During the past two decades, there have been many studies on the efficacy of competitive exclusion for the control of Salmonella in poultry. Undefined preparations of cultured fecal or cecal microflora generally reduce the prevalence of infected chicks upon challenge with a standard dose of Salmonella under laboratory conditions; in contrast, results under field conditions are more variable. The protective capacity of undefined cultures can be affected by several factors including the source of microflora, method for protective culture administration, presence of poultry feed additives, in-laboratory or natural environmental challenge, and hygienic practices on the farm. The formulation of effective defined cultures is most difficult because of insufficient knowledge on the underlying protective mechanism(s) and interactions between gut microflora. Defined cultures are less effective than undefined cultures under laboratory conditions and afford little protection against natural Salmonella challenge; their potency decreases upon storage and manipulation of single or mixtures of defined culture isolates.

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