Hartmannella amoeba and bacterium. Computer artwork of a Hartmannella vermiformis amoeba (green) engulfing a Legionella pneumophila bacterium (orange) with a pseudopodium, a foot- like extension of its cytoplasm. H. vermiformis is a single- celled organism that is found in soil and fresh- water and feeds on bacteria. L. pneumophila is the causative agent of Legionnaire's disease, a respiratory disease. It is able to resist digestion and replicate inside the amoeba. The passage through the amoeba makes the bacterium more virulent when it is inhaled by a human.