Analysis of Dot/Icm Type IVB Secretion System Subassemblies by Cryoelectron Tomography Reveals Conformational Changes Induced by DotB Binding

Type IV secretion systems (T4SSs) are sophisticated nanomachines used by many bacterial pathogens to translocate protein and DNA substrates across a host cell membrane. Although T4SSs have important roles in promoting bacterial infections, little is known about the biogenesis of the apparatus and the mechanism of substrate transfer. Here, high-throughput cryoelectron tomography (cryo-ET) was used to visualize Legionella pneumophila T4SSs (also known as Dot/Icm secretion machines) in both the whole-cell context and at the cell pole. These data revealed the distribution patterns of individual Dot/Icm machines in the bacterial cell and identified five distinct subassembled intermediates. High-resolution in situ structures of the Dot/Icm machine derived from subtomogram averaging revealed that docking of the cytoplasmic DotB (VirB11-related) ATPase complex onto the DotO (VirB4-related) ATPase complex promotes a conformational change in the secretion system that results in the opening of a channel in the bacterial inner membrane. A model is presented for how the Dot/Icm apparatus is assembled and for how this machine may initiate the transport of cytoplasmic substrates across the inner membrane.IMPORTANCE Many bacteria use type IV secretion systems (T4SSs) to translocate proteins and nucleic acids into target cells, which promotes DNA transfer and host infection. The Dot/Icm T4SS in Legionella pneumophila is a multiprotein nanomachine that is known to translocate over 300 different protein effectors into eukaryotic host cells. Here, advanced cryoelectron tomography and subtomogram analysis were used to visualize the Dot/Icm machine assembly and distribution in a single L. pneumophila cell. Extensive classification and averaging revealed five distinct intermediates of the Dot/Icm machine at high resolution. Comparative analysis of the Dot/Icm machine and subassemblies derived from wild-type cells and several mutants provided a structural basis for understanding mechanisms that underlie the assembly and activation of the Dot/Icm machine.

In vivo structure of the Legionella type II secretion system by electron cryotomography

The type II secretion system (T2SS) is a multiprotein envelope-spanning assembly that translocates a wide range of virulence factors, enzymes and effectors through the outer membrane of many Gram-negative bacteria1-3. Here, using electron cryotomography and subtomogram averaging methods, we reveal the in vivo structure of an intact T2SS imaged within the human pathogen Legionella pneumophila. Although the T2SS has only limited sequence and component homology with the evolutionarily related type IV pilus (T4P) system4,5, we show that their overall architectures are remarkably similar. Despite similarities, there are also differences, including, for example, that the T2SS-ATPase complex is usually present but disengaged from the inner membrane, the T2SS has a much longer periplasmic vestibule and it has a short-lived flexible pseudopilus. Placing atomic models of the components into our electron cryotomography map produced a complete architectural model of the intact T2SS that provides insights into the structure and function of its components, its position within the cell envelope and the interactions between its different subcomplexes.

In situ imaging of the bacterial flagellar motor disassembly and assembly processes

The self-assembly of cellular macromolecular machines such as the bacterial flagellar motor requires the spatio-temporal synchronization of gene expression with proper protein localization and association of dozens of protein components. In Salmonella and Escherichia coli, a sequential, outward assembly mechanism has been proposed for the flagellar motor starting from the inner membrane, with the addition of each new component stabilizing the previous one. However, very little is known about flagellar disassembly. Here, using electron cryo-tomography and sub-tomogram averaging of intact Legionella pneumophila, Pseudomonas aeruginosa, and Shewanella oneidensis cells, we study flagellar motor disassembly and assembly in situ. We first show that motor disassembly results in stable outer membrane-embedded sub-complexes. These sub-complexes consist of the periplasmic embellished P- and L-rings, and bend the membrane inward while it remains apparently sealed. Additionally, we also observe various intermediates of the assembly process including an inner-membrane sub-complex consisting of the C-ring, MS-ring, and export apparatus. Finally, we show that the L-ring is responsible for reshaping the outer membrane, a crucial step in the flagellar assembly process.

Seeing red; the development of pON.mCherry, a broad-host range constitutive expression plasmid for Gram-negative bacteria

The development of plasmid-mediated gene expression control in bacteria revolutionized the field of bacteriology. Many of these expression control systems rely on the addition of small molecules, generally metabolites or non-metabolized analogs thereof, to the growth medium to induce expression of the genes of interest. The paradigmatic example of an expression control system is the lac system from Escherichia coli, which typically relies on the Ptac promoter and the Lac repressor, LacI. In many cases, however, constitutive gene expression is desired, and other experimental approaches require the coordinated control of multiple genes. While multiple systems have been developed for use in E. coli and its close relatives, the utility and/or functionality of these tools does not always translate to other species. For example, for the Gram-negative pathogen, Legionella pneumophila, a causative agent of Legionnaires' Disease, the aforementioned Ptac system represents the only well-established expression control system. In order to enhance the tools available to study bacterial gene expression in L. pneumophila, we developed a plasmid, pON.mCherry, which confers constitutive gene expression from a mutagenized LacI binding site. We demonstrate that pON.mCherry neither interferes with other plasmids harboring an intact LacI-Ptac expression system nor alters the growth of Legionella species during intracellular growth. Furthermore, the broad-host range plasmid backbone of pON.mCherry allows constitutive gene expression in a wide variety of Gram-negative bacterial species, making pON.mCherry a useful tool for the greater research community.

