Minimum Intestinal Inoculum for Nurmi Cultures and a New Method for Determining Competitive Exclusion of Salmonella from Chicks

Abstract

Protection of chicks against Salmonella infection was obtained by oral administration of Nurmi cultures. The Nurmi cultures were prepared by inoculating adult chicken intestinal material (10⁻¹ to 10⁻¹¹ g) into anaerobic liquid medium and incubating for 3 d at 37°C. Excellent protection was obtained with cultures initiated with as little as 10⁻⁷ g of fecal or cecal content; progressively less protection with 10⁻⁸ to 5 × 10⁻¹⁰ g; and none with cultures initiated with 10⁻¹⁰ g. Protection was based on challenge of treated chicks with nalidixic acid-resistant (Nal⁺) Salmonella and enumeration of the Nal⁺ organisms in the chick cecal content 7 d later. To simplify the enumeration, a swab-plate method was developed. The method saves time and materials and gives relatively accurate (± 1 log₁₀) determinations of Nal⁺ Salmonella over the range of 10¹ to 10⁷ per g of cecal content.