Intracellular multiplication ofLegionella pneumophila in amoebae isolated from hospital hot water tanks

HeLa cells as a model to study the invasiveness and biology ofLegionella pneumophila

HeLa cells were established as a model system to study the invasiveness and biology of Legionella pneumophila. In this model, invasion could be distinguished from adherence; virulent strains of L. pneumophila were adherent and invasive, whereas nonvirulent strains were adherent but poorly invasive. Invasion was rapid and did not require de novo bacterial protein synthesis, suggesting that the invasion factor is constitutively expressed by virulent strains. Entry into HeLa cells required actin polymerization and an intact microtubule cytoskeleton and was only moderately inhibited by the presence of 100 mM glucose or galactose. Intracellular replication of virulent L. pneumophila took place in ribosome-studded complex endosomes and led to the formation of free bacteria-laden vesicles presumably released from lysed HeLa cells, These free vesicles (referred to as mature vesicles) were isolated in continuous density gradients of Percoll. The bacteria contained in the isolated mature vesicles had a unique envelope structure and were highly adherent to HeLa cells, characteristics that correlated with a bright red appearance after the Giménez stain (Giménez positive). Plate-grown legionellae and replicating legionellae, harboured in complex endosomes, displayed a typical Gram-negative envelope and stained green after the Giménez stain (Giménez negative). Chronically infected cultures of HeLa cells were also established that may be a useful tool for studying long-term interactions between virulent L. pneumophila and mammalian cells. HeLa cells constitute a valuable model system that offers unique opportunities to study parasite-directed endocytosis, as well as stage specific host-parasite interactions.Key words: Legionella pneumophila, HeLa cells, invasion mechanisms, intracellular pathogens.

Identification of linked Legionella pneumophila genes essential for intracellular growth and evasion of the endocytic pathway

Legionella pneumophila replicates within a specialized phagosome in cultured cells, a function necessary for its pathogenicity. The replicative phagosome lacks membrane marker proteins, such as the glycoprotein LAMP-1, that are indicators of the normal endocytic pathway. We describe the isolation of several Legionella genes essential for intracellular growth and evasion of the endocytic pathway, using a genetic and cell biological approach. We screened 4,960 ethyl methanesulfonate-mutagenized colonies for defects in intracellular growth and trafficking to the replicative phagosome. Six mutant strains of L. pneumophila that had severe intracellular growth defects in mouse bone marrow-derived macrophages were identified. All six mutants were found in phagosomes that colocalized with LAMP-1, indicating defects in intracellular trafficking. The growth defects of two of these strains were complemented by molecular clones from a bank constructed from a wild-type L. pneumophila strain. The inserts from these clones are located in a region of the chromosome contiguous with several other genes essential for intracellular growth. Three mutants could be complemented by single open reading frames placed in trans, one mutant by a gene termed dotH and two additional mutants by a gene termed dotO. A deletion mutation was created in a third gene, dotI, which is located directly upstream of dotH. The delta dotI strain was also defective for intracellular growth in macrophages, and this defect was complemented by a single open reading frame in trans. Based on sequence analysis and structural predictions, possible roles of dotH, dotI, and dotO in intracellular growth are discussed.

Heterogeneity in the attachment and uptake mechanisms of the Legionnaires' disease bacterium, Legionella pneumophila, by protozoan hosts

