How many Salmonella enteritidis cells are required to infect a chick at one day old?

• A. 1,000
• B. 10,000
• C. 100
• D. 100,000

Answer: (C)

Infectious dose and host susceptibility are being tested here. Day-old chicks are highly susceptible to Salmonella enteritidis, so only a relatively small number of bacteria is often enough to establish infection because their gastric acid barrier is not fully developed and their gut defenses are immature. This means the minimal infectious dose in such young birds can be quite low, and around one hundred cells can be sufficient to initiate infection under typical conditions. Larger inocula would also cause infection but aren’t as informative about the host’s vulnerability since they exceed what’s needed to start infection. Real-world numbers can vary with strain, exposure route, and the bird’s condition, but the concept being tested is that very young chicks can be infected by a relatively small number of organisms. Therefore, the best answer is about one hundred cells.