Activation of Ran GTPase by a Legionella effector promotes microtubule polymerization, pathogen vacuole motility and infection

The causative agent of Legionnaires' disease, Legionella pneumophila, uses the Icm/Dot type IV secretion system (T4SS) to form in phagocytes a distinct "Legionella-containing vacuole" (LCV), which intercepts endosomal and secretory vesicle trafficking. Proteomics revealed the presence of the small GTPase Ran and its effector RanBP1 on purified LCVs. Here we validate that Ran and RanBP1 localize to LCVs and promote intracellular growth of L. pneumophila. Moreover, the L. pneumophila protein LegG1, which contains putative RCC1 Ran guanine nucleotide exchange factor (GEF) domains, accumulates on LCVs in an Icm/Dot-dependent manner. L. pneumophila wild-type bacteria, but not strains lacking LegG1 or a functional Icm/Dot T4SS, activate Ran on LCVs, while purified LegG1 produces active Ran(GTP) in cell lysates. L. pneumophila lacking legG1 is compromised for intracellular growth in macrophages and amoebae, yet is as cytotoxic as the wild-type strain. A downstream effect of LegG1 is to stabilize microtubules, as revealed by conventional and stimulated emission depletion (STED) fluorescence microscopy, subcellular fractionation and Western blot, or by microbial microinjection through the T3SS of a Yersinia strain lacking endogenous effectors. Real-time fluorescence imaging indicates that LCVs harboring wild-type L. pneumophila rapidly move along microtubules, while LCVs harboring ΔlegG1 mutant bacteria are stalled. Together, our results demonstrate that Ran activation and RanBP1 promote LCV formation, and the Icm/Dot substrate LegG1 functions as a bacterial Ran activator, which localizes to LCVs and promotes microtubule stabilization, LCV motility as well as intracellular replication of L. pneumophila.

Gemfibrozil inhibits Legionella pneumophila and Mycobacterium tuberculosis enoyl coenzyme A reductases and blocks intracellular growth of these bacteria in macrophages

We report here that gemfibrozil (GFZ) inhibits axenic and intracellular growth of Legionella pneumophila and of 27 strains of wild-type and multidrug-resistant Mycobacterium tuberculosis in bacteriological medium and in human and mouse macrophages, respectively. At a concentration of 0.4 mM, GFZ completely inhibited L. pneumophila fatty acid synthesis, while at 0.12 mM it promoted cytoplasmic accumulation of polyhydroxybutyrate. To assess the mechanism(s) of these effects, we cloned an L. pneumophila FabI enoyl reductase homolog that complemented for growth an Escherichia coli strain carrying a temperature-sensitive enoyl reductase and rendered the complemented E. coli strain sensitive to GFZ at the nonpermissive temperature. GFZ noncompetitively inhibited this L. pneumophila FabI homolog, as well as M. tuberculosis InhA and E. coli FabI.

Temperature-regulated formation of mycelial mat-like biofilms by Legionella pneumophila

Fifty strains representing 38 species of the genus Legionella were examined for biofilm formation on glass, polystyrene, and polypropylene surfaces in static cultures at 25 degrees C, 37 degrees C, and 42 degrees C. Strains of Legionella pneumophila, the most common causative agent of Legionnaires' disease, were found to have the highest ability to form biofilms among the test strains. The quantity, rate of formation, and adherence stability of L. pneumophila biofilms showed considerable dependence on both temperature and surface material. Glass and polystyrene surfaces gave between two- to sevenfold-higher yields of biofilms at 37 degrees C or 42 degrees C than at 25 degrees C; conversely, polypropylene surface had between 2 to 16 times higher yields at 25 degrees C than at 37 degrees C or 42 degrees C. On glass surfaces, the biofilms were formed faster but attached less stably at 37 degrees C or 42 degrees C than at 25 degrees C. Both scanning electron microscopy and confocal laser scanning microscopy revealed that biofilms formed at 37 degrees C or 42 degrees C were mycelial mat like and were composed of filamentous cells, while at 25 degrees C, cells were rod shaped. Planktonic cells outside of biofilms or in shaken liquid cultures were rod shaped. Notably, the filamentous cells were found to be multinucleate and lacking septa, but a recA null mutant of L. pneumophila was unaffected in its temperature-regulated filamentation within biofilms. Our data also showed that filamentous cells were able to rapidly give rise to a large number of short rods in a fresh liquid culture at 37 degrees C. The possibility of this biofilm to represent a novel strategy by L. pneumophila to compete for proliferation among the environmental microbiota is discussed.

Legionella subvert the functions of Rab1 and Sec22b to create a replicative organelle

Legionella pneumophila is a bacterial pathogen that infects eukaryotic host cells and replicates inside a specialized organelle that is morphologically similar to the endoplasmic reticulum (ER). To better understand the molecular mechanisms governing transport of the Legionella-containing vacuole (LCV), we have identified host proteins that participate in the conversion of the LCV into a replicative organelle. Our data show that Rab1 is recruited to the LCV within minutes of uptake. Rab1 recruitment to the LCV precedes remodeling of this compartment by ER-derived vesicles. Genetic inhibition studies demonstrate that Rab1 is important for the recruitment of ER-derived vesicles to the LCV and that inhibiting Rab1 function abrogates intracellular growth of Legionella. Morphological studies indicate that the Sec22b protein is located on ER-derived vesicles recruited to the LCV and that Sec22b is delivered to the LCV membrane. Sec22b function was found to be important for biogenesis of the specialized organelle that supports Legionella replication. These studies demonstrate that Legionella has the ability to subvert Rab1 and Sec22b function to facilitate the transport and fusion of ER-derived vesicles with the LCV, resulting in the formation of a specialized organelle that can support bacterial replication.

Molecular and cell biology of Legionella pneumophila

Legionella pneumophila is a facultative intracellular pathogen that can replicate within phagocytic host cells such as protozoa and macrophages. Evasion of phagocytic killing is mediated by the type IV Dot/Icm secretion system, which exports bacterial effectors that modulate biogenesis of the phagosome to evade endocytic fusion and also to intercept vesicles derived from the endoplasmic reticulum. Bacterial replication is associated with activation of caspase-3 in infected macrophages and is culminated in apoptosis and pore formation-mediated cytolysis of the host.

Morphology of Legionella pneumophila according to their location within Hartmanella vermiformis

The morphology of Legionella pneumophila grown within Hartmanella vermiformis was studied by electron microscopy. Different morphologies were observed, including a replicative form and a mature intracellular form, that had previously been reported in Hela cells. L. pneumophila was also detected within the cyst wall of the amoeba.

