A study of a possible mode of action of antibiotics in chicks

Depression in the body weight of chicks (up to 2 weeks old) may be produced (a) by feeding small amounts of fresh ‘infected’ contents in the mash, (b) by holding chicks in close proximity to ‘infected’ chicks showing this depression in weight, (c) by oral or intranasal inoculation with filtered gut contents or (more particularly) with crude lung filtrates. The depression is overcome by addition of penicillin to the diet. The causative agent of the depression has not been found, but the results support the hypothesis of Coates et al. 12, 16‐18, 20 that the action of antibiotics is to overcome a sub‐clinical infection. Definite confirmation of this is not yet possible and the difficulties associated with this work are outlined.