Antimicrobial potential of bacteriocins in swine production

The routine use of antibiotics in agriculture has contributed to an increase in drug-resistant bacterial pathogens in animals that can potentially be transmitted to humans. Research is currently being carried out to identify alternative antimicrobial compounds for use in animal production. A number of studies, mostly in vitro, have provided evidence indicating that bacteriocins, which are antimicrobial peptides of bacterial origin, may be promising alternatives to conventional antibiotics in poultry and swine production. This review provides an update on bacteriocins and their potential for use in the poultry and swine industries.

Swine pathogens

• Post-weaning diarrhea is responsible for major economic losses in the swine industry. Enterotoxigenic E. coli (ETEC) is the major cause of this enteric disease in pigs, being responsible for approximately 50% of piglet mortality. Al Atya et al. [84] showed that combining colistin with bacteriocins (nisin, enterocin) from lactic acid bacteria enhances its in vitro antibacterial activity against planktonic and biofilm cultures of E. coli. They suggested that colistin disrupts the outer membrane of E. coli by acting on lipopolysaccharide, opening the way for the subsequent action of the bacteriocins. Colicins, a class of bacteriocins produced by and active against E. coli, have been investigated as a possible alternative to antibiotics in swine production. Colicin E1 inhibits the growth of E. coli strains that cause post-weaning diarrhea and edema disease in pigs, as shown in vitro. Furthermore, Cutler et al. showed that the addition of colicin E1 to the diet of piglets decreases the incidence and severity of experimental post-weaning diarrhea induced by an enterotoxigenic strain of E. coli and improved the growth performance of piglets. They also used a gene expression analysis (IL-β, TNF-β) to show that the inflammatory response occurring in ileal tissues leading to diarrhea was decreased. These promising results indicate that the use of colicins may have a positive impact on food safety since enterotoxigenic E. coli is considered to be an important foodborne pathogen.
• Haemophilus parasuis: Teixeira et al. reported the isolation of a low molecular mass bacteriocin from a reference strain of B. subtilis subsp. spizezinii (ATCC 6633). The bacteriocin was highly effective against approximately half the H. parasuis strains tested and may be a potential alternative to antibiotics for controlling infections caused by this pathogen.
• Streptococcus suis: The nisin-producing strain L. lactis subsp. lactis ATCC 11404 exerts antagonistic activity toward S. suis, suggesting that this bacterial species may represent a probiotic of interest for the control of S. suis infections. Moreover, all the S. suis isolates tested were susceptible to purified nisin, with MIC values ranging from 1.25 to 5 µg/mL. When nisin was combined with conventional antibiotics such as amoxicillin and ceftiofur, which are commonly used to treat S. suis infections, strong synergistic effects were obtained. These in vitro results provide support for the potential of nisin, a lantibiotic licensed as a food preservative, for preventing swine infections caused by S. suis. The purification and characterization of three lantibiotics, named suicins 90–1330, 3908, and 65, produced by three distinct strains of S. suis (serotype 2) have recently been reported. Interestingly, all three producing strains were non-virulent in mouse/pig infection models, and two of them were isolated from healthy carrier pigs. The distribution of suicin gene clusters in S. suis serotype 2 belonging to sequence type (ST) 25 and ST28, the two dominant STs in North America, was recently investigated. The gene clusters encoding suicin 65 (mostly in ST25 strains) and, to a lesser extent, suicin 90–1330 (exclusively in ST28 strains) are the most prevalent. Since all three suicins are bactericidal for highly virulent ST1 S. suis strains, which are mainly found in Eurasia, the use of the semi-purified bacteriocin preparations or the bacteriocin-producing strain may represent a valuable strategy for controlling S. suis infections and for reducing antibiotic use in the swine industry.
• In 2012, Riboulet-Bisson et al. evaluated the impact of Lactobacillus salivarius administration and, more specifically, the effect of bacteriocin production by this bacterium on the intestinal microbiota of healthy pigs. L. salivarius strain UCC118 is a well-known probiotic bacterium of human origin that produces a broad-spectrum class IIb bacteriocin. Administering the bacteriocin-producing L. salivarius resulted in the modulation of the Gram-negative bacterial population of the intestinal microflora, decreasing the levels of Bacteroidetes and Spirochaetes. Such an effect was not observed with a mutant lacking bacteriocin production. Although members of these two phyla are mostly commensals, under certain conditions they may become opportunistic pathogens in humans and animals. For example, Treponema spp. and Bacteroides spp. can cause colitis and diarrhea, respectively.
• Pediocin is a broad-spectrum class IIa bacteriocin produced by P. pentosaceus. Casadei et al. investigated the in vitro effects of pediocin A, on microbial metabolism in the small and large intestines of pigs. While pediocin A had no effect on the fermentation parameters of the small intestine, it significantly reduced the growth of harmful bacteria, including clostridia and coliforms, and increased the metabolic activity of cellulolytic bacteria. Based on these observations, the authors suggested that pediocin A could be an alternative to replace AGP and to improve the production of farm animals.

Conclusions

As more countries develop antibiotic-limiting policies, the need for alternative antimicrobials will likely become the main driving force behind the identification of novel bacteriocins and the testing of existing ones. The use of semi-purified bacteriocins or bacteriocin-producing bacteria in animal production is a field with enormous research and commercialization potential. Bacteriocins hold great promise for the prevention and/or treatment of bacterial diseases and may eventually be employed as alternatives to antibiotics