Intracellular growth of Legionella pneumophila serogroup 1 monoclonal antibody type 2 positive and negative bacteria

Population analysis of Legionella pneumophila reveals a basis for resistance to complement-mediated killing

Legionella pneumophila is the most common cause of the severe respiratory infection known as Legionnaires' disease. However, the microorganism is typically a symbiont of free-living amoeba, and our understanding of the bacterial factors that determine human pathogenicity is limited. Here we carried out a population genomic study of 902 L. pneumophila isolates from human clinical and environmental samples to examine their genetic diversity, global distribution and the basis for human pathogenicity. We find that the capacity for human disease is representative of the breadth of species diversity although some clones are more commonly associated with clinical infections. We identified a single gene (lag-1) to be most strongly associated with clinical isolates. lag-1, which encodes an O-acetyltransferase for lipopolysaccharide modification, has been distributed horizontally across all major phylogenetic clades of L. pneumophila by frequent recent recombination events. The gene confers resistance to complement-mediated killing in human serum by inhibiting deposition of classical pathway molecules on the bacterial surface. Furthermore, acquisition of lag-1 inhibits complement-dependent phagocytosis by human neutrophils, and promoted survival in a mouse model of pulmonary legionellosis. Thus, our results reveal L. pneumophila genetic traits linked to disease and provide a molecular basis for resistance to complement-mediated killing.

Legionnaires' disease case-finding algorithm, attack rates, and risk factors during a residential outbreak among older adults: an environmental and cohort study

Background

During a Legionnaires’ disease (LD) outbreak, combined epidemiological and environmental investigations were conducted to identify prevention recommendations for facilities where elderly residents live independently but have an increased risk of legionellosis.

Methods

Survey responses (n = 143) were used to calculate attack rates and describe transmission routes by estimating relative risk (RR) and 95% confidence intervals (95% CI). Potable water collected from five apartments of LD patients and three randomly-selected apartments of residents without LD (n = 103 samples) was cultured for Legionella.

Results

Eight confirmed LD cases occurred among 171 residents (attack rate = 4.7%); two visitors also developed LD. One case was fatal. The average age of patients was 70 years (range: 62–77). LD risk was lower among residents who reported tub bathing instead of showering (RR = 0.13, 95% CI: 0.02–1.09, P = 0.03). Two respiratory cultures were characterized as L. pneumophila serogroup 1, monoclonal antibody type Knoxville (1,2,3), sequence type 222. An indistinguishable strain was detected in 31 (74%) of 42 potable water samples.

Conclusions

Managers of elderly-housing facilities and local public health officials should consider developing a Legionella prevention plan. When Legionella colonization of potable water is detected in these facilities, remediation is indicated to protect residents at higher risk. If LD occurs among residents, exposure reduction, heightened awareness, and clinical surveillance activities should be coordinated among stakeholders. For prompt diagnosis and effective treatment, clinicians should recognize the increased risk and atypical presentation of LD in older adults.

Complex O-acetylation in Legionella pneumophila serogroup 1 lipopolysaccharide. Evidence for two genes involved in 8-O-acetylation of legionaminic acid

A putative gene encoding an O-acetyl transferase, lag-1, is involved in biosynthesis of the O-polysaccharide (polylegionaminic acid) in some Legionella pneumophila serogroup 1 strains. To study the effect of the presence and absence of the gene on the O-polysaccharide O-acetylation, lag-1 from strain Philadelphia 1 was expressed in trans in the naturally lag-1-negative OLDA strain RC1, and immunoblot analysis revealed that the lag-1-encoded O-acetyl transferase is active. O-Polysaccharides of different size were prepared from the lipopolysaccharides of wild-type and transformant strains by mild acid degradation followed by gel-permeation chromatography. Using NMR spectroscopy and MALDI-TOF mass spectrometry, it was found that O-acetylation of the first three legionaminic acid residues next to the core occurs in the short-chain O-polysaccharide (<10 sugars) from both strains. Hence, there is another O-acetyl transferase encoded by a gene different from lag-1. In the longer-chain O-polysaccharide, a legionaminic acid residue proximal to the core is N-methylated and could be further 8-O-acetylated in the lag-1-dependent manner. Only strains expressing a functional lag-1 gene were recognized in Western blot analysis by monoclonal antibody 3/1 requiring 8-O-acetylated polylegionaminic acid for binding. The highly O-acetylated outer core region of the lipopolysaccharide is involved in the epitope of another serogroup 1-specific monoclonal antibody termed LPS-1. The O-acetylation pattern of the L. pneumophila serogroup 1 core oligosaccharide was revised using MALDI-TOF mass spectrometry. lag-1-independent O-acetylation of the core and short-chain O-polysaccharide was found to be a common feature of L. pneumophila serogroup 1 strains. The biological importance of conserved lag-1-independent and variable lag-1-dependent O-acetylation is discussed.

