Inhibition of conidial growth of Venturia inaequalis by the extracellular protein fraction from the antagonistic bacterium Pseudomonas fluorescens Bk3

The present study investigated the inhibitory effect on the conidial germination of Venturia inaequalis by using living whole cells, extracellular protein fraction and individual proteins from the non-pathogenic antagonistic bacterium Pseudomonas fluorescens Bk3.The bacterial suspension of P. fluorescens Bk3 from growth in minimal medium showed up to 73% inhibition of conidial germination after 7 days of pre-incubation. Furthermore, this inhibitory effect could also be shown by the extra cellular protein fraction of P. fluorescens Bk3. The protein solution obtained from liquid culture in LB medium showed 100% inhibition of conidial germination after 1 day of pre-incubation. Since the solution contained at least 10 major proteins these proteins were extracted from the gel and subsequently re-natured for functional studies. After re-naturation individual proteins were applied on the conidia of V. inaequalis to see their impact on the conidial germination. Out of these 10 proteins three showed inhibitory effects (20–42%). De novo sequencing of these three proteins were carried out by ESI Q-ToF mass spectrometry and they were identified as an extracellular solute-binding protein, an extracellular alkaline metalloprotease and a peptidoglycan-associated lipoprotein. The proteolytic activity of the metalloprotease could also be confirmed with activity staining using casein as a substrate.