In vitro susceptibility of recent antibiotic-resistant urinary pathogens to ertapenem and 12 other antibiotics

A. Alhambra1, J. A. Cuadros2, J. Cacho3, J. L. Gómez-Garcés1 and J. I. Alós1,*

1 Department of Microbiology, Hospital de Móstoles, 28935 Móstoles, Madrid; 2 Department of Microbiology, Hospital Universitario Príncipe de Asturias, Alcalá de Henares, Madrid; 3 Department of Microbiology, Hospital Universitario de Getafe, Getafe, Madrid, Spain

Received 3 December 2003; returned 22 January 2004; revised and accepted 1 March 2004


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Background: The treatment of complicated urinary tract infections may require the use of a parenteral antibiotic with potent activity against the most common urinary pathogens. Ertapenem is a broad-spectrum 1ß-methyl carbapenem with a long plasma half-life that allows administration of a single daily dose.

Methods: The purpose of this work was to test the in vitro susceptibility to ertapenem, ampicillin, cefazolin, cefuroxime, cefotaxime, co-amoxiclav, piperacillin/tazobactam, imipenem, gentamicin, amikacin, fosfomycin, ciprofloxacin and co-trimoxazole of 482 strains of urinary pathogens of the family Enterobacteriaceae isolated from patients in the community of Madrid (40% from males). The distribution was as follows: Escherichia coli (n = 315), Proteus mirabilis (n = 42), Klebsiella spp. (n = 14) and AmpC-producing Enterobacteriaceae (n = 111). The strains studied were selected based on their resistance to quinolones and aminoglycosides, and their production of extended-spectrum ß-lactamases (ESBLs) or AmpC-type ß-lactamases.

Results: All the strains were susceptible to ertapenem, imipenem and amikacin. The MIC90 of ertapenem ranged from a minimum of 0.03 mg/L for Proteus vulgaris and a maximum of 1 mg/L for Enterobacter spp. Ertapenem was the most active of all drugs tested in all cases. On comparing antibiotic resistance among ESBL-producing strains of E. coli (n = 35) and E. coli strains not producing ESBLs (n = 280), statistically significant differences were obtained for ciprofloxacin (P = 0.002) and gentamicin (P = 0.011). Regarding ertapenem, only a slight increase in MIC50 was seen, the value being 0.015 mg/L for strains not producing ESBLs versus 0.03 mg/L for ESBL-producing strains.

Conclusions: In view of its significant antibiotic potency against antibiotic-resistant Enterobacteriaceae, ertapenem may constitute a good therapeutic alternative in urinary infections caused by these pathogens.

Keywords: carbapenems, antibiotic susceptibility, urinary tract infections


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Urinary tract infections (UTIs) account for ~7 million consultations and >1 million hospital admissions a year in the USA.1 Although uncomplicated cystitis in healthy young women is usually treated with oral antibiotics administered for 3–5 days, complicated UTIs may require a potent parenteral antibiotic, at least until a clinical response is obtained.

Ertapenem is a broad-spectrum 1ß-methyl carbapenem with a long plasma half-life that allows for administration of a single daily dose,2,3 while meropenem and imipenem must be administered three to four times a day. The 1ß-methyl substituent of the chemical structure of ertapenem and meropenem confers stability against renal dehydropeptidase I; as a result, co-administration with cilastatin is not required, unlike in the case of imipenem.4 Ertapenem has a broad-spectrum activity, and is active against Gram-positive pathogens most commonly involved in community-acquired infections, Enterobacteriaceae and anaerobes, but has a more limited activity against Pseudomonas aeruginosa, Acinetobacter spp., methicillin-resistant staphylococci and enterococci.58 Similar to the other carbapenems, ertapenem is a ß-lactam with an increased stability against the resistance mechanisms of Enterobacteriaceae, such as production of extended-spectrum ß-lactamases (ESBLs) or AmpC-type ß-lactamases.6,7

