMP/SMX

Amp

Mez

Oxa

Amp/Sul

Pip/Taz Cefuroxime Cefpodoxime

Cefotaxime

Ceftazidime Gentamicin

Total pathogens Gram-positive Gram-negative Enterobacteria P. aeruginosa Enterococci CNS S. aureus

24 43 12 9 32 46 58 14

22 22 24 19 60 14 38 5

47 27 56 53 97 5 72a 82a

36 27 40 37 74 3 72a 82a

nd nd nd nd nd nd 62 14

30 21 36 31 91 1 #b #b

8 23 2 1 8 1 #b #b

33 72 12 6 59 100 #b #b

28 72 2 2 0 100 #b #b

42 76 25 17 100 100 #b #b

38 74 18 10 100 100 #b #b

28 69 6 5 19 100 50 13

Cip: ciprofloxacin; TMP/SMX: trimethoprim/sulfamethoxazol; Amp: ampicillin; Mez: mezlocillin; Oxa: oxacillin; Amp/Sul: ampicillin/sulbactam; Pip/Taz: piperacillin/tazobaktam; CNS: coagulase-negative staphylococci. a b

Resistance rates as with penicillin. #, resistance rates as indicated with oxacillin; nd: not done.

ciprofloxacin also gradually increased from 4% in 1994 to 13% in 2002. Penicillin resistance of S. aureus increased from 55% in 1994 to 81% in 2003. Oxacillin resistance of S. aureus increased from 16% in 1994 to 24% in 2003. Ampicillin resistance in E. coli slightly increased from 29% in 1994 to 36% in 2003, as well as resistance to ampicillin protected with a beta-lactamase inhibitor (sulbactam) increased from 18% in 1995 to 26% in 2003. Resistance to cefuroxime increased in Proteus spp. from 9% in 1994 to 17% in 2003. Resistance to ceftazidime in Pseudomonas spp. remained stable between 0% to 8%. Resistance to gentamicin was low with enterobacteria between 0% to 8% and was gradually decreasing for Pseudomonas spp. from 50% in 1994 to 14% in 2003. High-level resistance to gentamicin (MIC > 500 mg/l) was evaluated for Enterococcus spp. and the rate was between 12% and 22%. There were no vancomycin or linezolid resistant enterococci or staphylococci in our study. In all the studies increasing resistance rates were found with species like E. coli, but not with all uropathogens. However, resistance rates may vary substantially between regions. Therefore local, hospital based surveillance of the bacterial spectrum and antibiotic sensitivity is paramount for a rational empiric therapy. Severe infections have lower mortality rates, when the empiric therapy has allready initially covered the causative bacteria [15,16]. 4. Antibiotic substances for the treatment of UTI In order to understand how bacteria become resistant against antibiotics, it is important to know, how anti-

biotics act against bacteria. Table 4 shows grouping and dosage of elected antibiotics for the treatment of complicated and nosocomial UTI. In order to affect the bacterial cell probably all antibiotics have to get into the cell. Basically antimicrobial substances used for the treatment of UTI can be distinguished in those acting on the baterial DNA, from those inhibiting proteinsynthesis and from those inhibiting peptidogylcan synthesis. The way urologically important antibiotics work are alluded to in the following passage. 4.1. Antibiotics that act on the DNA Fluoroquinolones act directly on the bacterial DNA. The target structures are the bacterial topoisomerases II and IV, which introduce so called negative ‘‘supercoils’’ into DNA and thus achieve a higher order by coiling. Four molecules of the quinolone substance form a ternary complex together with topoisomerases and DNA, which causes DNA double strand breakage [17]. Sulfonamides, such as sulfamethoxazole, lead to the formation of ineffective forms of tetrahydrofolate (dihydropteroic acid) from para-aminobenzoic acid and pteridine by substituting para-aminobenzoic acid. Tetrahydrofolate physiologically is further catalysed to dihydrofolic acid (folate) which is again further catalysed into tetrahydrofolic acid by dihydrofolatereductase. Pyrimethamines, such as trimethoprim, competitively inhibit the bacterial dihydrofolate-reductase. Purine synthesis is therefore inhibited [18]. 4.2. Antibiotics that inhibit protein synthesis The aminoglycosides bind irreversibly to distinct parts of the 30S- and 50S-subunits of the bacterial ribosomes. Consecutively the amino-acid translocation

