The development of the infant nasopharyngeal microbiome has an impact on early-life respiratory health. The composition of the early life nasopharyngeal microbiome may be influenced by bacterial interactions, together with host and environmental factors. In vitro models are useful to study bacterial-bacterial interactions. However, using nutrient rich laboratory media for in vitro studies is likely a poor surrogate for the nasopharyngeal microbial environment and interactions observed under nutrient rich conditions may not be the true reflection of in vivo bacterial relations. We aimed to observe interactions between clinical bacterial isolates from infant nasopharyngeal swab samples, in a medium that is reflective of the upper respiratory tract (URT) environment.
Synthetic nasal medium (SNM), which mimics the average metabolite concentration of human nasal secretions was used to simulate the URT environment. Bacterial growth in SNM was determined by growth curve assays and compared to the growth in nutrient rich brain heart infusion (BHI) media. Pairwise bacterial interactions were also studied in SNM, in which a 1:1 ratio of bacterial competitors was co-cultured in contact-dependent or contact-independent assays. Observed interactions were further investigated in conditioned media experiments.
SNM supported the growth of S. epidermidis, S. aureus, M. catarrhalis and C. propinquum, although to a lesser extent than BHI. S. aureus was an exception and grew better in SNM than in BHI. S. aureus growth was inhibited by S. epidermidis by an average of 167-fold (p=0.025) in contact-dependent co-culture experiments. However, no inhibition of S. aureus or S. epidermidis was observed in contact-independent co-culture experiments. We observed an 18-fold reduction (p = 0.0012) in S. aureus counts over the first 24 hours of growth in S. epidermidis conditioned media (that was prepared by growing S. epidermidis for 40 hours prior to the preparation of cell free conditioned media).
The results of this study indicate that bacterial growth is significantly varied when cultured in media that mimics the metabolome of the in vivo environment (SNM) compared with nutrient rich lab media (BHI). In addition, we found that S. aureus growth was inhibited by a strain of S. epidermidis in SNM via a contact-independent mechanism.