Legionella spp. development factors
The main condition that favors the development of Legionella spp. is the temperature. In fact, this bacterium is capable of proliferating within a wide temperature range (25-50 °C), so it is easy to be found in water systems. Below this range the bacterium survives but is not active and it is not able to proliferate, whereas above it the bacterium is killed.
On the contrary, the pH does not really influence its growth, in fact, Legionella spp. was isolated from pH 2.7 to pH 8.3.
Presence of Amoebas and other protozoa
Considering the fact that these are intracellular development bacteria, the presence of protozoa, in particular of amoebae, represents a determining factor for the growth and development of Legionella spp. Furthermore, they make it resistant to adverse environmental conditions, such as the presence of biocides. From a theoretical point of view, the amoebae should act as inhibitors of the development of Legionella spp., as they feed on bacteria. In reality, once ingested, Legionella spp. is able to release inhibitors that block their digestion done by the amoebae, this means they continue to proliferate within them until they "burst". This phase is the most critical one as it leads to a sudden increase in the circulating bacterial load that can cause the contraction of the disease by exposed people.
Presence of biofilm
Apart from representing an excellent growth medium, biofilm is able to protect Legionella from the disinfectant action of the drainage means and from physical factors. However, it is possible that even the biofilm can risk the breakage due to strong temperature changes, sudden turbulences or mechanical shocks, releasing large quantities of bacteria.
Presence of dead branches, stagnant conditions, sediments, iron oxides, plastic materials, wood, natural rubbers, rough surfaces (rusty, corroded, encrusted ...) etc.