Emergence of pathogenic and multiple-antibiotic-resistant Macrococcus caseolyticus in commercial broiler chickens

Area of Science:

• Microbiology
• Genomics
• Veterinary Pathology

Background:

• Macrococcus caseolyticus is traditionally considered non-pathogenic.
• A novel pathogenic strain (SDLY) has emerged, causing significant mortality in broiler chickens.
• This emergence necessitates understanding the genetic and pathological basis of its pathogenicity.

Purpose of the Study:

• To characterize the pathogenic M. caseolyticus SDLY strain.
• To compare its genome with a non-pathogenic strain.
• To investigate the genetic factors contributing to pathogenicity and antibiotic resistance.

Main Methods:

• Whole-genome sequencing and comparative genomics of the SDLY strain.
• Identification of mutations, insertions, deletions, and antibiotic resistance genes.
• Pathogenicity testing in SPF chickens and mice.
• Electron microscopy for capsule visualization.
• Antimicrobial susceptibility testing.

Main Results:

• The SDLY strain genome contains numerous mutations, antibiotic resistance genes, and virulence gene variations compared to the non-pathogenic JCSC5402 strain.
• A specific gene cluster encoding capsular polysaccharide synthesis was identified, associated with capsule formation and pathogenicity.
• Electron microscopy confirmed the presence of a distinct capsule in the SDLY strain.
• Pathogenicity tests demonstrated significant clinical signs and pathological changes in chickens and mice, mirroring field cases.
• The SDLY strain exhibited multi-drug resistance.

Conclusions:

• The emergence of pathogenic M. caseolyticus (SDLY strain) represents a significant threat to poultry health.
• Genomic analysis revealed key virulence factors, including a capsular polysaccharide synthesis gene cluster.
• The multi-drug resistant nature of this pathogen warrants increased attention for poultry and public health surveillance.