General Characteristics

All Legionella spp. are mesophilic (20° to 45° C), obligately aerobic, faintly staining, thin, gram-negative fastidious bacilli that require a medium supplemented with iron and L-cysteine, and buffered to pH 6.9 for optimum growth. The organisms utilize protein for energy generation rather than carbohydrates. The overwhelming majority of Legionella spp. are motile. As of this writing, more than 52 species belong to this genus. Nevertheless, the organism Legionella pneumophila predominates as a human pathogen within the genus and consists of 16 serotypes. In approximately decreasing order of clinical importance are L. pneumophila serotype 1 (about 70% to 90% of the cases of Legionnaires’ disease), L. pneumophila serotype 6, L. micdadei, L. dumoffii, L. anisa, and L. feeleii. Of note, many species of Legionella have only been isolated from the environment or recorded as individual cases. To date, 20 species of Legionella are documented as human pathogens in addition to L. pneumophila. Box 35-1 is an abbreviated list of some of the species of Legionella.

Pathogenesis and Spectrum of Disease

Legionella spp. can infect and multiply within some species of free-living amoebae (Hartmannella, Acanthamoeba, and Naegleria spp.), as well as within Tetrahymena spp., a ciliated protozoa, or within biofilms (well-organized microcolonies of bacteria usually enclosed in polymer matrices that are separated by water channels that remove wastes and deliver nutrients). This contributes to the organism’s survival in the environment. In addition, L. pneumophila exists in two well-defined, morphologically distinct forms in Hela cells: (1) a highly differentiated, cystlike form that is highly infectious, metabolically dormant, and resistant to antibiotics and detergent-mediated lysis and (2) a replicative intracellular form that is ultrastructurally similar to agar-grown bacteria. The existence of this cystlike form may account for the ability of L. pneumophila to survive for long periods between hosts (amoebae or humans).

Although the exact mechanisms by which L. pneumophila causes disease are not totally delineated, its ability to avoid destruction by the host’s phagocytic cells plays a significant role in the disease process. L. pneumophila is considered a facultative intracellular pathogen. Following infection, organisms are taken up by phagocytosis primarily in alveolar macrophages, where they survive and replicate within a specialized, membrane-bound vacuole by resisting acidification and evading fusion with lysosomes; it is still unknown how Legionella prevent vacuole acidification. Following replication, the organisms will kill the phagocyte releasing them into the lungs and will again be phagocytized by a mononuclear cell, and multiplication of the organism will increase.

The sequestering of legionellae within macrophages also makes it difficult to deliver and accumulate effective antimicrobials. Of significance, studies have shown that although certain antimicrobials can penetrate the macrophage and inhibit bacterial multiplication, L. pneumophila is not killed and, when drugs are removed, the organism resumes replicating. Therefore, a competent cell-mediated immune response is also important for recovery from Legionella infections. Humoral immunity appears to play an insignificant role in the defense against this organism.

In eukaryotic cells, most proteins secreted or transported inside vesicles to other cellular compartments are synthesized at the endoplasmic reticulum (ER) (Figure 35-1). Many bacterial pathogens use secretion systems as a part of how they cause disease. L. pneumophila possesses genes that are able to “trick” eukaryotic cells into transporting them to the endoplasmic reticulum; these virulence genes are called dot (defective organelle trafficking) or icm (intracellular multiplication). This dot/icm secretion system in L. pneumophila consists of 23 genes and is a type IV secretion system. Bacterial type IV secretion systems are bacterial devices that deliver macromolecules such as proteins across and into cells. After entry but before bacterial replication, L. pneumophila, residing in a membrane-bound vacuole, is surrounded by a ribosome-studded membrane derived from the host cell’s ER and mitochondria. Thus, by exploiting host cell functions, L. pneumophila is able to gain access to the lumen of the ER, which supports its survival and replication where the environment is rich in peptides. A second type II secretion system has also been implicated in the virulence of some strains of Legionella. The type II secretion system carries numerous genes for enzymatic degradation including lipases, proteinases, and a number of novel proteins. Mutations within the type II secretion system results in decreased infectivity of the organism. A number of additional bacterial factors have also been identified as crucial for intracellular infection; some of these are listed in Box 35-2.

(Modified from 2009annualreport.nichd.nih.gov/ump.html.)

Finally, several cellular components and extracellular products of L. pneumophila, such as an extracellular cytotoxin that impairs the ability of phagocytic cells to use oxygen and various enzymes (e.g., phospholipase C), have been purified and proposed as virulence factors. However, their exact role in the pathogenesis of Legionella infections is not completely clear.

Legionella spp. are associated with a spectrum of clinical presentations, ranging from asymptomatic infection to severe, life-threatening diseases. Serologic evidence exists for the presence of asymptomatic disease, because many healthy people surveyed possess antibodies to Legionella spp. Table 35-1 provides a more detailed description of the following three primary clinical manifestations:

Individuals at risk for pneumonia are those who are immunocompromised, older than age 60, or heavy smokers. The clinical manifestations following infection with a particular species are primarily caused by differences in the host’s immune response and perhaps by inoculum size; the same Legionella sp. gives rise to different expressions of disease in different individuals.

There are a number of bacteria that grow only within amoebae and are closely related phylogenetically based on 16S rRNA gene sequencing to Legionella species; these organisms are referred to as “Legionella-like amoeba pathogens” (LLAPs). Several LLAPs have been assigned to the Legionella genus. One LLAP has been isolated from the sputum of a patient with pneumonia after the specimen was incubated with the amoeba Acanthamoeba polyphaga. Serologic surveys of patients with community-acquired pneumonia suggest LLAPs may be occasional human pathogens.