Organic inhibitory substances

In the Accusphere test a freeze-dried sphere containing the test organism Streptococcus thermophilus and bromocresol purple as indicator disperses into the test milk sample. The acidification test is very similar the milk sample is heated to be further inoculated with a Streptococcus thermophilus culture containing yeast extract, bromocresol purple indicator, and trimethoprim. It is then incubated for 2.5 h at 45 C (33). In the presence of inhibitory substances, the organism growth is suppressed, acid production is reduced or eliminated, and the color of the indicator remains unchanged. Addition of penicillinase to a positive milk sample results in change of the color of the indicator from purple to yellow when only -lactams are present. 

In contrast to tire previous multiresidue tests, the Valio TlOl test uses a freeze-dried Streptococcus thermophilus TIO l-strain dispersed into the milk. During incubation, the test organisms grow and produce acid, which causes the pH indicator to change color from blue to yellow. In the presence of any inhibitory substances, the indicator remains blue or turns to a greenish color depending on the concentration of inhibitors. 

Extracts from the fruit or seed of several plant species will suppress the growth induced by gib-berellic acid. The inhibitory extract from immature fruit of Ceratonia siliqua L. has been partially purified and four inhibitory substances are now known to be present. One is an inorganic component which is inhibitory after ashing. Three organic inhibitors—A, B, and C—are responsible for most of the inhibitory activity of the extract. B and C are adsorbed by activated charcoal and can be separated in this way from organic inhibitor A, which is not adsorbed. C can be extracted with ethyl ether from aqueous solution at pH 5, while B is not removed under these conditions. 

The results shown in Table III reveal strong inhibition in the first pair of ethyl ether partitions and in the final water phase. The inhibition was progressively less through the remaining ether partitions, with none in the final pair of ether fractions. This evidence indicates the presence of two inhibitory substances. Organic inhibitor B is not extractable with ether at pH 5 organic inhibitor C is extractable with ether at pH 5. Both are adsorbed by activated charcoal. 

Nisin is an antibacterial polypeptide produced by some strains of Lactococcus lactis. Nisin-like substances are widely produced by lactic acid bacteria. These inhibitory substances are known as bacteriocins. Nisin has been called an antibiotic, but this term is avoided because nisin is not used for therapeutic purposes in humans or animals. Nisin-producing organisms occur naturally in milk. Nisin can be used as a processing aid against gram-positive organisms. Because its effectiveness decreases as the bacterial load increases, it is unlikely to be used to cover up unhygienic practices. 

Most inhibitory substances are degraded by strong biological activation of the soil the soil structure is improved and the soil is enriched with nitrogen and organic matter. 

The exact mechanism by which competitive exclusion operates is unknown. Mead and Barrow (1990) have suggested that the exclusion of Salmonella spp. may be by a variety of ways including competition for nutrients and/or adhesion sites on intestinal mucosa, production of inhibitory substances and alteration of pH. Inhibitory substances may include bacteriocins, hydrogen peroxide and/or organic acids (Juven et al., 1991). 

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