Pathogen defence

All of the cDNA clones isolated from tissue in which resistance has been induced encode proteins that can be described as PR-proteins or PR-like proteins (Bol et al., 1990 Carr Klessig, 1990). The appearance of PR-proteins in pathogen-infected tissue and in uninfected portions of the infected plant has been described (Hooft van Huijsduijnen et al., 1986 Metraux etal., 1988 Tuzun etal., 1989). However, a correlation between the induction of mRNA expression and the onset of systemic acquired resistance has not been conclusively demonstrated and the role of the PR-proteins in any type of pathogen defence remains questionable (Fraser, 1981 Fraser Clay, 1983). 

Lieberei, R., Pock, H. and Biehl, B. (1992) Cyanogenesis inhibits active pathogen defence in plants inhibition by gaseous HCN of photosynthetic CO2 fixation and respiration in intact leaves. /. Appl. Bot., 70, 230-38. 

Phytoalexins are low-molecular-mass antimicrobial compounds that are formed de novo after microbial infection (ii). They provide one strategy of the multi-component pathogen defence program. Many of the over 200,000 plant secondary metabolites are either constitutive or inducible antimicrobial agents. 

Possible functions of plant lipoxygenases in pathogen defence... 

Therefore, the ability of certain POs to bind with chitin is a widespread phenomenon and -possibly - connected with the defence reactions of the organisms to pathogen attacks. Since it was shown that some biogenic molecules - such as chitooligosaccharides or salicylic acid -can activate an anionic POs, we might suggest that an application of these compounds optimises the process of anionic PO isolation with a chitin. 

Pathogenic organisms possess certain properties which enable them to overcome these primary defences. They produce metabolic substances, often enzymic in nature, which facilitate the invasion of the body. The following are examples of these. 

Parasitism by T. spiralis has been a subject of scientific interest for over 150 years. Recently, considerable attention has been paid to the parasite by immunologists interested in immunity to nematodes in general, and mucosal immunity in particular. It has been shown that glycan-specific antibodies are highly effective mediators of host defence against intestinal 7. spiralis infection. Protective monoclonal antibodies have been used to elucidate mechanisms of worm expulsion, as well as to identify molecules that the parasite uses to create its niche. In the future, detailed characterization of these molecules and their functions should afford additional insights into parasitism by Trichinella spiralis, and possibly also by other types of pathogen. 

The objective of antibiotic treatments is to reduce the density of microbial pathogen in infected udder tissues and thereby improve the capacity of the animal s immune system to deal with the infection. The effect of a successful antibiotic treatment is therefore self-cure of mastitis (Hamann and Kromker, 1999). However, some antibiotics (e.g. tetracycline and gentamycine) may also have negative side effects on the animal s immune response to udder infection, as they have been shown to inhibit/reduce phagocytosis of the animal s own defence cells (Nickerson et al 1986). 

Dykhuizen R., Frazer R., Benjamin N., Duncan C., Smith C.C., Golden M. and Leifert C. (1996). Antimicrobial effect of acidified nitrite on gut pathogens the importance of dietary nitrate in host defence . J Antimicrob Agents Chemothe, 40, 1422-1425. 

---