Invasion and intracellular replication of Legionella pneumophila within protozoa in the environment plays a major role in the transmission of Legionnaires' disease. Intracellular replication of L. pneumophila within protozoa occurs in a rough endoplasmic reticulum (RER)-surrounded phagosome (Y. Abu Kwaik, Appl. Environ. Microbiol. 62:2022-2028, 1996). Since the subsequent fate of many intracellular pathogens is determined by the route of entry, we compared the mechanisms of attachment and subsequent uptake of L. pneumophila by the two protozoa Hartmannella vermiformis and Acanthamoeba polyphaga. Our data provide biochemical and genetic evidence that the mechanisms of attachment and subsequent uptake of L. pneumophila by the two protozoan hosts are, in part, different. First, uptake of L. pneumophila by H. vermiformis is completely blocked by the monovalent sugars galactose and N-acetyl-D-galactosamine, but these sugars partially blocked A. polyphaga. Second, attachment of L. pneumophila to H. vermiformis is associated with a time-dependent and reversible tyrosine dephosphorylation of multiple host proteins. In contrast, only a slight dephosphorylation of a 170-kDa protein of A. polyphaga is detected upon infection. Third, synthesis of H. vermiformis proteins but not of A. polyphaga proteins is required for uptake of L. pneumophila. Fourth, we have identified L. pneumophila mutants that are severely defective in attachment to A. polyphaga but which exhibit minor reductions in attachment to H. vermiformis and, thus, provide a genetic basis for the difference in mechanisms of attachment to both protozoa. The data indicate a remarkable adaptation of L. pneumophila to attach and invade different protozoan hosts by different mechanisms, yet invasion is followed by a remarkably similar intracellular replication within a RER-surrounded phagosome and subsequent killing of the host cell.

Intrapulmonary Hartmannella vermiformis: a potential niche for Legionella pneumophila replication in a murine model of legionellosis

The potential role of inhaled protozoa as a niche for intrapulmonary replication of Legionella pneumophila was investigated in vivo with mutant strains of L. pneumophila which have reduced virulence for the amoeba Hartmannella vermiformis. L. pneumophila AA488 and AA502 were derived from wild-type strain AA100 after transposon mutagenesis. These mutants have reduced virulence for H. vermiformis but are fully virulent for mononuclear phagocytic cells. A/J mice, which are susceptible to replicative L. pneumophila lung infections, were inoculated intratracheally with L. pneumophila AA100, AA488, or AA502 (10[6] bacteria per mouse) or were coinoculated with one of the L. pneumophila strains (10[6] bacteria per mouse) and uninfected H. vermiformis (10[6] amoebae per mouse). The effect of coinoculation with H. vermiformis on intrapulmonary growth of each L. pneumophila strain was subsequently assessed. In agreement with our previous studies, coinoculation with H. vermiformis significantly enhanced intrapulmonary growth of the parent L. pneumophila strain (AA100). In contrast, intrapulmonary growth of L. pneumophila AA488 or AA502 was not significantly enhanced by coinoculation of mice with H. vermiformis. These studies demonstrate that L. pneumophila virulence for amoebae is required for maximal intrapulmonary growth of the bacteria in mice coinoculated with H. vermiformis and support the hypothesis that inhaled amoebae may potentiate intrapulmonary growth of L. pneumophila by providing a niche for bacterial replication.

Colonization of transplant unit water supplies with Legionella and protozoa: precautions required to reduce the risk of legionellosis

Organ transplant recipients and other immunosuppressed patients are known to be at increased risk of nosocomial Legionnaires' disease. Although the ecology of Legionella in hospital water storage and distribution systems (including a protozoonotic relationship with free-living protozoa) has been well documented, little is known regarding the quality of water supplied to high-risk units. Hot- and cold-water samples (two first draw and one run to waste for 5 min) were taken from 69 (85%) of the 81 United Kingdom organ transplant units (31 renal, 24 bone marrow, nine cardiopulmonary and five liver transplant units) and cultured for Legionella and protozoa. Legionella spp. were isolated from the water supplies of 38 (55%) units and Legionella pneumophila from 31 (45%). The blue-white fluorescent group of Legionella (Legionella gormanii, Legionella bozemanii and others) was isolated from 18 (26%) units. Free-living protozoa were isolated from 47 units (68%) and genera of the protozoa known to permit the intracellular growth of Legionella (PGIGL), from 40 units (58%). Possible associations between Legionella and the variables Protozoa; PGIGL; water pH; and circulating water temperature (recorded after running to waste for 5 min) were examined by logistic regression analysis. In cold-water supplies, a significant association was found between the isolation of Legionella and PGIGL (P = 0.032; OR = 1.81; 95% CI 1.1-3.1). In hot-water supplies, an inverse association was found between the isolation of Legionella and circulating water temperature (P = 0.034; OR = 1.0719 per degree C; 95% CI 1.0052-1.1432). (We failed to isolate Legionella when the circulating hot water was > 58 degrees C. No other associations were significant. We recommend the active surveillance of water quality in high-risk patient areas, and that transplant units, either with a history of nosocomial Legionnaires' disease, or where active surveillance indicates a persistently high Legionella colony count, take remedial action. The quality of cold water may be improved by provision of a dedicated supply taken directly from the incoming mains; and of hot water by the use of a dedicated calorifier, able to maintain a minimum circulating hot water return temperature of 60 degrees C.