Ertapenem is excreted mainly in urine. After administration of ertapenem 1 g to healthy adults, 80% is recovered in urine.3

The presence of resistant or multiresistant Enterobacteriaceae is increasingly common in complicated UTIs, and even in uncomplicated infections, thereby making carbapenems a good treatment alternative. In adults with complicated UTIs requiring initial parenteral therapy followed by adequate oral treatment, ertapenem 1 g daily has been well tolerated and has shown good clinical results.9,10

The purpose of this work was to study the in vitro activity of ertapenem and 12 other antibiotics against recent antibiotic-resistant Enterobacteriaceae isolated from urine samples sent to the laboratory for the diagnosis of urinary infection.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
A total of 482 urinary pathogen strains from the microbiology laboratories of Hospital Universitario Príncipe de Asturias (Alcalá de Henares), Hospital Universitario de Getafe and Hospital de Móstoles, all located in the community of Madrid (Spain), were used in this study. The strains were collected from January to September 2003, and the majority were from hospital in-patients. A total of 289 strains were isolated from urine in females (60%) and 193 in males (40%).

The strains were selected based on the following criteria: Escherichia coli resistant to ciprofloxacin and/or resistant to gentamicin and/or resistant to cefotaxime and/or producing ESBLs; Proteus mirabilis, Klebsiella pneumoniae and Klebsiella oxytoca resistant to nalidixic acid and/or resistant to gentamicin and/or resistant to cefotaxime and/or producing ESBLs; and AmpC-producing Enterobacteriaceae (mainly Enterobacter, Citrobacter, Morganella, Providencia, Serratia and Proteus vulgaris). Each group was assigned a maximum number of bacteria, so that the final distribution of the 482 strains was as follows: E. coli (n = 315), P. mirabilis (n = 42), P. vulgaris (n = 10), Klebsiella spp. (n = 14), Morganella morganii (n = 39), Enterobacter spp. (n = 41), Citrobacter spp. (n = 6), Serratia spp. (n = 7), Providencia stuartii (n = 6) and Hafnia alvei (n = 2). In the case of multiple isolates from the same patient, only one strain was included.

Each bacterium was identified by the standard laboratory methods, and the MIC of each antibiotic was determined using the agar dilution method following the recommendations of the NCCLS.11 The antibiotics studied were: ampicillin, cefazolin, cefuroxime, cefotaxime, co-amoxiclav, piperacillin/tazobactam, imipenem, ertapenem, gentamicin, amikacin, fosfomycin, ciprofloxacin and co-trimoxazole. Mueller–Hinton agar (Mueller–Hinton Agar II; Becton Dickinson and Co., Cockeysville, MD, USA) was used as the culture medium, in which a dilution of the bacterial suspension was inoculated at a McFarland turbidity equivalent of 0.5, representing ~104 colony forming units (cfu) per drop applied with a Steers replicator (Craft Machine Inc., Chester, PA, USA). For the determination of fosfomycin MICs, glucose-6-phosphate (25 mg/L) was added to Mueller–Hinton agar. The plates were incubated for 18–24 h at 35°C.

The control strains were P. aeruginosa ATCC 27853, E. coli ATCC 25922 and ATCC 35218, Staphylococcus aureus ATCC 29213, and Enterococcus faecalis ATCC 29212.

The detection of ESBL production was based on the agar diffusion technique according to the standardized conditions of the NCCLS,11 using Etest strips (AB Biodisk, Solna, Sweden) of cefotaxime/cefotaxime clavulanate and ceftazidime/ceftazidime clavulanate, and cefoxitin discs.

Statistical analysis

The {chi}2-test and Fisher’s exact test were used. A two-tailed P value of <=0.05 was considered significant.


    Results
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Tables 1 and 2 show the MIC results and the susceptibility and resistance data of all strains tested. The established breakpoints are those recommended by the NCCLS11 for testing enterobacterial susceptibility using the agar dilution method.


