A Case Study: The American Legion, Cooling Towers, and a Novel, Phenotypically Plastic Bacterium

There are probably few organisms with a more interesting ecology (not to mention notoriety) than Legionella pneumophila, the bacterium that causes what became known as Legionnaires' disease or Legionellosis. This inconspicuous microbe achieved international fame when 149 of some 4400 delegates attending a convention of the American Legion in Philadelphia in the summer of 1976 became ill. All developed, to various degrees, similar symptoms including fever, cough, muscle aches, chills, diarrhea, and pneumonia. More cases were discovered within the local populace. Eventually 221 people were affected (including those not directly associated with the Legion event) and 34 died.

Interestingly, it was shown in due course that many hotel employees carried antibodies to the pathogen, implying that it had been present for some time. Reinvestigation of earlier cases attributed to nonspecific pneumonia showed that the Legionnaires' pathogen was infecting people at least as early as 1947 (reviewed by Gomez-Valero et al. 2009). Pathogenesis of the disease is reviewed in detail by Swanson and Hammer (2000).

The Philadelphia mystery was unraveled through intensive, persistent epidemiological and etiological studies (Fraser et al. 1977; McDade et al. 1977; Fraser and McDade 1979).

The story is a fascinating case study in insightful medical sleuthing (see excellent synopsis by Fraser and McDade 1979). An early, significant finding was that the victims had only one thing in common: they lived in, or spent time in or near, the hotel where the convention was held. Epidemiological work by the CDC did not support person-to-person transmission and there was no evidence from surveying subsequent hotel guests that the epidemic continued after the Legion event. Eventually, the causal organism was cultured by an unorthodox procedure for bacteria, identified—again by unusual methods in bacteriology—and shown to be a new species.

In particular, the culturing procedure was a pivotal development, not only in demonstrating the cause of the disease, but in establishing one aspect of the complex environmental relationships of the causal organism. L. pneumophila was found to have many distinctive properties, including fastidious nutritional requirements such that it grew in the laboratory only if culture media contained unusually high amounts of both cysteine and iron. (Subsequent work has elucidated over 50 species of Legionella. Some are closely related to L. pneumophila and about half are clinically important; Lau and Ashbolt 2009.)

Epidemiological evidence pointed to an airborne pathogen transmitted from a point source and eventually suggested strongly that the singular feature of that particular hotel was the cooling towers for the air conditioning system, which provided a mechanism for concentrating and distributing the bacterium. In later outbreaks (as well as in past episodes, known retrospectively from serological evidence to have been caused by L. pneumophila), such sources have been implicated unequivocally (as have other water systems subsequently, including domestic water supplies; Stout et al. 1985; Lau and Ashbolt 2009). The evidence includes patients known to have been exposed to drift from the source and culture of the pathogen from the cooling towers or evaporative condensers.

So, in terms of the environmental biology of this organism, we are faced initially with an intriguing paradox: How does a nutritionally demanding pathogen, which can be cultured in the laboratory only under the most exacting conditions, and that in its human host multiplies primarily in lung tissue, maintain substantial populations in water? A key later discovery was that Legionella could grow in ciliated protozoa and amoebae, which are the hosts in a commensal or parasitic, intracellular relationship (reviewed in Abu Kwaik et al. 1998; Swanson and Hammer 2000; Steinert et al. 2002).

This aspect of the life history of the pathogen is complex though epidemiologically important because it is an environmental reservoir for survival and multiplication. The bacterium exhibits several distinct phenotypes within its protist hosts (Gomez-Valero et al. 2009). In the aquatic environment, Legionella not only obtains nutrients from its associated microorganisms within biofilms, but is protected from desiccation while in transit. Fliermans et al. (1981) isolated the bacterium from diverse aquatic habitats (e.g., temperature range 5.7-63 °C; pH 5.0-8.5) although the concentration in lakes and rivers appears low other than in association with the protozoal host and biofilms.

L. pneumophila is now considered to be a thermotolerant component of freshwater microbial communities (Steinert et al. 2002). It also grows well in hot water tanks and distribution systems, particularly in areas where sediment (scale and organic floc) accumulate (Lau and Ashbolt 2009). Interestingly, the sediment seemingly acts by promoting growth of other bacteria, which in turn enhance growth of Legionella (Stout et al. 1985). (The occurrence of Legionella spp. in various water systems including drinking water is discussed by Lau and Ashbolt [2009].) When encased within amoebic cysts or protozoal vesicles, the bacterium resists desiccation and biocides.

It seems more than coincidental that amoebae share many attributes of alveolar macrophages, a key site of pathology in humans (Swanson and Hammer 2000), who evidently are incidental habitats in the life history of this opportunistic pathogen. The infection cycle is very similar in both amobae and human macrophages (Gomez-Valero et al. 2009; Escoll et al. 2013). Its long coevolutionary relationship with a eukaryotic, protozoal host likely preconditioned it via numerous virulence factors for pathogenesis in humans. The genomic sequence of the pathogen revealed genes for unexpected pathways and other attributes that in retrospect help explain the organism's ecology, including its broad host range (Chien et al. 2004; see also Burstein et al. 2016).

Tolerance of the pathogen for thermal aquatic environments, among other ecological attributes, explains why air conditioning systems can be ideal reservoirs and distribution systems for airborne infection. Their associated water cooling devices provide warm, wet habitats and often receive makeup water from sources containing protozoa and multiple other microorganisms. The cooling towers or, in other cases, evaporative condensers, also act as efficient scrubbers of microbes from the air. A typical tower of the size for a large hotel handles water at a rate of about 3800 L per minute.

This hot water from the compressor unit is sprayed over splash bars, cooled by evaporation, and returned to the compressor. Fresh air is pulled through the spray with a fan to enhance evaporation. Airborne microbes drawn in by the fan are entrained in the spray, as are any microbes originating from the water system itself. Many are expelled in minute water droplets and aerosols as drift in the discharge airstream (Fraser and McDade 1979).

In overview, as is true of the Case Studies in previous chapters, this example presents an interesting problem in basic ecology with obvious practical implications. The organism grows in more-or-less habitable sites as diverse as portions of the human body (even though the human host is a dead-end), in protists, and also in various forms principally as biofilms in natural or artificial water systems. Legionella clearly has the ability to dramatically change its environment; in turn, survival of the bacterium in nature seems to depend on a complex association with other microorganisms that provide the appropriate nutritional and protective milieu. In its various habitats and growth forms it exhibits multiple states and extreme phenotypic plasticity. Knowledge of the environmental biology of Legionella provides a basis for the intelligent application of controls, such as elevating the temperature of domestic hot water supplies, or chlorination, which can be directed at the localized inoculum reservoirs involved in amplification and dispersal of the pathogen.