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The bacterial plant pathogen Pseudomonas syringae causes disease in a wide range of plants. The associated decrease in crop yields results in economic losses and threatens global food security. Competition exists between the plant immune system and the pathogen, the basic principles of which can be applied to animal infection pathways. P. syringae uses a type III secretion system (T3SS) to deliver virulence factors into the plant that promote survival of the bacterium. The P. syringae T3SS is a product of the hypersensitive response and pathogenicity (hrp) and hypersensitive response and conserved (hrc) gene cluster, which is strictly controlled by the codependent enhancer-binding proteins HrpR and HrpS. Through a combination of bacterial gene regulation and phenotypic studies, plant infection assays, and plant hormone quantifications, we now report that Chp8 (i) is embedded in the Hrp regulon and expressed in response to plant signals and HrpRS, (ii) is a functional diguanylate cyclase, (iii) decreases the expression of the major pathogen-associated molecular pattern (PAMP) flagellin and increases extracellular polysaccharides (EPS), and (iv) impacts the salicylic acid/jasmonic acid hormonal immune response and disease progression. We propose that Chp8 expression dampens PAMP-triggered immunity during early plant infection., IMPORTANCE The global demand for food is projected to rise by 50% by 2030 and, as such, represents one of the major challenges of the 21st century, requiring improved crop management. Diseases caused by plant pathogens decrease crop yields, result in significant economic losses, and threaten global food security. Gaining mechanistic insights into the events at the plant-pathogen interface and employing this knowledge to make crops more resilient is one important strategy for improving crop management. Plant-pathogen interactions are characterized by the sophisticated interplay between plant immunity elicited upon pathogen recognition and immune evasion by the pathogen. Here, we identify Chp8 as a contributor to the major effort of the plant pathogen Pseudomonas syringae pv. tomato DC3000 to evade immune responses of the plant.

Staphylococcus (S.) aureus can cause serious infections toxinoses and life threatening illnesses. Staphylocoagulase production represents the major criterion for detection of S. aureus isolates. As coagulase deficient clinical isolates of S. aureus have been described additional use of chromogenic media in S. aureus detection was postulated to represent a highly specific (97) and sensitive (99 after 48h) tool in the identification of the organism. In this study we further investigated genotypic and phenotypic characteristics of a coagulase and a glucosidase deficient variant of S. aureus obtained from bovine mastitis milk. MSSA_129 was identified as S. aureus by species specific PCR MALDI TOF (ID 99.90) and DNA microarray and could be assigned to spa type t543 clonal complex CC479 agr type II and capsule type 8. Still no opaque zone was visible on rabbit plasma fibrinogen agar (RPF Oxoid Cambridge United Kingdom). On S. aureus ID agar (SAID Biomérieux La Balme les Grottes France) bright yellow colonies instead of the characteristic green colonies were formed. Determination of the nucleotide sequence of coa revealed a deletion at nucleotide position 653 within the D2 region of the gene leading to a frameshift. Although coa is known to represent a highly polymorphic region it can be divided into six common regions: the signal sequence the D1 and D2 regions enabling contact with prothrombin the central region a repeat region and the C terminal sequence. The detected frameshift results in the reading of a premature stop codon at amino acid position 224 thus rendering the polypeptide abnormally short and most likely not functional. The a glucosidase gene of MSSA_129 and its upstream region were compared to sequences available at Genbank. Although no strain possesses an a glucosidase gene exactly identical over its full length to the one found in MSSA_129 no unique amino acid changes were found. However variabilty of amino acid sequences on 12 positions (position 43 83 88 133 439 448 454 462 492 514 519 and 539) within the available sequences was noticed. Considering amino acids found at these positions strain MSSA_129 showed a unique combination. Directly upstream of the a glucosidase gene (malA) we identified a sequence homologous to malR a putative transcriptional regulator involved in maltose transport in Staphylococcus xylosus. In S. xylosus the malRA genes are cotranscribed and constitute an essential locus for maltosaccharide utilization. MSSA_129 malR exhibited several unique amino acid changes. Expression levels of malA were compared among MSSA_129 and two phenotypically a glucosidase positive strains LRA1 and HG003 by quantitative Real Time PCR. As MSSA_129 expressed malA in similar levels compared to HG003 and even in significantly higher levels than in LRA1 (p = 0.01) the hypothesis that the yellow phenotype may be due to a negative effect of malR polymorphisms on malA expression was dismissed. Complementation of MSSA_129 with the a glucosidase gene of strain HG003 on a plasmid rescued the green phenotype on SAID agar. Complementation of MSSA_129 with its own malA or an empty vector did not alter the phenotype on SAID. We therefore suggest the atypical phenotype on SAID to be due to the sequence polymorphisms discovered in the MSSA_129 a glucosidase gene. Considering our findings screening for S. aureus by only RPF and SAID agar harbors the risk of false negative results potentially leading to severe therapeutic mistakes.