Legionella pneumophila

Submitted by amh10 on 10 August, 2006 - 12:52

After the recent reports of an oncology patient's death related to a "dirty shower", I thought that I would talk about Legionella pneumophila.

Legionella pneumophila is a thin, pleomorphic, flagellated Gram-negative bacterium of the genus Legionella. L. pneumophilia is non-acid-fast, non-sporulating, and morphologically a non-capsulated rod-like bacteria. Aerobic and unable to hydrolyse gelatin or produce urease, they are also non-fermentative. L. pneumophila is neither pigmented nor does it autofluoresce. It is oxidase- and catalase-positive, and produces beta-lactamase.

L. pneumophilia is the primary human pathogen in this group and is responsible for legionellosis or Legionnaires' disease. Legionella are common in many environments, with at least 48 species and 70 serogroups identified. While L. pneumophila is categorized as a Gram-negative organism, it stains poorly due to its unique lipopolysaccharide-content in the outer leaflet of the outer cell membrane. On the side-chains of the cell wall are carried the bases for the somatic antigen specifity of these organisms. The chemical composition of these side chains both with respect to components as well as arrangement of the different sugars determines the nature of the somatic or O-antigen determinants, which are such important means of serologically classifying many Gram-negative bacteria. At least 14 different serovars of L. pneumophila have been described as well as several other species being subdivided into a number of serovars. Sera have been used both for slide agglutination studies as well as for direct detection of bacteria in tissues using fluorescent-labelled antibody. Specific antibody in patients can be determined by the indirect fluorescent antibody test. ELISA and microagglutination tests have also been successfully applied.
 
L. pneumophila is a facultative intracellular parasite that can invade and replicate inside amoebae and, in humans, in macrophages. The internalisation of the bacteria can be enhanced by the presence of antibody and complement but is not absolutely required. A pseudopod coils around the bacterium in this unique form of phagocytosis. Once internalised, the bacteria surround themselves in a membrane-bound vacuole that does not fuse with lysosomes that would otherwise degrade the bacteria. In this protected compartment the bacteria multiply. The bacteria use a Type IVB Secretion System known as Icm/Dot to inject effector proteins into the host. These effectors are involved in increasing the bacteria's ability to survive inside the host cell. They also secrete a 39kDa metalloprotease into culture fluids, which is cytotoxic for some cultured tissue culture cells.
 
The pathogenic nature of L. pneumophila was first recognized after a 1976 outbreak among a group of elderly men attending an American Legion convention in Philadelphia, Pennsylvania (hence the name Legionaires' disease). This outbreak affected over 200 individuals, with 34 fatalities. It is worth noting that person-to-person transmission of L. pneumophila has not been demonstrated. Common sources of Legionella include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems, domestic hot water systems, fountains, and similar disseminators that draw upon a public water supply. Natural sources include freshwater ponds and creeks.
 
Symptoms include pleural effusion (~50%), pleuritic pain (~30%), hyponatremia (~45%). The CXR usually shows multilobular involvement, usually without sputum production. There is a high incidence of GI upset, together with abnormal LFTs.

Isolation of the microbe from  sputum or bronchoalveolar lavage fluid on selective media is the most reliable means of diagnosis. Buffered charcoal-yeast extract (BCYE) agars with added antibiotics to suppress commensal flora are available commercially, but these media often have decreased sensitivity for isolation of non-pneumophila strains; cefamandole is especially inhibitory. Legionella species lacking β-lactamase, such as L. micdadei and L. bozemanii, will not grow on BCYE formulations containing cephalosporins. A more sensitive medium consists of BCYE with added vancomycin, anisomycin, and polymyxin B. The non-pneumophila strains are easily missed in clinical and environmental specimens if dye-containing media are not used. Colonies of L. micdadei and L. maceachernii are blue on culture media containing bromocresol purple and bromthymol blue dyes, whereas the colonies of other species are yellow-green to apple green; the dyes color the organism, making detection easier. Direct fluorescent antibody (DFA) stains for the visualization of Legionella species in clinical specimens are commercially available for a limited number of species. The sensitivity and specificity of DFA staining for species other than L. pneumophila is not precisely known. A Legionella DNA probe can detect the presence of multiple Legionella spp. Legionella urinary antigen detects only L. pneumophila serogroup 1; it is not useful for other Legionella species. Detection of Legionella spp. in clinical specimens by DNA amplification is a promising technique that has been applied in a limited number of cases. species, but does not differentiate among species. The DNA probe appears to have fewer false-positive reactions than does DFA staining; it is no longer commercially available. Antibody seroconversion in diagnosing infection caused by non-pneumophila species is of uncertain specificity. Reports of infection based on seroconversion alone should be viewed with skepticism.

There are no randomized trials of therapy for Legionella infection; the majority of reported clinical experience concerns infection with L. pneumophila. In vitro susceptibility data and more limited clinical experience indicate that response to the therapy of infection with other species should be similar. Legionella species are susceptible in vitro to erythromycin, macrolides, tetracyclines, trimethoprim-sulfamethoxazole, rifampacin, and fluoroquinolones. The newer macrolide agents are more active than erythromycin both in vitro and intracellularly against the non-pneumophila species. They offer a number of other clinical advantages over erythromycin, including better penetration into tissue and alveolar macrophages, and improved pharmacokinetics permitting once daily dosing. The fluoroquinolones are considerably more active than erythromycin. Based on these factors, the newer macrolides or quinolones are the therapy of choice for infection caused by Legionella species. Patients who are immunocompromised or who are hospitalized with potentially life-threatening infection should receive intravenous therapy with either a macrolide or a fluoroquinolone. Quinolones are preferable when treating transplant patients receiving cyclosporin, because macrolides interfere with the metabolism of these antirejection agents. Erythromycin has been the historical drug of choice based on the observation of clinical response in the majority of patients. However, there are a number of case reports of erythromycin failure in highly immunocompromised patients. Failure of erythromycin may be due to the fact that erythromycin is bacteriostatic rather than bactericidal against intracellular Legionella. The optimal duration of therapy with these agents is uncertain. Data from clinical trials of community-acquired pneumonia suggest that, in immunocompetent patients, 5 to 10 days of therapy with macrolide therapy or 10 to 14 days of therapy with a fluoroquinolone constitutes adequate therapy. Immunocompromised patients should receive longer courses of therapy (14 to 21 days) in order to prevent relapse. Oral therapy may be used as initial treatment in immunocompetent patients who are not seriously ill. Patients receiving initial parenteral therapy may be switched to oral therapy once a clinical response is apparent.

Tag: 
Bacteriology