Amoebal endosymbiont Neochlamydia protects host amoebae against Legionella pneumophila infection by preventing Legionella entry

Phylogenomic, structural, and cell biological analyses reveal that Stenotrophomonas maltophilia replicates in acidified Rab7A-positive vacuoles of Acanthamoeba castellanii

Acanthamoeba species are clinically relevant free-living amoebae (FLA) ubiquitously found in soil and water bodies. Metabolically active trophozoites graze on diverse microbes via phagocytosis. However, functional studies on Rab GTPases (Rabs), which are critical for controlling vesicle trafficking and maturation, are scarce for this FLA. This knowledge gap can be partly explained by the limited genetic tools available for Acanthamoeba cell biology. Here, we developed plasmids to generate fusions of A. castellanii strain Neff proteins to the N- or C-termini of mEGFP and mCherry2. Phylogenomic and structural analyses of the 11 Neff Rab7 paralogs found in the RefSeq assembly revealed that eight of them had non-canonical sequences. After correcting the gene annotation for the Rab7A ortholog, we generated a line stably expressing an mEGFP-Rab7A fusion, demonstrating its correct localization to acidified macropinocytic and phagocytic vacuoles using fluorescence microscopy live cell imaging (LCI). Direct labeling of live Stenotrophomonas maltophilia ESTM1D_MKCAZ16_6a (Sm18) cells with pHrodo Red, a pH-sensitive dye, demonstrated that they reside within acidified, Rab7A-positive vacuoles. We constructed new mini-Tn7 delivery plasmids and tagged Sm18 with constitutively expressed mScarlet-I. Co-culture experiments of Neff trophozoites with Sm18::mTn7TC1_Pc_mScarlet-I, coupled with LCI and microplate reader assays, demonstrated that Sm18 underwent multiple replication rounds before reaching the extracellular medium via non-lytic exocytosis. We conclude that S. maltophilia belongs to the class of bacteria that can use amoeba as an intracellular replication niche within a Stenotrophomonas-containing vacuole that interacts extensively with the endocytic pathway.IMPORTANCEDiverse Acanthamoeba lineages (genotypes) are of increasing clinical concern, mainly causing amoebic keratitis and granulomatous amebic encephalitis among other infections. S. maltophilia ranks among the top 10 most prevalent multidrug-resistant opportunistic nosocomial pathogens and is a recurrent member of the microbiome hosted by Acanthamoeba and other free-living amoebae. However, little is known about the molecular strategies deployed by Stenotrophomonas for an intracellular lifestyle in amoebae and other professional phagocytes such as macrophages, which allow the bacterium to evade the immune system and the action of antibiotics. Our plasmids and easy-to-use microtiter plate co-culture assays should facilitate investigations into the cellular microbiology of Acanthamoeba interactions with Stenotrophomonas and other opportunistic pathogens, which may ultimately lead to the discovery of new molecular targets and antimicrobial therapies to combat difficult-to-treat infections caused by these ubiquitous microbes.

Chlamydiae in corals: shared functional potential despite broad taxonomic diversity

Cnidarians, such as corals and sea anemones, associate with a wide range of bacteria that have essential functions, including nutrient cycling and the production of antimicrobial compounds. Within cnidarians, bacteria can colonize all microhabitats including the tissues. Among them are obligate intracellular bacteria of the phylum Chlamydiota (chlamydiae) whose impact on cnidarian hosts and holobionts, especially corals, remain unknown. Here, we conducted a meta-analysis of previously published 16S rRNA gene metabarcoding data from cnidarians (e.g. coral, jellyfish, and anemones), eight metagenome-assembled genomes (MAGs) of coral-associated chlamydiae, and one MAG of jellyfish-associated chlamydiae to decipher their diversity and functional potential. While the metabarcoding dataset showed an enormous diversity of cnidarian-associated chlamydiae, six out of nine MAGs were affiliated with the Simkaniaceae family. The other three MAGs were assigned to the Parasimkaniaceae, Rhabdochlamydiaceae, and Anoxychlamydiaceae, respectively. All MAGs lacked the genes necessary for an independent existence, lacking any nucleotide or vitamin and most amino acid biosynthesis pathways. Hallmark chlamydial genes, such as a type III secretion system, nucleotide transporters, and genes for host interaction, were encoded in all MAGs. Together these observations suggest an obligate intracellular lifestyle of coral-associated chlamydiae. No unique genes were found in coral-associated chlamydiae, suggesting a lack of host specificity. Additional studies are needed to understand how chlamydiae interact with their coral host, and other microbes in coral holobionts. This first study of the diversity and functional potential of coral-associated chlamydiae improves our understanding of both the coral microbiome and the chlamydial lifestyle and host range.