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Table 1. In vitro susceptibility of resistant and multiresistant Enterobacteriaceae causing urinary infections to 13 antibiotics (MIC in mg/L)
 

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Table 2. In vitro susceptibility of 111 AmpC-producing Enterobacteriaceaea causing urinary infections to 13 antibiotics (MIC in mg/L)
 
Based on the resistance criteria described above, the strains selected for this study were all susceptible to ertapenem, imipenem and amikacin. In addition, all P. vulgaris strains tested were susceptible to piperacillin/tazobactam, cefotaxime and gentamicin; all P. mirabilis strains were susceptible to cefotaxime and piperacillin/tazobactam; and all Enterobacter spp. were susceptible to gentamicin.

The MIC90 of ertapenem ranged from a minimum of 0.03 mg/L against P. vulgaris to a maximum of 1 mg/L against Enterobacter spp., and ertapenem was the most active drug tested in all cases. Imipenem also showed a good activity, with MIC90s ranging from 0.5 to 4 mg/L.

The highest MIC90s of imipenem were found with Enterobacter (2 mg/L) and Morganella (4 mg/L) species. Ertapenem always showed an MIC equal to or less than that of imipenem, except for four strains of Enterobacter cloacae, three E. coli strains and a single strain of K. pneumoniae, which yielded MICs one to two dilutions greater for ertapenem compared with imipenem.

None of the bacteria studied showed resistance to the tested carbapenems, and only nine strains of M. morganii showed intermediate susceptibility (MIC 4 mg/L) to imipenem.

Antibiotics routinely used to treat UTIs, such as co-trimoxazole, showed high resistance values, particularly against E. coli (92.4% resistance), P. mirabilis (88%) and P. vulgaris (60%). In contrast, Enterobacter spp. were highly susceptible, with a resistance rate of only 2.4%. Fosfomycin had an MIC90 > 128 mg/L for all groups tested, except for Enterobacter spp., where the MIC90 was 128 mg/L, and E. coli, where it was 8 mg/L.

On comparing antibiotic resistance among ESBL-producing strains of E. coli (n = 35) and E. coli strains not producing ESBLs (n = 280), statistically significant differences were obtained for ciprofloxacin (P = 0.002) and gentamicin (P = 0.011). As regards ertapenem, only a slight increase in MIC50 was noted, the value being 0.015 mg/L for bacteria not producing ESBLs and 0.03 mg/L for ESBL-producing bacteria.


    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
This study tested the susceptibility to ertapenem and 12 additional antibiotics of a series of Enterobacteriaceae isolated from urine samples collected in three health care areas of the community of Madrid (Spain), selected on the basis of their resistance phenotype to quinolones, aminoglycosides and ß-lactams. The true levels of resistance of E. coli in 2003 in our laboratories were as follows: cefotaxime 3%; imipenem 0%; gentamicin 5.6%; and ciprofloxacin 19.2%.

In routine practice, urinary infection isolates are much more frequent in females than in males. However, of the total bacteria studied (n = 482), 40% were isolated from male urine. This high percentage is due to the selection of resistant bacteria, which are more often present in complicated UTIs, which are in turn more common among males.

All the strains were susceptible to ertapenem, imipenem and amikacin, and ertapenem was the most potent of all the tested antibiotics. This same conclusion has been reached in other susceptibility studies involving non-selected Enterobacteriaceae in Australia, Europe and the USA.7,8,12 None of the bacteria studied was resistant to ertapenem, and the highest MIC90 recorded was 1 mg/L, for Enterobacter spp.

In our experience, the MIC90 for ESBL-producing strains of E. coli compared with strains not producing ESBLs was only a double step higher for piperacillin/tazobactam, amikacin and ertapenem, but not for co-amoxiclav. However, the ESBL-producing strains were significantly more resistant to ciprofloxacin and to gentamicin than those that did not produce ESBLs. For ESBL-producing Klebsiella spp. and ertapenem, other authors have found the MIC90 to range from 0.016 to 0.12 mg/L.7

In view of its important antibiotic potency against antibiotic-resistant Enterobacteriaceae, ertapenem may constitute a good therapeutic alternative for urinary infections caused by these pathogens, particularly in the hospital setting, where these conditions are increasingly common.


