Sensitivity of Escherichia coli to Seaweed (Ascophyllum nodosum) Phlorotannins and Terrestrial Tannins

Z Xu,S J Bach,Y Wang,T A Mcallister

doi:10.5713/ajas.2009.80213

Abstract

Pure culture experiments were conducted to assess the bacteriostatic and bactericidal effects of phlorotannins (PT) isolated from Ascophyllum nodosum (brown seaweed) on Escherichia coli O157:H7. In Exp. 1, one non-0157:H7 strain (25922) and three strains of E. coli O157:H7 (3081, EDL933 and E318N) were cultured in M9 medium with PT included at 0 (control), 25, 50 or 100 μg/ml (n = 3). Bacterial growth was monitored by OD 600 at 0, 4, 6, 12 and 24 h, and by dilution plating at 0, 4, 6 and 24 h. All strains were inhibited (p<0.001) by PT to varying degrees. At 50 or 100 μg/ml, PT prevented growth of all four strains. At 25 μg PT/ml, growth of 25922, 3081, E318N and EDL933 was inhibited for 6, 12 and 24 h, respectively, but 25922 and 3081 resumed growth by 12 and 24 h. Direct plating confirmed bactericidal effects of PT on all four strains at 100 mg/ml, and on EDL933 and E318N at 50 μg/ml. In Exp. 2, strains 25922 and 3081 were incubated with no tannins or with 50 μg/ml of PT, purified condensed tannins (CT) from Quebracho (Schinopsis balansaei), or purified tannic acid from Rhus semialata (Anacardiaceae) as hydrolysable tannins (HT). Strain 3081 was unaffected by HT or CT, but was completely inhibited (p<0.001) by PT at 4, 6 and 24 h. Strain 25922 was unaffected by HT, slightly inhibited by CT, and almost eradicated by PT at 4 and 6 h. Transmission electron microscopy revealed tannin-mediated alterations to bacterial cell walls. Phlorotannins from A. nodosum exhibit growth-inhibiting and bactericidal effects in vitro against the strains of E. coli O157:H7 investigated. Anti-E. coli efficacy of A. nodosum PT is superior to that of terrestrial tannins purified from Quebracho and from Rhus semialata.

Highlights

Food-borne illnesses caused by enterohaemorrhagic Escherichia coli, in particular serotype O157:H7 (Wells et al, 1995; Paton and Paton, 1998; Bower, 1999), are a significant health concern in North America, with associated health care costs estimated at over USD 200 million per year
Ruminants are the major reservoir of E. coli O157:H7 and their fecal shedding of E. coli O157:H7 is affected by various factors such as animal health conditions, diet, and season (Stanford et al, 2005; Kobayashi and El-Sawy, 2007)
Recent research conducted with seaweeds has shown that brown seaweed (Ascophyllum nodosum) reduces the prevalence and shedding of E. coli O157:H7 in cattle before harvest (Barham et al, 2001; Braden et al, 2004; Bach et al, 2008)

Summary

INTRODUCTION

Food-borne illnesses caused by enterohaemorrhagic Escherichia coli, in particular serotype O157:H7 (Wells et al, 1995; Paton and Paton, 1998; Bower, 1999), are a significant health concern in North America, with associated health care costs estimated at over USD 200 million per year. Recent research conducted with seaweeds has shown that brown seaweed (Ascophyllum nodosum) reduces the prevalence and shedding of E. coli O157:H7 in cattle before harvest (Barham et al, 2001; Braden et al, 2004; Bach et al, 2008). 1958; Nakamura et al, 1996) and anti-bacterial (Targett and (resistant to 50 μg/ml nalidixic acid; bovine isolate, PT14), Arnold 1998; Horikawa et al, 1999; Nagayama et al, 2002) and strain EDL933 (ATCC isolate) were obtained from the activities. The anti-E. coli O157:H7 activity of this Inspection Agency, Lethbridge, AB, Canada). The strains marine source of PT was compared to terrestrial were maintained separately in tryptic soy broth, tannins from two sources

MATERIALS AND METHODS

RESULTS

DISCUSSION