Phenotypic detection of presumptive AmpC β-lactamase production among cefoxitin screen-positive gram-negative bacilli in a tertiary care teaching hospital

Vijay Prakash Singh; Anju Rani; Varuna Gupta

doi:10.18203/2320-6012.ijrms20263543

Authors

Vijay Prakash Singh Department of Microbiology, Autonomous State Medical College (ASMC), Gonda, Uttar Pradesh, India
Anju Rani Department of Microbiology, K.D. Medical College, Mathura, Uttar Pradesh, India
Varuna Gupta Department of Microbiology, K.D. Medical College, Mathura, Uttar Pradesh, India

DOI:

https://doi.org/10.18203/2320-6012.ijrms20263543

Keywords:

AmpC β-lactamase, Cefoxitin screening, Gram-negative bacilli, Phenotypic detection, Antimicrobial resistance, CC-DDST

Abstract

Background: AmpC β-lactamase-mediated resistance among Gram-negative bacilli is an increasingly important therapeutic challenge because it reduces the effectiveness of several β-lactam antibiotics and limits treatment options. In resource-limited laboratories, reliable phenotypic methods remain essential for the routine evaluation of cefoxitin screen-positive isolates for presumptive AmpC β-lactamase production.

Methods: This prospective descriptive observational study was conducted at a tertiary care teaching hospital in Western Uttar Pradesh, India, from January to December 2025. A total of 2,860 clinical specimens yielded 1,486 clinically significant, non-duplicate Gram-negative isolates. Cefoxitin screen-positive isolates (inhibition zone diameter ≤18 mm) were evaluated using the Cefoxitin–Cloxacillin Double Disk Synergy Test (CC-DDST), Phenylboronic Acid Disc Test (PBA), and Disk Approximation Test (DAT) in accordance with CLSI M100, 35th edition (2025). Frequencies, percentages, and 95% confidence intervals were calculated.

Results: Among the 1,486 Gram-negative isolates, 626 (42.1%; 95% CI: 39.6–44.6%) were cefoxitin screen-positive. Escherichia coli (40.6%) and Klebsiella pneumoniae (28.6%) were the predominant cefoxitin screen-positive isolates. CC-DDST identified the highest proportion of presumptive AmpC β-lactamase-positive isolates (303/626; 48.4%; 95% CI: 44.5–52.3%), followed by PBA (285/626; 45.5%; 95% CI: 41.6–49.4%) and DAT (206/626; 32.9%; 95% CI: 29.3–36.7%).

Conclusions: A considerable proportion of Gram-negative isolates were cefoxitin screen-positive. Among the evaluated phenotypic methods, CC-DDST showed the highest positivity rate for presumptive AmpC β-lactamase production and appears to be a practical and cost-effective method for routine evaluation of cefoxitin screen-positive Gram-negative isolates in laboratories where molecular diagnostic facilities are unavailable.

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