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Table 4 Groups and dosages of elected antibiotics for the treatment of nosocomial and complicated UTI Antibiotic groups

Beta-lactams Aminopenicillin þ BLI Acylureidopenicillin þ BLI Cephalosporin Gr. 1 Cephalosporin Gr. 2

Cephalosporin Gr. 3 Cephalosporin Gr. 3a Cephalosporin Gr. 3b Cephalosporin Gr. 4 Carbapenem Gr. 1 Carbapenem Gr. 2 Fluoroquinolones Fluoroquinolone Gr. 2 Fluoroquinolone Gr. 3 Fluoroquinolone Gr. 4

Antimicrobial substance

Dosage Oral

i.v./i.m.

Ampicillin/Sulbactam Amoxicillin/Clavulanic acid Piperacillin/Tazobactam Piperacillin/Combactam Cephalexin Cefuroxime axetil Cefuroxime Cefotiam Cefpodoxim proxetile Ceftibuten Cefotaxime Cetriaxone Ceftazidime Cefepime Imipenem Meropenem Ertapenem

2  750 mg 2 1g – – For prophylaxis only 2  250–500 mg – – 2  200 mg 1  200–400 mg – – – – – – –

3  0.75–3 g 3  1.2–2.2 g 3  2.5–4.5 g 3  5g – – 3  0.75–1.5 g 2–3  1–2 g – – 2–3  1–2 g 1  1–2 g 2–3  1–2 g 2  2g 3  500 mg 3  500 mg 1  400 mg

Ciprofloxacin Levofloxacin Gatifloxacin

2  500–750 mg 1–2  500 mg 1  500 mg

2  400–600 mg 1–2  500 mg 1  500 mg

Pyrimethamines Trimethoprim Trimethoprim þ sulfamethoxazole Aminoglycosides Aminoglycoside

Oxazolidinones Oxazolidinone Glycopeptides Glycopeptide

2  200 mg 2  160 mg þ 2  800 mg Gentamicin Tobramycin Amikacin

– – –

1  5–7 mg/KG 1  5–7 mg/KG 1  15 mg/KG

Linezolid

2  600 mg

2  600 mg

Vancomycin Teicoplanin

– –

2  1000 mg 1  400 mg

BLI: beta-lactamase inhibitor; Gr: group according to PEG [42]; KG: body weight.

is inhibited by preventing the binding of elongation factor G. The elongation stage in bacterial translation is therefore inhibited [18]. Oxazolidinones bind to the 23S-rRNA of the 50Ssubunit of the bacterial ribosomes and thus inhibit the formation of the 70S-initiation complex. The initiation stage of bacterial translation is therefore inhibited [18]. 4.3. Antibiotics that inhibit peptidoglycan synthesis Beta-lactams inhibit the last stage of peptidoglycansynthesis. They bind to penicillin-binding proteins (PBP), which act as enzymes (i.e. transpeptidases, carboxypeptidases and endopeptidases) in the formation and preservation of parts of the bacterial cell wall. The affinity of distinct penicillines to certain PBPs

might be very different. Penicillines inhibit the process of cross-linking of the long polysaccharide chains by short polypeptides. They are analogous substrates of the acyl-D-alanyl-D-alanine moieties and acylate transpeptidases. Consecutively the peptidoglycan wall is weakened [18]. The glycopeptide antibiotics bind to the acyl-Dalanyl-D-alanine moieties and inhibit the process of transglycosilation of the peptidoglycan wall [18].