Persistence of Legionella pneumophila in a hospital's water system: a 13-year survey

OBJECTIVE

To describe the molecular epidemiology of Legionella pneumophila infections in the University of Iowa Hospitals and Clinics (UIHC).

DESIGN

Molecular epidemiological study using pulsed-field gel electrophoresis (PFGE).

SETTING

A large university teaching hospital. ISOLATES: All surviving isolates obtained from culture-proven nosocomial L. pneumophila infections and all surviving isolates obtained from the University of Iowa Hospital and Clinics' water supply between 1981 and 1993.

RESULTS

Thirty-three isolates from culture-proven nosocomial cases of L. pneumophila pneumonia were available for typing. PFGE of genomic DNA from the clinical isolates identified six different strains. However, only strain C (16 cases) and strain D (13 cases) caused more than 1 case. Strain C caused clusters of nosocomial infection in 1981, 1986, and 1993 and also caused 4 sporadic cases. Strain D caused a cluster in 1987 and 1988 plus 4 sporadic cases. Of the six strains causing clinical infections, only strains C and D were identified in water samples. PFGE identified three strains in the water supply, of which strains C and D caused clinical disease and also persisted in the water supply during most of the study period.

CONCLUSION

Specific strains of L. pneumophila can colonize hospital water supplies and cause nosocomial infections over long periods of time.

Discovery of virulence genes of Legionella pneumophila by using signature tagged mutagenesis in a guinea pig pneumonia model

Legionella pneumophila is the cause of Legionnaires' disease, which is a form of potentially fatal pneumonia. To identify genes required for virulence of the bacterium, a library of 1,386 L. pneumophila signature tagged transposon mutants was studied for guinea pig virulence. The mutants were screened in pools of 96 each in a guinea pig model of L. pneumophila pneumonia. Sixteen unique mutant clones were determined to have attenuated virulence after being screened twice in the animal model. All 16 mutants failed to multiply in both lungs and spleens. Four of the sixteen had no apparent defect for intracellular multiplication in macrophages. Partial DNA sequences of the interrupted genes adjacent to the transposon insertions showed that six of them had mutations in five known L. pneumophila virulence genes: dotB, dotF/icmG, dotO/icmB, icmX, and proA. Three of the sequenced clones contained mutations in genes without known homology to other published bacterial genes, and seven clones appeared to be homologous to five different known bacterial genes but are still being characterized. With this methodology, we demonstrate the existence of L. pneumophila genes responsible for non-macrophage-related virulence. The discovery of L. pneumophila virulence genes indicates the utility of the signature tagged mutagenesis technique for pulmonary pathogens.

Molecular characterization of a virulence-associated epitope on the lipopolysaccharide of Legionella pneumophila serogroup 1

For identification of lipopolysaccharide (LPS)-associated epitopes of Legionella pneumophila serogroup 1, LPS of strain Philadelphia 1 was investigated using monoclonal antibodies (MAbs). The O-specific chain of LPS is a homopolymer of 5-acetamidino-7-acetamido-8-O-acetyl-3,5,7,9-tetradeoxy-D-glycero- L-galacto- nonulosonic acid. At least four immunoaccessible epitopes were recognized by different MAbs on the intact LPS. After O-deacetylation of LPS, the reactivity of one of the MAbs (MAb 3/1) was lost, indicating thus that the corresponding epitope is associated with the 8-O-acetyl group. Since the reactivity pattern of the MAb 3/1 is identical with those of the MAb 2 which was considered as a virulence marker for serogroup 1, this epitope may be involved in mediating virulence in L. pneumophila. Four MAbs specific to strains of serogroup 1 other than the monoclonal subtype Philadelphia recognized epitopes on the O-deacetylated LPS of strain Philadelphia 1 and, therefore, the virulence-associated epitope blocks recognition of the immunodeterminants that are accessible on the intact LPS of the strains lacking this epitope.