The molecular ecology of legionellae

Legionella pneumophila is the most highly characterized member of a genus of bacteria that survive as intracellular parasites of freshwater protozoa. These bacteria can also multiply intracellularly in human phagocytic cells and cause respiratory disease in humans. Comparison of the invasive strategies of L. pneumophila in mammalian and protozoan cells and study of the interactions between Legionella and protozoa should prove useful in development of strategies for the prevention of legionellosis.

Association of flagellum expression and intracellular growth of Legionella pneumophila

We examined the role of the flagella of Legionella pneumophila in the infection of amoebae and human monocyte-like cells. Insertional mutants were constructed with mini-Tn10. Ten mutants (F-) which did not react with polyclonal L. pneumophila antiflagellar antisera were identified. Ten randomly selected mutants (F+) that did react with the polyclonal antiflagellar antiserum were also identified. The infectivity of these 20 mutants in Hartmannella vermiformis and human U937 cells was characterized. Seven of the 10 F- mutants were attenuated in their ability to multiply in the amoebae during the first 3 days of coincubation and failed to multiply in U937 cells. Three of the 10 F- mutants multiplied as well as the wild-type parent strain did in amoebae and to a limited degree in U937 cells. None of the 10 F+ mutants were attenuated in either the amoebae or U937 cells. While the flagellar structure is not essential for virulence, the ability of L. pneumophila to infect amoebae and human phagocytic cells appears to be linked to flagellar expression. We believe that the attenuated F- mutants contain insertions in genes critical to both flagellum expression and the infection process.

The Legionella micdadei flagellin: expression in Escherichia coli K 12 and DNA sequence of the gene

To study the structure and function of the Legionella flagellum, we screened a genomic L. micdadei library in Escherichia coli for expression of the flagellin (Fla) subunit. One recombinant clone, JM105 (pHI5588), producing a truncated Fla protein of 40.5 kDa was identified. The plasmid pHI5588 carried a L. micdadei DNA insert of 5 kb, containing ca 95% of the fla gene. The complete DNA sequence of the L. micdadei fla gene was obtained by combining sequence data from pHI5588 with results using a polymerase chain reaction-based system for genome walking (vectorette PCR). The L. micdadei fla gene shared a high degree of homology with other flagellin genes in the amino- and carboxy termini, whereas the central region was found to be nonconserved. The fla sequence will facilitate the cloning of Fla proteins from other Legionella species and the study of flagella in the pathogenesis of Legionnaires' disease.

Field trial of biocides for control of Legionella in cooling towers

Legionella and amebae populations in 16 cooling towers were challenged with three commercially available biocide formulations. The active agents were: a chlorinated phenolic thioether (CPTE), bromo-nitro-propane-diol (BNPD), and bromo-chloro-dimethylhydantoin (BCD, in briquette form). The towers were dosed with these biocides for approximately 4 weeks. BCD was effective against Legionella in each of nine challenge experiments, and CPTE in eight of nine challenges. BNPD was effective in only five of 11 challenges. None of the biocides had any significant effect in reducing planktonic amebae concentrations during the challenges.