Ultrastructural analysis of differentiation in Legionella pneumophila

Legionella pneumophila is an adaptive pathogen that replicates in the intracellular environment of fundamentally divergent hosts (freshwater protozoa and mammalian cells) and is capable of surviving long periods of starvation in water when between hosts. Physiological adaptation to these quite diverse environments seems to be accompanied by morphological changes (Garduño et al., p. 82-85, in Marre et al., ed., Legionella, 2001) and conceivably involves developmental differentiation. In following the fine-structural pathway of L. pneumophila through both in vitro and in vivo growth cycles, we have now discovered that this bacterium displays an unprecedented number of morphological forms, as revealed in ultrathin sections and freeze-fracture replicas for transmission electron microscopy. Many of the forms were identified by the obvious ultrastructural properties of their cell envelope, which included changes in the relative opaqueness of membrane leaflets, vesiculation, and/or profuse invagination of the inner membrane. These changes were best documented with image analysis software to obtain intensity tracings of the envelope in cross sections. Also prominent were changes in the distribution of intramembranous particles (clearly revealed in replicas of freeze-fractured specimens) and the formation of cytoplasmic inclusions. Our results confirm that L. pneumophila is a highly pleomorphic bacterium and clarify some early observations suggesting sporogenic differentiation in L. pneumophila. Since morphological changes occurred in a conserved sequence within the growth cycle, our results also provide strong evidence for the existence of a developmental cycle in L. pneumophila that is likely accompanied by profound physiological alterations and stage-specific patterns of gene expression.

Intracellular growth of Legionella pneumophila gives rise to a differentiated form dissimilar to stationary-phase forms

When Legionella pneumophila grows in HeLa cells, it alternates between a replicative form and a morphologically distinct "cyst-like" form termed MIF (mature intracellular form). MIFs are also formed in natural amoebic hosts and to a lesser extent in macrophages, but they do not develop in vitro. Since MIFs accumulate at the end of each growth cycle, we investigated the possibility that they are in vivo equivalents of stationary-phase (SP) bacteria, which are enriched for virulence traits. By electron microscopy, MIFs appeared as short, stubby rods with an electron-dense, laminar outer membrane layer and a cytoplasm largely occupied by inclusions of poly-beta-hydroxybutyrate and laminations of internal membranes originating from the cytoplasmic membrane. These features may be responsible for the bright red appearance of MIFs by light microscopy following staining with the phenolic Giménez stain. In contrast, SP bacteria appeared as dull red rods after Giménez staining and displayed a typical gram-negative cell wall ultrastructure. Outer membranes from MIFs and SP bacteria were equivalent in terms of the content of the peptidoglycan-bound and disulfide bond cross-linked OmpS porin, although additional proteins, including Hsp60 (which acts as an invasin for HeLa cells), were detected only in preparations from MIFs. Proteomic analysis revealed differences between MIFs and SP forms; in particular, MIFs were enriched for an approximately 20-kDa protein, a potential marker of development. Compared with SP bacteria, MIFs were 10-fold more infectious by plaque assay, displayed increased resistance to rifampin (3- to 5-fold) and gentamicin (10- to 1,000-fold), resisted detergent-mediated lysis, and tolerated high pH. Finally, MIFs had a very low respiration rate, consistent with a decreased metabolic activity. Collectively, these results suggest that intracellular L. pneumophila differentiates into a cyst-like, environmentally resilient, highly infectious, post-SP form that is distinct from in vitro SP bacteria. Therefore, MIFs may represent the transmissible environmental forms associated with Legionnaires' disease.

The road less traveled: transport of Legionella to the endoplasmic reticulum

Phagosomes containing the bacterial pathogen Legionella pneumophila are transported to the ER after macrophage internalization. To modulate phagosome transport, Legionella use a specialized secretion system that injects bacterial proteins into eukaryotic cells. This review will focus on recent studies that have identified bacterial proteins and host processes that play a concerted role in transporting Legionella to the ER.

How the parasitic bacterium Legionella pneumophila modifies its phagosome and transforms it into rough ER: implications for conversion of plasma membrane to the ER membrane

Within five minutes of macrophage infection by Legionella pneumophila, the bacterium responsible for Legionnaires’ disease, elements of the rough endoplasmic reticulum (RER) and mitochondria attach to the surface of the bacteria-enclosed phagosome. Connecting these abutting membranes are tiny hairs, which are frequently periodic like the rungs of a ladder. These connections are stable and of high affinity - phagosomes from infected macrophages remain connected to the ER and mitochondria (as they were in situ) even after infected macrophages are homogenized. Thin sections through the plasma and phagosomal membranes show that the phagosomal membrane is thicker (72±2 Å) than the ER and mitochondrial membranes (60±2 Å), presumably owing to the lack of cholesterol, sphingolipids and glycolipids in the ER. Interestingly, within 15 minutes of infection, the phagosomal membrane changes thickness to resemble that of the attached ER vesicles. Only later (e.g. after six hours) does the ER-phagosome association become less frequent. Instead ribosomes stud the former phagosomal membrane and L. pneumophila reside directly in the rough ER. Examination of phagosomes of various L. pneumophila mutants suggests that this membrane conversion is a four-stage process used by L. pneumophila to establish itself in the RER and to survive intracellularly. But what is particularly interesting is that L. pneumophila is exploiting a poorly characterized naturally occuring cellular process.

Role of biofilms in the survival of Legionella pneumophila in a model potable-water system

Legionellae can infect and multiply intracellularly in both human phagocytic cells and protozoa. Growth of legionellae in the absence of protozoa has been documented only on complex laboratory media. The hypothesis upon which this study was based was that biofilm matrices, known to provide a habitat and a gradient of nutrients, might allow the survival and multiplication of legionellae outside a host cell. This study determined whether Legionella pneumophila can colonize and grow in biofilms with and without an association with Hartmannella vermiformis. The laboratory model used a rotating disc reactor at a retention time of 6.7 h to grow biofilms on stainless steel coupons. The biofilm was composed of Pseudomonas aeruginosa, Klebsiella pneumoniae and a Flavobacterium sp. The levels of L. pneumophila cells present in the biofilm were monitored for 15 d, with and without the presence of H. vermiformis, and it was found that, although unable to replicate in the absence of H. vermiformis, L. pneumophila was able to persist.

