The right answer is Bacillus.
The pH-sensitive green fluorescent dye conjugates (available for E.coli and Staphylococcus aureus), give an accurate and faster results than other phagocytosis assay.
These conjugates are non-fluorescent in the extracellular medium (at neutral pH), but fluoresce will be brightly green at acidic pH such as in phagosomes, which makes it an ideal tool for phagocytosis assay.