Phenotypic detection and associated factors of AmpC beta-lactamase among clinical isolates of Escherichia coli, Klebsiella species, and Proteus species at the National Hospital of Sri Lanka

Introduction: Bacteria that express AmpC beta-lactamases are important in clinical practice as they are resistant to most beta-lactam antibiotics. This study aimed to determine the rate of positivity of AmpC beta-lactamase-producing Enterobacterales isolated from blood and sterile fluids at the National Hospital of Sri Lanka. Further, we compared AmpC phenotypic detection methods and evaluated AmpC and extended-spectrum beta-lactamase (ESBL) co-existence.  
Methods: A total of 141 samples of Escherichia coli, Proteus species, and Klebsiella species were tested for two screening tests and three confirmatory tests. The cefoxitin-cloxacillin double disc was used as the phenotypic reference method. For ESBL detection, the Clinical & Laboratory Standards Institute guidelines (CLSI) were followed. 
Results: Out of 141 isolates, 34 (24.1%) were AmpC producers. AmpC test was positive in 26.3% of blood culture isolates (30/114) and 14.8% of other samples (4/27). AmpC-ESBL co-existence was observed in 20.6% (29/141) of total isolates.  
All 34 AmpC-positive isolates were cefoxitin screening positive, while 31 were positive for cefotetan screening. Both screening methods produced five false positives. Among the 34 confirmed AmpC producers, all were positive with the cefoxitin–boronic acid test, 31 with the cefotetan–boronic acid test, and 31 for the EDTA test, with respective false positives of one, one, and none.  
Conclusion: A relatively high prevalence of AmpC beta-lactamase production was observed in this clinical setting. AmpC and ESBL co-existed in approximately one-fifth of total isolates. Cefoxitin is a superior screening method, while the EDTA test demonstrated good confirmatory performance in addition to the cefoxitin-cloxacillin double disc test.