Lipid-dependent inhibition of Streptococcus pneumoniae by Corynebacterium nostril isolates

Objectives: Non-pathogenic Corynebacterium spp. are common constituents of human nostril/skin microbiota, yet their function within these communities is virtually unknown. Streptococcus pneumoniae, an important pathobiont and common cause of pneumonia, otitis media and meningitis, colonizes ~50% of children ages 6 months to 7 years. Colonization corresponds to an increased risk of infection. Children without S. pneumoniae nasal colonization have increased relative abundance of Corynebacterium spp. in their nasal microbiota. We, therefore, hypothesized that antagonism exists between Corynebacterium spp. and S. pneumoniae. Our objective was to elucidate the mechanism(s) involved if this antagonism occurs in cultivation.

Methods: We used co-cultivation to detect Corynebacterium-produced diffusible molecules that inhibit S. pneumoniae growth and vice-versa. We performed organic solvent extraction of Corynebacterium cell-free conditioned medium (CoryneCFCM) and tested extracts for anti-pneumococcal activity using filter disc assays.

Results: Co-cultivation of S. pneumoniae and Corynebacterium nostril isolates revealed mutual antagonism. Not unexpectedly, pneumococcal-produced hydrogen peroxide inhibited Corynebacterium. Surprisingly, Corynebacterium spp. that are not lipid auxotrophs required the medium component Tween 80 (polyethylene glycol sorbitan monooleate) to inhibit pneumococcus, both in co-cultivation and in extracts of CoryneCFCM. Preliminary data indicated that the skin/nostril surface lipid triolein can substitute for Tween 80. This lipid-dependent anti-pneumococcal activity was detected in all 7 Corynebacterium species tested. We speculate that Corynebacterium spp. hydrolyze skin surface lipids containing oleic acid esters, e.g., triolein, releasing free oleic acid, which has antimicrobial properties. Filter disc assays confirmed that oleic acid inhibits pneumococcal growth in cultivation.

Conclusions: In cultivation, Corynebacterium spp. exhibited lipid-dependent inhibition of S. pneumoniae. We postulate that in vivo free fatty acids released by Corynebacterium spp. from skin surface lipids modify the local nostril habitat impeding pneumococcal colonization/growth. Verifying this will increase understanding of pneumococcal colonization dynamics and uncover alternative approaches to prevent pneumococcal colonization and infection.