Plants are known to secrete chemical compounds that can change the behavior of rhizosphere-inhabiting bacteria. We investigated the effects of extracts from legume host plants on the swarming behavior of Rhizobium leguminosarum bv. viciae. We also investigated the effects on swarming when Rhizobium is exposed to extracts from an ancestor to vascular plants, the model bryophyte Physcomitrella patens. Lentil and faba bean seed exudates enhanced and inhibited swarming motility, respectively, whereas pea seed exudates had no observable effect on swarming. Swarming was also enhanced by the moss extracts. Exposure to lentil seed exudates and the moss extract increased flaA expression 2-fold, while faba bean seed exudates exposure decreased expression 3-fold, suggesting that the swarming effect could, in part, be due to regulation of flagellin gene expression. However, the exudates and extracts did not significantly affect flaA gene expression in planktonic motile cells, indicating that the response to flagellar regulation is specific to a physiology unique to the swarming cell. Transmission electron microscopy demonstrated that addition of the lentil seed exudate and the moss extract results in earlier differentiation into swarmer cells, which could contribute to the development of a larger swarming surface area. To gain further mechanistic insight into the effect of the moss extract on swarming, a moss strigolactone-deficient mutant (Ppccd8⌬) was tested. A reduction in the promotive effect was observed, suggesting that the plant hormone strigolactone may be a signalling molecule activating swarming motility in R. leguminosarum.Key words: Rhizobium, swarming motility, plant extracts. Résumé :Les plantes sont reconnues pour sécréter des composés chimiques qui modifient le comportement des bactéries vivant en rhizosphère. Nous avons examiné l'impact d'extraits de plantes légumineuses hôtes sur l'essaimage de Rhizobium leguminosarum bv. viciae. Nous avons également examiné la modulation de l'essaimage lorsque Rhizobium était exposé à des extraits d'un ancêtre des plantes vasculaires, la bryophyte modèle Physcomitrella patens. Des exsudats de graines de lentille et de féveroles ont respectivement stimulé et inhibé l'essaimage, alors que des exsudats de graines de pois n'ont pas eu d'effet notable. L'essaimage a aussi été stimulé par des extraits de mousse. L'exposition aux exsudats de graines de lentille et à l'extrait de mousse a doublé l'expression de flaA alors que les exsudats de graines de féveroles l'ont diminué des deux tiers, ce qui indique que l'effet sur l'essaimage pourrait être partiellement attribuable à la régulation de l'expression du gène de la flagelline. Or, les exsudats et extraits n'ont pas eu d'incidence significative sur l'expression du gène flaA chez les cellules planctoniques motiles, ce qui indique que la réponse en termes de régulation flagellaire n'opérerait que grâce à une physiologie particulière de la cellule en essaimage. La microscopie électronique par transmission a démontré que ...

The phage P106B (vB_RglS_P106B) is a Siphoviridae phage with a narrow spectrum of infectivity, which has been isolated from soils with a history of pea cultivation. The trapping host of P106B is an indigenous strain of Rhizobium gallicum (SO14B-4) isolated from soils associated with Vicia cracca. Phenotypic characterization of the phage revealed that P106B has an approximate burst size of 21 p.f.u. per infected cell with 60 min and 100 min eclipse and latent periods, respectively. Phage P106B was unable to transduce under the conditions tested. The genome of P106B is 56 024 bp in length with a mean DNA G+C content of 47.9 %. The complete genome sequence contains 95 putative ORFs and a single tRNA gene coding for leucine with the anticodon TTA. Putative functions could only be assigned to 22 of the predicted ORFs while a significant number of ORFs (47) shared no sequence similarities to previously characterized proteins. The remaining 26 putative protein-coding genes exhibited a sequence resemblance to other hypothetical proteins. No lysogeny-related genes were found in the P106B genome.

