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Journal of Clinical Microbiology, 12 1995, 3146-3149, Vol 33, No. 12
M Osterblad, T Leistevuo and P Huovinen
Replica plating can be used for the detection of antibiotic resistance in
normal flora. We have evaluated this application of the replica plating
method by comparing it with a five-colony method. The replica plating
method uses a single plate for each antibiotic, with a concentration just
above that for borderline resistance. By the five- colony method, five
colonies per sample were picked, chosen to represent all different colony
morphologies present, and MICs were determined by a standard agar dilution
method. The gram-negative, aerobic floras of 131 fecal samples were
screened for resistance to ampicillin, cefuroxime, nalidixic acid,
trimethoprim, sulfamethoxazole, and tetracycline by both methods. The rate
of resistance detection by the two methods did not differ statistically for
any of the antibiotics tested. The breakpoint concentrations used for the
replica plates in the study gave results similar to those produced by the
agar dilution method and the breakpoint values of the National Committee
for Clinical Laboratory Standards and can thus be recommended. As the only
currently used resistance detection method, replica plating facilitates an
exact determination of the percentage of resistant colonies/total number of
colonies (between 1 and 100%) in a sample. This revealed an uneven
distribution, with only 23% of the samples having resistance frequencies in
the range of 10 to 85%; usually, the resistant flora either was a small
minority or was very dominant in samples with resistance. This phenomenon
was present for all of the antibiotics.
Copyright © 1995 by the American Society for Microbiology. All rights reserved.
Screening for antimicrobial resistance in fecal samples by the replica plating method
Antimicrobial Research Laboratory, National Public Health Institute, Turku, Finland. Monica.Osterblad@utu.fi
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