Acanthamoeba sp. from the Philippines: electron microscopy studies on naturally occurring bacterial symbionts

The isolation of two plasmid-like circular DNAs, measuring 52 and 42 kbp, from an Acanthamoeba sp. from the Philippines has led to the demonstration of a bacterial endosymbiont occurring in this free-living amoeba. The 52-kbp band hybridized with a short sequence of cytochrome b gene and was identified as the mitochondrial DNA, whereas the 42-kbp band was identified as plasmid DNA of the bacterial symbionts on the basis of electron microscopy. The endosymbionts are gram-negative, rod-shaped bacteria measuring approximately 1.3 x 0.43 microns and numbering about eight to ten cells per section. They are randomly distributed in both cysts and trophozoites and are surrounded neither by a phagolysosomal membrane nor by a clear or electron-translucent region. The endosymbiont membrane appears to have a close association with ribosomes, which are seen to be more concentrated within the vicinity of the symbionts than elsewhere within the cytoplasm. Attempts to grow the symbionts and the amoebae separately have failed.

Non-culturable Legionella pneumophila associated with Acanthamoeba castellanii: detection of the bacterium using DNA amplification and hybridization

The intracellular localization of Legionella pneumophila serogroup 1 within Acanthamoeba castellanii rendered the bacteria non-culturable on supplemented BCYE agar. DNA amplification, using two 19-mer primers, and hybridization using a 25-mer oligonucleotide probe, permitted detection of Leg. pneumophila in approximately 81% (29/36) of samples where the bacteria could not be detected using culture. A combination of co-cultivation of samples with Leg. pneumophila-naive A. polyphaga or Hartmannella vermiformis, incubation in a defined liquid medium or use of catalase indicated that approximately 31% (9/29) of the samples contained Leg. pneumophila which were viable although not culturable.

Cloning and sequencing of the Legionella pneumophila fur gene

Iron is required for the intracellular and extracellular growth of Legionella pneumophila (Lp). In addition, variations in iron levels may serve as a signal for changes in gene expression. In a number of bacterial pathogens, the regulation of gene expression by iron is usually mediated by the Fur (ferric uptake regulation) repressor protein. Through complementation of an Escherichia coli fur mutation and nucleotide sequence analysis, we have cloned and characterized the Lp fur gene. Lp fur encoded a 15.0-kDa protein whose repressive activity was, as expected, highest in bacteria grown in iron-rich media. Computer analysis determined that Lp Fur had an amino-acid identity of over 54% and a similarity of over 72% to the Fur of E. coli, Yersinia pestis, Vibrio species and Pseudomonas aeruginosa. The promoter region of Lp fur contained sequences homologous to the Fur-binding site, suggesting that fur is autoregulated in Lp. Finally, Southern blot hybridizations demonstrated that fur is conserved among Lp strains and Legionella species.

Protein expression by the protozoan Hartmannella vermiformis upon contact with its bacterial parasite Legionella pneumophila

Legionella pneumophila is ingested by both human macrophages and amoebae, and it multiplies within similar endocytic compartments in both eukaryotic species. Inhibitors of eukaryotic protein synthesis, such as cycloheximide and emetine, had no effect on the uptake of L. pneumophila by macrophages but completely abolished ingestion by the amoeba Hartmannella vermiformis. Therefore, host cell protein synthesis is required for the bacterium to infect the amoeba but not human macrophages. To identify proteins expressed by H. vermiformis upon contact with L. pneumophila, we radiolabeled amoebal proteins after contact with bacteria in bacteriostatic concentrations of tetracycline to inhibit bacterial protein synthesis. We analyzed protein expression by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and found that 33 amoebal proteins were induced; 12 of these were not detected in resting amoebae. Eleven other amoebal proteins were repressed; four of them became undetectable. In contrast, no phenotypic changes were observed in H. vermiformis upon contact with Escherichia coli or heat-killed L. pneumophila. An isogenic, avirulent variant of L. pneumophila, incapable of infecting either macrophages or amoebae, induced a different pattern of protein expression upon contact with H. vermiformis. Our data showed that amoebae manifested a specific phenotypic response upon contact with virulent L. pneumophila. This phenotypic modulation may be necessary for uptake of the bacteria into an endocytic compartment that permits bacterial survival and multiplication.

