*Legionella pneumophila* **and Protozoan Hosts: Implications for the Control of Hospital and Potable Water Systems**

#### **Muhammad Atif Nisar, Kirstin E. Ross, Melissa H. Brown, Richard Bentham and Harriet Whiley \***

College of Science and Engineering, Flinders University, GPO Box 2100, Adelaide 5001, Australia;Muhammadatif.Nisar@flinders.edu.au (M.A.N.); Kirstin.Ross@flinders.edu.au (K.E.R.); Melissa.Brown@flinders.edu.au (M.H.B.); Richard.Bentham@flinders.edu.au (R.B.)

**\*** Correspondence: Harriet.Whiley@flinders.edu.au; Tel.: +61-8-7221-8580

Received: 21 March 2020; Accepted: 14 April 2020; Published: 15 April 2020

**Abstract:** *Legionella pneumophila* is an opportunistic waterborne pathogen of public health concern. It is the causative agen<sup>t</sup> of Legionnaires' disease (LD) and Pontiac fever and is ubiquitous in manufactured water systems, where protozoan hosts and complex microbial communities provide protection from disinfection procedures. This review collates the literature describing interactions between *L. pneumophila* and protozoan hosts in hospital and municipal potable water distribution systems. The effectiveness of currently available water disinfection protocols to control *L. pneumophila* and its protozoan hosts is explored. The studies identified in this systematic literature review demonstrated the failure of common disinfection procedures to achieve long term elimination of *L. pneumophila* and protozoan hosts from potable water. It has been demonstrated that protozoan hosts facilitate the intracellular replication and packaging of viable *L. pneumophila* in infectious vesicles; whereas, cyst-forming protozoans provide protection from prolonged environmental stress. Disinfection procedures and protozoan hosts also facilitate biogenesis of viable but non-culturable (VBNC) *L. pneumophila* which have been shown to be highly resistant to many water disinfection protocols. In conclusion, a better understanding of *L. pneumophila*-protozoan interactions and the structure of complex microbial biofilms is required for the improved managemen<sup>t</sup> of *L. pneumophila* and the prevention of LD.

**Keywords:** *Legionella pneumophila*; protozoa; *Vermamoeba*; *Acanthamoeba*; potable water; hospital water; water disinfection; legionellosis
