Isolation and characterisation of a bacterial strain with control activity against post-harvest mould from citrus

Abstract

The susceptibility of citrus fruits to microbial pathogens, especially green and blue moulds, is one of the difficulties that face citrus fruits suppliers to keep these fruits in good condition for consumers. Therefore, there is an urgent need for a safe and efficient method to preserve them during transportation and storage. This study focused on searching for a bacterial strain that have antifungal activity against moulds that attack citrus fruits. Environmental samples were screened for microorganisms with antifungal activity against Penicillium fungi that were isolated from infected lemon fruits. Twenty out of 500 isolated bacterial strains showed inhibitory activity against Penicillium, 16 of these strains were excluded from further study because they belonged to the genus Bacillus which has been widely studied as a bio-agent in the biological control. The results of phenotyping and the genotyping characteristics on the other active strain (4 isolates) proved that they were Pseudomonas koreensis.

These four isolates differed in their antifungal activity, isolate B had the best suppression activity either with or without living cells (in concentrated spent media) which was mainly linked to the difference in lytic enzyme production capability (protease, chitinase and others). In addition, the active strain was able to produce siderophores and biosurfactants which could take part in the suppression activity. The antifungal activity in broth culture is produced during the log phase of the bacterial growth.

The primary characterization of the antifungal activity showed it was heat sensitive, and not stable against pH changes (especially raising the pH), was sensitive to the treatment with proteinase K and appeared to have a molecular weight >3000 Da which suggested a protein nature for the active molecule. In addition, Tn5 mutagenesis was employed to identify the gene(s) that is responsible for the inhibition activity, the obtained results from one of the mutants showed that the antifungal activity may be linked in some way to the acquisition of iron, making it unavailable for the fungal growth.