Aging and cellular defense mechanisms: age-related changes in resistance of mice to Listeria monocytogenes.

Age-related changes in resistance of mice to infection with Listeria monocytogenes were investigated. One-month-old mice exhibited the least resistance, and the resistance level increased over the first few months to reach a maximum by 8 months. Increase in age thereafter was accompanied by a slow but progressive decrease in resistance. Thus, 50% lethal doses for 1-, 8-, and 24-month-old mice were 10(4.2), 10(6.6), and 10(5.2), respectively. In spite of differences in resistance, the growth of Listeria in the organs of mice of different age groups was identical during the first 48 h of infection, regardless of the size of the inoculum. Moreover, both young (3- to 8-month-old) and old (22-month-old or older) mice inoculated with a small dose of Listeria were equally capable of inactivating the bacterial load in their spleens and livers within 8 to 10 days of infection. However, a difference in bacterial growth after day 2 of infection was observed when different age groups of mice were inoculated with a large dose of Listeria. These results suggest that the decreased capacity of aged mice to resist infection with Listeria is not due to deficiency in the innate mechanisms of antibacterial resistance, but instead is due to age-related decline in the capacity to acquire immunologically specific antibacterial immunity.