An outbreak of Legionnaires' disease in a Swedish hospital

We report a nosocomial outbreak of Legionella pneumophila serogroup (sg) 1 infection at the general hospital, Värnamo, Sweden. From December 1990 to February 1991, 28 patients and 3 staff fell ill with pneumonia and 3 died. L. pneumophila sg 1 together with several other Legionellae were isolated from the hot water supply to 17 of 20 hospital wards, probably being spread by aerosolization via shower nozzles. Raising the hospital's hot water temperature from 45 degrees C to 65 degrees C, together with heat disinfection of the shower equipment, arrested the outbreak within a week. Keeping the hot water temperature > or = 60 degrees C without chlorination eliminated L. pneumophila from > 75% of the wards. During a period of 2 years after the outbreak we have diagnosed only 1 case of nosocomial legionellosis at the hospital despite an active surveillance program.

Impact of Chlorine and Heat on the Survival of Hartmannella vermiformis and Subsequent Growth of Legionella pneumophila

Hartmannella vermiformis , a common amoebal inhabitant of potable-water systems, supports intracellular multiplication of Legionella pneumophila and is probably important in the transportation and amplification of legionellae within these systems. To provide a practical guide for decontamination of potable-water systems, we assessed the chlorine and heat resistance of H. vermiformis. H. vermiformis cysts and trophozoites were treated independently with chlorine at concentrations of 2.0 to 10.0 ppm for 30 min and then cocultured with L. pneumophila. Both cysts and trophozoites were sensitive to concentrations between 2.0 and 4.0 ppm and above (trophozoites somewhat more so than cysts), and 10.0 ppm was lethal to both forms. Hartmannellae treated with chlorine up to a concentration of 4.0 ppm supported the growth of legionellae. To determine whether heat would be an effective addendum to chlorine treatment of amoebae, hartmannellae were subjected to temperatures of 55 and 60°C for 30 min and alternatively to 50°C followed by treatment with chlorine at a concentration of 2 ppm. Fewer than 0.05% of the amoebae survived treatment at 55°C, and there were no survivors at 60°C. Pretreatment at 50°C appeared to make hartmannella cysts more susceptible to chlorine but did not further reduce the concentration of trophozoites.

Stimulatory effect of cooling tower biocides on amoebae

Two species of amoebae were isolated from the cooling tower of an air-conditioning system and examined for effects of exposure to four cooling tower biocides, a thiocarbamate compound, tributyltin neodecanoate mixed with quaternary ammonium compounds, another quaternary ammonium compound alone, and an isothiazolin derivative. The amoebae isolated were Acanthamoeba hatchetti and a Cochliopodium species. Two other amoeba cultures, an A. hatchetti culture and Cochliopodium bilimbosum, were obtained from the American Type Culture Collection (ATCC) and were also tested. The cooling tower isolates were more resistant to most of the biocides than the ATCC isolates were. The isothiazolin derivative was the least inhibitory to all four amoeba isolates, and tributyltin neodecanoate mixed with quaternary ammonium compounds was the most inhibitory to three of the four isolates. After exposure to lower concentrations of the biocides, including for one strain the manufacturer's recommended concentration of one biocide, the cooling tower amoeba populations increased significantly compared with unexposed controls, whereas the ATCC isolates were not stimulated at any of the concentrations tested. In some cases, concentrations which stimulated cooling tower amoebae inhibited the growth of the ATCC isolates. These results suggest that cooling tower amoebae may adapt to biocides, underscoring the need to use freshly isolated cooling tower organisms rather than organisms from culture collections for testing the efficacy of such biocides. The stimulatory effect of biocides on amoeba populations is an alarming observation, since these organisms may be reservoirs for legionellae. Biocides used to control microbial growth may actually enhance populations of host organisms for pathogenic bacteria.

