Did new transmission cycles in anthropogenic, dense, host populations encourage the emergence and speciation of pathogenic Bordetella?

Genomic evidence and virulence properties decipher the extra-host origin of Bordetella bronchiseptica

Until recently, members of the classical Bordetella species comprised only pathogenic bacteria that were thought to live exclusively in warm-blooded animals. The close phylogenetic relationship of Bordetella with Achromobacter and Alcaligenes, which include primarily environmental bacteria, suggests that the ancestral Bordetellae were probably free-living. Eventually, the Bordetella species evolved to infect and live within warm-blooded animals. The modern history of pathogens related to the genus Bordetella started towards the end of the 19th century when it was discovered in the infected respiratory epithelium of mammals, including humans. The first identified member was Bordetella pertussis, which causes whooping cough, a fatal disease in young children. In due course, B. bronchiseptica was recovered from the trachea and bronchi of dogs with distemper. Later, a second closely related human pathogen, B. parapertussis, was described as causing milder whooping cough. The classical Bordetellae are strictly host-associated pathogens transmitted via the host-to-host aerosol route. Recently, the B. bronchiseptica strain HT200 has been reported from a thermal spring exhibiting unique genomic features that were not previously observed in clinical strains. Therefore, it advocates that members of classical Bordetella species have evolved from environmental sources. This organism can be transmitted via environmental reservoirs as it can survive nutrient-limiting conditions and possesses a motile flagellum. This study aims to review the molecular basis of origin and virulence properties of obligate host-restricted and environmental strains of classical Bordetella.

Natural History and Ecology of Interactions Between Bordetella Species and Amoeba

A variety of bacteria have evolved the ability to interact with environmental phagocytic predators such as amoebae, which may have facilitated their subsequent interactions with phagocytes in animal hosts. Our recent study found that the animal pathogen Bordetella bronchiseptica can evade predation by the common soil amoeba Dictyostelium discoideum, survive within, and hijack its complex life cycle as a propagation and dissemination vector. However, it is uncertain whether the mechanisms allowing interactions with predatory amoebae are conserved among Bordetella species, because divergence, evolution, and adaptation to different hosts and ecological niches was accompanied by acquisition and loss of many genes. Here we tested 9 diverse Bordetella species in three assays representing distinct aspects of their interactions with D. discoideum. Several human and animal pathogens retained the abilities to survive within single-celled amoeba, to inhibit amoebic plaque expansion, and to translocate with amoebae to the fruiting body and disseminate along with the fruiting body. In contrast, these abilities were partly degraded for the bird pathogen B. avium, and for the human-restricted species B. pertussis and B. parapertussis. Interestingly, a different lineage of B. parapertussis only known to infect sheep retained the ability to interact with D. discoideum, demonstrating that these abilities were lost in multiple lineages independently, correlating with niche specialization and recent rapid genome decay apparently mediated by insertion sequences. B. petrii has been isolated sporadically from diverse human and environmental sources, has acquired insertion sequences, undergone genome decay and has also lost the ability to interact with amoebae, suggesting some specialization to some unknown niche. A genome-wide association study (GWAS) identified a set of genes that are potentially associated with the ability to interact with D. discoideum. These results suggest that massive gene loss associated with specialization of some Bordetella species to a closed life cycle in a particular host was repeatedly and independently accompanied by loss of the ability to interact with amoebae in an environmental niche.

Probing Immune-Mediated Clearance of Acute Middle Ear Infection in Mice

Acute otitis media (AOM) is commonly caused by bacterial pathobionts of the nasopharynx that ascend the Eustachian tube to cause disease in the middle ears. To model and study the various complexities of AOM, common human otopathogens are injected directly into the middle ear bullae of rodents or are delivered with viral co-infections which contribute to the access to the middle ears in complex and partially understood ways. Here, we present the novel observation that Bordetella bronchiseptica, a well-characterized respiratory commensal/pathogen of mice, also efficiently ascends their Eustachian tubes to colonize their middle ears, providing a flexible mouse model to study naturally occurring AOM. Mice lacking T and/or B cells failed to resolve infections, highlighting the cooperative role of both in clearing middle ear infection. Adoptively transferred antibodies provided complete protection to the lungs but only partially protected the middle ears, highlighting the differences between respiratory and otoimmunology. We present this as a novel experimental system that can capitalize on the strengths of the mouse model to dissect the molecular mechanisms involved in the generation and function of immunity within the middle ear.

Evolution and Conservation of Bordetella Intracellular Survival in Eukaryotic Host Cells

The classical bordetellae possess several partially characterized virulence mechanisms that are studied in the context of a complete extracellular life cycle in their mammalian hosts. Yet, classical bordetellae have repeatedly been reported within dendritic cells (DCs) and alveolar macrophages in clinical samples, and in vitro experiments convincingly demonstrate that the bacteria can survive intracellularly within mammalian phagocytic cells, an ability that appears to have descended from ancestral progenitor species that lived in the environment and acquired the mechanisms to resist unicellular phagocytic predators. Many pathogens, including Mycobacterium tuberculosis, Salmonella enterica, Francisella tularensis, and Legionella pneumophila, are known to parasitize and multiply inside eukaryotic host cells. This strategy provides protection, nutrients, and the ability to disseminate systemically. While some work has been dedicated at characterizing intracellular survival of Bordetella pertussis, there is limited understanding of how this strategy has evolved within the genus Bordetella and the contributions of this ability to bacterial pathogenicity, evasion of host immunity as well as within and between-host dissemination. Here, we explore the mechanisms that control the metabolic changes accompanying intracellular survival and how these have been acquired and conserved throughout the evolutionary history of the Bordetella genus and discuss the possible implications of this strategy in the persistence and reemergence of B. pertussis in recent years.