QS-regulated Phenotypes Research Articles

Characterization of Three Types of Quorum-Sensing Mutants in Burkholderia glumae Strains Isolated in Japan

Quorum sensing (QS) has been believed to be essential for the production of virulence factor such as toxoflavin, oxalate and the motility in Burkholderia glumae. Two types of QS mutant (tofI mutant) were recently reported in geographically different isolates of B. glumae. The tofI mutant derived from B. glumae BGR1 isolated in Korea had lost toxoflavin production but not the one derived from B. glumae 336gr-1 isolated in the United States. In the present study, we generated tofI mutants from 14 different B. glumae strains isolated in Japan and investigated the QS-regulated phenotypes. All tofI mutants failed to produce N-acyl-L-homoserine lactones. However, tofI mutants from 11 out of 14 strains (Group I) retained toxoflavin productivity both on a solid and in a liquid medium like B. glumae 336gr-1 tofI mutant, whereas those from 2 strains (Group II) lost toxoflavin productivity both on a solid and in a liquid medium similar to B. glumae BGR1 tofI mutant. The other strain (Group III) showed novel phenotype because toxoflavin production was observed only on a solid medium. Furthermore, Group II tofI mutants lost oxalate productivity and exhibited a severe reduction in motility while Group I tofI mutants slightly produced oxalate and showed higher motility than Group II tofI mutants. These data suggest that the difference between Group I and II is not limited to toxoflavin production but also observed other QS-regulated functions. On the other hand, R1-types of morphological mutants frequently emerge from S-type colonies during the subculture for B. glumae (Tsushima et al., 1991). This study revealed that Group I wild-type strains displayed the S-type morphology and those of Group II showed the R1-type morphology. The results suggest that colony type is involved in the QS-regulated functions of Group I and II tofI mutants.

Anti-quorum sensing activity of flavonoid-rich fraction from Centella asiatica L. against Pseudomonas aeruginosa PAO1.

Inhibition of quorum sensing (QS), a cell-density dependent regulation of gene expression in bacteria by autoinducers is an attractive strategy for the development of antipathogenic agents. In this study, the anti-QS activity of the ethanolic extract of the traditional herb Centella asiatica was investigated by the biosensor bioassay using Chromobacterium violaceum CV026. The effect of ethyl acetate fraction (CEA) from the bioassay-guided fractionation of ethanol extract on QS-regulated violacein production in C. violaceum ATCC12472 and pyocyanin production, proteolytic and elastolytic activities, swarming motility, and biofilm formation in Pseudomonas aeruginosa PAO1 were evaluated. Possible mechanism of QS-inhibitory action on autoinducer activity was determined by measuring the acyl homoserine lactone using C. violaceum ATCC31532. Anti-QS compounds in the CEA fraction were identified using thin layer chromatography biosensor overlay assay. Ethanol extract of C. asiatica showed QS inhibition in C. violaceum CV026. Bioassay-guided fractionation of ethanol extract revealed that CEA was four times more active than the ethanol extract. CEA, at 400μg/mL, completely inhibited violacein production in C. violaceum ATCC12472 without significantly affecting growth. CEA also showed inhibition of QS-regulated phenotypes, namely, pyocyanin production, elastolytic and proteolytic activities, swarming motility, and biofilm formation in P. aeruginosa PAO1 in a concentration-dependent manner. Thin layer chromatography of CEA with biosensor overlay showed anti-QS spot with an Rf value that corresponded with that of standard kaempferol. The anti-QS nature of C. asiatica herb can be further exploited for the formulation of drugs targeting bacterial infections where pathogenicity is mediated through QS.

Quorum quenching by Bacillus cereus U92: a double-edged sword in biological control of plant diseases

In the present survey, quorum quenching activity was examined from a biocontrol point of view. Acyl-homoserine lactone (AHL) degrading bacteria were isolated from tomato rhizosphere using two standard bioreporter strains and different synthetic AHLs and then identified according to 16S rDNA sequences. Five isolates capable of inactivating both short and long 3oxo-substituted AHLs showed high similarity with the genera Bacillus, Microbacterium and Arthrobacter, and thereby Bacillus cereus U92 was determined as the most efficient quorum quencher strain. In the quantitative experiments, this strain remarkably inactivated all synthetic AHLs up to 80%. In the laboratory co-cultures, B. cereus U92 efficiently quenched QS-regulated phenotypes in Agrobacterium tumefaciens, Pseudomonas aeruginosa, Pseudomonas chlororaphis and Chromobacterium violaceum. The strain successfully reduced the frequency of Ti-plasmid conjugal transfer in A. tumefaciens by about 99% in the binary cultures. Meanwhile, in a more natural environment, this strain acted as a biocontrol agent, efficient in alleviating QS-regulated crown gall incidence on tomato roots (up to 90%) as well as attenuating Pectobacterium soft rot on potato tubers (up to 60%). On the other hand, reducing phenazine production in P. chlororaphis operated as a suppressor of its QS-regulated biocontrol activity and also inhibited pyocyanin production in P. aeruginosa, a plant growth-promoting bacterium, by 75%. In general, B. cereus U92 seems very promising in the biological control of pathogenic bacteria; however, its broad AHL-degrading activity has a detrimental role on beneficial microbes which should not be neglected.