Bacteriophages may play an important role in regulating population size and diversity of the root nodule symbiont Rhizobium leguminosarum, as well as participating in horizontal gene transfer. Although phages that infect this species have been isolated in the past, our knowledge of their molecular biology, and especially of genome composition, is extremely limited, and this lack of information impacts on the ability to assess phage population dynamics and limits potential agricultural applications of rhizobiophages. To help address this deficit in available sequence and biological information, the complete genome sequence of the Myoviridae temperate phage PPF1 that infects R. leguminosarum biovar viciae strain F1 was determined. The genome is 54,506 bp in length with an average G+C content of 61.9 %. The genome contains 94 putative open reading frames (ORFs) and 74.5 % of these predicted ORFs share homology at the protein level with previously reported sequences in the database. However, putative functions could only be assigned to 25.5 % (24 ORFs) of the predicted genes. PPF1 was capable of efficiently lysogenizing its rhizobial host R. leguminosarum F1. The site-specific recombination system of the phage targets an integration site that lies within a putative tRNA-Pro (CGG) gene in R. leguminosarum F1. Upon integration, the phage is capable of restoring the disrupted tRNA gene, owing to the 50 bp homologous sequence (att core region) it shares with its rhizobial host genome. Phage PPF1 is the first temperate phage infecting members of the genus Rhizobium for which a complete genome sequence, as well as other biological data such as the integration site, is available.

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<i>Mesorhizobium</i> phage vB_MloS_Cp1R7A-A1 was isolated from soil cultivated to chickpea in Saskatchewan and is dissimilar in sequence and morphology to previously described rhizobiophages. It is a B3 morphotype virus with a distinct prolate capsid and belongs to the tailed phage family <i>Siphoviridae</i>. Its genome has a GC content of 60.3% and 238 predicted genes. Putative functions were predicted for 57 genes, which include 27 tRNA genes with anticodons corresponding to 18 amino acids. This represents the highest number of tRNA genes yet reported in a rhizobiophage. The gene arrangement shows a partially modular organization. Most of the structural genes are found in one module, whereas tRNA genes are in another. Genes for replication, recombination and nucleotide metabolism form the third module. The arrangement of the replication module resembles the replication module of Enterobacteria phage T5, raising the possibility that it uses a recombination-based replication mechanism. Phage termini appear to be long direct repeats of length just over 12 kb. Phylogenetic analysis revealed that Cp1R7A-A1 is more closely related to phiCbK-like <i>Caulobacter</i> phages and other B3 morphotype phages than to other rhizobiophages sequenced thus far.

Bacterial soft rot disease, caused by Pectobacterium spp., can lead to significant losses in carrot yields. As current control measures involving the use of chemicals or antibiotics are not recommended in many countries, bacteriophage-mediated biocontrol strategies are being explored for the successful control of these phytopathogens.

Listeria monocytogenes is a food-borne pathogen that can cause severe invasive infection called ‘listerosis’ in humans. Development of antibiotic resistance is a major setback in the management of conditions caused by Listeria in both human and veterinary medicine. In this study, antibiotic resistance of fifty L. monocytogenes strains isolated from raw milk samples collected from farms in Polonnaruwa district, Sri Lanka was determined for four commonly used antibiotics; penicillin, ampicillin, streptomycin and tetracycline. The strains were also tested for the presence of selected antibiotic resistant genes (penA, ampC, strA, strB, tetA and tetB). L monocytogenes isolates showed resistance to ampicillin (60%), penicillin (40%) streptomycin (16%) and tetracycline (8%) in diffusion assays. Phenotypic multidrug resistance was exhibited by twenty isolates. The tetracycline resistant gene (tetA) was detected in seven isolates, while tetB was not detected in any. Presence of streptomycin resistant genes (strA or strB) was confirmed in seven isolates. Ampicillin (ampC) and penicillin (penA) resistant genes were not detected in any of the tested isolates. Except from the samples collected from Sungavila area, isolates from other sampling areas showed resistance to at least one of the antibiotics tested, suggesting that raw milk samples are prone to be contaminated with L. monocytogenes strains with different antibiotic resistant profiles. Therefore, necessary hygienic precautions are recommended to avoid any potential public health threats and to safeguard the health of raw milk consumers.

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