Immunolocalization of the Mip protein of intracellularly and extracellularly grown Legionella pneumophila

The macrophage infectivity potentiator (Mip) protein is an important factor in the optimal intracellular survival of Legionella pneumophila in protozoa and human cell lines. In this study we have localized the Mip protein in Legionella grown on buffered charcoal yeast extract (BCYE) agar as well as in Legionella which were ingested by Acanthamoeba castellanii. Immunogold techniques have shown that Mip is exposed on the cell surface of extracellularly grown bacteria. In A. castellanii infected with Legionella the Mip protein was also detected on host membranes which exhibited a multilamellar structure. The morphology of these structures is similar to that of respirable vesicles of amoebas by which live legionellas may be transmitted to humans. It can be assumed that the accumulation of Mip protein in the multilamellar host membranes increases the infection potential.

Identification of Icm protein complexes that play distinct roles in the biogenesis of an organelle permissive for Legionella pneumophila intracellular growth

Legionella pneumophila is a bacterial pathogen that can enter the human lung and grow inside alveolar macrophages. To grow within phagocytic host cells, the bacteria must create a specialized organelle that restricts fusion with lysosomes. Biogenesis of this replicative organelle is controlled by 24 dot and icm genes, which encode a type IV-related transport apparatus. To understand how this transporter functions, isogenic L. pneumophila dot and icm mutants were characterized, and three distinct phenotypic categories were identified. Our data show that, in addition to genes that encode the core Dot/Icm transport apparatus, subsets of genes are required for pore formation and modulation of phagosome trafficking. To understand activities required for virulence at a molecular level, we investigated protein-protein interactions. Specific interactions between different Icm proteins were detected by yeast two-hybrid and gel overlay analysis. These data support a model in which the IcmQ-IcmR complex regulates the formation of a translocation channel that delivers proteins into host cells, and the IcmS-IcmW complex is required for export of virulence determinants that modulate phagosome trafficking.

Surface-associated hsp60 chaperonin of Legionella pneumophila mediates invasion in a HeLa cell model

HeLa cells have been previously used to demonstrate that virulent strains of Legionella pneumophila (but not salt-tolerant avirulent strains) efficiently invade nonphagocytic cells. Hsp60, a member of the GroEL family of chaperonins, is displayed on the surface of virulent L. pneumophila (R. A. Garduño et al., J. Bacteriol. 180:505-513, 1988). Because Hsp60 is largely involved in protein-protein interactions, we investigated its role in adherence-invasion in the HeLa cell model. Hsp60-specific antibodies inhibited the adherence and invasiveness of two virulent L. pneumophila strains in a dose-dependent manner but had no effect on the association of their salt-tolerant avirulent derivatives with HeLa cells. A monospecific anti-OmpS (major outer membrane protein) serum inhibited the association of both virulent and avirulent strains of L. pneumophila to HeLa cells, suggesting that while both Hsp60 and OmpS may mediate bacterial association to HeLa cells, only virulent strains selectively displayed Hsp60 on their surfaces. Furthermore, the surface-associated Hsp60 of virulent bacterial cells was susceptible to the action of trypsin, which rendered the bacteria noninvasive. Additionally, pretreatment of HeLa cells with purified Hsp60 or precoating of the plastic surface where HeLa cells attached with Hsp60 reduced the adherence and invasiveness of the two virulent strains. Finally, recombinant Hsp60 covalently bound to latex beads promoted the early association of beads with HeLa cells by a factor of 20 over bovine serum albumin (BSA)-coated beads and competed with virulent strains for association with HeLa cells. Hsp60-coated beads were internalized in large numbers by HeLa cells and remained in tight endosomes that did not fuse with other vesicles, whereas internalized BSA-coated beads, for which endocytic trafficking is well established, resided in more loose or elongated endosomes. Mature intracellular forms of L. pneumophila, which were up to 100-fold more efficient than agar-grown bacteria at associating with HeLa cells, were enriched for Hsp60 on the bacterial surface, as determined by immunolocalization techniques. Collectively, these results establish a role for surface-exposed Hsp60 in invasion of HeLa cells by L. pneumophila.

Identification of putative cytoskeletal protein homologues in the protozoan host Hartmannella vermiformis as substrates for induced tyrosine phosphatase activity upon attachment to the Legionnaires' disease bacterium, Legionella pneumophila

The Legionnaires' disease bacterium, Legionella pneumophila, is a facultative intracellular pathogen that invades and replicates within two evolutionarily distant hosts, free living protozoa and mammalian cells. Invasion and intracellular replication within protozoa are thought to be major factors in the transmission of Legionnaires' disease. We have recently reported the identification of a galactose/N-acetyl-D-galactosamine (Gal/GalNAc) lectin in the protozoan host Hartmannella vermiformis as a receptor for attachment and invasion by L. pneumophila (Venkataraman, C., B.J. Haack, S. Bondada, and Y.A. Kwaik. 1997. J. Exp. Med. 186:537-547). In this report, we extended our studies to the effects of bacterial attachment and invasion on the cytoskeletal proteins of H. vermiformis. We first identified the presence of many protozoan cytoskeletal proteins that were putative homologues to their mammalian counterparts, including actin, pp125(FAK), paxillin, and vinculin, all of which were basally tyrosine phosphorylated in resting H. vermiformis. In addition to L. pneumophila-induced tyrosine dephosphorylation of the lectin, bacterial attachment and invasion was associated with tyrosine dephosphorylation of paxillin, pp125(FAK), and vinculin, whereas actin was minimally affected. Inhibition of bacterial attachment to H. vermiformis by Gal or GalNAc monomers blocked bacteria-induced tyrosine dephosphorylation of detergent-insoluble proteins. In contrast, inhibition of bacterial invasion but not attachment failed to block bacteria-induced tyrosine dephosphorylation of H. vermiformis proteins. This was further supported by the observation that 10 mutants of L. pneumophila that were defective in invasion of H. vermiformis were capable of inducing tyrosine dephosphorylation of H. vermiformis proteins. Entry of L. pneumophila into H. vermiformis was predominantly mediated by noncoated receptor-mediated endocytosis (93%) but coiling phagocytosis was infrequently observed (7%). We conclude that attachment but not invasion by L. pneumophila into H. vermiformis was sufficient and essential to induce protein tyrosine dephosphorylation in H. vermiformis. These manipulations of host cell processes were associated with, or followed by, entry of the bacteria by a noncoated receptor-mediated endocytosis. A model for attachment and entry of L. pneumophila into H. vermiformis is proposed.

