Silver nanoparticles as a potential antimicrobial additive for weaned pigs
Introduction
In the last two decades, several types of additives have been proposed in pig production as alternatives to the use of antibiotics as growth promoters, such as organic acids, oligosaccharides, plant extracts or probiotics (Cowan, 1999, Naughton et al., 2001, Gardiner et al., 2004, Franco et al., 2005). However, their effect in preventing digestive disorders at weaning and promoting higher productive performances is variable and in most cases below the magnitude previously reached with the use of antibiotics.
Silver compounds are currently used as antimicrobial agents in a variety of applications, including coating of catheters, dental resin composites, burn wounds and homeopathic medicine, with a minimal risk of toxicity in humans (Lansdown, 2006). The small size of nanoparticles of metallic silver (below 200 nm) in solid or colloidal state allows for a higher microbiological effect than silver salts (Atiyeh et al., 2007). Besides, metallic silver is potentially less toxic (Wadhera and Fung, 2005) and it would be deactivated at a slower rate by gastric HCl (Atiyeh et al., 2007) than silver salts. In the 1950s, silver nanoparticles were used as additive in poultry nutrition, but the price could not compete with that of antibiotics. Nowadays, a more efficient industrial production process makes it potentially interesting as prebiotic.
Silver is toxic to microorganisms by poisoning respiratory enzymes and components of the microbial electron transport system, and it also binds to bacterial surface, altering the membrane function (Percival et al., 2005), and to DNA bases, thus inhibiting replication (Wright et al., 1994). Other metal compounds used as growth promoters in pig nutrition such as zinc oxide or copper sulphate also modulate digestive microbiota and reduce post-weaning diarrhoea (Jensen-Waern et al., 1998, Broom et al., 2006), but their role on the gut microbial ecosystem (Hojberg et al., 2005), in enhancing activity of pancreatic enzymes or in maintaining intestinal morphology (Hedemann et al., 2006) is not yet clear. There is no information available about negative effects of silver on pigs in productive conditions. The minimum silver level observed to have an adverse effect on chicken, rat and mice has been 300 μg/kg feed; on this basis, NRC (1980) set the maximum tolerable level of silver for poultry and swine at 100 μg/kg feed.
The current study studied the effect of different dietary doses of metallic silver nanoparticles in diets for weaned pigs on the digestive microbiota and gut morphology (Experiments 1 and 2), and in productive performances and silver retention in tissues (Experiment 3).
Section snippets
Experiment 1: Effect on digestive microbiota in vitro
An initial experiment was carried out in vitro to estimate the dose effect of metallic silver on digestive microbiota of weaned piglets. Four 21-day-old (Pietrain x Large White) × (Large White × Landrace) piglets were used as donors of inoculum. Animals were weaned and received a mixed feed as meal, whose ingredient and chemical composition is shown in Table 1 as diet 0–2 weeks after weaning. After 7 days of ad libitum feeding, pigs were slaughtered after previous stunning with CO2 and after three
Experiment 1
Average concentration of coliforms and lactobacilli for the different concentrations of metallic silver is shown in Table 2. Total concentration of coliforms was linearly reduced with the concentration of metallic silver in the medium (P=0.003). Similarly, coliform proportions regarding the control (without silver) were linearly reduced by silver concentration (P=0.046). A linear reduction in the concentration of lactobacilli was also observed (P=0.041), but no effect was observed in the
Discussion
To the author’s knowledge, there are no data available on the use of silver nanoparticles as feed additive in pig diets. The antimicrobial activity of silver is manifested by blocking the electron transport system, altering the function of the bacterial membrane and inhibiting the DNA replication (Wright et al., 1994, Percival et al., 2005). Although these effects have been demonstrated experimentally against E. coli (Zhao and Stevens, 1998, Sondi and Salopek-Sondi, 2004), our first concern was
Conclusions
Results from the present work indicate that low doses of metallic silver nanoparticles given as dietary additive could improve intake and growth of weaned piglets, although results from Experiment 3 do not allow to clearly state this. The effect of silver could be mediated through its antimicrobial properties, either by acting against certain bacterial groups or just reducing the microbial load of the small intestine; however, other beneficial effects over the host metabolism cannot be
Acknowledgements
This work was financed by Laboratorios Argenol, S.L. (Zaragoza, Spain) through the Project OTRI 2006/0279. Thanks are given to the Food Microbial Sciences Unit from the School of Food Biosciences of the University of Reading (Reading, Berkshire, UK) for their kind help in FISH analysis. Dr. Leticia Abecia was financed by a post-doctoral fellowship from the Gobierno Vasco (Spain).
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