A covert contaminant of cultured plant cells: elimination of a Hyphomicrobium sp. from cultures of Datura innoxia (Mill.)

We have discovered a bacterial contaminant in some cell cultures of Datura innoxia (Mill.). The bacterium was tentatively identified as a species of Hyphomicrobium on the basis of its morphology and life cycle, and was isolated and grown in pure culture on a defined medium. The contaminant was not macroscopically observable in plant cell cultures. It caused neither a reduction of plant cell growth nor a noticeable increase in culture turbidity. Furthermore, it was not readily detectable by many standard assays for culture contamination: it would not grow alone in plant culture medium or yeast extract potato dextrose medium, and grew only very slowly on nutrient agar or beef-peptone medium. Repeated treatments with a combination of streptomycin (100 g/ml) and carbenicillin (100 g/ml) eliminated the contaminant from D. innoxia cell cultures without harming the plant cells.