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

<b>Background</b>: Bovine mastitis (BM) is a major disease affecting dairy herds (DHs), with <i>Serratia marcescens</i> (<i>S. marcescens</i>) being increasingly implicated as a causative agent. The growing concern over antimicrobial resistance (AMR) extends to BM-associated <i>S. marcescens</i> isolates, where resistance patterns are emerging. <b>Methods</b>: 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. <b>Results</b>: Phenotypically, isolates 1-DH1, 2-DH1, and 4-DH3 were identified as <i>S. marcescens</i>, and 3-DH2 as <i>Serratia odorifera</i> (confirmed as <i>S. marcescens</i> 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 <i>AAC(6')-Ic</i>, <i>aac(6')-Ic_1</i>, <i>aac(6')-Ial</i>, <i>H-NS</i>, <i>SRT-2</i>, <i>oqxB</i>, <i>oqxB_1</i>, <i>oqxB25</i>, <i>mexI</i>, <i>CRP,</i> and <i>blaSST-1</i>, and <i>flgH</i>, <i>fliP</i>, <i>fliM,</i> and <i>fliG</i> VF genes were identified in the whole genome of the <i>S. marcescens</i> 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 <i>S. marcescens</i> prevenient from environmental sources. <b>Conclusions</b>: This study reports, for the first time, AMR resistance to tobramycin, cefuroxime, colistin, and nitrofurantoin in BM <i>S. marcescens</i> 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 <i>S. marcescens</i> isolates is more closely related to environmental <i>S. marcescens</i> strains.