The large orange bit is an amoeba and it's using a false foot called a pseudopod to reel in a bacterium, green bit. The bacterium is Legionella pneumophila and is the cause of Legionnaires' Disease but instead of being digested by the amoeba, it stays safely inside this 'trojan horse'.
Scientists began using the term about 15 years ago to describe Legionella pneumophila, the bacteria eventually traced as the cause of the mysterious disease that broke out among people attending an American Legion convention in Philadelphia in 1976. It sickened 221 people and killed 34.
Epidemiologists traced the cause back to water in air-conditioning tanks on the roof of the building where the Legionnaires were meeting, but they still had trouble finding Legionella within that water. It turns out that high concentrations of Legionella were living within the protists that lived in the tanks.
Legionella were reproducing within these protists and coming out in small aggregates that became airborne in water droplets. Susceptible people breathed in high concentrations of Legionella that had just come out of their hosts and were looking for new hosts.
The image is from the Centres for Disease Control and Prevention and Dr. Barry S. Fields.