Hidden from plain sight: Novel Simkaniaceae and Rhabdochlamydiaceae diversity emerging from screening genomic and metagenomic data

Chlamydiota are an ancient and hyperdiverse phylum of obligate intracellular bacteria. The best characterized representatives are pathogens or parasites of mammals, but it is thought that their most common hosts are microeukaryotes like Amoebozoa. The diversity in taxonomy, evolution, and function of non-pathogenic Chlamydiota are slowly being described. Here we use data mining techniques and genomic analysis to extend our current knowledge of Chlamydiota diversity and its hosts, in particular the Order Parachlamydiales. We extract one Rhabdochlamydiaceae and three Simkaniaceae Metagenome-Assembled Genomes (MAGs) from NCBI Short Read Archive deposits of ciliate and algal genome sequencing projects. We then use these to identify a further 14 and 8 MAGs respectively amongst existing, unidentified environmental assemblies. From these data we identify two novel clades with host associated data, for which we propose the names "Sacchlamyda saccharinae" (Family Rhabdochlamydiaceae) and "Amphrikana amoebophyrae" (Family Simkaniaceae), as well as a third new clade of environmental MAGs "Acheromyda pituitae" (Family Rhabdochlamydiaceae). The extent of uncharacterized diversity within the Rhabdochlamydiaceae and Simkaniaceae is indicated by 16 of the 22 MAGs being evolutionarily distant from currently characterised genera. Within our limited data, there was great predicted diversity in Parachlamydiales metabolism and evolution, including the potential for metabolic and defensive symbioses as well as pathogenicity. These data provide an imperative to link genomic diversity in metagenomics data to their associated eukaryotic host, and to develop onward understanding of the functional significance of symbiosis with this hyperdiverse clade.

Proteomic analysis of Acanthamoeba castellanii response to Legionella pneumophila infection

Legionella pneumophila is an opportunistic pathogen responsible for Legionnaires' disease or Legionellosis. This bacterium is found in the environment interacting with free-living amoebae such as Acanthamoeba castellanii. Until now, proteomic analyses have been done in amoebae infected with L. pneumophila but focused on the Legionella-containing vacuole. In this study, we propose a global proteomic analysis of the A. castellanii proteome following infection with L. pneumophila wild-type (WT) or with an isogenic ΔdotA mutant strain, which is unable to replicate intracellularly. We found that infection with L. pneumophila WT leads to reduced levels of A. castellanii proteins associated with lipid homeostasis/metabolism, GTPase regulation, and kinase. The levels of organelle-associated proteins were also decreased during infection. Legionellapneumophila WT infection leads to increased levels of proteins associated with polyubiquitination, folding or degradation, and antioxidant activities. This study reinforces our knowledge of this too little explored but so fundamental interaction between L. pneumophila and A. castellanii, to understand how the bacterium could resist amoeba digestion.

Genomic diversity and biosynthetic capabilities of sponge-associated chlamydiae

Sponge microbiomes contribute to host health, nutrition, and defense through the production of secondary metabolites. Chlamydiae, a phylum of obligate intracellular bacteria ranging from animal pathogens to endosymbionts of microbial eukaryotes, are frequently found associated with sponges. However, sponge-associated chlamydial diversity has not yet been investigated at the genomic level and host interactions thus far remain unexplored. Here, we sequenced the microbiomes of three sponge species and found high, though variable, Chlamydiae relative abundances of up to 18.7% of bacteria. Using genome-resolved metagenomics 18 high-quality sponge-associated chlamydial genomes were reconstructed, covering four chlamydial families. Among these, Candidatus Sororchlamydiaceae shares a common ancestor with Chlamydiaceae animal pathogens, suggesting long-term co-evolution with animals. Based on gene content, sponge-associated chlamydiae resemble members from the same family more than sponge-associated chlamydiae of other families, and have greater metabolic versatility than known chlamydial animal pathogens. Sponge-associated chlamydiae are also enriched in genes for degrading diverse compounds found in sponges. Unexpectedly, we identified widespread genetic potential for secondary metabolite biosynthesis across Chlamydiae, which may represent an unexplored source of novel natural products. This finding suggests that Chlamydiae members may partake in defensive symbioses and that secondary metabolites play a wider role in mediating intracellular interactions. Furthermore, sponge-associated chlamydiae relatives were found in other marine invertebrates, pointing towards wider impacts of the Chlamydiae phylum on marine ecosystems.

