Growth of ‘Difficult-to-Culture’ Prevotella Strain is Stimulated by Specific Siderophores

Objectives: Approximately half of oral bacteria are as-yet-uncultivated in isolation and may depend on neighbouring bacteria for growth. Although many bacteria produce iron-chelating siderophores for solubilisation and uptake of iron, an essential metabolite, the ability to synthesise these factors may have been lost in ‘unculturable’ bacteria. This study aimed to investigate the ability of six siderophore-like/iron-chelating agents to stimulate the growth of Prevotella HOT376 strain E7_34, which is dependent on co-culture for growth.
Methods: Fastidious Anaerobe Agar plates were inoculated with a lawn of Prevotella E7_34 and test agents were placed in a central well in each plate. Plates were incubated anaerobically, after which the diameter of any zone of growth was recorded. Siderophores desferricoprogen, pyoverdines-Fe-complex and ferrichrome-Fe-free; siderophore-like molecules 2,3-dihydroxybenzoic acid and salicylic acid; and iron chelator ferric citrate were added to the wells in 1.5, 15 or 150 μg amounts. The positive control was filtered supernatant of a 72-hr broth culture of F. nucleatum and negative controls were the agent diluents.
Results: The positive control stimulated growth with a mean growth zone radius of 13.4 mm at eight days. Pyoverdines-Fe-complex at 0.1 mg/ml stimulated growth more strongly than the positive control, with a zone of stimulation of 22.7 mm. 2,3-dihydroxybenzoic acid exhibited no growth stimulation. The other test agents enhanced the growth of E7_34 compared to the negative control, but the stimulation zone was incomplete and could not be reliably measured. The growth stimulation was ranked relative to positive (++++) and negative controls (-) as follows: ferric citrate ++, desferricoprogen +, ferrichrome-Fe-free +, salicylic acid +.
Conclusions: A range of compounds with siderophore activity stimulated the growth of the difficult-to-culture strain Prevotella E7_34. Pyoverdines-Fe-complex, in particular, has potential for use as a growth-enhancing supplement for bacterial culture media in the characterisation of the oral microbiome.