Institute of Plant Protection, ul. Miczurina 20, Poznan, Poland

Background and objectives
Chitin is the second most abundant polymer occurring in nature. Chitosan - a derivative of chitin - is obtained by a chemical deacetylation of crustacean chitin. Chitin and chitosan exist widely in nature as a structural material of insects and fungi, but they do not occur in higher plant tissues. However, both polymers induce a broad spectrum of defensive plant responses. Polycationic chitin derivatives showed antifungal [1], antiviral [2] and antiviroid [3] activity. We report here our study on antibacterial activity of chitin derivatives.

Materials and methods
The plant pathogenic bacteria used in this study were: Clavibacter michiganense subsp. michiganense, C. michiganense subsp. insidiosum, Pseudomonas syringae pv. phaseolicola, P. syringae pv. tomato, Erwinia carotovora subsp. carotovora and Xanthomonas campestris pv. pelargonii. Water soluble chitin oligomers, chitosan (solution in acetic acid), low molecular-deaminated chitosan, microcrystalline chitosan, chitosan oligomers, carboxymethylchitosan and sulphate chitosan were examined for antibacterial activity. The effect of chitin derivatives on the bacterial growth were determinated by minimal inhibitory concentration (MIC) method. Drops of chitin derivatives of different concentrations were applied to the surface of agarose plates containing culture of bacteria in nutrient dextrose medium.

For determination of inhibitory effect on the plant infection by bacteria, tomato or bean plants before inoculation with Pseudomonas syringae pv. tomato or P. syringae pv. phaseolicola, respectively, were sprayed with chitin derivatives. The effect of chitosan on bacterial infection was calculated as the percentage of reduction of the number of local lesions induced by pathogen on the chitosan treated leaves in comparision to control.

Results and conclusions
Anionic chitin derivatives, i.e. carboxymethylchitosan and sulphate chitosan, did not display antibacterial activity. Positively charged chitin derivatives, i.e. chitin oligomers, chitosan, and deaminated chitosan, manifested their antibacterial activity dually: by inhibition of the bacterial growth in vitro and by inhibition of infection of plants by bacteria. Chitin derivatives inhibited growth of both Gram-positive and Gram-negative bacteria, however, Gram-positive bacteria were more sensitive to inhibition. During short-time incubation in liquid medium chitin derivatives showed bacteriostatic action, later bactericidal activity. This may suggest that the inhibition of the bacterial infection might be due to the presence of residual amounts of chitin derivatives, which can act directly on bacteria. It is important to note that chitosan oligomers, which lost the ability to inhibit the growth of bacteria and microcrystalline chitosan, a very weak inhibitor, also protected plants from the bacterial infection.

These results suggested that the inhibition of the bacterial infection was due to induction of plant resistance to the infection rather than direct inactivation of bacteria. The efficiency of chitin derivatives in the inhibition of bacterial infection varied from 50-70% and depended on their type, concentration and mode of application.

1. Benhamon N, Thierauft G, 1992. Physiological and Molecular Plant Pathology 41, 33-52.
2. Pospieszny H, Chirkov S, Atabekov JG, 1991. Plant Science 79, 63-68.
3. Pospieszny H, 1997. Crop Protection 16, 105-106.