A PCR-denaturing gradient gel electrophoresis (DGGE) method was used to examine on-farm sources of Clostridium cluster I strains in four dairy farms over 2 years. Conventional microbiological analysis was used in parallel to monitor size of clostridial populations present in various components of the milk production chain (soil, forage, grass silage, maize silage, dry hay, and raw milk). PCR amplification with Clostridium cluster I-specific 16S rRNA gene primers followed by DGGE separation yielded a total of 47 operational taxonomic units (OTUs), which varied greatly with respect to frequency of occurrence. Some OTUs were found only in forage, and forage profiles differed according to farm location (southern or northern Québec). More clostridial contamination was found in maize silage than in grass silage. Milk represented a potential environment for certain OTUs. No OTU was milk specific, indicating that OTUs originated from other environments. Most (83%) of the OTUs detected in raw milk were also found in grass or maize silage. Milk DGGE profiles differed according to farm and sampling year and fit into two distinct categories. One milk profile category was characterized by the presence of a few dominant OTUs, the presence of which appeared to be more related to farm management than to feed contamination. OTUs were more varied in the second profile category. The identities of certain OTUs frequently found in milk were resolved by cloning and sequencing. Clostridium disporicum was identified as an important member of clostridial populations transmitted to milk. Clostridium tyrobutyricum was consistently found in milk and was widespread in the other farm environments examined.

The diets of six groups of weaned mice were supplemented with ultra high temperature (UHT) milk containing a washed suspension of lactic acid bacteria (mixture of 8 strains) or with UHT milk fermented by the same strains and heat-treated or not. Control groups received physiological saline or UHT milk only. The mice were infected intranasally by Klebsiella pneumoniae AD-1 on the 13th d of feeding. The effect on the immune system (specific and nonspecific) before and after infection was evaluated by measuring the phagocytosis of alveolar macrophages (using zymosan particles) and by measuring of total immunoglobulin G and A levels in serum and in pulmonary fluid (using the enzyme-linked immunosorbent assay method). Postinfection survival was 0.7 d longer for mice receiving fermented milk than for the saline control group. The percent phagocytosis did not vary significantly, while serum immunoglobulin G levels differed between mice fed fermented milk and those fed bacterial suspensions in unfermented milk. Fermentation appears to be essential for the beneficial effects on the immune system and survival time; this effect no longer occurs after pasteurization of fermented milk.