Environmental (Saprozoic) Pathogens of Engineered Water Systems: Understanding Their Ecology for Risk Assessment and Management.

Nicholas Ashbolt

doi:10.3390/pathogens4020390

Abstract

Major waterborne (enteric) pathogens are relatively well understood and treatment controls are effective when well managed. However, water-based, saprozoic pathogens that grow within engineered water systems (primarily within biofilms/sediments) cannot be controlled by water treatment alone prior to entry into water distribution and other engineered water systems. Growth within biofilms or as in the case of Legionella pneumophila, primarily within free-living protozoa feeding on biofilms, results from competitive advantage. Meaning, to understand how to manage water-based pathogen diseases (a sub-set of saprozoses) we need to understand the microbial ecology of biofilms; with key factors including biofilm bacterial diversity that influence amoebae hosts and members antagonistic to water-based pathogens, along with impacts from biofilm substratum, water temperature, flow conditions and disinfectant residual—all control variables. Major saprozoic pathogens covering viruses, bacteria, fungi and free-living protozoa are listed, yet today most of the recognized health burden from drinking waters is driven by legionellae, non-tuberculous mycobacteria (NTM) and, to a lesser extent, Pseudomonas aeruginosa. In developing best management practices for engineered water systems based on hazard analysis critical control point (HACCP) or water safety plan (WSP) approaches, multi-factor control strategies, based on quantitative microbial risk assessments need to be developed, to reduce disease from largely opportunistic, water-based pathogens.

Highlights

John Snow was possibly the first to ascribe cholera as a drinking water disease in modern times [1](noting first description of the agent by Pacini [2]), and the subsequent work of Robert Koch cemented the germ theory of disease [3]
This review describes recent information on non-enteric, environmental pathogens and their primary growth niche, the biofilms that form on pipe surfaces and sediment within engineered water systems; noting recent reviews in this field [6,7,8]
This paper takes a risk assessment perspective intended to assist with potential management options—of particular value for hazard analysis critical control point (HACCP) or water safety plans like those recently drafted by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc

Summary

Introduction

John Snow was possibly the first to ascribe cholera as a drinking water disease in modern times [1]. For environmental studies, quantitative polymerase chain reaction (qPCR) methods are clearly superior in obtaining positive detects for legionellae (72% by qPCR vs culture’s 34% from a review of papers over the last ten years) [96], with similar findings for NTM [97] In part, this loss of culturability may be related to the formation of a cyst-like state of the infectious form of L. pneumophila [98], or the slower growth rate/poorer competitiveness on artificial media of target cells [99]. A critical realization is that while various legionellae may grow freely within engineered biofilms, strains that grow within free-living amoebae appear to have enhanced pathogenicity [18] and would allow for rapid development of the high cell densities thought necessary for infections via aerosolized water [16]. It is unclear what characteristics may identify pathogenic strains within the MAC

Risk Assessment and Risk Management of Engineered Water Systems

Management of Sapronoses from Engineered Water Systems

Conclusions