Antagonistic activity of Lactobacillus casei strain Shirota against gastrointestinal Listeria monocytogenes infection in rats

Abstract

In the present study, the effect of ingested viable Lactobacillus casei Shirota strain YIT9029 on oral infection with the enteric pathogen Listeria monocytogenes in Wistar rats was investigated. Rats were orally infected with 10⁹ viable L. monocytogenes. Starting 3 days before the infection, rats received a daily dosage of 10⁹ viable L. casei. It was shown that supplementation of L. casei significantly reduced the numbers of L. monocytogenes in stomach, caecum, faeces, spleen and liver, 2 days after L. monocytogenes infection. The number of L. monocytogenes in the mesenteric lymph nodes was not affected by the ingestion of L. casei. In comparison with control animals, the levels of the liver-specific alanine aminotransferase were lower in L. casei-fed rats. Histological analysis of spleen and liver revealed no differences between the experimental and control animals. In a parallel study with orally L. monocytogenes infected rats, it was shown that L. casei was able to increase cellular immunity significantly as determined with the delayed-type hypersensitivity response against heat-killed L. monocytogenes. In conclusion, in the present study it was shown that orally administered L. casei is able to enhance host resistance against oral L. monocytogenes infection. In the gastrointestinal tract, as well as in the spleen and liver, L. monocytogenes numbers were reduced. Furthermore, it is concluded that the enhancement of this anti-Listeria activity might be, at least partly, due to increased cell-mediated immunity.

Introduction

Parenteral Listeria monocytogenes infections in rodents are frequently applied to study cellular immunity against intracellular bacteria (Shen et al., 1998). Additionally, oral infections with L. monocytogenes have been used in animal models of foodborne pathogens Schlech et al., 1993, Marco et al., 1997. In these models, L. monocytogenes invades the host through its natural portal of entry, the alimentary tract. Different studies have shown that L. monocytogenes may translocate across the mucosal barrier via Peyer's patch dependent and independent pathways MacDonald and Carter, 1980, MacDonald et al., 1982, Pron et al., 1998. After translocation, L. monocytogenes disseminates via mesenteric lymph nodes to spleen and liver. Both cell-mediated immunity and the intestinal bacterial flora have been reported to play important roles in the host resistance against oral L. monocytogenes infection Zachar and Savage, 1979, Czuprynski and Balish, 1981, Okamoto et al., 1994.

L. monocytogenes infections are primarily the result of the ingestion of contaminated food-products and can lead to severe clinical diseases. Unique among all food-borne pathogens, a significantly high percentage of cases can lead to death (Lorber, 1997). Elderly individuals, immunosuppressed adults, newborns and gestating women are especially prone to acquiring L. monocytogenes infections. In pregnant women, L. monocytogenes may cause abortion, stillbirth and/or neonatal meningitis. Since the society is confronted with an expanding population of elderly people and immunocomprised individuals, L. monocytogenes still forms a serious threat in medical and economical terms. An additional problem is that mankind is confronted with an increasing number of pathogens, which are multiple antibiotic-resistant. This would favor the application of anti-infection strategies other than antibiotics (Bengmark, 1998).

Health-improving functional foods containing so-called probiotic bacteria form serious candidates to prevent and/or reduce intestinal infections. A probiotic can be defined as a living microbial food ingredient that has health-promoting activities (Salminen et al., 1998). Various strains of lactic acid bacteria and bifidobacteria have been claimed to confer health-stimulating activities (Naidu et al., 1999). Numerous studies have reported that oral administration of specific lactobacilli strains stimulate immunity and improve the resistance to tumors and infections Kato et al., 1998, Shida et al., 1998, Takagi et al., 1999, Gill et al., 2000.

The objective of the current study was to evaluate the effects of ingested Lactobacillus casei strain Shirota on clinical, bacteriological and immunological parameters in orally L. monocytogenes infected Wistar rats.