Host specificity of bacterial pathogens

Abstract

Most pathogens are able to infect multiple hosts but some are highly adapted to a single-host species. A detailed understanding of the basis of host specificity can provide important insights into molecular pathogenesis, the evolution of pathogenic microbes, and the potential for pathogens to cross the species barrier to infect new hosts. Comparative genomics and the development of humanized mouse models have provided important new tools with which to explore the basis of generalism and specialism. This review will examine host specificity of bacterial pathogens with a focus on generalist and specialist serovars of Salmonella enterica.

Figure 1.

Host range of members of the genus Salmonella. The genus Salmonella consists of two species, S. enterica and S. bongori. S. enterica is further subdivided into seven subspecies, designated I, II, IIIa, IIIb, IV, VI, and VII. Serovars of S. bongori and S. enterica subspecies II, IIIa, IIIb, IV, VI, and VII are largely reptile associated and can be occasionally transmitted from this reservoir to humans. The majority of serovars belonging to S. enterica subspecies I are generalists that have reservoirs in mammalian, avian, and reptilian species. Specialists with a more restricted host range have independently evolved from this group several times.

Figure 2.

Humanized mouse model of typhoid fever. Newborn immunodeficient NOD-scid IL2rγ^null mice are irradiated before transplantation of human CD34⁺ hematopoietic stem cells from T-cell-depleted umbilical cord blood. Engrafted mice that have been repopulated with human SCID repopulating cells (hu-SRC) are now susceptible to lethal systemic infection with the human-adapted pathogen Salmonella Typhi.