5. Mechanisms of antibiotic resistance in uropathogens Antibiotic resistance is defined, if bacteria can still grow under achievable therapeutic concentra-

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tions of antibiotic substances at the site of infection. Resistance is distinguished into primary or inherent resistance of bacteria, if bacteria are constitutively resistant against an antibacterial substance and secondary or acquired resistance, if resistance emerges in intrinsically susceptible bacteria. Epidemiologically important is transferable resistance located on plasmids (extrachromosomal autonomous mobile genetic element transferable to other cells) or transposons (mobile genetic element transposable to plasmids or other chromosomal sites). 5.1. Alterations of permeability and efflux mechanisms Probably all antibiotics have to get into the bacterial cells in order to act. Intrinsic resistance for example of Gram-negative bacteria against macrolides is due to impermeability of the outer membrane to these hydrophilic compounds. Enterococcus spp. shows decreased permeability towards aminoglycosides and is therefore intrinsically low-level resistant to aminoglycosides. On the other hand permeability can also be altered by altered production of outer membrane proteins, for example in E. coli, leading to decreased susceptibility to fluoroquinolones or beta-lactam antibiotics. Efflux mechanisms can potentially pump antibiotic substances, such as quinolones or tetracyclines out of the cell. There are five superfamilies of efflux transport systems known so far: ABC (ATP-binding cassette), MFS (major facilitator superfamily), RND (resistancenodulation-division), SMR (small multidrug resistance) and MATE (multidrug and toxic compound extrusion) family. Efflux systems are responsible for low level resistance, and thus may promote selection of mutations responsible for higher level resistance [18,19]. A powerful efflux mechanism in Pseudomonas spp. is one constitutively produced system (MexAB-OprM– RND superfamily) that generates intrinsic resistance against most beta-lactams, quinolones, tetracycline, chloramphenicol, trimethoprim and sulfamethoxazole. On the other side non-constitutive systems (i.e. MexCD-OprJ; MexEF-OprN) can be expressed by mutation. In other species like S. aureus, coagulase-negative staphylococci or C. freundii efflux is also an important mechanism of clinical resistance against quinolones [20–23]. 5.2. Alterations of target structures Target structures can be altered by mutations, acquisition of genetic material or inactivation of antibiotics by enzymatic modification [18].

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5.2.1. Mutations Fluoroquinolone resistance is mediated by target modifications (DNA gyrase and/or topoisomerase IV) and/or decreased intracellular accumulation [20]. Whereas in Gram-negative bacteria like E. coli the DNA gyrase is the primary target, in Gram-positive bacteria like S. aureus topoisomerase IV is the primary target for some but not all quinolones [21]. With clinically relevant concentrations newer quinolones like moxifloxacin and gemifloxacin inhibit both targets, the DNA gyrase as well as topoisomerase IV. Ampicillin resistance in enterococci is associated with overproduction of a low-affinity penicillin-binding protein (PBP) that is called (PBP)-5. High-level ampicillin resistance in E. faecium is associated with intrinsic overproduction of a modified (PBP)-5 that further lowers the penicillin-binding capability [24]. Vancomycin resistance in enterococci is due to the manufacture of a peptidoglycane side chain from Dalanyl-D-lactate, which is incorporated into the peptidoglycan cell wall instead of the vancomycin target Dalanyl-D-alanyl. The D-alanyl-D-lactate chain shows dramatically lowered affinity to vancomycin. Vancomycin resistance is conferred by five genes located on a transposable element [25]. 5.2.2. Acquisition of genetic material Resistance to TMP/SMX arises from a variety of mechanisms, involving enzyme alteration, cellular impermeability, enzyme overproduction, inhibitor modification or loss of binding capacity. The mechanism of greatest clinical importance is the production of plasmid encoded, trimethoprim resistant forms of dihydrofolate reductase [26–28]. Resistance in methicilline resistant S. aureus (MRSA) is mediated by an additional penicillin-binding protein (PBP)-2a, which has unusual low affinity for all beta-lactam antibiotics. The (PBP)-2 and (PBP)2a belong to a family of bifunctional proteins with an N-terminal transglycosylase and C-terminal transpeptidase domain. In case of blockage of (PBP)-2 by betalactam antibiotics, (PBP)-2a takes over the enzymatic activity [18]. (PBP)-2a is encoded by a mecA-gen that has been incorporated into the chromosomal DNA of S. aureus and coagulase negative staphylococci strains. 5.3. Inactivation of antibiotics Beta-lactamases are enzymes produced by bacteria that inactivate beta-lactam antibiotics by cleavage of the beta-lactam ring. More than 80 different enzymes

