This study aimed to develop a convenient model to investigate the senescence of host defenses and the influence of food and nutrition. A small soil nematode, Caenorhabditis elegans, was grown for 3 days from hatching on a lawn of Escherichia coli OP50 as the normal food source, and subsequently some of the nematodes were fed lactic acid bacteria (LAB). The life spans of worms fed LAB were significantly longer than the life spans of those fed OP50. To investigate the effect of age on host defenses, 3-to 7-day-old worms fed OP50 were transferred onto a lawn of Salmonella enterica serovar Enteritidis for infection. The nematodes died over the course of several days, and the accumulation of salmonella in the intestinal lumen suggested that the worms were infected. The 7-day-old worms showed a higher death rate during the 5 days after infection than nematodes infected at the age of 3 days; no clear difference was observed when the worms were exposed to OP50. We then investigated whether the LAB could exert probiotic effects on the worms' host defenses and improve life span. Seven-day-old nematodes fed LAB from the age of 3 days were more resistant to salmonella than worms fed OP50 until they were infected with salmonella. This study clearly showed that LAB can enhance the host defense of C. elegans and prolong life span. The nematode appears to be an appropriate model for screening useful probiotic strains or dietetic antiaging substances.

Lactobacilli and bifidobacteria are probiotic bacteria that modify host defense systems and have the ability to extend the lifespan of the nematode Caenorhabditis elegans. Here, we attempted to elucidate the mechanism by which bifidobacteria prolong the lifespan of C. elegans. When the nematode was fed Bifidobacterium infantis (BI) mixed at various ratios with the standard food bacterium Escherichia coli strain OP50 (OP), the mean lifespan of worms was extended in a dose-dependent manner. Worms fed BI displayed higher locomotion and produced more offspring than control worms. The growth curves of nematodes were similar regardless of the amount of BI mixed with OP, suggesting that BI did not induce prolongevity effects through caloric restriction. Notably, feeding worms the cell wall fraction of BI alone was sufficient to promote prolongevity. The accumulation of protein carbonyls and lipofuscin, a biochemical marker of aging, was also lower in worms fed BI; however, the worms displayed similar susceptibility to heat, hydrogen peroxide, and paraquat, an inducer of free radicals, as the control worms. As a result of BI feeding, loss-of-function mutants of daf-16, jnk-1, aak-2, tol-1, and tir-1 exhibited a longer lifespan than OP-fed control worms, but BI failed to extend the lifespan of pmk-1, skn-1, and vhp-1 mutants. As skn-1 induces phase 2 detoxification enzymes, our findings suggest that cell wall components of bifidobacteria increase the average lifespan of C. elegans via activation of skn-1, regulated by the p38 MAPK pathway, but not by general activation of the host defense system via DAF-16.

The survival times of Caenorhabditis elegans worms infected with Legionella pneumophila from day 7.5 or later after hatching were shorter than those of uninfected worms. However, nematodes fed bifidobacteria prior to Legionella infection were resistant to Legionella. These nematodes may act as a unique alternative host for Legionella research.Legionella pneumophila, an environmental bacterium naturally found in fresh water, is the major causative agent of Legionnaires' disease (7). Fresh water amoebas, a natural host of Legionella, have been used as an infection model to study invasion of Legionella into human macrophages and subsequent intracellular growth (15). However, analyses using these protozoa have inevitably concentrated on the intracellular lifestyle of L. pneumophila. The fate of Legionella organisms in nonmammalian metazoans had not been described (10) until a very recent report by Brassinga et al. (6).Numerous authors have reported Caenorhabditis elegans to be a suitable model to investigate virulence-associated factors of human pathogens (2,8,11,14,16,20,23,24,30,31,33). In the present study, we examined whether C. elegans can serve as an alternative host for L. pneumophila. Although the nematocidal activity of Legionella has been described recently, the nematodes in the previous study were infected with the pathogen on buffered charcoal yeast extract (BCYE) agar plates, which can support Legionella growth (6). In contrast, our experiments were independently performed on simple agar plates to exclude the possibility that the inoculated pathogen would have proliferated regardless of whether it had successfully infected the nematodes and derived nutrition from the hosts. Garsin et al. showed that nutrition available in agar plates does influence the virulence of pathogens on the medium (9). Furthermore, some pathogens produce toxic metabolites on nutrient medium in situ (3), and thus, we also avoided this possibility. Moreover, we focused on the effects of worm age, since Legionella is prone to infect elderly people.Age at infection is likely one of the most important determinants of disease morbidity and mortality (18). Since Legionella organisms are prone to infect elderly people opportunistically, infections in young and older nematodes were compared. Furthermore, survival curves were compared between worms fed Escherichia coli OP50 (OP), an international standard food for these organisms, and those fed bifidobacteria prior to infection with Legionella organisms, since lactic acid bacteria exert beneficial effects on human and animal health (21).Nematocidal assays. As a standard strain of C. elegans, Bristol N2 was maintained and propagated on nematode growth medium (NGM) according to standard techniques using OP (25). Tryptone soya agar (Oxoid, Basingstoke, United Kingdom) was used to culture OP at 37°C. The virulent and attenuated L. pneumophila strains used in this study are listed in Table 1. All attenuated strains were produced by transposon insertion into the Icm/Dot (intracellular multiplic...

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