Neonatal Group B streptococcus (GBS) sepsis and pneumonia result in lung injury and remain a major cause of morbidity and mortality in the newborn. Increased lung hyaluronan (HA) content is an important component of the lung's early response to damage in diseases such as adult respiratory distress syndrome (ARDS), infant respiratory distress syndrome (IRDS), and bleomycin-induced fibrosis. It is known, however, that GBS virulence factors include specific secretory enzymes such as hyaluronidase, an enzyme which breaks down HA. We therefore hypothesized that in lobar GBS pneumonia, lung HA would be decreased compared with normal values, and that in lobar pneumonia with atelectasis, lung HA would be further decreased because of increased substrate availability. The right lower lobes (RLL) and left lower lobes (LLL) of anesthetized piglets 16 +/- 2 d old were each selectively inoculated with 1 x 10(9) colony-forming units (CFU) GBS via an endobronchial catheter (n = 7). The LLL was subsequently collapsed by endobronchial occlusion following 10 min of 100% O2. Control animals (n = 6) was anesthetized, instrumented, and ventilated without exposure to GBS. At 4 h, lungs were removed and HA extracted and assayed using a competitive inhibition assay. HA extracted from areas of lobar pneumonia was significantly decreased (27 +/- 6.6 micrograms/g wet lung, p < 0.005) when compared with control values of control piglets (51 +/- 19.6 micrograms/g wet lung). Atelectasis plus lobar pneumonia further decreased lung HA to 10 +/- 13.3 micrograms/g wet lung, p < 0.0001. We conclude that lobar GBS decreases lung HA and that this process is augmented by collapsed lung regions, and speculate that this departure from the usual early lung response to injury contributes to GBS invasion of lung parenchyma.