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

Abstract

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.