Out of 39 isolates of rhizobacteria, recovered from economic plants grown in 8 locations in Egypt, 6 isolates were able to produce Hydrogen Cyanide (HCN). 16S rRNA sequence analysis identified these isolates as: Pseudomonas japonica strain NBRC 103040, Bacillus megaterium strain CtST3.5, Pseudomonas sp. strain Gamma-81, P. tolaasii strain ATCC 33618, P. chlororaphis strain Lzh-T5, and P. mosselii strain CV25. These HCN producers were able to inhibit growth of Agrobacterium tumefaciens and affect viability of Meloidogyne incognita juveniles in vitro. The isolates of P. japonica and Pseudomonas sp. Gamma-81 prevented the gall formation on tomato plants by A. tumefaciens, regardless of the presence of M. incognita. The isolates of B. megaterium, P. chlororaphis, P. tolaasii, and P. mosselii decreased the weight and number of galls produced by A. tumefaciens in the presence or absence of M. incognita. The 6 HCN producers decreased the population of M. incognita and the number of nematode galls than the positive control, when used against M. incognita. A similar effect was achieved against mixed infections with M. incognita and A. tumefaciens. The HCN-producing rhizobacteria, in the presence of A. tumefaciens and/or M. incognita, caused obvious increment in all growth parameters of tomato than the negative control and healthy plants. The only exception was found in case of Pseudomonas sp. Gamma-81 against M. incognita and against mixed infection, where growth parameters of tomato were decreased. Although the isolates were naturally isolated from the rhizosphere of economic plants, it must be cautiously considered since the isolate identified as P. japonica has been reported as a human pathogen. Also, P. tolaasii was reported causing a bacterial blotch on cultivated mushrooms under certain environmental conditions. Further investigations are needed.