Complexities of Inferring Symbiont Function: Paraburkholderia Symbiont Dynamics in Social Amoeba Populations and Their Impacts on the Amoeba Microbiota

The relationship between the social amoeba Dictyostelium discoideum and its endosymbiotic bacteria Paraburkholderia provides a model system for studying the development of symbiotic relationships. Laboratory experiments have shown that any of three species of the Paraburkholderia symbiont allow D. discoideum food bacteria to persist through the amoeba life cycle and survive in amoeba spores rather than being fully digested. This phenomenon is termed "farming," as it potentially allows spores dispersed to food-poor locations to grow their own. The occurrence and impact of farming in natural populations, however, have been a challenge to measure. Here, we surveyed natural D. discoideum populations and found that only one of the three symbiont species, Paraburkholderia agricolaris, remained prevalent. We then explored the effect of Paraburkholderia on the amoeba microbiota, expecting that by facilitating bacterial food carriage, it would diversify the microbiota. Contrary to our expectations, Paraburkholderia tended to infectiously dominate the D. discoideum microbiota, in some cases decreasing diversity. Similarly, we found little evidence for Paraburkholderia facilitating the carriage of particular food bacteria. These findings highlight the complexities of inferring symbiont function in nature and suggest the possibility that Paraburkholderia could be playing multiple roles for its host. IMPORTANCE The functions of symbionts in natural populations can be difficult to completely discern. The three Paraburkholderia bacterial farming symbionts of the social amoeba Dictyostelium discoideum have been shown in the laboratory environment to allow the amoebas to carry, rather than fully digest, food bacteria. This potentially provides a fitness benefit to the amoebas upon dispersal to food-poor environments, as they could grow their food. We expected that meaningful food carriage would manifest as a more diverse microbiota. Surprisingly, we found that Paraburkholderia tended to infectiously dominate the D. discoideum microbiota rather than diversifying it. We determined that only one of the three Paraburkholderia symbionts has increased in prevalence in natural populations in the past 20 years, suggesting that this symbiont may be beneficial, however. These findings suggest that Paraburkholderia may have an alternative function for its host, which drives its prevalence in natural populations.

Defensive symbiosis against giant viruses in amoebae

Protists are important regulators of microbial communities and key components in food webs with impact on nutrient cycling and ecosystem functioning. In turn, their activity is shaped by diverse intracellular parasites, including bacterial symbionts and viruses. Yet, bacteria–virus interactions within protists are poorly understood. Here, we studied the role of bacterial symbionts of free-living amoebae in the establishment of infections with nucleocytoplasmic large DNA viruses (Nucleocytoviricota). To investigate these interactions in a system that would also be relevant in nature, we first isolated and characterized a giant virus (Viennavirus, family Marseilleviridae) and a sympatric potential Acanthamoeba host infected with bacterial symbionts. Subsequently, coinfection experiments were carried out, using the fresh environmental isolates as well as additional amoeba laboratory strains. Employing fluorescence in situ hybridization and qPCR, we show that the bacterial symbiont, identified as Parachlamydia acanthamoebae, represses the replication of the sympatric Viennavirus in both recent environmental isolates as well as Acanthamoeba laboratory strains. In the presence of the symbiont, virions are still taken up, but viral factory maturation is inhibited, leading to survival of the amoeba host. The symbiont also suppressed the replication of the more complex Acanthamoeba polyphaga mimivirus and Tupanvirus deep ocean (Mimiviridae). Our work provides an example of an intracellular bacterial symbiont protecting a protist host against virus infections. The impact of virus–symbiont interactions on microbial population dynamics and eventually ecosystem processes requires further attention.

