Abstract

Background: Antimicrobial resistance mediated by ESBL-producing Klebsiella pneumoniae is an emerging concern in both human and veterinary medicine, with companion animals increasingly considered relevant within the One Health framework. This study aimed to investigate the fecal carriage of ESBL-producing K. pneumoniae complex in cats from Italy and to characterize the strains by the phenotypic and genetic profile of ESBL production, virulent pathotypes, antibiotic resistance profile and biofilm production. Methods: Fecal samples collected from cats admitted to the Veterinary Teaching Hospital of Milan (Italy) in 2020–2026 were bacteriologically and genetically analyzed. Results: All the Klebsiella pneumoniae strains isolated [4/200 (2%, 95% CI: 0.06–3.94%)] were ESBL-producing K. pneumoniae complex isolates harboring blaCTX-M-15, blaSHV, and blaTEM genes. The isolates were detected with higher presence in cats with diarrhea and were found only in cats treated with antibiotics and hospitalized. All four ESBL-producing isolates were classified as the classical K. pneumoniae pathotype based on the negative string test results, the absence of reliable virulence genes used for pathotype identification (peg-344, iucA, rmpA and rmpA2), and the lack of K1 and K2 serotypes, despite the detection of terB and irp2 virulence genes in one and two isolates, respectively. All four ESBL-producing K. pneumoniae complexes were classified as multidrug-resistant, with resistance mainly observed to β-lactams, fluoroquinolones, quinolones and folate antagonists. All four ESBL-producing K. pneumoniae complexes demonstrated biofilm-forming abilities, with two isolates showing weak adhesion, one moderate adhesion, and one strong adhesion. Conclusions: The detection of ESBL genes together with the MDR pattern, biofilm-forming capacity and selected virulence determinants suggests the potential epidemiological relevance of cats in the dissemination of antimicrobial-resistant K. pneumoniae complexes, underscoring the need for strengthened surveillance and prevention strategies in veterinary settings to provide information to pet cat owners and children who may interact with stray cats, in full implementation of the One Health approach.

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