    Acknowledgements
 
This work was supported by MSD, Madrid, Spain.


    Footnotes
 
* Corresponding author. Tel: +34-91-6648695; Fax: +34-91-6471917; E-mail: nachoalos{at}microb.net Back


    References
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
1 . Stamm, W. E. & Hooton, T. M. (1993). Management of urinary tract infections in adults. New England Journal of Medicine 329, 1328–34.[Free Full Text]

2 . Gill, C. J., Jackson, J. J., Gerckens, L. S. et al. (1998). In vivo activity and pharmacokinetic evaluation of a novel long-acting carbapenem antibiotic, MK-826 (L-749,345). Antimicrobial Agents and Chemotherapy 42, 1996–2001.[Abstract/Free Full Text]

3 . Cunha, B. A. (2002). Ertapenem. A review of its microbiologic, pharmacokinetic and clinical aspects. Drugs of Today (Barcelona, Spain: 1998) 38, 195–213.

4 . Edwards, J. R. & Betts, M. J. (2000). Carbapenems: the pinnacle of the ß-lactam antibiotics or room for improvement? Journal of Antimicrobial Chemotherapy 45, 1–4.[Abstract/Free Full Text]

5 . Fuchs, P. C., Barry, A. L. & Brown S. D. (1999). In-vitro antimicrobial activity of a carbapenem, MK-0826 (L-749,345) and provisional interpretive criteria for disc tests. Journal of Antimicrobial Chemotherapy 43, 703–6.[Abstract/Free Full Text]

6 . Kohler, J., Dorso, K. L., Young, K. et al. (1999). In vitro activities of the potent, broad-spectrum carbapenem MK-0826 (L-749,345) against broad-spectrum ß-lactamase and extended-spectrum ß-lactamase-producing Klebsiella pneumoniae and Escherichia coli clinical isolates. Antimicrobial Agents and Chemotherapy 43, 1170–6.[Abstract/Free Full Text]

7 . Livermore, D. M., Carter M. W., Bagel, S. et al. (2001). In vitro activities of ertapenem (MK-0826) against recent clinical bacteria collected in Europe and Australia. Antimicrobial Agents and Chemotherapy 45, 1860–7.[Abstract/Free Full Text]

8 . Fuchs, P. C., Barry, A. L. & Brown, S. D. (2001). In vitro activities of ertapenem (MK-0826) against clinical bacterial isolates from 11 North American medical centers. Antimicrobial Agents and Chemotherapy 45, 1915–8.[Abstract/Free Full Text]

9 . Tomera, K. M., Burdmann, E. A., Pamo Reyna, O. G. et al. (2002). Ertapenem versus ceftriaxone followed by appropriate oral therapy for treatment of complicated urinary tract infections in adults: results of a prospective, randomized, double-blind multicenter study. Antimicrobial Agents and Chemotherapy 46, 2895–900.[Abstract/Free Full Text]

10 . Jimenez-Cruz, F., Jasovich, A., Cajigas, J. et al. (2002). A prospective, multicenter, randomized, double-blind study comparing ertapenem and ceftriaxone followed by appropriate oral therapy for complicated urinary tract infections in adults. Urology 60, 16–22.[CrossRef][ISI][Medline]

11 . National Committee for Clinical Laboratory Standards. (2003). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically—Sixth Edition: Approved Standard M7-A6. NCCLS, Wayne, PA, USA.

12 . Pelak, B. A., Citron, D. M., Motyl, M. et al. (2002). Comparative in vitro activities of ertapenem against bacterial pathogens from patients with acute pelvic infection. Journal of Antimicrobial Chemotherapy 50, 735–41.[Abstract/Free Full Text]