Mycobacterium avium bacilli grow saprozoically in coculture with Acanthamoeba polyphaga and survive within cyst walls

Protozoans are gaining recognition as environmental hosts for a variety of waterborne pathogens. We compared the growth of Mycobacterium avium, a human pathogen associated with domestic water supplies, in coculture with the free-living amoeba Acanthamoeba polyphaga with the growth of M. avium when it was separated from amoebae by a 0.1-micron-pore-size polycarbonate membrane (in a parachamber). Although viable mycobacteria were observed within amoebal vacuoles, there was no significant difference between bacterial growth in coculture and bacterial growth in the parachamber. This suggests that M. avium is able to grow saprozoically on products secreted by the amoebae. In contrast, Legionella pneumophila, a well-studied intracellular parasite of amoebae, multiplied only in coculture. A comparison of amoebae infected with L. pneumophila and amoebae infected with M. avium by electron microscopy demonstrated that there were striking differences in the locations of the bacteria within amoebal cysts. While L. pneumophila resided within the cysts, M. avium was found within the outer walls of the double-walled cysts of A. polyphaga. These locations may provide a reservoir for the bacteria when environmental conditions become unfavorable.

HeLa cells as a model to study the invasiveness and biology of Legionella 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-parasite interactions.

Identification and temperature regulation of Legionella pneumophila genes involved in type IV pilus biogenesis and type II protein secretion

Previously, we had isolated by transposon mutagenesis a Legionella pneumophila mutant that appeared defective for intracellular iron acquisition. While sequencing in the proximity of the mini-Tn10 insertion, we found a locus that had a predicted protein product with strong similarity to PilB from Pseudomonas aeruginosa. PilB is a component of the type II secretory pathway, which is required for the assembly of type IV pili. Consequently, the locus was cloned and sequenced. Within this 4-kb region were three genes that appeared to be organized in an operon and encoded homologs of P. aeruginosa PilB, PilC, and PilD, proteins essential for pilus production and type II protein secretion. Northern blot analysis identified a transcript large enough to include all three genes and showed a substantial increase in expression of this operon when L. pneumophila was grown at 30 degrees C as opposed to 37 degrees C. The latter observation was then correlated with an increase in piliation when bacteria were grown at the lower temperature. Southern hybridization analysis indicated that the pilB locus was conserved within L. pneumophila serogroups and other Legionella species. These data represent the first isolation of type II secretory genes from an intracellular parasite and indicate that the legionellae express temperature-regulated type IV pili.

The ultrastructure of stressed Legionella pneumophila

Legionellae are able to survive in natural environments which kill many other organisms. The reason for this stress resistance is not well understood nor have its morphological correlates been investigated. The ultrastructure of Legionella pneumophila subjected to stress by heat, acid and starvation, was studied by transmission electron microscopy. Heat-treated cells showed little change although cell shape tended to be distorted. Acid-treated cells showed severe shrinkage of cytoplasmic contents. Starvation caused a reduction in cell size, and after 1 month of starvation cells became fragile, although viability was not reduced.

Entry and intracellular localization of Legionella dumoffii in Vero cells

Organisms of some Legionella species are known to internalize and multiply within epithelial cell lines. During the study on interaction between Legionella spp. and Vero cells, we found that L. dumoffii Tex-KL (ATCC 33343) can enter into Vero cells approximately four to 20 times more often than five other strains of four species of legionella. The mode of entry between L. dumoffii Tex-KL and L. pneumophila Philadelphia-1 was compared and studied by treating Vero cells with reagents which inhibit phagocytosis and endocytosis. Monodansylcadaverine, cytochalasin D and nocodazol were used as inhibitors of receptor-mediated endocytosis, microfilament-dependent phagocytosis and polymerization of microtubules, respectively. The uptake of L. dumoffii Tex-KL required receptor-mediated endocytosis by Vero cells, while the uptake of L. pneumophila Philadelphia-1 used mainly microfilament-dependent phagocytosis. Polymerization of microtubules was necessary for Vero cells for the uptake of both strains of legionella. An electron microscopic examination revealed that some organisms of the L. dumoffii strain Tex-KL escaped from endosomal vacuoles into cytoplasm in the early stage of infection, and proliferated in the cytoplasm. At that period, most of the bacteria were surrounded by rough endoplasmic reticula. In contrast, L. pneumophila Philadelphia-1 proliferated only within ribosome-lined endosome. We suggest that L. dumoffii Tex-KL internalize and proliferate in Vero cells in a different way to L. pneumophila Philadelphia-1, and that there is a variety of the mode of interaction between Legionella spp. and epithelial cells.

Immunolocalization of Hsp60 in Legionella pneumophila

One of the most abundant proteins synthesized by Legionella pneumophila, particularly during growth in a variety of eukaryotic host cells, is Hsp60, a member of the GroEL family of molecular chaperones. The present study was initiated in response to a growing number of reports suggesting that for some bacteria, including L. pneumophila, Hsp60 may exist in extracytoplasmic locations. Immunolocalization techniques with Hsp60-specific monoclonal and polyclonal antibodies were used to define the subcellular location and distribution of Hsp60 in L. pneumophila grown in vitro, or in vivo inside of HeLa cells. For comparative purposes Escherichia coli, expressing recombinant L. pneumophila Hsp60, was employed. In contrast to E. coli, where Hsp60 was localized exclusively in the cytoplasm, in L. pneumophila Hsp60 was predominantly associated with the cell envelope, conforming to a distribution pattern typical of surface molecules that included the major outer membrane protein OmpS and lipopolysaccharide. Interestingly, heat-shocked L. pneumophila organisms exhibited decreased overall levels of cell-associated Hsp60 epitopes and increased relative levels of surface epitopes, suggesting that Hsp60 was released by stressed bacteria. Putative secretion of Hsp60 by L. pneumophila was further indicated by the accumulation of Hsp60 in the endosomal space, between replicating intracellular bacteria. These results are consistent with an extracytoplasmic location for Hsp60 in L. pneumophila and further suggest both the existence of a novel secretion mechanism (not present in E. coli) and a potential role in pathogenesis.

