Plant secondary metabolites, which include a wide variety of phytochemicals, have always been constituents of the diets of man and other animals. Although a high proportion of these phytochemicals have been considered to be of little value in plants (although this view is changing), they have frequently been shown to have adverse effects on animals when ingested. The effects depend to a great extent on the chemistry of the compounds, their concentration in the diet and the amount consumed, and are further dependent on the health status of the animals. Traditionally, most studies of the effects of these compounds on animals have focused on their adverse effects and how to alleviate them. However, recent public concern about the use of synthetic compounds in animal diets to enhance performance and health and welfare issues, coupled with changes in regulations on the use of synthetic medicaments, has stimulated interest and research in the use and effects of phytochemicals in the diets of farmed animals. Phytochemicals vary in their chemistry but can be divided into hydrophilic and hydrophobic compounds, of which a wide variety of polyphenolic and terpenoid compounds, as well as alkaloids, carbohydrates and non-protein amino acids, invoke special interest. The chemistry, biochemistry and mechanisms of action of these compounds in plants and their effects in animals when ingested will be explored. Phytochemicals: Effects in animalsAs plant secondary metabolites (PSM) are an extremely large group of compounds, a comprehensive overview of their biochemistry, bioactivity and chemistry is not possible in a relatively short review paper. The importance of PSM in ecology, human foods and animal feeds, and as pharmaceuticals with chemical and biochemical attributes has already been described in some detail

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An undescribed bacterium capable of clearing tannic acid‐protein complexes has been isolated from ruminal contents of feral goats browsing tannin‐rich Acacia species. The bacterium is a Gram‐positive facultative anaerobe, characterized as a Streptococcus, but DNA‐DNA hybridization and 16S rDNA sequencing show that it is distinct from the common ruminal species Strep. bovis. We propose the name Streptococcus caprinus for this species. The type strain is Strep. caprinus 2.3, Australian Collection of Microorganisms (ACM) 3969. The bacterium grows in media containing at least 2.5% w/v tannic acid or condensed tannin and produces zones of clearing around colonies on nutrient agar plates with added tannic acid. Streptococcus caprinus is not a major inhabitant of domestic livestock, but is found in feral goats browsing tannin‐rich Acacia species, at a population of up to 2 times 106 cfu ml‐1 of rumen fluid.

Streptococcus gallolyticus (S. caprinus) was resistant in vitro to at least 7 % (w/v) tannic acid and 4 % (w/v) acacia condensed tannin, levels 10-fold greater than those tolerated by S. bovis. Growth of S. gallolyticus in liquid medium was characterized by a lag period which increased, and a growth rate which decreased, with increasing tannin concentration. S. gallolyticus was also more tolerant to the presence of simple phenolic acid monomers than was S. bovis, but the lag period was still concentration dependent. Gallate decarboxylase activity in S. gallolyticus was elevated in the presence of tannic acid or gallic acid but not with other phenolic acids. Scanning electron microscopic analysis showed that both the size and shape of S. gallolyticus and S. bovis changed in response to tannin but only S. gallolyticus was surrounded by an extracellular polysaccharide matrix which accumulated in a tannin-concentration-dependent fashion. Washing of the cells to remove extracellular polysaccharide increased the lag period of S. gallolyticus in the presence of 1 % (w/v) tannic acid from 4 h to 6 h. In contrast, increasing extracellular polysaccharide synthesis in S. bovis did not increase its tolerance to tannic acid. These data demonstrate that S. gallolyticus has developed a number of mechanisms to reduce the potential effect of tannins on cell growth, and that these mechanisms provide the organism with a selective advantage over S. bovis when grown in the presence of tannins.

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