Secreted prokaryotic effector proteins have evolved to modulate the cellular functions of specific eukaryotic hosts. Generally, these proteins are considered virulence factors that facilitate parasitism. However, in certain plant and insect eukaryotic/prokaryotic relationships, effector proteins are involved in the establishment of commensal or symbiotic interactions. In this study, we report that the AvrA protein from Salmonella typhimurium, a common enteropathogen of humans, is an effector molecule that inhibits activation of the key proinflammatory NF-κB transcription factor and augments apoptosis in human epithelial cells. This activity is similar but mechanistically distinct from that described for YopJ, an AvrA homolog expressed by the bacterial pathogen Yersinia. We suggest that AvrA may limit virulence in vertebrates in a manner analogous to avirulence factors in plants, and as such, is the first bacterial effector from a mammalian pathogen that has been ascribed such a function.

bAgrobacterium tumefaciens is a facultative plant pathogen and the causative agent of crown gall disease. The initial stage of infection involves attachment to plant tissues, and subsequently, biofilms may form at these sites. This study focuses on the periplasmic ExoR regulator, which was identified based on the severe biofilm deficiency of A. tumefaciens exoR mutants. Genome-wide expression analysis was performed to elucidate the complete ExoR regulon. Overproduction of the exopolysaccharide succinoglycan is a dramatic phenotype of exoR mutants. Comparative expression analyses revealed that the core ExoR regulon is unaffected by succinoglycan synthesis. Several findings are consistent with previous observations: genes involved in succinoglycan biosynthesis, motility, and type VI secretion are differentially expressed in the ⌬exoR mutant. In addition, these studies revealed new functional categories regulated by ExoR, including genes related to virulence, conjugation of the pAtC58 megaplasmid, ABC transporters, and cell envelope architecture. To address how ExoR exerts a broad impact on gene expression from its periplasmic location, a genetic screen was performed to isolate suppressor mutants that mitigate the exoR motility phenotype and identify downstream components of the ExoR regulatory pathway. This suppression analysis identified the acidsensing two-component system ChvG-ChvI, and the suppressor mutant phenotypes suggest that all or most of the characteristic exoR properties are mediated through ChvG-ChvI. Subsequent analysis indicates that exoR mutants are simulating a response to acidic conditions, even in neutral media. This work expands the model for ExoR regulation in A. tumefaciens and underscores the global role that this regulator plays on gene expression.

SummaryAgrobacterium tumefaciens-is a plant pathogen that transfers a segment of its own DNA into host plants to cause Crown Gall disease. The infection process requires intimate contact between the infecting bacteria and the host tissue. A. tumefaciens attaches efficiently to plant tissues and to abiotic surfaces, and can establish complex biofilms at colonization sites. The dominant mode of attachment is via a single pole in contact with the surface. Several different appendages, adhesins and adhesives play roles during attachment, and foster the transition from free-swimming to sessile growth. This polar surface interaction reflects a more fundamental cellular asymmetry in A. tumefaciens that influences and is congruent with its attached lifestyle.

Background Recurrent Clostridioides difficile infection (rCDI) is associated with loss of microbial diversity and microbe-derived secondary bile acids, which inhibit C. difficile germination and growth. SER-109, an investigational microbiome drug of donor-derived, purified spores, reduced recurrence in a dose-ranging, phase (P) 1 study in subjects with multiple rCDIs. Methods In a P2 double-blind trial, subjects with clinical resolution on standard-of-care antibiotics were stratified by age (< or ≥65 years) and randomized 2:1 to single-dose SER-109 or placebo. Subjects were diagnosed at study entry by PCR or toxin testing. Safety, C. difficile–positive diarrhea through week 8, SER-109 engraftment, and bile acid changes were assessed. Results 89 subjects enrolled (67% female; 80.9% diagnosed by PCR). rCDI rates were lower in the SER-109 arm than placebo (44.1% vs 53.3%) but did not meet statistical significance. In a preplanned analysis, rates were reduced among subjects ≥65 years (45.2% vs 80%, respectively; RR, 1.77; 95% CI, 1.11–2.81), while the <65 group showed no benefit. Early engraftment of SER-109 was associated with nonrecurrence (P < .05) and increased secondary bile acid concentrations (P < .0001). Whole-metagenomic sequencing from this study and the P1 study revealed previously unappreciated dose-dependent engraftment kinetics and confirmed an association between early engraftment and nonrecurrence. Engraftment kinetics suggest that P2 dosing was suboptimal. Adverse events were generally mild to moderate in severity. Conclusions Early SER-109 engraftment was associated with reduced CDI recurrence and favorable safety was observed. A higher dose of SER-109 and requirements for toxin testing were implemented in the current P3 trial. Clinical Trials Registration NCT02437487, https://clinicaltrials.gov/ct2/show/NCT02437487?term=SER-109&draw= 2&rank=4.

The ubiquitous plant pathogen Agrobacterium tumefaciens attaches efficiently to plant tissues and abiotic surfaces and can form complex biofilms. A genetic screen for mutants unable to form biofilms on PVC identified disruptions in a homologue of the exoR gene. ExoR is a predicted periplasmic protein, originally identified in Sinorhizobium meliloti, but widely conserved among alphaproteobacteria. Disruptions in the A. tumefaciens exoR gene result in severely compromised attachment to abiotic surfaces under static and flow conditions, and to plant tissues. These mutants are hypermucoid due to elevated production of the exopolysaccharide succinoglycan, via derepression of the exo genes that direct succinoglycan synthesis. In addition, exoR mutants have lost flagellar motility, do not synthesize detectable flagellin and are diminished in flagellar gene expression. The attachment deficiency is, however, complex and not solely attributable to succinoglycan overproduction or motility disruption. A. tumefaciens ExoR can function independently of the ChvG–ChvI two component system, implicated in ExoR-dependent regulation in S. meliloti. Mutations that suppress the exoR motility defect suggest a branched regulatory pathway controlling succinoglycan synthesis, motility and biofilm formation.