Department of Medical Microbiology, University Hospital Maastricht, PO Box 5800, 6202 AZ Maastricht, The Netherlands
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Abstract |
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Introduction |
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Many studies have reported data on therapeutic antibiotic use and antibiotic resistance in hospitalized patients. Mostly the prevalence of antibiotic resistance has been described for clinical isolates only such as methicillinresistant Staphylococcus aureus and vancomycin-resistant enterococci.2,3 These data may show only the tip of the antibiotic-resistance iceberg. A large reservoir of antibiotic-resistance genes can be present in the intestinal bacterial flora, which can be transferred to potentially pathogenic bacteria. The prevalence and degree of antibiotic resistance found in indicator bacteria in the faecal flora are considered to be a good indicator for the antibiotic resistance to be expected in pathogenic microorganisms.4
In the present study the prevalence and degree of resistance of Escherichia coli and enterococci was determined in faecal samples from patients on admission and from patients at discharge to analyse the influence of prophylactic use of cefazolin and hospitalization on antibiotic resistance of these microorganisms.
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Material and methods |
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Faecal samples from two groups of patients were analysed for the prevalence and degree of antibiotic resistance in faecal E. coli and enterococci. The first group comprised faecal samples from patients within 24 h of admission for cardiac surgery, and the second group comprised faecal samples from patients after hospitalization also for cardiac surgery obtained within 1 week of discharge. After collection, the samples were sent to the laboratory within 12 h, diluted (10-1) in 0.9% saline containing 20% (v/v) glycerol and stored at -20°C until examination.
During hospitalization all patients received cefazolin as antibiotic prophylaxis (2 g iv just before surgery and then thrice 1 g iv every 6 h). Patients with therapeutic use of antibiotics during hospitalization were excluded. Patient characteristics including gender, age and type of cardiac operation were registered. On admission, additional information about hospitalization and antibiotic use in the 3 months prior to hospitalization was obtained using a questionnaire.
Bacteriological analysis
Ten-fold serial dilutions of the faecal samples (10-210-4) were made in physiological saline and inoculated using a spiral plater (Salm and Kip, Utrecht, The Netherlands) on selective eosinmethylene blue agar plates (Oxoid CM69, Basingstoke, UK) with and without antibiotics for E. coli and on KFStreptococcus agar plates (Oxoid CM701) with and without antibiotics for isolation of Enterococcus spp. Colonies were counted and identified as previously described.5
In addition, colonies of E. coli and enterococci isolated from the agar plates without antibiotics were used for susceptibility testing using a microbroth dilution method in IsoSensitest Broth (Oxoid CM473). Breakpoints of resistance used were based on NCCLS guidelines. Furthermore, from each antibiotic-containing agar plate with bacterial growth, one colony was randomly picked to confirm antibiotic resistance by determination of MIC values according to the NCCLS guidelines.
The prevalence of antibiotic resistance was defined as the percentage of faecal samples showing growth of E. coli or enterococci on antibiotic-containing agar plates, divided by the total number of samples tested x 100%. The degree of resistance was calculated for each sample as the percentage of the total number of colonies that was resistant divided by the total number of colonies on the control plate x 100%. Two degrees of antibiotic resistance were distinguished: the proportion of faecal samples with less than 50% (low degree) and with more than 50% (high degree) of the organisms being resistant.
Statistical analysis
For analysis of the differences in prevalence a 2 test was performed. Continuous variables were analysed by the non-parametric MannWhitney U-test. A P value <0.05 was considered to be significant.
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Results |
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In total, 180 faecal samples were collected from patients on admission and 108 samples from patients at discharge. The admitted and discharged patients were comparable with regard to age (62.8 ± 10.2 versus 60.8 ± 10.5 years), gender (67% male versus 78% male) and type of operation (83% versus 85% coronary artery bypass grafts, 13% versus 13% valve replacements and 4% versus 2% other cardiac operations). Information on antibiotic use and prior hospitalization was available for 121 and 134 of the patients on admission, respectively: 14 of 121 patients (11.6%) were treated with antibiotics and 86 of 134 (64.2%) were hospitalized in the 3 months before hospitalization for cardiac surgery, mostly because of cardiac failure. Patients from whom faecal samples were obtained at discharge had a mean hospital stay of 10 ± 5 days (median 8 days).
E. coli
The prevalence and high degree of antibiotic-resistant E. coli in faecal samples from patients at admission and at discharge are shown in Table 1. For cefazolin, amoxicillin and oxytetracycline the prevalence was higher for discharged patients compared with those on admission, being significant for amoxicillin and cefazolin (P < 0.05). In contrast to the prevalence, a tendency towards a decrease in the high degree of resistance was observed for amoxicillin, oxytetracycline and trimethoprim.
