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Group B Streptococcal Lung Infection in Neonatal Rabbits

Summary: The interaction of alveolar macrophages (AMs) and group B streptococci (GBS) was investigated in 1 and 2-day-old rabbits following infection in an exposure chamber containing 107 streptococci per cubic foot of air. The % of streptococci within AMs was similar at 0 and 4 h (36 and 65%) after infection for the two groups of rabbits. Twenty-four h after infection, the 2-day-old rabbits had a significantly higher % of ingested GBS (86 versus 68%). Sixty % of inspired GBS were inactivated by the older rabbits within 4 h after infection. This clearance persisted throughout the 72 h experimental period. Inspired GBS proliferated in the younger rabbits until 48 h (mean negative clearance of –17, –276, and –79% at 4, 24, and 48 h) before their numbers were reduced by inflammation. Sixty of 78 1-day-old rabbits had inflammatory responses between 24 and 72 h versus only 5 of 50 older rabbits. At 24 h after infection, AMs of 1-day-old rabbits contained significantly increased numbers of intracellular GBS microcolonies (17/ 20) than did AMs from 2-day-old rabbits (5/19). These observations suggest that the enhanced susceptibility to GBS infection in the immediate postnatal period is caused at least in part by ineffective intracellular killing by AMs.Speculation: If the ability of macrophages to inactivate intrapulmonary group B streptococci in the immediate postpartum period is due to an underdeveloped oxygen-dependent bactericidal system, then the bactericidal abnormality may be even more profound in the prenatal period when macrophages are exposed to lower in utero alveolar oxygen tensions. As a consequence of this reduced activity, group B streptococci aspirated prenatally are likely to proliferate uncontrollably, and either prenatal stimulation of the alveolar macrophage or early detection of intrauterine streptococcal infection followed by delivery of the infected fetus may be life saving.