## 3 Factors Governing Survival and Occurrence of Opportunistic Pathogens in Premise Plumbing Systems
### Legionella Pneumophila  
_Legionella_ is part of the bacterial genus that includes several well-known pathogenic species including _Legionella pneumophilia_. It was first isolated as the causative agent in an outbreak of severe respiratory illness following the 1976 American Legion convention in Philadelphia, PA [73, 74]. Around 16% of cases acquired from this convention resulted in fatality, and the illness henceforth became known as Legionnaires' disease [75]. At the time, it was hypothesised that the bacterium was harbored in cooling towers and air conditioning systems within proximity to the affected population. For decades, _Legionella_ and Legionnaires' disease were strongly associated with these origins. In more recent times, with developments in detection and analytical methods, premise plumbing has become recognised as another important source [13, 76]. Reported outbreaks of Legionnaires' diseases have been linked to water systems in hotels, cruise ships, industrial facilities, public buildings, and residences; however, the majority have occurred in hospitals, healthcare facilities, and nursing homes [77]. The growth and survival of _Legionella_ in premise plumbing is not the sole requisite for disease. Rather, the organism must penetrate the deep alveolar region of the lungs within a susceptible host [6, 78]. When airborne, virulent _Legionella_ are inhaled into this region; they infect and replicate within alveolar macrophages to cause disease. To become airborne, _Legionella_ must enter the bulk water and exit the system as bioaerosol. Aerosolisation is considered the primary mode of _Legionella_ transmission [79, 80]. Microscopic water droplets created at outlets can readily evaporate to yield only small infectious particles. These particles can travel great distances (up to 3 km under extreme circumstances), having complex dispersion patterns that are a function of many variables [76, 81]. [PERSON] et al. [6] suggests that Legionella-parasitised protozoa and Legionella-containing protozoan vacuoles may be similarly released and disseminated. It has been shown that changes in operation of the premise system can dislodge _Legionella_ colonised biofilms to increase the concentration of _Legionella_ in bulk water and dispersed aerosols by association [12, 82]. Aspiration and the instillation of contaminated water into the lung during respiratory tract manipulation also present possible routes for infection [6, 13]. Legionella can be introduced into premise plumbing systems in relatively small numbers carried via high-quality finished water, or in relatively large numbers via contamination from non-potable sources (e.g., backflow from fixtures, poorly installed or maintained cross-connections, etc.) and from disruptions in the supply water distributions system [6; 13]. Once introduced, premise plumbing provides conditions unique from main water distribution systems that stimulate colonisation. These conditions include high surface to volume ratios, excessive water age, water temperatures within optimal growth ranges, and inadequate or absent disinfect residuals [83; 84]. The role of biofilms, which develop on most surfaces in contact with non-sterile water, is considered fundamental for the chronic colonisation of premise plumbing by _Legionella_[85]. Biofilms containing amoebic host organisms facilitate replication of _Legionella_ by providing protection from chemical and heat treatments [86; 87]. Legionella can withstand a wide range of temperatures depending on system conditions and available nutrients. For example, [PERSON] et al. [88] documented survival between 20 and 50 \({}^{\circ}\)C in the presence of iron and L-cysteine. [PERSON] and [PERSON] [84] observed optimal growth with low levels of nutrients, low flow, and stagnant water between 32 and 42 \({}^{\circ}\)C. Tepid hot water systems resulting from improper operation in attempt to reduce energy consumption are particularly vulnerable to colonisation [6].