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Although prevalence tended to be higher after discharge, no significant increases in prevalence and high degree of resistance were found in patients at discharge compared with those on admission (Table 1). In the discharged patient group two amoxicillin-resistant enterococci were found. Both were Enterococcus faecium isolates with MIC values of 16 mg/L.
The isolated enterococci were Enterococcus faecalis and E. faecium in 39% and 45%, respectively, on admission and 22% and 75%, respectively, at discharge.
MIC90 values of the enterococci did not differ more than one dilution step between both patient groups, except for cotrimoxazole (Table 3).
Influence of hospital stay and prior antibiotic use
The prevalence and degree of resistance in discharged patients with a short (7 days) or long (>7 days) hospital stay are given in Table 2
. The prevalence of E. coli resistant to cefazolin, amoxicillin, oxytetracycline and trimethoprim was higher in short-versus long-term hospitalized patients being significant for amoxicillin only (P < 0.05). Although the prevalence of resistance to dalfopristinquinupristin and vancomycin was slightly higher for long-compared with short-term hospitalized patients, no significant differences were found for the enterococci.
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Discussion |
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A methodological limitation of the study is the lack of a cross-over design. However, as the groups of patients on admission and at discharge were comparable with regard to patient characteristics, they were used to analyse the influence of short-term hospitalization with limited antibiotic use on the prevalence of antibiotic-resistant faecal indicator bacteria.
No significant differences were found between either the prevalence or high degree of antibiotic-resistant enterococci of the patients on admission compared with at discharge. The data obtained from the samples on admission were in line with the results obtained from healthy volunteers from the same region.6 However, a shift from E. faecalis towards E. faecium was observed in the patient group at discharge compared with the group on admission. A similar observation, indicating antibiotic selective pressure, has been described before.7
The prevalence of vancomycin-resistant enterococci increased from 4% in the samples on admission to 8% in the samples at discharge (P > 0.05). Although the prevalence of vancomycin-resistant enterococci is still rather low in European hospitals (08.5%), enterococci are increasingly isolated as causative agents of infections, notably in hospitalized patients.8 Monitoring of vancomycin-resistant enterococci in hospitalized patients is important as they can pose a serious health risk since alternative therapeutic options are limited.9
Prevalence and high degree of resistance for E. coli isolates in the admitted patient group were comparable with those from healthy volunteers.6 The higher prevalence of E. coli resistant to cefazolin, amoxicillin and oxytetracycline after discharge compared with the prevalence on admission might be because of the use of cefazolin as surgical prophylaxis. Both the penicillins and cephalosporins contain a ß-lactam ring and high prevalence of both amoxicillin/ampicillin and cefazolin have been described in a population with community-acquired urinary tract infections.10 The high degree of resistance of E. coli to amoxicillin, oxytetracycline and trimethoprim decreased slightly at discharge. Furthermore, a lower prevalence of cefazolin, amoxicillin and oxytetracycline resistant to E. coli was found in patients with a long hospital stay compared with patients with a short stay. This difference suggests that cefazolin prophylaxis has a short-term effect on the prevalence of cefazolin-, amoxicillin- and oxytetracycline-resistant E. coli.
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Notes |
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References |
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van den Bogaard, A. E., London, N. & Stobberingh, E. E. (2000). Antimicrobial resistance in pig faecal samples from the Netherlands (five abattoirs) and Sweden. Journal of Antimicrobial Chemotherapy 45, 66371.
6 . Bruinsma, N., Hutchinson, J. M., Giamarellou, H., Degener, J., de Smet, P., van den Bogaard, A. et al. (2000). Importance of population density on the prevalence of antibiotic resistance in the fecal flora of healthy volunteers. In Program and Abstracts of the Fortieth Interscience Conference on Antimicrobial Agents and Chemotherapy, Toronto, 2000. Abstract 101. American Society for Microbiology, Washington, DC.
7 . Hanberger, H., Hoffmann, M., Lindgren, S. & Nilsson, L. E. (1997). High incidence of antibiotic resistance among bacteria in 4 intensive care units at a University Hospital in Sweden. Scandinavian Journal of Infectious Disease 29, 60714.[ISI][Medline]
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Schouten, M. A., Voss, A. & Hoogkamp-Korstanje, J. A. (1999). Antimicrobial susceptibility patterns of Enterococci causing infections in Europe. The European VRE Study Group. Antimicrobial Agents and Chemotherapy 43, 25426.
9 . Zimmermann, U. P., Klare, I., Witte, W. & Höffler, D. (1998). Occurrence and significance of Enterococcus faecium resistant to vancomycin. Antiinfective Drugs and Chemotherapy 16, 1479.
10 . Finkelstein, R., Kassis, E., Reinhertz, G., Gorenstein, S. & Herman, P. (1998). Community-acquired urinary tract infection in adults: a hospital viewpoint. Journal of Hospital Infection 38, 193202.[ISI][Medline]
Received 29 May 2001; returned 17 September 2001; revised 12 November 2001; accepted 3 December 2001