Fig. 1

Experimental outline. In the first round of the experiment, fertilized eggs were collected for a week from ten broiler breeder hens and incubated until hatching. Individual fecal samples were collected from the hens during the egg-collection week. Chicks were raised for 14 days, and individual chick fecal samples were collected on days 2, 7 and 14. This was repeated twice more with the same hens, with a different intervention performed on half of the hens or their eggs in each round. In the second round of the experiment, eggs laid by half of the hens were disinfected before incubation, while eggs from the other hens remained untreated. In the third round, half of the hens were treated with an antibiotic cocktail for two weeks, including the egg-collection week. Internal organs were sampled from the hens at the end of the experiment