Legionella
Thanks for taking the time to clarifying your position re the occurrence of Legionella, Ladiesman. That does indeed make me feel special! Per your request, here is my understanding regarding the topic.
It is important to recognize that any water distribution system where the temperature ranges between 70 to 120F is a potential source for Legionella regardless of the type of water heater used. In warmer climates, it is not uncommon for the temperature of the cold water distribution system to be within the growth range. Piping materials used, the proximity of the distribution piping to heat sources, dead ends, etc are also variables to be considered.
Even at temperatures in excess of 130F, Legionella can survive 5 to 6 hours. Instantaneous kill is not achieved until 160F.
In the Canadian study sited earlier, the temperature in the bottom of the electric water heaters where Legionella was observed was typically between 85F and 105F. No reason was given for maintaining the temperature that low or if the heaters in question where single element. The recommendation was to raise the bottom temp in those heaters 140F.
I should note that in the preceding I have rounded the temperatures given for ease of discussion and that the actual temperatures would be slightly different.
Of course, the preceding discussion completely ignores the effect of chlorine on different types of bacteria, including Legionella. Many variables come into play, such as pH, temperature, residence time, and the amount of chlorine present. I could address that as well if anyone is truly interested.
The point of all of this is that the use of a tankless system does not preclude the presence or growth of Legionella. Your statement that "no water storage in a tankless means no water is stored at the optimum temperature needed to create a breeding ground for bacteria either" is false. I hope I have helped both you and other readers of this forum understand some of the reasons why.
That's all the leisure I've got for today!