AmpC Disk Test Controls Coming Soon
MicroBioLogics® will soon have available the positive controls for the AmpC Test. The AmpC Disk Test is used for detecting AmpC beta-lactamases. AmpC β-lactamases are found on a chromosome of several gram negative bacteria such as Enterobacter cloacae and Serratia marcescens. They may also be found on transmissible plasmids in bacterial species that lack an inducible chromosomal enzyme such as E. coli, Klebsiella ssp., Salmonella ssp. and Proteus mirabilis. Plasmid-mediated AmpC β-lactamases can be harder to detect. They may give false susceptible results when tested with cephalosporins.
Below is a brief outline of the AmpC disk test as described in the Journal of Clinical Microbiology article, "AmpC Disk Test for Detection of Plasmid-Mediated AmpC β-lactamases in Enterobacteriaceae Lacking Chromosomal AmpC β-lactamases":
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Make a suspension of E. coli, ATCC® 25922™*, equivalent to a 0.5 McFarland standard.
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Inoculate a Mueller Hinton plate with the suspension.
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Inoculate a Tris-EDTA disk with several colonies of the test microorganism. The Tris-EDTA is able to permeabilize bacterial cells and release β-lactamases into the environment.
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Place a cefoxitin disk close to the Tris-EDTA disk.
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Incubate Mueller Hinton
plate at 35°C.
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Examine Mueller Hinton plate
after incubation. A positive test will show indentation or flattening of the zone of inhibition, indicating enzymatic inactivation of cefoxitin.
A negative test will show no distortion of the zone of inhibition.
The controls needed for the test are listed below:
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E. cloacae, ATCC® BAA-1143™*, MicroBioLogics® #1018, the strong positive control, produces a high level of chromosomal AmpC β-lactamase.
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K. pneumoniae, ATCC® BAA-1144™*, MicroBioLogics® #1019, the weakly positive control, produces DHA-1, an inducible plasmid-mediated AmpC.
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E. coli, ATCC® 25922™*, MicroBioLogics® #0335, is the negative control.
A picture of the AmpC disk
diffusion test is shown below:
References
Black, J. A., Moland, E.S. and Thomson, K.S. 2005. AmpC Disk Test for Detection of Plasmid-Mediated AmpC β-lactamases in Enterobacteriaceae Lacking Chromosomal AmpC β-lactamases. Journal of Clinical Microbiology
43:3110-3112
Murray, P.R., Baron, E.J., Jorgensen, J.H., Landry, M.L., Pfaller, M.A. 2007. Manual of Clinical Microbiology. 1182
Jacoby, G.A., AmpC β-Lactamases. 2009. Clinical Microbiology Reviews, January 2009, p. 161-182, Vol. 22, No. 1.
 *The
ATCC Licensed Derivative Emblem, the ATCC Licensed
Derivative word mark and the ATCC catalog marks are
trademarks of ATCC. MicroBioLogics, Inc. is licensed to
use these trademarks and to sell products derived from
ATCC® cultures.
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