Potential probiotic approaches to control Legionella in engineered aquatic ecosystems

Opportunistic pathogens belonging to the genus Legionella are among the most reported waterborne-associated pathogens in industrialized countries. Legionella colonize a variety of engineered aquatic ecosystems and persist in biofilms where they interact with a multitude of other resident microorganisms. In this review, we assess how some of these interactions could be used to develop a biological-driven “probiotic” control approach against Legionella. We focus on: (i) mechanisms limiting the ability of Legionella to establish and replicate within some of their natural protozoan hosts; (ii) exploitative and interference competitive interactions between Legionella and other microorganisms; and (iii) the potential of predatory bacteria and phages against Legionella. This field is still emergent, and we therefore specifically highlight research for future investigations, and propose perspectives on the feasibility and public acceptance of a potential probiotic approach.

Novel Chlamydiae and Amoebophilus endosymbionts are prevalent in wild isolates of the model social amoeba Dictyostelium discoideum

Amoebae interact with bacteria in multifaceted ways. Amoeba predation can serve as a selective pressure for the development of bacterial virulence traits. Bacteria may also adapt to life inside amoebae, resulting in symbiotic relationships. Indeed, particular lineages of obligate bacterial endosymbionts have been found in different amoebae. Here, we screened an extensive collection of Dictyostelium discoideum wild isolates for the presence of these bacterial symbionts using endosymbiont specific PCR primers. We find that these symbionts are surprisingly common, identified in 42% of screened isolates (N = 730). Members of the Chlamydiae phylum are particularly prevalent, occurring in 27% of the amoeba isolated. They are novel and phylogenetically distinct from other Chlamydiae. We also found Amoebophilus symbionts in 8% of screened isolates (N = 730). Antibiotic-cured amoebae behave similarly to their Chlamydiae or Amoebophilus-infected counterparts, suggesting that these endosymbionts do not significantly impact host fitness, at least in the laboratory. We found several natural isolates were co-infected with multiple endosymbionts, with no obvious fitness effect of co-infection under laboratory conditions. The high prevalence and novelty of amoeba endosymbiont clades in the model organism D. discoideum opens the door to future research on the significance and mechanisms of amoeba-symbiont interactions.

Distribution of amoebal endosymbiotic environmental chlamydia Neochlamydia S13 via amoebal cytokinesis

We previously isolated a symbiotic environmental amoeba, harboring an environmental chlamydia, Neochlamydia S13. Interestingly, this bacterium failed to survive outside of host cells and was immediately digested inside other amoebae, indicating bacterial distribution via cytokinesis. This may provide a model for understanding organelle development and chlamydial pathogenesis and evolution; therefore, we assessed our hypothesis of Neochlamydia S13 distribution via cytokinesis by comparative analysis with other environmental Chlamydiae (Protochlamydia R18 and Parachlamydia Bn₉ ). Dual staining with 4',6-diamidino-2-phenylindole and phalloidin revealed that the progeny of Neochlamydia S13 and Protochlamydia R18 existed in both daughter cells with a contractile ring on the verge of separation. However, in contrast to other environmental Chlamydiae, little Neochlamydia S13 16S ribosomal DNA was amplified from the culture supernatant. Interestingly, Neochlamydia S13 failed to infect aposymbiotic amoebae, indicating an intimate interaction with the host cells. Furthermore, its infectious rates in cultures expanded from a single amoeba were always maintained at 100%, indicating distribution via cytokinesis. We concluded that unlike other environmental Chlamydiae, Neochlamydia S13 has a unique ability to divide its progeny only via host amoebal cytokinesis. This may be a suitable model to elucidate the mechanism of cell organelle distribution and of chlamydial pathogenesis and evolution.

Peculiar Paramecium Hosts Fail to Establish a Stable Intracellular Relationship With Legionella pneumophila

Legionella pneumophila, an intracellular human pathogen, establishes intracellular relationships with several protist hosts, including Paramecium caudatum. L. pneumophila can escape the normal digestion process and establish intracellular relationships in Paramecium. In this study, we identify new Paramecium strains that significantly reduce the number of L. pneumophila during infection. As a result, stable intracellular relationships between L. pneumophila and these Paramecium strains were not observed. These digestion-type Paramecium also showed high efficiency for Escherichia coli elimination compared to other strains of Paramecium. These results suggest that the digestion-type strains identified have high non-specific digestion activity. Although we evaluated the maturation process of Legionella-containing vacuoles (LCVs) in the Paramecium strains using LysoTracker, there were no discriminative changes in these LCVs compared to other Paramecium strains. Detailed understanding of the mechanisms of high digestion efficiency in these strains could be applied to water purification technologies and L. pneumophila elimination from environmental water.

