Microbiological Tests Detect AmpC β-Lactamase Producing Bacteria

A modification of existing tests allows the detection of plasmid-mediated AmpC-producing Escherichia coli and Klebsiella pneumoniae that are not readily identified in clinical laboratories.

Antibiotic susceptibility by standard microbiological procedures can be carried out on automatic platforms and further tested by the modified Hodge test (MHT) to ascertain the presence or absence Gram-negative bacteria producing AmpC ß-lactamase.

Scientists at the University of Florida (Gainesville, FL, USA), tested between January 2007 and June 2008, 753 gram-negative bloodstream isolates for AmpC by using the ethylenediaminetetraacetic acid (EDTA) synergy disk test and the MHT on 172 samples and the MHT alone on the other 581 isolates. All isolates were identified and tested for antibiotic susceptibility by standard microbiological procedures using MicroScan (Siemens HealthCare, Deerfield Park, IL, USA).

The 26 patients of the AmpC group were selected on the basis on the degree of AmpC positivity on the MHT, with a growth of the indicator strain along the test isolate streak of 3 mm or more. In the control group, there were had 52 isolates that had no or minimal distortion of the cefoxitin zone and therefore were considered negative. These non-AmpC producers were matched in a 2:1 ratio by bacterial species. Mortality within 30 days was reported in 9% of the AmpC group and in 6% for the control group. Patients in the AmpC group were significantly more likely to be in an intensive care unit and to have been intubated than patients in the control group were at the time of the positive blood culture. Moreover, patients in the AmpC group were significantly less likely to have received an effective antibiotic within the first 48 hours after the blood culture was obtained than were patients in the control group.

The authors concluded that there was a very good correlation between the EDTA disk test and the MHT in a subset of their isolates, although the correlation was not perfect and the EDTA disk test identified more positives. The MHT and the EDTA disk test did not seem to identify a subset of patients at risk for a poor outcome from AmpC with the antibiotic use patterns in their hospital. The study was published in April 2011 in the American Journal of Clinical Pathology.

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University of Florida
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