Production of respirable vesicles containing live Legionella pneumophila cells by two Acanthamoeba spp

Two Acanthamoeba species, fed at three temperatures, expelled vesicles containing living Legionella pneumophila cells. Vesicles ranged from 2.1 to 6.4 microns in diameter and theoretically could contain several hundred bacteria. Viable L. pneumophila cells were observed within vesicles which had been exposed to two cooling tower biocides for 24 h. Clusters of bacteria in vesicles were not dispersed by freeze-thawing and sonication. Such vesicles may be agents for the transmission of legionellosis associated with cooling towers, and the risk may be underestimated by plate count methods.

Induced expression of the Legionella pneumophila gene encoding a 20-kilodalton protein during intracellular infection

The eukaryotic protein synthesis inhibitor cycloheximid has been used by many investigators to selectively radiolabel intracellular bacteria. Although cycloheximide has no direct effect on bacterial gene expression, there are concerns that long-term inhibition of the host cell protein synthesis may have secondary effects on bacterial gene expression. Therefore, prior to further identification and cloning of the macrophage-induced (MI) genes of Legionella pneumophila, the effects of cycloheximide on L. pneumophila-infected U937 cells were evaluated by transmission electron microscopy. Inhibition of protein synthesis of the host cell for 6 h had no major effect on the ultrastructure of the host cell, on the formation of rough endoplasmic reticulum-surrounded replicative phagosome, or on initiation of intracellular bacterial replication. In contrast, by 15 h of cycloheximide treatment, there was profound deterioration in the host cell as well as in the phagosome. To examine protein synthesis by L. pneumophila during the intracellular infection, U937 macrophage-like cells were infected with L. pneumophila, and intracellular bacteria were radiolabeled during a 2-h cycloheximide treatment or following 12 h of cycloheximide treatment. Comparison by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protein profile of radiolabeled in vitro-grown L. pneumophila to that of intracellularly radiolabeled bacteria showed that 23 proteins were induced in response to the intracellular environment during 2 h of inhibition of host cell protein biosynthesis. Twelve MI proteins of L. pneumophila were artifactually induced due to prolonged inhibition of the host cell protein synthesis. The gene encoding a 20-kDa MI protein was cloned by a reverse genetics technique. Sequence analysis showed that the cloned gene encoded a protein that was 80% similar to the enzyme inorganic pyrophosphatase. Studies of promoter fusion to a promoterless lacZ gene showed that compared to in vitro-grown bacteria, expression of the pyrophosphatase gene (ppa) was induced fourfold throughout the intracellular infection. There was no detectable induction in transcription of the ppa promoter during exposure to stress stimuli in vitro. The ppa gene of L. pneumophila is the first example of a regulated ppa gene which is selectively induced during intracellular infection and which may reflect enhanced capabilities of macromolecular biosynthesis by intracellular L. pneumophila. The data indicate caution in the long-term use of inhibition of host cell protein synthesis to selectively examine gene expression by intracellular bacteria.

The phagosome containing Legionella pneumophila within the protozoan Hartmannella vermiformis is surrounded by the rough endoplasmic reticulum

Legionella pneumophila is an intracellular parasite of protozoa and human phagocytes. To examine adaptation of this bacterium to parasitize protozoa, the sequence of events of the intracellular infection of the amoeba Hartmannella vermiformis was examined. The previously described uptake phenomenon of coiling phagocytosis by human monocytes was not detected. A 1 h postinfection with wild-type strain AA100, mitochondria were observed within the vicinity of the phagosome. At 2.5 h postinfection, numerous vesicles surrounded the phagosomes and mitochondria were in close proximity to the phagosome. At 5 h postinfection, the bacterium was surrounded by a ribosome-studded multilayer membrane. Bacterial multiplication was evident by 8 h postinfection, and the phagosome was surrounded by a ribosome-studded multilayer membrane until 15 h postinfection. The recruitment of organelles and formation of the ribosome-studded phagosome was defective in an isogenic attenuated mutant of L. pneumophila (strain AA101A) that failed to replicate within amoebae. At 20 h postinfection with wild-type strain AA100, numerous bacteria were present in the phagosome and ribosome were not detected around the phagosome. These data showed that, at the ultrastructural level, the intracellular infection of protozoa by L. pneumophila is highly similar to that of infection of macrophages. Immunocytochemical studies provided evidence that at 5 h postinfection the phagosome containing L. pneumophila acquired an abundant amount of the endoplasmic reticulum-specific protein (BiP). Similar to phagosomes containing heat-killed wild-type L. pneumophila, the BiP protein was not detectable in phagosomes containing the mutant strain AA101A. In addition to the absence of ribosomes and mitochondria, the BiP protein was not detected in the phagosomes at 20 h postinfection with wild-type L. pneumophila. The data indicated that the ability of L. pneumophila to establish the intracellular infection of amoebae is dependent on its capacity to reside and multiply within a phagosome surrounded by the rough endoplasmic reticulum. This compartment may constitute a rich source of nutrients for the bacteria and is probably recognized as cellular compartment. The remarkable similarity of the intracellular infections of macrophages and protozoa by L. pneumophila strongly supports the hypothesis that adaptation of the bacterium to the intracellular environment of protozoa may be the mechanism for its ability to adapt to the intracellular environment of human alveolar macrophages and causes pneumonia.

Porcine interferon-gamma inhibits the growth of Legionella pneumophila in WiREF cells in vitro

The intracellular growth of Legionella pneumophila in WiREF (Wistar rat embryonal fibroblast) cells was inhibited by porcine interferon-gamma. The effect was compared with that of different human interferons (alpha and gamma). The growth inhibition was dose-dependent and required the pretreatment of WiREF cells with interferon. The development of an antibacterial state of the cells was observed. When interferon was added together with bacteria or 1 d after the infection there was no inhibition. Also, there was no direct antibacterial effect of the interferon. In addition, cell pretreatment with a combination of interferon and antibiotics failed to show a synergistic effect.

Effect of monensin on the invasiveness and multiplication of Legionella pneumophila

The polyether antibiotic monensin exhibited bacteriostatic activity against a clinical isolate of Legionella pneumophila in vitro. Experiments designed to test the effect of the compound on the invasiveness and multiplication of L. pneumophila in HeLa cells showed that, in the presence of the antibiotic, legionellas that penetrated the cells did not multiply. However, monensin did not alter the characteristics of phagosomes that contained ingested legionellas. In the presence of monensin, infected cells exhibited extensive vacuolation and a noticeable reduction in the number of intracellular micro-organisms was evident a few hours after infection.