Chlamydiae in the Environment

Chlamydiae have been known for more than a century as major pathogens of humans. Yet they are also found ubiquitously in the environment where they thrive within protists and in an unmatched wide range of animals. This review summarizes recent advances in understanding chlamydial diversity and distribution in nature. Studying these environmental chlamydiae provides a novel perspective on basic chlamydial biology and evolution. A picture is beginning to emerge with chlamydiae representing one of the evolutionarily most ancient and successful groups of obligate intracellular bacteria.

Highly Reduced Genomes of Protist Endosymbionts Show Evolutionary Convergence

Genome evolution in bacterial endosymbionts is notoriously extreme: the combined effects of strong genetic drift and unique selective pressures result in highly reduced genomes with distinctive adaptations to hosts [1-4]. These processes are mostly known from animal endosymbionts, where nutritional endosymbioses represent the best-studied systems. However, eukaryotic microbes, or protists, also harbor diverse bacterial endosymbionts, but their genome reduction and functional relationships with their hosts are largely unexplored [5-7]. We sequenced the genomes of four bacterial endosymbionts from three species of diplonemids, poorly studied but abundant and diverse heterotrophic protists [8-12]. The endosymbionts come from two bacterial families, Rickettsiaceae and Holosporaceae, that have invaded two families of diplonemids, and their genomes have converged on an extremely small size (605-632 kilobase pairs [kbp]), similar gene content (e.g., metabolite transporters and secretion systems), and reduced metabolic potential (e.g., loss of energy metabolism). These characteristics are generally found in both families, but the diplonemid endosymbionts have evolved greater extremes in parallel. They possess modified type VI secretion systems that could function in manipulating host metabolism or other intracellular interactions. Finally, modified cellular machinery like the ATP synthase without oxidative phosphorylation, and the reduced flagellar apparatus present in some diplonemid endosymbionts and nutritional animal endosymbionts, indicates that intracellular mechanisms have converged in bacterial endosymbionts with various functions and from different eukaryotic hosts across the tree of life.

Symbiont-Mediated Defense against Legionella pneumophila in Amoebae

Bacterial pathogens are generally investigated in the context of disease. To prevent outbreaks, it is essential to understand their lifestyle and interactions with other microbes in their natural environment. Legionella pneumophila is an important human respiratory pathogen that survives and multiplies in biofilms or intracellularly within protists, such as amoebae. Importantly, transmission to humans occurs from these environmental sources. Legionella infection generally leads to rapid host cell lysis. It was therefore surprising to observe that amoebae, including fresh environmental isolates, were well protected during Legionella infection when the bacterial symbiont Protochlamydia amoebophila was also present. Legionella was not prevented from invading amoebae but was impeded in its ability to develop fully virulent progeny and were ultimately cleared in the presence of the symbiont. This study highlights how ecology and virulence of an important human pathogen is affected by a defensive amoeba symbiont, with possibly major consequences for public health.

Legionella pneumophila is an important opportunistic pathogen for which environmental reservoirs are crucial for the infection of humans. In the environment, free-living amoebae represent key hosts providing nutrients and shelter for highly efficient intracellular proliferation of L. pneumophila, which eventually leads to lysis of the protist. However, the significance of other bacterial players for L. pneumophila ecology is poorly understood. In this study, we used a ubiquitous amoeba and bacterial endosymbiont to investigate the impact of this common association on L. pneumophila infection. We demonstrate that L. pneumophila proliferation was severely suppressed in Acanthamoeba castellanii harboring the chlamydial symbiont Protochlamydia amoebophila. The amoebae survived the infection and were able to resume growth. Different environmental amoeba isolates containing the symbiont were equally well protected as different L. pneumophila isolates were diminished, suggesting ecological relevance of this symbiont-mediated defense. Furthermore, protection was not mediated by impaired L. pneumophila uptake. Instead, we observed reduced virulence of L. pneumophila released from symbiont-containing amoebae. Pronounced gene expression changes in the presence of the symbiont indicate that interference with the transition to the transmissive phase impedes the L. pneumophila infection. Finally, our data show that the defensive response of amoebae harboring P. amoebophila leaves the amoebae with superior fitness reminiscent of immunological memory. Given that mutualistic associations between bacteria and amoebae are widely distributed, P. amoebophila and potentially other amoeba endosymbionts could be key in shaping environmental survival, abundance, and virulence of this important pathogen, thereby affecting the frequency of human infection.

