## 3 Factors Governing Survival and Occurrence of Opportunistic Pathogens in Premise Plumbing Systems
### Common Features of Opportunistic Premise Plumbing Pathogens
\begin{table}
\begin{tabular}{c c} \hline \hline
**Characteristics** & **Characteristics** \\ \hline Infection of human hosts—particularly the young, elderly, immunosuppressed, and immunocompromised \\ Disinfectant/chlorine resistance \\ Persistence in drinking water distribution systems (i.e., mains and premise plumbing) \\ Slow growth and regrowth in drinking water distribution systems \\ Growth within amoebae (i.e., resistance to phagocytosis) \\ Biofilm formation \\ Thermal tolerance \\ Survival at low oxygen during periods of stagnation \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of features common to opportunistic premise plumbing pathogens. such as _Escherichia coli_ will fall as they move from the source due to the dilution and the absence of growth [102; 141]. Conversely, OPPPs are native to drinking water distribution systems, and their numbers will increase beyond the treatment plant. To further complicate the matter, numbers of OPPPs do not correspond with numbers of _Escherichia coli_, faecal coliforms, or other measures of microbial water quality and can otherwise be difficult to measure. Slow growth is considered to be an attribute that contributes to this lack of OPPP detection and its persistence in drinking water distribution systems. For example, it takes up to 14 days for the first appearance of _Mycobacterium avium_ colonies incubated at 37 \({}^{\circ}\)C [72]. This is considered as an advantage in the sense that a slow growth means a slow death. This is supported by Table 6, which shows increased chlorine resistance for cells of _Legionella pneumophila_ and _Mycobacterium avium_, which are adapted to drinking water. Additional common features include biofilm formation, resistance to killing by phagocytic amoebae, thermal tolerance, and survival under stagnancy (i.e., low levels of oxygen). Attachment to biofilms and growth within amoebae appear to provide OPPPs with increased protection against these harsh conditions. Biofilm formation also prevents microorganisms from being washed out of flowing pipe systems [142]. Some opportunistic pathogens, such as _Mycobacterium avium_, can withstand assimilable organic carbon concentrations as low as 50 \(\upmu\)g/L [142]. The ability of _Legionella_ to grow in low organic concentrations is believed to depend largely on its relationship with host amoebae. On the other hand, _Pseudomonas aeruginosa_ is so good as persisting without readily available nutrients that it has been noted to grow in distilled water. It is important to acknowledge that the overall performance of these pathogens are not the same. [PERSON] [2] writes that _Mycobacterium avium_ is clearly the most resistant to chlorine and only _Pseudomonas aeruginosa_ can grow under anaerobic conditions [143]. Overall, a review of common features and trends in the occurrence of opportunistic pathogens supports the view that they are well suited for growth in premise plumbing systems. Efforts to reduce or eliminate their incidence might best focus on these common features and on conditions within the premise plumbing environment. 4 Evaluation of Specific Engineered Controls for Opportunistic Pathogens in Premise Plumbing Systems  
In order to reduce or eliminate the risk of disease caused by OPPPs, it is necessary to minimise their concentrations in the affected systems [144]. With this considered, the following sections evaluate the effectiveness of specific treatment methods and approaches.