Antagonistic action of the bacterium Bacillus licheniformis M-4 toward the amoeba Naegleria fowleri

Free-living amoebae belonging to the species Naegleria fowleri are known to be the etiological agents for a form of fulminant meningoencephalitis that is generally fatal (primary amoebic meningoencephalitis). In a broad bacterial screening from soil and water we have isolated three strains (M-4, D-13 and A-12) belonging to the species Bacillus licheniformis that have remarkable amoebicidal activity against Naegleria sp. and also against different Gram-positive and Gram-negative bacteria. Physical-chemical characteristics, partial purification and biological activities of a substance produced by the M-4 strain have been investigated. This substance (m-4) is stable at high temperature (up to 100 degrees C) and extremes of pH (2.5-9.5) and also at -20 degrees C for months. Its production is greatly influenced by oxygenation of the cultures and is probably related to the sporulation process of the bacterium. Scanning electron microscope observations reveal that amoebae are lysed after a few minutes contact with m-4.

Analysis of Legionella pneumophila serogroup 6 strains isolated from a hospital warm water supply over a three-year period by using genomic long-range mapping techniques and monoclonal antibodies

Over a period of 3 years, Legionella pneumophila serogroup 6 strains were isolated from warm water outlets and dental units in the Dental Faculty and from the Surgery and Internal Medicine Clinics at the University of Dresden, Dresden, Germany. In the bacteriological unit of the above-mentioned facility, L. pneumophila serogroups 3 and 12 were grown from warm water specimens. The medical facilities are located in separate buildings connected with a ring pipe warm water system. All L. pneumophila serogroup 6 strains isolated from the warm water supply reacted with a serogroup-specific monoclonal antibody, but not with two other monoclonal antibodies which are subgroup specific, reacting with other serogroup 6 strains. The NotI genomic profiles obtained by pulsed-field gel electrophoresis of 25 serogroup 6 strains isolated from the Dental Faculty over a 3-year period, 1 isolate from the Internal Medicine Clinic, and 4 strains from the Surgery Clinic were identical. Furthermore, all these strains hybridized with a 300-kb NotI fragment when a legiolysin (lly)-specific DNA probe was used. The NotI pattern, however, differed from those of six serogroup 6 strains of other origins, one serogroup 12 strain from the bacteriological unit, and another six unrelated strains of serogroups other than serogroup 6. L. pneumophila serogroup 6 strains which can be divided into only two subgroups by the use of monoclonal antibodies are differentiated in at least six NotI cleavage types obtained by pulsed-field electrophoresis.

Risk factors for contamination of domestic hot water systems by legionellae

To assess risk factors associated with the contamination of the domestic environment by legionellae, 211 houses in the Quebec City area were randomly selected and water samples were collected from the hot water tank, the shower heads, and the most frequently used faucet. After centrifugation, concentrated samples were seeded in triplicate on BCYE and GPV media. Data on the characteristics of the hot water system and plumbing in the house and on the personal habits of the occupants were collected for each house. Among these 211 houses, hot water was provided by either an oil or gas heater in 33 and by an electric heater in 178. Legionellae were isolated from none of the samples from houses with oil or gas heaters and from 39% (69 of 178) of those with electric water heaters (P less than 0.0001). This association remained highly significant after control for water temperature and other variables in a stratified analysis. In the 178 houses with an electric heater, 12% of the faucets, 15% of the shower heads, and 37% of the water heaters were contaminated. Legionella pneumophila serogroups 2 and 4 were the most frequently isolated strains. Logistic regression showed that factors associated with electric water heater contamination were (i) location of the house in older districts of the city (P less than 0.0001), (ii) old age of the water heater (P = 0.003), and (iii) low water temperature (P = 0.05). Contamination of the water heater was the only factor significantly associated with the contamination of peripheral outlets (P less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Multiplication of Legionella spp. in tap water containing Hartmannella vermiformis