Long-term persistence of infectious Legionella with free-living amoebae in drinking water biofilms

Prolific growth of pathogenic Legionella pneumophila within engineered water systems and premise plumbing, and human exposure to aerosols containing this bacterium results in the leading health burden of any water-related pathogen in developed regions. Ecologically, free-living amoebae (FLA) are an important group of the microbial community that influence biofilm bacterial diversity in the piped-water environment. Using fluorescent microscopy, we studied in-situ the colonization of L. pneumophila in the presence of two water-related FLA species, Willaertia magna and Acanthamoeba polyphaga in drinking water biofilms. During water flow as well as after periods of long-stagnation, the attachment and colonization of L. pneumophila to predeveloped water-biofilm was limited. Furthermore, W. magna and A. polyphaga showed no immediate interactions with L. pneumophila when introduced to the same natural biofilm environment. A. polyphaga encysted within 5-7 d after introduction to the tap-water biofilms and mostly persisted in cysts till the end of the study period (850 d). W. magna trophozoites, however, exhibited a time delay in feeding on Legionella and were observed with internalized L. pneumophila cells after 3 weeks from their introduction to the end of the study period and supported putative (yet limited) intracellular growth. The culturable L.pneumophila in the bulk water was reduced by 2-log over 2 years at room temperature but increased (without a change in mip gene copies by qPCR) when the temperature was elevated to 40 °C within the same closed-loop tap-water system without the addition of nutrients or fresh water. The overall results suggest that L. pneumophila maintains an ecological balance with FLA within the biofilm environment, and higher temperature improve the viability of L. pneumophila cells, and intracellular growth of Legionella is possibly cell-concentration dependent. Observing the preferential feeding behavior, we hypothesize that an initial increase of FLA numbers through feeding on a range of other available bacteria could lead to an enrichment of L. pneumophila, and later force predation of Legionella by the amoeba trophozoites results in rapid intracellular replication, leading to problematic concentration of L. pneumophila in water. In order to find sustainable control options for legionellae and various other saprozoic, amoeba-resisting bacterial pathogens, this work emphasizes the need for better understanding of the FLA feeding behavior and the range of ecological interactions impacting microbial population dynamics within engineered water systems.

Evolution of the Arsenal of Legionella pneumophila Effectors To Modulate Protist Hosts

Within the human host, Legionella pneumophila replicates within alveolar macrophages, leading to pneumonia. However, L. pneumophila is an aquatic generalist pathogen that replicates within a wide variety of protist hosts, including amoebozoa, percolozoa, and ciliophora. The intracellular lifestyles of L. pneumophila within the two evolutionarily distant hosts macrophages and protists are remarkably similar. Coevolution with numerous protist hosts has shaped plasticity of the genome of L. pneumophila, which harbors numerous proteins encoded by genes acquired from primitive eukaryotic hosts through interkingdom horizontal gene transfer. The Dot/Icm type IVb translocation system translocates ∼6,000 effectors among Legionella species and >320 effector proteins in L. pneumophila into host cells to modulate a plethora of cellular processes to create proliferative niches. Since many of the effectors have likely evolved to modulate cellular processes of primitive eukaryotic hosts, it is not surprising that most of the effectors do not contribute to intracellular growth within human macrophages. Some of the effectors may modulate highly conserved eukaryotic processes, while others may target protist-specific processes that are absent in mammals. The lack of studies to determine the role of the effectors in adaptation of L. pneumophila to various protists has hampered the progress to determine the function of most of these effectors, which are routinely studied in mouse or human macrophages. Since many protists restrict L. pneumophila, utilization of such hosts can also be instrumental in deciphering the mechanisms of failure of L. pneumophila to overcome restriction of certain protist hosts. Here, we review the interaction of L. pneumophila with its permissive and restrictive protist environmental hosts and outline the accomplishments as well as gaps in our knowledge of L. pneumophila-protist host interaction and L. pneumophila's evolution to become a human pathogen.