## 3 Factors Governing Survival and Occurrence of Opportunistic Pathogens in Premise Plumbing Systems
### Pseudomonas aeruginosa  
_Pseudomonas aeruginosa_ is a naturally occurring bacterium in soil and water; however, it has also been isolated from drinking water distribution systems, antimicrobial soaps, and disinfectants [117; 118; 119; 120; 121]. Although rarely carried by healthy individuals, _Pseudomonas aeruginosa_ has been recovered in up to 60% of hospitalised patients and is therefore considered an opportunistic pathogen [122]. _Pseudomonas aeruginosa_ has been identified as the cause of hospital- and community-acquired cases of life-threatening pneumonia. Community-acquired infections, within a relatively healthy and normal population, are more likely linked with recreation water (e.g., contaminated swimming pools, hot tubs, and whirlpools) and characterised by relatively minor eye, ear, and skin conditions.  
Tap water and premise plumbing have been established as a source of _Pseudomonas aeruginosa_ disease, particularly in healthcare settings [123; 124]. _Pseudomonas aeruginosa_ strains present within tap water samples from sinks within intensive care units have been indistinguishable by molecular typing to strains identified within infected patients [125]. In addition, _Pseudomonas aeruginosa_ has been found to colonise faucets for more than two years despite its absence from the mains supplying the sinks. Several studies have demonstrated that the pathogen is transmissible via direct contact and/or indirect contact. Direct contact may occur through bathing or the ingestion of contaminated water. Indirect contact may occur through contamination of a device or fomite, inhalation of aerosols dispersed from contaminated water sources, and aspiration of contaminated water or aerosols [126; 127; 128; 13].  
The minimal nutritional requirements of _Pseudomonas aeruginosa_ enable its survival in a range of natural and artificial environments. It persists in distilled or deionised waters, as well as aquatic environments of moderate salinity and even in high nutrient environments [129; 130]. The growth of _Pseudomonas aeruginosa_ is dependent on system conditions including temperature, pH, and oxygen conditions system. Although able to survive a wide range of temperatures (from 4 to 42 \({}^{\circ}\)C) common in premise plumbing, virulence decreases below 30 \({}^{\circ}\)C. Optimal growth in rich medium and suspended form is between 30 and 37 \({}^{\circ}\)C [131]. _Pseudomonas aeruginosa_ can grow well in microaerobic (i.e., not completely anaerobic) conditions expected in stagnant waters associated with low flows in premise plumbing. Growth also becomes significantly limited under acidic conditions.  
_Pseudomonas aeruginosa_ displays relative resistance against common disinfectants used in water treatment [130; 132]. In hospital and care facility settings, infections have been linked to the pathogen's presence in solutions used for surface sterilisation, bronchoscopes washed with non-sterile water or disinfectant solutions, and in-dwelling catheters [133].  
The persistence and growth of _Pseudomonas aeruginosa_ in engineered distribution systems and premise plumbing is largely credited for its habitation in biofilms [134; 135]. Through this mechanism, _Pseudomonas aeruginosa_ colonise biofilm in plumbing fixtures, including faucets and showerheads, where risk of exposure to susceptible persons is high [123; 136]. The production of extracellular polymeric substances (EPS) by _Pseudomonas aeruginosa_ affords it additional protection from environmental stressors by aiding the colonisation of and organisation within biofilms.