Molecular characterization of the capsule locus from non-typeable Staphylococcus aureus

Area of Science:

• Microbiology
• Vaccinology
• Molecular Biology

Background:

• Staphylococcus aureus commonly expresses serotype 5 or 8 capsular polysaccharide (CP).
• A significant proportion of human (20-25%) and bovine (up to 86%) S. aureus isolates are non-typeable (NT), meaning they do not react with antibodies to common CP types.
• A vaccine targeting S. aureus CP would not protect against these NT strains.

Purpose of the Study:

• To investigate the molecular basis for the lack of serotype 5 or 8 capsule production in non-typeable Staphylococcus aureus isolates.
• To characterize NT S. aureus strains at the molecular level to understand their capsule deficiency.

Main Methods:

• Molecular characterization of non-typeable Staphylococcus aureus isolates.
• Analysis of the cap5(8) locus, its transcript, and associated genes (cap5A, RNAIII, arlR).
• Investigated IS257 insertion as a cause for locus replacement in bovine isolates.

Main Results:

• The cap5(8) locus was present in most NT isolates, but mutations within essential capsule genes were identified in strains expressing the cap5(8) transcript.
• Reduced cap5(8) transcript levels were associated with mutations in the cap5A promoter, decreased RNAIII levels, or a truncated arlR gene product.
• IS257 insertion replaced the cap5(8) locus in a substantial number of NT bovine isolates (13/21).

Conclusions:

• Multiple genetic mechanisms, including mutations in essential capsule genes and IS257-mediated locus replacement, explain the lack of capsule expression in non-typeable Staphylococcus aureus.
• The findings suggest that capsule serotypes beyond 1, 2, 5, and 8 do not exist in S. aureus.