Lactic acid bacteria as antagonists of phytopathogenic bacteria

Abstract

Lactic acid bacteria were isolated from plants and plant-associated products. These isolates were identified and tested as possible biocontrol agents of certain phytopathogenic bacteria, namely: 2 strains of Xanthomonas campestris, 2 strains of Pseudomonas syringae and 1 strain of Erwinia carotovora. In agar diffusion experiments, in vitro antagonism occurred and the majority of the lactic acid bacterial isolates formed clear inhibition zones. In associative broth cultures of lactic acid bacteria and phytopathogenic bacteria, the phytopathogens were completely killed within 12-36h. In vivo experiments were carried out with bean plants grown greenhouse. The application of a suspension of lactic acid bacteria 24h before treating the plants with the halo blight pathogen, P. syrin-gae, led to a significant reduction in disease incidence. The mechanism of antagonism was investigated and it was concluded that the lactic acid produced by the lactic acid bacteria was the most important inhibitory substance. Slight inhibition still occurred in broth with CaC03 added as buffer, but the phytopathogenic bacteria were not killed as in the case of unbuffered media. Hydrogen peroxide production by the lactic acid bacteria did not have an important effect on their antagonistic activity. Preliminary trials were carried out to determine the survival rates of certain antagonistic lactic acid bacteria on the phylloplane. The lactic acid bacterial numbers decreased by approximately 1 log colony forming units/g per week. Thus, it seems as if repeated applications of the antagonists might be necessary, unless the environment or antagonists can be manipulated to ensure better survival rates. Electron microscopy bf the leaves of bean plants indicated that the lactic acid bacteria occur in the grooves between epidermal cells. Field trials were not included in this study. Therefore it cannot yet be claimed that the biological control of phytopathogenic bacteria by means of lactic acid bacteria is possible.