Replicative Legionella pneumophila lung infection in intratracheally inoculated A/J mice. A murine model of human Legionnaires' disease

The role of host immune responses in the pathogenesis of Legionnaires' disease is incompletely understood, due in part to the current lack of an animal model that is both susceptible to replicative Legionella pneumophila-induced lung infection and for which species-specific immunological reagents are available. We have developed a model of replicative L. pneumophila lung infection in intratracheally inoculated A/J mice. L. pneumophila was obtained in the exponential growth phase and inoculated into the trachea of 6- to 8-week-old female A/J mice. Microbiological and histopathological evidence of infection was demonstrated in mice inoculated with 10(6) colony-forming units. Development of an acute pneumonia that resembled human Legionnaires' disease coincided with exponential growth of the bacteria in the lung 24 to 48 hours after intratracheal inoculation of L. pneumophila. This was associated with increased plasma levels of interferon-gamma at 24 hours after inoculation. After 48 hours, the bacteria were gradually eliminated from the lung over the next 5 days, corresponding with resolution of the inflammatory response in the lung, thereby mimicking the outcome frequently seen in the immunocompetent human host. Treatment of animals with anti-interferon-gamma antibody enhanced bacterial replication and disease progression, indicating an important role of host immune response in resolution of the infection. Because of the availability of murine-specific reagents, this model of replicative L. pneumophila lung infection in A/J mice after intrapulmonary inoculation of L. pneumophila potentially provides an important tool for future studies investigating the role of host immune responses in the pathogenesis of Legionnaires' disease in the immunocompetent host.

Intracellular multiplication of Legionella pneumophila in Tetrahymena thermophila

It has been shown that Legionella pneumophila proliferates intracellularly in more than ten species of protozoa, but the fate of the bacteria in Tetrahymena thermophila has not been reported. We investigated the multiplication of L. pneumophila Philadelphia-1 strain in micronucleated T. thermophila, and the effects of temperature and numbers of the bacteria ingested by the protozoa after in vitro feeding were studied. T. thermophila preyed actively upon the bacteria. After being ingested, the fate of the bacteria was affected by both temperature and the number of bacteria ingested. When the number of ingested bacteria was 30 per protozoon, the bacteria proliferated intracellularly at 35 degrees C. The bacteria, however, could not proliferate at 28 degrees C or 32 degrees C though they survived in the protozoa. When the ingested bacteria was 10 per protozoon, the bacteria were killed in the protozoa at all of the temperatures tested. Electron microscopic examination revealed that the protozoa ingesting the bacteria remarkably swelled and that protozoan food vacuoles which contained L. pneumophila were studded with ribosomes.

Altered intracellular targeting properties associated with mutations in the Legionella pneumophila dotA gene

Legionella pneumophila dot mutations cause defects in intracellular targeting of the microorganism within cultured macrophages. Each of the previously characterized dot mutations was shown to be complemented by a single open reading frame designated dotA. The defects caused by the mutations appear to be due to disrupted function of the predicted 1048-amino-acid residue DotA protein, and not by polarity effects on a downstream gene. Complementation studies indicated that the product of the dotA53 mutation results in a partially functional DotA protein, consistent with a stable N-terminal fragment having biological activity.

Influence of temperature and plumbing material selection on biofilm formation and growth of Legionella pneumophila in a model potable water system containing complex microbial flora

Survival and growth of Legionella pneumophila in both biofilm and planktonic phases were determined with a two-stage model system. The model used filter-sterilized tap water as the sole source of nutrient to culture a naturally occurring mixed population of microorganisms including virulent L. pneumophila. At 20 degrees C, L. pneumophila accounted for a low proportion of biofilm flora on polybutylene and chlorinated polyvinyl chloride, but was absent from copper surfaces. The pathogen was most abundant on biofilms on plastics at 40 degrees C, where it accounted for up to 50% of the total biofilm flora. Copper surfaces were inhibitory to total biofouling and included only low numbers of L. pneumophila organisms. The pathogen was able to survive in biofilms on the surface of the plastic materials at 50 degrees C, but was absent from the copper surfaces at the same temperature. L. pneumophila could not be detected in the model system at 60 degrees C. In the presence of copper surfaces, biofilms forming on adjacent control glass surfaces were found to incorporate copper ions which subsequently inhibited colonization of their surfaces. This work suggests that the use of copper tubing in water systems may help to limit the colonization of water systems by L. pneumophila.

Opsonin-independent adherence and intracellular development of Legionella pneumophila within U-937 cells

Legionella pneumophila adhered to and multiplied intracellularly in the human histiocytic lymphoma U-937 cell line. The infectious process was evaluated by viable bacterial cell colony counts and documented by transmission and scanning electron microscopy. In the absence of opsonins, wash-resistant bacterial adherence to host cells occurred within 1 h and attachment of 1 or 2 organisms per U-937 host cell involved close surface interactions at the prokaryotic and eukaryotic membranes. Intracellular multiplication of bacteria was maximal by 24 h after inoculation of cell monolayers. Release of L. pneumophila from these cells appeared as a lytic process that resulted in an increase in the numbers of microorganisms in the extracellular fluids and a concomitant decline in the number of intracellular bacteria. The course of cellular infection was completed by 72 h. The cellular and ultrastructural events of L. pneumophila adherence and uptake by U-937 cells in the absence of antibody or complement have been defined. In addition, this work further establishes the U-937 cell as a suitable model for investigating Legionella--host cell interactions.

Attachment and entry of Legionella pneumophila in Hartmannella vermiformis

Legionella pneumophila is an intracellular parasite of Hartmannella vermiformis. Attachment to the amebae and entry of L. pneumophila were studied by two quantitative assays: One used plate counts to measure the number of bacteria attaching to amebae at 4 degrees C; the other determined the number of intracellular bacteria by use of transmission electron microscopy (TEM). The attachment assay showed that L. pneumophila are inefficient in attachment to amebae. About 0.05% of the bacteria were bound after 1 h with a 10- to 40-fold increase over the next 11 h. Attachment of both virulent and avirulent strains of L. pneumophila occurred at a similar rate. Uptake of L. pneumophila was measured by counting intracellular bacteria using TEM. Limited numbers of virulent L. pneumophila were found intracellularly before 4 h, but the numbers increased logarithmically after this time. The number of amebae containing virulent L. pneumophila increased linearly during the 12-h co-incubation. Avirulent L. pneumophila were rarely detected within amebae throughout the 12-h incubation. Results indicate that entry, not attachment, of virulent L. pneumophila is the limiting step in infection of axenically grown H. vermiformis.