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so far are identified and the substrates comprise penicillines, cephalosporins or other beta-lactam antibiotics [18]. Resistance to penicillin is mediated by a penicillinase that hydrolyses the beta-lactam ring of penicillin. More than 90% of S. aureus isolates are penicillinase producer. This resistance can be overcome with penicillinase-stable penicillins such as oxacillin [29]. A frequent resistance mechanism in E. coli and Proteus spp. is production of TEM-1, a plasmidmediated beta-lactamase, that is inhibitor resistant [30]. It therefore confers resistance in strains having acquired the resistance plasmid, for example to ampicillin as well as ampicillin/sulbactam. The SHV-1 beta-lactamase of Klebsiella pneumoniae, as well as the K1-beta lactamase of Klebsiella oxytoca spp. are chromosomally encoded but are inhibitor-sensitive [31]. It therefore encodes intrinsic resistance in all Klebsiella strains, for example to ampicillin but not to ampicillin/sulbactam. Enterobacter spp. possesses a chromosomally encoded ampC beta-lactamase that inactivates penicillins and cephalosporins and is not inhibitor-sensitive. Resistance, however, results only if the beta-lactamase is hyperproduced. Ampicillin is a strong inducer of this enzyme, Mezlocillin is less suitable to induce hyperproduction of this beta-lactamase [32]. The genus Citrobacter comprises such species (Citrobacter freundii group) that behave like Enterobacter spp. and those that produce other less extended beta-lactamases (Citrobacter koseri/diversus). In Proteus spp. a wide diversity of beta-lactamases can be produced, serving as a possible beta-lactamaseencoding reservoir [33]. Plasmid encoded extended-spectrum beta-lactamase production (ESBL) is important in K. pneumoniae, E. coli, Proteus spp. and C. diversus. Other resistances, such as aminoglycoside and trimethoprim-sulfamethoxazole resistance, are often cotransferred on the same plasmid [34]. As indicated above, Enterobacter spp., C. freundii, Serratia spp., K. oxytoca, M. morganii and Providencia spp. possess a chromosomally encoded beta-lactamase that can be induced to hyperproduction by mutation or derepression [35]. This hyperproduced beta-lactamase also causes a resistance phenotype, comparable to ESBL, although no ESBL is produced. Other inactivating enzymes for example can inactivate aminoglycosides or macrolides. The expression of a bifunctional aminoglycoside inactivating enzyme, 60 -N-aminoglycoside acetyltransferrase-20 -O-aminoglycoside phosphotransferase, is the most important mechanism of high level aminoglycoside resistance in

Staphylococcus spp. and Enterococcus spp. [36]. Enterococci are intrinsically low level resistant, in the case of high level resistance, aminoglycoside combination therapy would be ineffective. Among Enterobacteriaceae combinations of gentamicin-modifying enzymes are common In Pseudomonas spp. the combination of gentamicin-modifying enzymes and decreased permeability is common [37]. Bacteria exhibit an enormous repertoire of different resistance mechanisms. Unspecific mechanisms such as reduced permeability or efflux alter the tolerance to antibiotic substances less than specific mechanisms, such as inactivation of the antbiotic for example, however the antibiotic spectrum targeted is much more extensive. On the other hand unspecific mechanisms can also be induced by non-antibiotic substances such as salicylates. Low level resistance can thus be confered and give bacteria a selection advantage.

6. Causes of increasing antimicrobial resistance Antibiotic resistance in general is related to the amount of application of an antibiotic substance or a related substance or an unrelated substance with an identical resistance mechanism. 6.1. Frequent application of antimicrobials in clinical practice It is generally assumed that the antibiotic prescription policy of a hospital has a significant impact on emergence of bacterial resistance. In a three-year surveillance study it could be shown that the consumption of imipenem correlated significantly with betalactam resistance in nosocomial Pseudomonas aeruginosa isolates, while consumption of ceftazidime or piperacillin/tazobactam had no apparent association with resistance [38]. The authors could show that periods of extensive imipenem use were associated with significant increases in resistance and thus support the concept that a written antibiotic policy which balances the use of various antibiotic classes may be helpful. Another example is the resistance rate of E. coli against fluoroquinolones in isolates of hopitalised children, which is ten-fold less than in adults, because fluoroquinolones are generally not used for children. 6.2. Pharmacokinetic parameters of antimicrobials It has been shown that resistant mutants are selectively enriched in the concentration range between the

