Ultra-structure and localisation of formazan formed by human neutrophils and amoebae phagocytosing virulent and avirulent Legionella pneumophila

Abstract Legionella pneumophila (LP) strains of differing virulence were incubated with a solution of nitroblue-tetrazolium (NBT) at a concentration of 1 mg · ml⁻ in the presence of Acanthamoeba polyphaga or human polymorphonuclear neutrophils (PMN). Reduction of NBT to formazan occurred at a faster rate in the presence of virulent strains. Reduction appeared to be temperature dependent; at 37°C the reaction rate was higher than at 20°C. On microscopic examination, deposits of formazan around Legionella cells were observed inside amoebae similar to those deposited in human neutrophils. Electron microscopy revealed electron-dense particles surrounding virulent legionellae, which appeared to be associated with formazan foundation. Formazan formation inside amoebae may suggest the presence of a respiratory burst against LP, which is more intense with virulent strains.