a Antimicrobial Research Laboratory, National Public Health Institute, PO Box 57, FIN-20521 Turku b Municipal Food Laboratory of Turku, Kirkkotie 13, 20540 Turku, Finland
Sir,
Antibiotic resistance among the members of the family Enterobacteriaceae is often high and can cause major clinical problems. In the normal flora they have also been found to harbour resistant strains at high frequencies. Some of this resistance could originate from food. 1 In a study of enterobacteria on vegetables, we found very low resistance levels, including plasmid-mediated. 1 We hypothesized that the corresponding population on meat might be different, since the risk of faecal contamination is much greater, and resistance in animal faecal flora could be high as a consequence of antibiotic use in animal husbandry. In contrast to vegetables, meat is not normally consumed raw, but can easily cross-contaminate other foods.
Samples of minced meat, collected by the local food surveillance laboratory (six beef, 20 beefpork, locally produced), were plated onto MacConkey agar plates for the selective cultivation of enterobacteria. All colonies of different morphology were streaked for purity. Gram-negative, oxidase-negative, glucose-fermenting bacilli were further identified to at least genus level by a set of 22 standard biochemical tests. MICs were done by a standard agar dilution method, using NCCLS breakpoints. Since most enterobacteria have intrinsic resistance to penicillins and older cephalosporins, results for these are not presented. In all, 150 isolates were tested. Duplicate isolates from a sample which appeared to be the same based on biochemical and MIC profiles, were excluded from the analysis, leaving 131 isolates.
The most common genera were Serratia (n = 37), Hafnia (n = 22), Yersinia (n = 13) and 10 each of Citrobacter, Klebsiella and unnamed 2 Enteric Groups. Only four Escherichia coli were found, which would indicate that the Enterobacteriaceae found were of environmental, not animal faecal, origin.
The resistance levels were of the same magnitude as in our earlier study on vegetables. 1 Resistance was not found to piperacillin/tazobactam, cefotaxime, aztreonam, imipenem, gentamicin, nalidixic acid and ciprofloxacin. Most of the cefuroxime resistance found (Table) was caused by endogenous chromosomal ß-lactamases; 15 of 18 resistant, and 15 of 17 intermediately resistant isolates were among Serratia spp. Serratia spp. have an AmpC enzyme that, when produced at high levels, also gives resistance to newer cephalosporins. We also saw very little evidence of the kind of transferable plasmid-mediated multiresistance that is common in human strains. Only three strains with probable plasmid-mediated multiresistance were found (sulphamethoxazole-trimethoprim-streptomycin-tetracycline, Klebsiella oxytoca; sulphamethoxazole-chloramphenicol-tetracycline, Hafnia alvei; sulphamethoxazole-trimethoprim, Enterobacter sp.).
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Enterobacteria from minced meat do not seem to contribute to the relatively high levels of antimicrobial resistance of enterobacteria we have found in human faecal flora in Finland. Thus excluding food, and indirectly the environment, as an important source of resistance, the evidence points ever more strongly towards the human use of antibiotics as the most important factor maintaining high resistance levels in our normal flora. The resistance might arise elsewhere too, e.g. in animal husbandry, and occasionally transfer to humans, where antibiotic use would favour its spread. In countries where the use of antimicrobials in animal husbandry is greater than in Finland, and contamination of meat by faecal matter at the time of slaughter perhaps more common, the results of a study such as this might be very different.
Notes
* Corresponding author. Tel: +358-2-2519255; Fax: +358-2-2519254.
References
1
.
Österblad, M., Pensala, O., Peterzéns, M.,
Helenius, H. & Huovinen, P. (1999). Antimicrobial susceptibility of
Enterobacteriaceae isolated from vegetables. Journal of Antimicrobial Chemotherapy 43, 5039.
2 . Farmer, J. J. (1995). Enterobacteriaceae: introduction and identification. In Manual of Clinical Microbiology, 6th Edn (Murray, P. R., Baron, E. J., Pfaller, M. A., Tenover, F. C. & Yolken, R. H., Eds), pp. 4389. ASM Press, Washington, DC.
3 . Persson, L., Olsson, B. & Franklin, A. (1980). Antibiotic resistance patterns of coliform bacteria isolated from food. Scandinavian Journal of Infectious Diseases 12, 28994.[ISI][Medline]
4 . Österblad, M., Leistevuo, T. & Huovinen, P. (1995). Screening for antimicrobial resistance in fecal samples by the replica plating method. Journal of Clinical Microbiology 33, 31469.[Abstract]
5 . Österblad, M., Leistevuo, J., Leistevuo, T., Järvinen, H., Pyy, L., Tenovuo, J. et al. (1995). Antimicrobial and mercury resistance in aerobic Gram-negative bacilli in fecal flora among persons with and without dental amalgam fillings. Antimicrobial Agents and Chemotherapy 39 , 2499502.[Abstract]
6 . Platt, D. J., Chesham, J. S. & Kristinsson, K. G. (1986). R-plasmid transfer in vivo: a prospective study. Journal of Medical Microbiology 21, 32530.[Abstract]