AmpC Type beta-lactamase

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Bacteriology • Antimicrobial Resistance • Digital Learning Chapter

AmpC β-Lactamases

AmpC β-lactamases: mechanism, chromosomal and plasmid-mediated forms, detection, phenotypic tests and molecular identification.

AmpC β-lactamaseAMRLaboratory IdentificationM4S Student Notes

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Contents Overview Location Chromosomal & plasmid-mediated Identification Phenotypic tests Boronic acid disk test References

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Overview

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AmpC type β-lactamases are such type of beta-lactamases which are not inhibited by clavulanic acid but are inhibited by boronic acid. These are clinically important cephalosporinases encoded on the chromosomes of many Enterobacteriaceae and a few other organisms. In such organisms they mediate resistance to cephalothin, cefazolin, cefoxitin, most penicillins, and β-lactamase inhibitor/β-lactam combinations.

The plasmid-borne AmpC beta-lactamases were found in bacteria such as Escherichia coli and Klebsiella spp. in the late 1980s [1,2]. The source also states that the first bacterial enzyme reported to destroy penicillin was the AmpC β-lactamase of Escherichia coli, dating back to 1940, before the specific name AmpC β-lactamase was used. Systematic study of the genetics of penicillin resistance in E. coli began in 1965.

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Cellular Location

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With the exception of the AmpC β-lactamase of Psychrobacter immobilis, which is secreted mainly into the external medium, the majority of the enzymes are located in the bacterial periplasm [3].

DNA

Chromosomal and Plasmid-Mediated AmpC

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AmpC type β-lactamases can be mediated either by chromosome or plasmid. The plasmid mediated AmpC β-lactamases can hydrolyze all β-lactam except cefepime and carbapenem, according to the supplied source. These genes are derived from inducible chromosomal genes being mobilized in various organisms.

Commonly reported genotypes in the source
ACC, FOX, MOX, DHA, CMY, CIT, EBC

Source note: The statements in this chapter preserve the claims made in the supplied source. No additional susceptibility interpretation has been added.

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Identification and Resistance Considerations

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The identification technique of AmpC β-lactamase-producing isolates is still evolving and is not yet optimized for the clinical laboratory, according to the source. The source notes that this has contributed to underestimation of these resistance mechanisms.

Carbapenems can usually be used to treat infections due to AmpC-producing bacteria, but carbapenem resistance can arise in some organisms by mutations. These mutations either reduce influx through outer membrane porin loss or enhance efflux through efflux pump activation.

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Phenotypic Detection Tests

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The supplied source lists the following phenotypic procedures:

Phenotypic test	Purpose / source description
Three-dimensional extract test	Listed as a routine phenotypic testing procedure for AmpC detection.
AmpC disk test	Listed as a routine phenotypic testing procedure for AmpC detection.
Boronic acid disk test	Listed as a routine phenotypic testing procedure; the source illustrates a positive result.
Disk approximation test	Listed as a routine phenotypic testing procedure for AmpC detection.

These phenotypic tests do not differentiate between chromosomal and plasmid-mediated AmpC β-lactamases. Hence, according to the source, plasmid-mediated AmpC β-lactamases are mostly detected with multiplex AmpC PCR involving specific primers for the genes that induce the resistance.

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Boronic Acid Disk Test — Figure 1

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Fig. 1: Representation of positive boronic acid disk test. Left: 30 μg cefoxitin disk supplemented with 300 μg phenyl boronic acid. Right: 30 μg cefoxitin disk alone. The supplied source states that an organism demonstrating a defined increase (≥5-mm) in zone diameter around the antibiotic disk with added boronic acid is considered an AmpC producer.

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References

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1. Phillppon A, Arlet G, Jacoby G. Plasmid-determined AmpC-type beta-lactamases. Antimicrob Agents Chemother. 2002 Jan;46(1):1-11.

2. Poirel L, Pitout JD, Nordmann P. Carbapenemases: molecular diversity and clinical consequences. Future Microbiol. 2007 Oct;2(5):501-12.

3. Feller G, Zekhnini Z, Lamotte-Brasseur J, Gerday C. Enzymes from cold-adapted microorganisms. The class C β-lactamase from the antarctic psychrophile Psychrobacter immobilis A5. Eur J Biochem. 1997;244:186–191.

4. Jacoby GA. AmpC β-lactamase. Clinical Microbiology Reviews. ASM. 2009;22(1):161-182.

5. Hanson ND. AmpC beta-lactamases: What do we need to know for the future? J Antimicrob Chemother. 2003;52:2–4.

6. Pérez-Pérez FJ, Hanson ND. Detection of plasmid-mediated AmpC beta-lactamase genes in clinical isolates by using multiplex PCR. J Clin Microbiol. 2002;40:2153–62.

7. Papanicolaou GA, Medeiros AA, Jacoby GA. Novel plasmid-mediated beta-lactamase (MIR-1) conferring resistance to oxyimino- and alpha-methoxy beta-lactams in clinical isolates of Klebsiella pneumoniae. Antimicrob Agents Chemother. 1990;34:2200–.

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AmpC β-Lactamases • Digital Learning Chapter
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