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minimal inhibitory concentraton of wild-type cells and the mutant prevention concentration. This concentration window is called mutant selection window and has a function over the time. The longer bacterial cells are exposed to the mutant selection window, the higher is the possibility of selection of resistant mutants [39]. Therefore optimal kinetics show those antibiotics that pass rapidly through the mutant selection window at start of therapy, stay above the mutant prevention concentration the entire time of therapy and return rapidly again through this window at the end of therapy. Antibiotics with extensive long half lives exhibit a slow return through the window and have therefore a higher propensity to select resistant mutants. On the other hand compliance of patients is improved with reduced dosing by antibiotics with longer half lives. 6.3. Increasing numbers of patients with severe or chronic diseases or immune disorders Patients with severe or chronic diseases or immune disorders are continuously becoming more frequent in the hospital landscape. Baquero et al. have presented a mathematical model in which the propability of survival of a resistant bacterial population has been calculated in respect to the host immune status [40]. A resistant bacterial population had a 44% probability to be selected by antibiotics in the immunocompetent host, and of 90% in the immunodepressed patient. Therefore the patient population is a significant factor in the emergence of antibiotic resistance.

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6.4. Antibacterial substances other than for treatment of human infections Antibiotics are not only used for treatment of human infections. A great proportion of antibiotic substances or related compounds is used in animal husbandry as food adjunctive or in household products, such as aseptic soaps or lotions. Avoparcin for example is a glycopeptide antibiotic and therefore related to vancomycin, used exclusively in animal husbandry. Avoparcin was prohibeted to be used in husbandry in Denmark since 1995. Consecutively the percentage of vancomycin resistant E. faecium isolates in broiler decreased from 80% in 1995 to less than 10% in 1999 [41]. 7. Conclusion Antibiotic resistance is an increasing problem in urological practice. Especially nosocomial uropathogens may exhibit resistances to multiple antibiotics and pose problems for empiric therapy. In order to chose the right antibiotic for empiric therapy it is necessary to consider the bacterial spectrum and the antibiotic susceptibility of the uropathogens. Therefore each institution must be able to have its own local and recent evaluation. To combat the development of antibiotic resistance a basic understanding of antibiotic action and resistance mechanisms is helpful. In the future the rate of antibiotic resistance will possibly continue to increase. Strategies to decrease this trend, such as antibiotic policies, will need to become developed and incorporated in urological practice.

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CME question Please visit http://www.uroweb.org/updateseries to answer these CME questions on-line. The CME credits will then be attributed automatically. 1. Which of the following answers is correct: In a recent study of 2478 uropathogenic E. coli isolates from female outpatients with UTI in European

countries and Canada, the rate of TMP/SMX resistant E. coli in Europe averaged about A. 14%; B. 28%; C. 7%; D. 50%.

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2. Which of the following answers is not correct: Basically antimicrobial substances used for the treatment of UTI can be distinguished in those A. acting on the baterial DNA; B. inhibiting protein synthesis; C. inhibiting peptidogylcan synthesis; D. inhibiting mitochondrial synthesis. 3. Which of the following answers is correct: The target structures of the fluoroquinolones are A. efflux pumps; B. the bacterial topoisomerases II and IV; C. cell wall synthesis; D. telomerases.

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4. Which of the following statements to enterococci is not correct: A. Ampicillin resistance in enterococci is associated with overproduction of a low-affinity penicillinbinding protein (PBP) that is called (PBP)-5. B. High level ampicillin resistance in E. faecium is associated with intrinsic overproduction of a modified (PBP)-5 that further lowers the penicillin-binding capability. C. Enterococci are generally sensitive to cephalosporins. D. Vancomycin resistance in enterococci is due to the manufacture of a peptidoglycane side chain from D-alanyl-D-lactate.