Serratia marcescens Isolates from Bovine Mastitic Milk: Antimicrobial Resistance and Virulence Features

Background: Bovine mastitis (BM) is a major disease affecting dairy herds (DHs), with Serratia marcescens (S. marcescens) being increasingly implicated as a causative agent. The growing concern over antimicrobial resistance (AMR) extends to BM-associated S. marcescens isolates, where resistance patterns are emerging. Methods: Here, four BM Gram-negative isolates were investigated: 1-DH1, 2-DH1, 3-DH2, and 4-DH3. Phenotypic characterization was performed using the Neg-Urine-Combo98 panel on a MicroScan WalkAway Plus system. Whole-genome sequencing (WGS) was performed to characterize and identify AMR and virulence factors (VF) genes and plasmids in isolates 1-DH1, 3-DH2, and 4-DH3, and phylogenomic analyses were conducted for a visual comparison of the genomes. Results: Phenotypically, isolates 1-DH1, 2-DH1, and 4-DH3 were identified as S. marcescens, and 3-DH2 as Serratia odorifera (confirmed as S. marcescens by WGS). A 28.00% (n = 25) prevalence of phenotypic AMR for isolates 1-DH1, 2-DH1, and 4-DH3 against Aug-E, AM, To, Cfx, Crm, Cl, and Fd was shown, and 24.00% (n = 25) for isolate 3-DH2 against Aug-E, AM, To, Crm, Cl, and Fd. The AMR genes AAC(6')-Ic, aac(6')-Ic_1, aac(6')-Ial, H-NS, SRT-2, oqxB, oqxB_1, oqxB25, mexI, CRP, and blaSST-1, and flgH, fliP, fliM, and fliG VF genes were identified in the whole genome of the S. marcescens sequenced isolates 1-DH1, 2-DH1, and 4-DH3. In addition, a phylogenomic analysis of these three isolates revealed that WGS genomes are more closely related to S. marcescens prevenient from environmental sources. Conclusions: This study reports, for the first time, AMR resistance to tobramycin, cefuroxime, colistin, and nitrofurantoin in BM S. marcescens isolates. Genomic analysis revealed the presence of multiple AMR and VF genes, further highlighting the pathogenic potential of these isolates. Phylogenomic analysis revealed that the genome of the three BM S. marcescens isolates is more closely related to environmental S. marcescens strains.