B. Duffy

Characterization of spontaneous gacS and gacA regulatory mutants of Pseudomonas fluorescens biocontrol strain CHA0

In Pseudomonas fluorescens strain CHA0, the response regulator gene gacA controls expression of extracellular enzymes and antifungal secondary metabolites, which are important for this strains biocontrol activity in the plant rhizosphere. Two Tn5 insertion mutants of strain CHA0 that had the same pleiotropic phenotype as gacA mutants were complemented by the gacS sensor kinase gene of P. syringae pv. syringae as well as that of P. fluorescens strain Pf-5, indicating that both transposon insertions had occurred in the gacS gene of strain CHA0. This conclusion was supported by Southern hybridisation using a gacS probe from strain Pf-5. Overexpression of the wild-type gacA gene partially compensated for the gacS mutation, however, the overexpressed gacA gene was not stably maintained, suggesting that this is deleterious to the bacterium. Strain CHA0 grown to stationary phase in nutrient-rich liquid media for several days accumulated spontaneous pleiotropic mutants to levels representing 1.25% of the population; all mutants lacked key antifungal metabolites and extracellular protease. Half of 44 spontaneous mutants tested were complemented by gacS, the other half were restored by gacA. Independent point and deletion mutations arose at different sites in the gacA gene. In competition experiments with mixtures of the wild type and a gacA mutant incubated in nutrient-rich broth, the mutant population temporarily increased as the wild type decreased. In conclusion, loss of gacA function can confer a selective advantage on strain CHA0 under laboratory conditions.

Comparative genomic analysis of Salmonella enterica subsp. enterica serovar Weltevreden foodborne strains with other serovars

Salmonella enterica subsp. enterica serovar Weltevreden is a dominant serovar associated with foodborne gastroenteritis in South-East Asia and emerging in Europe associated with fresh vegetables. Here we compared the genome of strain 2007-60-3289-1 linked to an alfalfa sprout outbreak in Scandinavia with a S. Weltevreden strain isolated from scallops in the USA and with other S. enterica serovars. A unique plasmid pSW82 was identified for S. Weltevreden carrying a two-component type II non-ribosomal peptide synthase/polyketide synthase. Analysis of all available complete S. enterica genomes identified differences for presence of type VI secretion systems and carbohydrate metabolic pathways. Differential transcription thereof was observed when S. Weltevreden strains were grown in vitro or on sprouts.

Misidentification slanders Pantoea agglomerans as a serial killer

Pantoea agglomerans is an ecologically versatile species that has important beneficial applications in agriculture as a biological alternative to antibiotic use for orchard protection against phytopathogenic bacteria. Disease-suppressive, biocontrol isolates are among the most effective tools available to manage fire blight, an invasive threat to global apple and pear production. P. agglomerans-based products are registered for biocontrol in the USA, Canada and New Zealand, but clinical reports have been a regulatory obstacle in Europe. Thus, the recent report of a fatal bacteraemia outbreak attributed to P. agglomerans is alarming and would have significant socio-economic impact, if the conclusion of clinical pathogenicity was correct. There is mounting evidence that many, if not most, P. agglomerans clinical reports are based upon inaccurate isolate identification, resulting from investigators’ reliance on inadequate identification methods and/or obsolete nomenclature. The P. agglomeranseEnterobacter agglomeranseErwinia herbicola complex has experienced several taxonomic revisions, and both biochemical profiling as well as 16S rRNA gene sequencing lack the resolution needed to discriminate P. agglomerans sensu stricto isolates. Despite conclusive demonstration using gyrB sequencing and matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) that a number of clinical strains archived in reputable culture collections do not belong to the genus Pantoea and even less to the species P. agglomerans, most remain erroneously catalogued. Inaccurate identification has been shown for 95.5% of ‘P. agglomerans’ clinical strains in the American Type Culture Collection (ATCC), a reference used to identify the Brazilian outbreak pathogen. The authors correctly recognized the difficulties involved in routine biochemical identification of P. agglomerans and appropriately applied gyrB sequencing in their characterization of isolates from a nosocomial outbreak in Brazil. They reported their isolates as P. agglomerans based on a 100% sequence identity of their gyrB sequences with that of strain ATCC 27990, which was deposited in GenBank by our laboratory (GenBank nucleotide database: FJ617358). However, this strain is not P. agglomerans but rather belongs to an unnamed

Trace Mineral Amendments in Agriculture for Optimizing the Biocontrol Activity of Plant-Associated Bacteria

There is growing interest in the large-scale application of plant-associated, nonpathogenic bacteria for the biological control of soilborne plant diseases. This is largely due to a lack of other control options for soilborne diseases but also to agricultural trends towards sustainability and to public concern over the hazards of synthetic fungicides and fumigants. In the last twenty years, a diversity of plant-associated bacteria, particularly fluorescent pseudomonads, have been shown to effectively control a wide-spectrum of soilborne diseases in greenhouse trials and field plots of small to moderate scale. Commercialization of most biocontrol agents, however, has been hindered by variable performance and only a handful of bacterial products have been registered for agricultural application (Cook, 1993). The level and reliability of disease suppression must be optimized for biocontrol to become commercially feasible on a large-scale.