Legionella pneumophila replicates within rat alveolar epithelial cells

Legionella pneumophila replicates in the distal pulmonary airspace, causing legionnaires' pneumonia. Legionella organisms replicate within alveolar macrophages and recruited blood monocytes; however, when these cells are activated, they become potent inhibitors of L. pneumophila proliferation. L. pneumophila may replicate in other cells and thereby avoid the host defenses of macrophages. Experiments demonstrated that L. pneumophila replicate within primary cultures of rat pulmonary alveolar epithelial cells. Double-label immunofluorescent and electron microscopy demonstrated L. pneumophila within epithelial cells. Replication of L. pneumophila required similar numbers of alveolar epithelial cells or alveolar macrophages, required viable epithelial cells, and took place intracellularly. While replication of L. pneumophila occurred in both serum-free and serum-containing media, it was enhanced in the presence of serum. Pulmonary alveolar epithelial cells may represent an alternative site for replication of Legionella species in the terminal airspace and thus clarify some previously unexplained aspects of the pathogenesis of legionnaires' disease.

Physiology and morphology of Legionella pneumophila in continuous culture at low oxygen concentration

Two strains of Legionella pneumophila serogroup 1 monoclonal subgroup Pontiac were grown for the first time in continuous culture using a chemically defined medium. The influence of temperature on physiology and morphology was investigated by fixing the growth rate (equal to the dilution rate, D) at 0.08 h-1 and controlling the pH and dissolved oxygen concentration of the culture. Serine provided the principal source of carbon and energy but growth was limited by tyrosine. The bacterium behaved as a microaerophile in this medium, with maximal growth occurring at 0.31 (mg O2)I-1 (equivalent to a dissolved oxygen tension of 4% (v/v) air saturation at 30 degrees C). The cultures consisted of flagellated, short rods at 24 degrees C, but exhibited an increased level of pleomorphism and the loss of flagella as the temperature was increased to 37 degrees C. The presence of intracellular granules was noted, and their abundance was temperature-dependent. Polyhydroxybutyrate was present in L. pneumophila, and the proportion of the cell dry weight that it accounted for varied with temperature, being maximal at 24 degrees C. The ratio of saturated to unsaturated fatty acids in the cells decreased as the temperature was reduced towards 24 degrees C, so as to maintain membrane fluidity at low growth temperature.

Membrane sorting during phagocytosis: selective exclusion of major histocompatibility complex molecules but not complement receptor CR3 during conventional and coiling phagocytosis

We have used immunocytochemical techniques and enzyme cytochemistry to examine the distribution of plasma membrane proteins during coiling phagocytosis of Legionella pneumophila and conventional phagocytosis of Escherichia coli. Whereas class I and class II major histocompatibility complex (MHC) molecules are relatively excluded from nascent phagosomes during conventional and coiling phagocytosis, the CR3 complement receptor persists in nascent phagosomes. The staining pattern for alkaline phosphatase activity resembles that of MHC molecules, with a marked exclusion of phosphatase activity from L. pneumophila coils and nascent phagosomes. The staining pattern for 5'-nucleotidase activity, on the other hand, resembles that of CR3 with intense staining in the inner layers of L. pneumophila coils. These results demonstrate that the cell has the ability to exclude selectively certain membrane proteins from the nascent phagosome during phagocytosis, thereby producing a phagosomal membrane markedly different from the plasma membrane from which it is derived.

Legionnaires' disease and acute renal failure: case report and review

The case of a 26-year-old man with pneumonia due to Legionella pneumophila associated with acute renal failure is presented, and the English-language literature on legionnaires' disease is reviewed. For this review, acute renal failure was defined as rapid deterioration in renal function indicated by a rise in levels of blood urea nitrogen and creatinine with or without the presence of oliguria. Our patient experienced renal failure and underwent hemodialysis. His condition gradually improved after treatment of legionnaires' disease with erythromycin. Biopsy of the kidney showed acute tubulointerstitial nephritis. Immunofluorescence microscopy demonstrated the presence of L. pneumophila serogroup 1. The laboratory findings suggested rhabdomyolysis. To our knowledge, this is the first case report of a patient with legionnaires' disease who recovered from acute renal failure and in whom the presence of L. pneumophila was demonstrated, and we believe it is the first case in which morphology of the kidney demonstrated the presence of L. pneumophila in a patient with legionnaires' disease, rhabdomyolysis, and renal failure.

[The effect of interferon preparations on the ultrastructural organization of Legionella pneumophila]

The influence of the preparations of interferon on morphological changes in L. pneumophila on the ultrastructural level has been studied. Disturbances in the ultrastructure of L. pneumophila result from the direct bactericidal action of interferons without any interference of immune mechanisms. These disturbances are manifested by damages in the cell wall, plasma membrane, nuclear and ribosomal apparatuses of microbial cells. Leukinferon exhibits pronounced anti-Legionella activity, both in vitro in a liquid culture medium and in ovo, than reaferon.

Decreased biocide susceptibility of adherent Legionella pneumophila

In a study of the in vitro effectiveness of biocides against Legionella pneumophila, some aspects of the cooling tower environment were replicated in the laboratory, paying particular attention to water hardness and pH. Pieces of Douglas fir and polyvinyl chloride were colonized in a recirculating system and the comparative efficacy of two biocides (Bronopol and Kathon) against the sessile and planktonic populations was examined. While the biocides were relatively effective against the planktonic L. pneumophila population over a short period of time (minimum 9-12 h), substantially longer periods of time (maximum greater than 48 h) were required to reduce the number of cultivable bacteria to below detectable levels in the adherent population. The results indicate that failure to monitor the sessile population of L. pneumophila in laboratory studies of biocides may result in the use of incorrect dosages and/or contact times in field trials with apparently reduced in situ efficacy.