Legionella-like amebal pathogens--phylogenetic status and possible role in respiratory disease.

Legionella-like amebal pathogens (LLAPs) are nonculturable, protozoonotic, gram-negative bacilli. They were named LLAPs because of their ability to infect and multiply intracellularly within amebae (Figure 1) in the same way that legionellae infect and multiply in protozoa and human alveolar mac-rophages (1-3). However, unlike other known legionellae, LLAPs do not grow routinely on buff-ered charcoal yeast extract agar (BCYE), or on any other known bacteriologic media. The first known LLAP was isolated from Polish soil by Drozanski in 1954 (4). Until 1991 when it was described and given the name Sarcobium lyticum (5), it was simply referred to as an obligate intracellular bacterial parasite of free-living ame-bae (6, 7). The next isolation of an LLAP was in England more than 20 years later. Since then, LLAPs have been commonly isolated from various sources and found to infect a variety of amebae (Table 1). Because one LLAP cannot be differentiated from another, each strain is given the name LLAP suffixed by a designated number. Except for S. lyticum, LLAP-1, LLAP-5, and LLAP-3, which was a clinical isolate, all LLAPs were originally isolated from sources associated with confirmed cases or outbreaks of Legionnaires' disease (Table 1). Amebae are natural hosts for legionellae in the environment (2) and have been used to isolate legionellae from clinical samples (8). The relationship between these organisms is unique in that amebae, which generally use other bacteria as food, are parasitized by legionellae. The mechanism by which legionellae multiply and evade host defenses in amebae simulates the mechanism of infection in humans (9,10). The ability of Legionella pneumophila to infect epithelial cells in vitro is also enhanced by prior cultivation of the bacteria in ame-bae (11). This suggests that there may be a connection between the pathogenicity of legionellae for mammalian cells and animal models and their pathogenicity for amebae. Whereas members of the genus Legionella, particularly L. pneumophila, are recognized as important etiologic agents of pneumonia (12), little is known about the clinical relevance of LLAPs; nevertheless, some evidence suggests that they may be an unrecognized and possibly significant cause of respiratory disease. LLAPs' inability to grow or multiply in the absence of amebae makes it difficult to isolate or identify them in clinical samples by conventional techniques. No serologic reagents are currently available for the detection or identification of LLAPs in clinical specimens. It is particularly noteworthy that one LLAP strain was originally isolated by …