Salmonella penetration through eggshells of chickens of different genetic backgrounds

Eggs have been identified as a source of salmonellosis, making the transmission of Salmonella to eggs of great concern to the poultry industry. The goal of this experiment was to determine the ability of Salmonella to penetrate the eggshell of 5 different breeds of noncommercial chicken, Barred Plymouth Rock, White Leghorn, Brown Leghorn, Fayoumi, and Light Sussex, and 1 commercial Lohmann LSL-Lite. Egg weight, breaking force, shell weight, and shell thickness measurements were taken for 30 eggs per breed. A 1 cm in diameter hole was cut out from the narrow end of 30 additional eggs per breed. The shells were filled with plate count agar containing tetracycline and 0.1% 2,3,5-triphenyl terazolium chloride and sealed with paraffin wax. Agar-filled eggs were submerged for 1 min in an overnight culture of tetracycline-resistant Salmonella Heidelberg and incubated at 37°C for 40 h. Eggs were candled and visual colonies were counted and reported as cfu per egg and cfu per gram of shell. The SAS mixed model was used to evaluate differences between breeds for egg quality characteristics and the number of cfu per egg and per gram of shell. Commercial layers (62.6 g) and Barred Plymouth Rock (61.5 g) produced the largest eggs, whereas Fayoumi (47.1 g) produced the smallest (P < 0.05). Force to break the shell was lowest (P < 0.05) for Barred Plymouth Rock (3.6 kg) and greatest for the commercial (4.4 kg), White Leghorn (4.4 kg), and Fayoumi (4.2 kg). Bacteria penetrating the shell was lowest (P < 0.05) for Barred Plymouth Rock (10.7 cfu/g) and highest for Light Sussex (27.7 cfu/g) and Brown Leghorn (27.2 cfu/g), with other breeds intermediate. These results indicate that there are breed-specific influences on the ability of an egg to resist Salmonella, which cannot be explained by shell quality measurements. Further investigations are warranted to determine the contributing factors to shell penetration by bacteria. This study highlights the value in maintaining heritage chicken breeds as a genetic resource for the future.