A model was developed to study the multiplication of various Legionella spp. in tap water containing Hartmannella vermiformis. Tap water cultures prepared with the following components were suitable for the multiplication studies: Legionella spp., 10(3) CFU/ml; H. vermiformis, 10(4.4) cysts per ml; and killed Pseudomonas paucimobilis, 10(9) cells per ml. Cocultures were incubated at 37 degrees C for at least 1 week. The following legionellae multiplied in tap water cocultures in each replicate experiment: L. bozemanii (WIGA strain), L. dumoffii (NY-23 and TX-KL strains), L. micdadei (two environmental strains), and L. pneumophila (six environmental strains and one clinical isolate). Growth yield values for these strains were 0.6 to 3.5 log CFU/ml. Legionellae which did not multiply in replicate cocultures included L. anisa (one strain), L. bozemanii (MI-15 strain), L. micdadei (a clinical isolate), L. longbeachae, (one strain), and L. pneumophila (Philadelphia 1 strain). L. gormanii and an environmental isolate of L. pneumophila multiplied in only one of three experiments. None of the legionellae multiplied in tap water containing only killed P. paucimobilis. The mean growth yield (+/- standard deviation) of H. vermiformis in the cocultures was 1.2 +/- 0.1 log units/ml. H. vermiformis supports multiplication of only particular strains of legionellae, some of which are from diverse origins.

The intracellular multiplication of Legionella pneumophila in protozoa from hospital plumbing systems

Between October 1987 and March 1989, we tested 144 water samples obtained from the plumbing and cooling tower systems of 5 Paris hospitals for the presence of legionellae and amoebae. Of the samples tested for Legionella, 67 out of 144 (46.5%) were positive, and 82 out of 116 tested for amoebae (70.7%) were positive. The ability of protozoa to support the multiplication of legionella was shown by incubating samples at 35.5 degrees C for 7-15 days. Prior to determining the presence of legionellae and amoebae, 51 of the 144 samples were incubated. After incubation, 22 out of 25 (88%) samples which were positive for the presence of both Legionella and amoebae showed multiplication of Legionella. In 3 out of the 25 (12%) samples containing Legionella and amoebae, Legionella failed to multiply. Six out of the 51 (11.8%) samples which were negative in direct culture for Legionella but positive for amoebae, became positive after incubation. Legionella did not multiply in samples negative for amoebae, nor was there proliferation in samples after filtration through a 1.2-microns membrane followed by incubation for the same period and temperature. Strains of Legionella pneumophila serogroup 1 and serogroup 6 (SG1 and SG6), including 3 patient isolates and 2 environmental isolates, were cocultured with 2 strains of amoebae and Tetrahymena pyriformis. Plate counts, Gimenez staining and electron microscopy demonstrated that intracellular legionellae proliferation occurred.

Characterization of an axenic strain of Hartmannella vermiformis obtained from an investigation of nosocomial legionellosis

A free-living amoeba identified as Hartmannella vermiformis was isolated from a water sample obtained during an investigation of nosocomial legionellosis. Hartmannella vermiformis is known to support the intracellular multiplication of Legionella pneumophila. This strain of H. vermiformis, designated CDC-19, was cloned and established in axenic culture to develop a model for the study of the pathogenicity of legionellae. Isoenzyme patterns of axenically-cultivated strain CDC-19 were compared with two strains of H. vermiformis derived from the type strain, one axenic (ATCC 50236) and the other grown in the presence of bacteria (ATCC 30966). Enzyme patterns suggested that all three strains are assignable to the species H. vermiformis. Axenic H. vermiformis strain CDC-19 has been deposited with the American Type Culture Collection (ATCC 50237) and should prove useful in the study of protozoan-bacterial interaction.

The Legionella pneumophila major secretory protein, a protease, is not required for intracellular growth or cell killing

The Legionella pneumophila major secretory protein (Msp) is a Zn2+ metalloprotease whose function in pathogenesis is unknown. The structural gene for the Msp protease, mspA, was isolated from an L. pneumophila genomic library. In Escherichia coli which contain plasmids with the mspA gene, Msp protein and activity are found in the periplasmic space and the cytoplasm. Transposon mutagenesis with Tn9 of an mspA-containing plasmid in E. coli yielded mutants which no longer expressed protease activity and others with increased protease activity. These results suggested that mspA expression might be regulated. Msp was shown to be produced at a much higher level in L. pneumophila grown in rich compared to semidefined media. A Tn9 insertion which abolishes Msp expression was introduced into the L. pneumophila genome. This mspA::Tn9 L. pneumophila strain showed no detectable production of Msp by immunoblot analysis, and it had less than 0.1% of the protease activity found in the wild-type strain. This mutant was fully capable of growing within and killing human macrophages derived from the HL-60 cell line.

