Legionella pneumophila, a waterborne bacterium, is the primary cause of Legionnaires’ disease, a severe form of pneumonia contracted through the inhalation of contaminated aerosols. While this study focuses on L. pneumophila, other species, such as Legionella longbeachae, can also cause the disease, particularly in Australia and New Zealand. Cooling towers are a predominant source of Legionnaires' outbreaks, as they support the growth of both sessile and planktonic microbial communities and facilitate their aerosolisation. Therefore, microbial growth within these structures must be rigorously controlled through a variety of control measures. Currently, the efficacy of these control measures is assessed by routine monitoring of bulk water samples using culture-based methods. However, this reliance on planktonic samples combined with culture methods, typically delays remedial actions, allowing outbreaks to occur. This thesis aims to minimise the delay between the detection of Legionella and the implementation of remedial actions to prevent outbreaks. It hypothesises that biofilm samples could potentially serve as lead indicators for the rapid detection of L. pneumophila, thereby enabling timely corrective measures. Additionally, this research explores how biofilm samples enhance our understanding of interactions between biofilms, planktonic communities, and physicochemical parameters, potentially revealing key factors and trends that influence Legionella growth and lead to outbreaks.
To investigate whether biofilm samples could serve as lead indicators for Legionella detection, a novel and optimised biofilm sampling technique was developed and subsequently employed. This method was complemented by independently assessed emerging detection techniques and applied across four strategically selected operational cooling towers over an 18-month period. Additionally, the impact of microbial concentration, viability, and bacterial community structure was examined using flow cytometry and 16S rRNA bacterial community analysis, respectively. This approach significantly enhanced the likelihood of identifying precursor events indicative of Legionella presence.
The research revealed that biofilm samples exclusively identified L. pneumophila during the study period, with Legionella spp. found in higher concentrations in biofilms compared to bulk water. This underscores biofilms as a crucial ecological niche for Legionella spp. Although bacterial concentrations, viability, and physicochemical parameters were found to be insufficient predictors of Legionella proliferation, the composition of the bacterial community within biofilms played a pivotal role. Biofilms, while sharing a core set of taxa with bulk water, exhibited a unique community dominated by low-abundance and rare taxa that significantly influenced microbial interactions and ecological dynamics. These dynamics include the introduction of additional organic compounds and nitrogen, as well as the potential reduction of competitive bacterial populations, which can foster the growth of L. pneumophila. These findings suggest that biofilms can serve as a lead indicator for L. pneumophila, highlighting the need for more comprehensive monitoring policies, practices, and regulations. Such measures should include the assessment of biofilm samples alongside traditional planktonic samples. Prioritising biofilm samples provides a more accurate and holistic understanding of the dynamics of Legionella, revealing crucial insights into the microbial communities that foster conditions conducive to the presence of L. pneumophila. Therefore, integrating biofilm sampling into current monitoring strategies will not only enhance our understanding of Legionella proliferation but also allow for the earlier implementation of remedial actions. Ultimately, this approach is expected to reduce the risk of Legionella outbreaks, thereby safeguarding public health.
