Final answer:
The upper respiratory tract hosts a variety of normal flora including Staphylococcus epidermidis and Corynebacterium spp., while the lower tract is less populated but may contain transient bacteria such as Pseudomonas.
Sinusitis and otitis media are often caused by Streptococcus pneumoniae and other bacteria, which can take hold following viral infections.
Step-by-step explanation:
The upper respiratory tract is home to a rich and diverse array of microorganisms, including Staphylococcus epidermidis, viridans group streptococci (VGS), Corynebacterium spp., Propionibacterium spp., and Haemophilus spp.
Occasionally, opportunistic pathogens such as Staphylococcus aureus, Streptococcus, and Neisseria also inhabit this area in healthy individuals. These bacteria employ a range of virulence factors to overcome immune defense mechanisms such as the mucociliary escalator.
In sharp contrast, the lower respiratory tract features fewer microbes. The few that are identified, such as Pseudomonas, Streptococcus, Prevotella, Fusobacterium, and Veillonella, may be just transients rather than a consistent normal microbiota.
With regard to the sinuses and the middle ear, which includes acute otitis media (AOM) and bacterial rhinosinusitis, common causative agents include Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. These infections often follow a viral infection, which may impede immune responses and aid bacterial colonization.