For quantification of the siderophore coprogen, the complete 50 ml culture supernatants were filtered and lyophilised to dryness.
The siderophore quantification of S. aureus-culture-supernatants is based on Chrome Azurol Sulphonate (CAS assay) according to Schwyn and Neilands (1987).
The orange halo formation is a reflection of siderophore release into the medium as a consequence of the siderophore action.
As siderophores can be taken up by any cell with the cognate receptor, regardless of its own levels of siderophore production, they represent a public good: while individually metabolically costly to produce, siderophores confer a group-level benefit [22], [26].
This xenosiderophore scavenging will thus allow the acquisition of siderophore-bound iron without the energy cost of siderophore production [ 35].
This makes siderophore secretion open to invasion by non-producing 'cheats', who pay none of the costs of siderophore production [ 2, 3], but can still use siderophores produced by nearby cells.
A crucial feature of siderophore production is that it is a form of cooperation: siderophores potentially benefit all bacteria within the locality that are capable of taking up the siderophore, but are metabolically costly to the producer [ 8].
Pyoverdine knockout mutants WCS358-PVD− and WCS417-PVD− are completely abolished in siderophore activity in the universal siderophore assay on CAS agar medium (Fig. 5) [ 73– 75], indicating that both strains produce only one type of siderophore.
Inactivation of these pumps results in decreased efflux of the fully synthesized siderophore, but increased extrusion of siderophore precursor products [ 14, 15].
This makes siderophore production open to invasion by non-producing 'cheats', who pay none of the costs of siderophore production, but can still take up siderophores produced by nearby cells [ 8, 9, 17].
