SUMMARYThirty-two different types of anaerobic bacteria isolated from chickens have been tested for anti-salmonella activity in vitro. Under the conditions of the test only Bacteroides hypermegas and a Bifidobacterium sp. were shown to inhibit the salmonellas and this was attributed to the production of volatile fatty acids (VFA's) coupled with a low pH. When these organisms were tested in newly hatched chicks no irnhibition of S. typhimurium occurred. Possible explanations for this observation are considered.The pH value and concentration of VFA's in the caecal material were determined in chicks from 0-84 days. In vitro tests with S. typhimurium indicated that, whilst the organism would be able to multiply at the pH and concentration of VFA's found during the first few days after hatching, the rapid increase inVFA concentration during the first 21 days would make this increasingly difficult. The significance of the developing caecal flora in relation to VFA production and pH is discussed.Because certain feed additives are known to influence the carriage of salmonellas, the sensitivity of various caecal anaerobes to these compounds was determined in vitro, generally at 1, 10 and 100 ltg/ml. The additives tested included flavomycin, furazolidone, nitrovin, tetracycline, tylosin, sulphaquinoxaline, virginiamycin and zinc bacitracin. All the organisms tested were inhibited by 100 ,ugfml furazolidone; none were inhibited by 500 ,tg/ml sulphaquinoxaline.Changes occurring in the VFA concentration, pH value and microflora of the caeca of chicks fed for 49 days or longer on a normal starter diet or the same diet containing 10 or 100 mg/kg nitrovin have been compared. When the chicks were fed on the diet containing 100 mg/kg nitrovin, the Gram-negative non-sporing anaerobes were eliminated as a significant part of the caecal flora. However, the VFA concentration combined with a low pH in chicks from 2 weeks onwards was still sufficient to inhibit salmonella multiplication. Other possibly interrelated factors which might lead to an increased salmonella carrier rate in the nitrovintreated chickens are discussed.

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