Abstract
Condensed tannins (CT) from purple prairie clover (PPC; Dalea purpurea Vent.) and sainfoin (SF; Onobrychis viciifolia) were assessed for anti-Escherichia coli activity by comparing their ability to react with proteins and liposome, cause cell aggregation, and alter outer membrane (OM) morphology and permeability. The PPC CT had greater (P < 0.01) protein-precipitating capacity than SF CT using either bovine serum albumin or ribulose 1,5-disphosphate carboxylase as model proteins. Minimum inhibitory concentration of PPC CT for two strains of E. coli and five strains of E. coli O157:H7 was four to six times lower than that of SF CT. E. coli exposed to 10 µg/mL of both CT had higher (P < 0.05) OM permeability than controls and was greater (P < 0.05) for PPC than for SF CT. Addition of both CT at 50 and 200 µg/mL caused cell aggregation which was more evident (P < 0.05) for PPC than for SF CT. Transmission electron microscopy showed electron dense material on the cell surface when cells were exposed to 50 µg/mL of PPC CT. The greater anti-E. coli activity of PPC than SF CT was due to its enhanced ability to precipitate protein that increased OM permeability and promoted cell aggregation.
Highlights
Ruminants are a major reservoir of Escherichia coli serotype O157:H7, a food-borne pathogen that costs millions of dollars annually in food processing and health care in North America
Condensed tannins (CT) or proanthocyanidins isolated from purple prairie clover (PPC) possess stronger anti-E. coli O157:H7 activity in vitro than condensed tannins (CT) isolated from SF [6]
Ribulose 1,5-disphosphate carboxylase (Rubisco) protein was completely precipitated by ≥500 μg PPC CT, whereas only 73% (2.2/3.0) of Rubisco was precipitated by SF CT at the highest concentration tested (1,500 μg)
Summary
Ruminants are a major reservoir of Escherichia coli serotype O157:H7, a food-borne pathogen that costs millions of dollars annually in food processing and health care in North America. Some plant tannins have been shown to possess anti-E. coli O157:H7 activities and their efficacy depends on the tannin sources [4,5,6]. Condensed tannins (CT) or proanthocyanidins isolated from PPC possess stronger anti-E. coli O157:H7 activity in vitro than CT isolated from SF [6]. The reason why these two CT differ in their inhibition of E. coli and E. coli O157:H7 is unknown. Anti-microbial activity of CT against gram-positive bacteria has been reported to be greater than against
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