Gluconobacter oxydans from apple surface as biocontrol agent against Penicillium expansum

Blue mold is the most important postharvest decay of apples in the world. The losses from this disease can be significant but can be substantially reduced by following proper sanitation and control measures. The fungus, Penicillium expansum, not only causes fruit decay but also produces the carcinogenic mycotoxin patulin. This toxin may rise to unacceptable levels in fruit destined for processing and may also result in off flavors. Postharvest treatment of fruit with fungicides has been traditionally the most common method of combating blue mold. However, this decay can be reduced by using antagonistic microorganisms such as a bacterium Gluconobacter oxydans. The genus Gluconobacter, whose taxonomy is at present under worldwide study, is made up of five different species that do not show health risks and are commonly used in food manufacturing. The purpose of this study was to isolate the natural microbial population from apples and investigate the biocontrol abilities of Gluconobacter oxydans strain against postharverst infecting fungi, Penicillium expansum. Classical microbiological method and MALDI-TOF MS analysis were used for isolation and identification of the microbial population from apples. Tests were performed in two ways. First, the inhibition of colony growth was determined by measuring the lesions diameter (mm) induced on YPM plates and apple fruits after wound inoculation with the respective isolate. In another variant, the amount of biomass was determined gravimetrically in a liquid media (apple juice). The highest antifungal activity of this bacterium was observed between 7 and 9 days with no colony growth, while after 12 days colony diameter was reduced to 42, 3 % respectively. Although the Gluconobacter oxydans was not completely inhibited mold growth in apple juice, they showed high level of efficiency (72-78%). Gluconobacter oxydans partially to completely inhibit mold growth depending on the time and conditions of cultivation.