The stealth episome: suppression of gene expression on the excised genomic island PPHGI-1 from Pseudomonas syringae pv. phaseolicola

Pseudomonas syringae pv. phaseolicola is the causative agent of halo blight in the common bean, Phaseolus vulgaris. P. syringae pv. phaseolicola race 4 strain 1302A contains the avirulence gene avrPphB (syn. hopAR1), which resides on PPHGI-1, a 106 kb genomic island. Loss of PPHGI-1 from P. syringae pv. phaseolicola 1302A following exposure to the hypersensitive resistance response (HR) leads to the evolution of strains with altered virulence. Here we have used fluorescent protein reporter systems to gain insight into the mobility of PPHGI-1. Confocal imaging of dual-labelled P. syringae pv. phaseolicola 1302A strain, F532 (dsRFP in chromosome and eGFP in PPHGI-1), revealed loss of PPHGI-1::eGFP encoded fluorescence during plant infection and when grown in vitro on extracted leaf apoplastic fluids. Fluorescence-activated cell sorting (FACS) of fluorescent and non-fluorescent PPHGI-1::eGFP F532 populations showed that cells lost fluorescence not only when the GI was deleted, but also when it had excised and was present as a circular episome. In addition to reduced expression of eGFP, quantitative PCR on sub-populations separated by FACS showed that transcription of other genes on PPHGI-1 (avrPphB and xerC) was also greatly reduced in F532 cells harbouring the excised PPHGI-1::eGFP episome. Our results show how virulence determinants located on mobile pathogenicity islands may be hidden from detection by host surveillance systems through the suppression of gene expression in the episomal state

The Family Campylobacteraceae

The Campylobacteraceae is the largest and most diverse family in the phylogenetically distinct Epsilonproteobacteria, presently comprising the genera Campylobacter (30 taxa), Arcobacter (17 taxa), and Sulfurospirillum (7 taxa). Individual species may be able to grow in microaerobic, anaerobic, and/or aerobic conditions, in temperatures from 25°C to 42°C; free-living, commensal, or pathogenic; motile or aflagellate; and able to colonize the oral cavity, intestine, stomach, or reproductive tracts of humans, large production animals (such as sheep, cattle, and deer), birds, and reptiles. Some species are known to be among the most frequent causes of human gastroenteritis, others are significant threats to bovine and ovine fertility, and many pose an as-yet unknown, or no, role in human or animal diseases. The taxonomy of the Campylobacteraceae has evolved extensively since its beginnings in 1963. This chapter outlines key events in the family's taxonomic history; reviews general phenotypic traits of each genus, including their isolation; outlines the clinical (including pathogenicity studies and antimicrobial resistance traits) and/or ecological significance of constituent species; and describes current approaches and challenges for species identification and epidemiological subtyping