Epidemiology of free-living ameba infections

Small free-living amebas belonging to the genera Acanthamoeba and Naegleria occur world-wide. They have been isolated from a variety of habitats including fresh water, thermal discharges of power plants, soil, sewage and also from the nose and throats of patients with respiratory illness as well as healthy persons. Although the true incidence of human infections with these amebas is not known, it is believed that as many as 200 cases of central nervous system infections due to these amebas have occurred worldwide. A majority (144) of these cases have been due to Naegleria fowleri which causes an acute, fulminating disease, primary amebic meningoencephalitis. The remaining 56 cases have been reported as due either to Acanthamoeba or some other free-living ameba which causes a subacute and/or chronic infection called granulomatous amebic encephalitis (GAE). Acanthamoeba, in addition to causing GAE, also causes nonfatal, but nevertheless painful, vision-threatening infections of the human cornea, Acanthamoeba keratitis. Infections due to Acanthamoeba have also been reported in a variety of animals. These observations, together with the fact that Acanthamoeba spp., Naegleria fowleri, and Hartmannella sp. can harbor pathogenic microorganisms such as Legionella and or mycobacteria indicate the public health importance of these amebas.

Relationship between free amoeba and Legionella: studies in vitro and in vivo

In 1980, Robowtham demonstrated that Legionella multiplies in free amoeba cytoplasm and hypothesized that the amoeba could act as a reservoir of virulent bacteria. In this paper we report various aspects of the relationship between amoeba and Legionella. A liquid medium co-culture method was applied to Acanthamoeba sp. and Legionella pneumophila serogroup 1. Within 4 days, Legionella growth increased by 2 log s CFU/ml. Using a direct immunofluorescence assay and electron microscopy, Legionella was shown to grow abundantly inside phagosomes, and bacteria and/or antigen were present on the cytoplasmic membrane of the amoeba. These aspects are very similar to those observed with Legionella-infected alveolar macrophages. The morphology and structure of Legionella cells were modified after 20 days of co-culture: - viable bacteria showed large fatty cytoplasmic inclusions, - gas liquid chromatography analysis demonstrated a decrease in the i16:0 fatty acid ratio. Cystic forms of amoeba were abundant but none contained viable Legionella. In an in-vivo study using a guinea-pig aerosol infection model, we compared the virulence of Legionella in co-culture with Legionella grown on charcoal dialysed yeast extract (CDYE) agar medium. The Legionella obtained by co-culture had an LD 50 (50% lethal dose) similar to that obtained for those grown on CDYE, showing that bacterial virulence is preserved in the cellular model.

Legionella pneumophila grows adherent to surfaces in vitro and in situ

Legionella pneumophila continues to play a role in both community- and nosocomially-acquired pneumonia. We investigated the ability of L pneumophila to adhere to various types of materials such as those found in the hospital air-cooling and portable water distribution systems. Through the use of a unique sampling apparatus, we were able to regularly acquire planktonic and sessile samples and determine the numbers of bacteria present in both populations, in vitro and in situ. Portions of these apparatuses could be aseptically removed for examination by scanning electron microscopy, or for the determination of the number of viable adherent L pneumophila. The number of bacteria present in each sample was determined by direct plate count, with presumptive L pneumophila colonies being positively identified by direct fluorescent antibody staining techniques. The results demonstrated that not only are legionellae capable of colonizing various metallic and nonmetallic surfaces but that they are preferentially found on surfaces. Surface-adherent bacteria may play a profound role as a reservoir of these potential pathogens in aquatic environments. Furthermore, these results suggest that any comprehensive legionella monitoring program must include not only water samples but also an examination of the adherent populations.