Articles
CHARACTERIZATION AND CONTROL OF BACTERIAL CONTAMINANTS IN IN VITRO CULTURES OF BANANA (MUSA SPP.)
The test involves streaking of the basal part of the shoot tip on to nutrient agar, enriched with yeast extract (5 g l-1) and glucose (10 g l-1). 38 bacterial strains were isolated from 851 banana accessions, maintained between 3 and 15 years in the IPGRI/INIBAP in vitro Musa collection.
Isolates were identified as Gram-positive endospore forming Bacillus sp. (65 %) and slow growing Mycobacterium sp. (30 %). The selection of an alternative culture medium, Middlebrook 7H10, which meets the nutritional requirements of these mycobacteria made it possible to detect and grow these contaminants more rapidly.
Two approaches to decontaminate in vitro cultures have been investigated.
The first one involved the use of antibiotics in shoot tip cultures.
Ten antibiotics with activity against Gram-positive bacteria were tested against predominant Bacillus sp. and Mycobacterium sp. to determine their minimal bactericidal concentration (MBC). Rifampicin (100 mg l-1) added in liquid culture was found suitable for controlling Bacillus sp. and Mycobacterium sp. in banana shoot tips, without causing phytotoxic effects.
The second approach involved isolation of small explants.
Culturing of 1 mm meristems from contaminated in vitro plants or from contaminated plants grown in a greenhouse proved to be effective in eliminating bacteria.
Because of the relative simplicity of the technique, meristem culture has been adopted routinely to control bacterial contamination in the banana collection.