Age-dependent susceptibility of neonatal rats to group B streptococcal type III infection: correlation of severity of infection and response of myeloid pools.

A distinct age-dependent susceptibility to group B streptococcus type III (GBS) was demonstrated, utilizing a neonatal rat model. The most dramatic changes in susceptibility occurred within the first 7 days of postnatal life. To further investigate this susceptibility, experiments were performed utilizing two age groups of rats: (i) animals within the first 24 h of life (NB) and (ii) 7-day-old animals (7d). The infective dosage used was 10(4) GBS per g of body weight, a dose lethal to 100% of NB but only to 15% of 7d. The responses of the myeloid cells in the peripheral blood, spleen, and bone marrow were evaluated at intervals during the first 24 h post-GBS infection. The susceptibility of the NB to GBS appeared to be associated with a number of events, including smaller base-line levels of myeloid elements particularly in the bone marrow, a lag of at least 2 h in their initial response to infection, and an inability to maintain the myeloid pools. The band form of neutrophils appeared to be the predominant cell type in both total number and rapidity of response to infection. Moreover, an initial depletion of this band form was seen in both groups, which returned to base-line levels with recovery in 7d but persisted until death in NB animals. Similarly, shifts in numbers of peripheral nucleated erythrocytes appeared to reflect changes in the myeloid storage pools, with numbers of nucleated erythrocytes significantly decreasing in 7d animals with recovery in contrast to persistence in NB until death. Therefore, shifts in these cells in peripheral blood during infection appear to reflect the state of myeloid storage pools which parallel disease outcome.