Defined cultures and prospects.

Resistance of young chicks to Salmonella colonization can be increased by treating them with suspensions or anaerobic cultures of gut content from healthy, adult birds. Unfortunately, such treatments have an unknown bacterial composition and are therefore not acceptable to regulatory agencies in some countries. Efforts are continuing to identify components in the gut microflora that are involved in the protective process. The ultimate aim is to identify bacteria that may be used, either alone or in mixtures (defined cultures), as a prophylactic treatment of chicks against Salmonella colonization. Although much research has been done in this area, a defined culture treatment with a potency and stability equivalent to that of undefined culture has not been developed. Since the mechanism of protection is not clear, there are no reliable criteria for selecting potentially protective strains. The most effective mixtures contain large numbers of bacterial cultures (about 50 isolates). These mixtures are comparable to undefined faecal cultures in protecting chicks against challenge levels of up to 10(4) Salmonella/chick. However, they are not protective for turkey poults. Mixtures containing lower number of strains are generally less protective. It appears that obligate and facultative anaerobes from different genera are required for protection. Mixtures containing strains from only one genus, e.g. Lactobacillus or Bifidobacterium, are generally not effective. Maintenance of intestinal isolates on laboratory media, as well as in storage at -70 degrees C, results in a progressive decrease in their protective ability. The protective ability of mixtures or individual strains can be partially restored by in vivo passage. A promising criterion for isolating and/or screening potentially protective isolates is based on the adherence of native microflora to the caecal wall of treated chicks. Despite substantial efforts and some progress in the development of defined cultures, the prospects are not very encouraging. Until the exact mechanism of protection is elucidated, the basis for selecting the required strains will be difficult to define.