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Introduction  and Clinical Significance of AmpC

AmpC β-lactamases are enzymes of gram -negative bacteria like many Enterobacteriaceae and a few other bacteria. It is clinically important cephalosporinases encoded on the chromosomes of these bacteria  where they mediate resistance to cephalothin,  cefoxitin, cefazolin, most penicillins, and β-lactamase inhibitor or β-lactam combinations. The increasing antibiotic resistance is a notable example of how bacteria can procure, maintain and express new genetic information that can confer resistance to one or several antibiotics. Detection of bacteria producing these enzymes can be difficult, because their presence does not always produce a resistant phenotype on conventional disc diffusion or automated antibiotics  susceptibility testing methods. The false susceptibility to AmpC β-lactamases  laboratory report  is potentially fatal . These enzymes show tremendous variation in geographic distribution and therefore , their accurate detection and characterization are important for  epidemiological as well as  clinical, laboratory, and infection control point of view.

Test for AmpC Production in Gram- Negative Isolates

Screening for AmpC β-lactamase production was performed by Cefoxitin disk test. Isolates that yielded a zone diameter less than 18 mm (Positive in screening) were further subjected to confirmatory testing. These positive on Screen isolates were swabbed on to a MHA plate. Cefoxitin (30ug) and Cefotaxime (30ug) disks were placed at 20 mm distance between the disks. Blunting of zone of inhibition around Cephotaxime disk at the side of Cefoxitin disk after overnight incubation at 37°C were considered as positive for AmpC production by the isolates.

 In brief,

Ampc detection method (Phenotypic)

Screening method

Zone of inhibition (ZOI) of Cefoxitin (30 µg)  should be less than 18 mm.

Confirmatory method

Put cefoxitin (30 µg) and combination of cefoxitin and phenylboronic acid(PBA) 10  µg disks  with  distance 20 mm from  disk to disk on Muller-Hinton agar as shown above picture.

After over night incubation, ZOI of combined disk( cefoxitin+PBA) should be equal and greater than 5 mm that of cefoxitin.

The organisms (Gram negative bacteria ) is AmpC producer as shown above image.

Betalactamase detection|ESBL| MBL| KPC |AmpC |Oxacillinase|MRSA|D Zone test positive| iMLSB strain as shown below-

References

1. Conejo MC, García I, Martínez-Martínez L, Picabea L, Pascual Á. Zinc eluted from siliconized latex urinary catheters decreases OprD expression, causing carbapenem resistance in Pseudomonas aeruginosa. Antimicrobial agents and chemotherapy. 2003;47(7):2313-5
2. Al-Bayssari C, Dabboussi F, Hamze M, Rolain J-M. Detection of expanded-spectrum β-lactamases in Gram-negative bacteria in the 21st century. Expert review of anti-infective therapy. 2015;13(9):1139-58.
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3969634/
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1169113/
5. https://cmr.asm.org/content/22/1/161