Role of distinct type-IV-secretion systems and secreted effector sets in host adaptation by pathogenic Bartonella species

Abstract

The α-proteobacterial genus Bartonella comprises a large number of facultative intracellular pathogens that share a common lifestyle hallmarked by hemotrophic infection and arthropod transmission. Speciation in the four deep-branching lineages (L1-L4) occurred by host adaptation facilitating the establishment of long lasting bacteraemia in specific mammalian reservoir host(s). Two distinct type-IV-secretion systems (T4SSs) acquired horizontally by different Bartonella lineages mediate essential host interactions during infection and represent key innovations for host adaptation. The Trw-T4SS confined to the species-rich L4 mediates host-specific erythrocyte infection and likely has functionally replaced flagella as ancestral virulence factors implicated in erythrocyte colonisation by bartonellae of the other lineages. The VirB/VirD4-T4SS translocates Bartonella effector proteins (Bep) into various host cell types to modulate diverse cellular and innate immune functions involved in systemic spreading of bacteria following intradermal inoculation. Independent acquisition of the virB/virD4/bep locus by L1, L3, and L4 was likely driven by arthropod vectors associated with intradermal inoculation of bacteria rather than facilitating direct access to blood. Subsequently, adaptation to colonise specific niches in the new host has shaped the evolution of complex species-specific Bep repertoires. This diversification of the virulence factor repertoire of Bartonella spp. represents a remarkable example for parallel evolution of host adaptation.

Keywords: Bartonella; Bep effector proteins; Trw; VirB/VirD4; host adaptation; type-IV-secretion systems.

Figure 1

Phylogeny of Bartonella and distribution of key virulence factors. Phylogeny of the genus Bartonella with the ant‐specific species (a) Candidatus Tokpelaia hoelldoblerii as outgroup taxon. The phylogenetic pattern resembles the tree topology from (Segers, Kesnerova, Kosoy, & Engel, 2017) and shows the three Bartonella clades composed of the honeybee symbiont (b) Bartonella apis, pathogenic Bartonella tamiae, and the eubartonellae. Eubartonellae are further separated into four lineages and Bartonella australis (d). The phylogenetic tree was inferred based on a concatenated alignment of five core protein sequences. Indicated are arthropod (C‐confirmed vectors) and reservoir hosts, as well as the zoonotic potential of Bartonella spp. The presence and absence of key virulence factors is indicated by full and empty circles, respectively. In contrast to chromosomally encoded VbhT T4SSs, the plasmid encoded counterparts are indicated with an (e) next to the full circle. BaGTA: Bartonella gene transfer agent; T4SS: type‐IV‐secretion system; Bep: Bartonella effector protein; nd: not determined

Figure 2

Parallel evolution of VbhT and Bep repertoires. Independent fusion of FIC‐domains (from intrabacterial toxin‐antitoxin (TA) modules) to a relaxase derived type‐IV‐secretion (T4S) signal leading to VbhT and a primordial toxin. This T4S signal is composed of a C‐terminal BID domain and a positive tail (+++).The primordial, interbacterial toxin evolved to an interkingdom effector—the ancestral Bep. The three Bep repertoires of Bartonella ancashensis (L1), L3, and L4 likely evolved from this ancestral Bep independently via gene duplication, followed by recombination and fixation of adaptive mutations. The majority of Beps (and VbhT) possess the FIC‐BID architecture; however, Beps with a derived domain composition evolved in all three lineages. Indicated are the catalytic FIC‐motif (*): conserved and canonical (HPFX[D/E]GNGRXXR; blue vertical line), conserved, but not canonical (CPFX[G/A]GNECTQX for Bep4 orthologues; purple vertical line) and not conserved (XPFXXGNXXTXX; for BepA orthologues, black vertical line) among orthologues. Tyrosine‐phosphorylation motifs are highlighted with an Y