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Encapsulation of Staphylococcus aureus isolated from bovine milk
Abstract:
Evidence is presented that nearly all Staphylococcus aureus infections of the bovine mammary gland are by encapsulated organisms (94% of isolates examined). This observation is based on the demonstration of diffuse growth in serum-soft agar, of cultures taken directly from mastitic milk without subculture on artificial media. Only milks containing pure cultures of S. aureus were examined. It was previously reported that only 6% had capsules. Evidence is presented that as few as 3 or 4 subcultures and/or a short storage on artificial media results in loss of encapsulation of most S. aureus of bovine origin. Antisera raised against encapsulate strains inhibit the expression of capsule formation on bacteria that are encapsulated in the presence of normal serum.
Insights
Nearly all Staphylococcus aureus bovine mastitis infections involve encapsulated bacteria. Subculturing or storage causes rapid loss of this capsule, impacting diagnostic accuracy for Staphylococcus aureus.
Area of Science:
- Veterinary Microbiology
- Bacterial Pathogenesis
- Immunology
Background:
- Staphylococcus aureus is a primary cause of bovine mastitis.
- Capsular expression in Staphylococcus aureus is critical for virulence and immune evasion.
- Previous studies indicated low capsular prevalence in Staphylococcus aureus isolates.
Purpose of the Study:
- To investigate the prevalence of encapsulated Staphylococcus aureus in bovine mastitis.
- To determine the impact of laboratory procedures on bacterial encapsulation.
- To explore the role of antisera in inhibiting capsule formation.
Main Methods:
- Direct examination of S. aureus from mastitic milk using serum-soft agar.
- Culturing and storage of isolates on artificial media.
- Testing antisera against encapsulated strains for inhibition of capsule formation.
Main Results:
- 94% of S. aureus isolates from bovine mastitis were encapsulated when examined directly from milk.
- As few as 3-4 subcultures or short storage led to significant loss of encapsulation.
- Antisera against encapsulated strains inhibited capsule expression in the presence of normal serum.
Conclusions:
- Encapsulation is a near-universal characteristic of S. aureus in bovine mastitis infections.
- Standard laboratory techniques can rapidly lead to loss of capsule expression, potentially masking true prevalence.
- Capsule formation is an active process that can be modulated and targeted.