The haem pigment of the oral anaerobes Prevotella nigrescens and Prevotella intermedia is composed of iron(III) protoporphyrin IX in the monomeric form.

The haem pigment of Porphyromonas gingivalis is composed of μ-oxo bishaem, [Fe(III)PPIX]2O, but the nature of that generated by Prevotella species has not been established. Mossbauer, Raman and UV-visible spectrophotometry were used to characterize the haem pigment of Prevotella intermedia and Prevotella nigrescens. Mossbauer and Raman spectroscopy revealed the major haem species to be monomeric iron protoporphyrin IX, Fe(III)PPIX.OH (haematin). The terminal growth pH of both species on blood agar was between 5·8 and 6·0, which favours the formation and maintenance of monomeric Fe(III)PPIX.OH. Incubation of Pr. nigrescens and Pr. intermedia with oxyhaemoglobin at pH 6·5 resulted in formation of aquomethaemoglobin which was degraded to generate Fe(III)PPIX.OH which in turn became cell-associated, whilst incubation at pH 7·5 resulted in formation of [Fe(III)PPIX]2O. It is concluded that both Prevotella species degrade oxyhaemoglobin to form [Fe(III)PPIX]2O as an intermediate, which is converted to Fe(III)PPIX.OH through a depression in pH. The low pH encourages cell-surface deposition of insoluble Fe(III)PPIX.OH which would act as a barrier against oxygen and reactive oxygen species, and also protect against H2O2 through its inherent catalase activity.