A finding of routine respiratory flora means the normal and expected collection of microorganisms residing in the respiratory tract has been detected. This collection, which includes various bacteria, fungi, and sometimes viruses, does not cause disease in a healthy individual. The presence of this flora is generally an indicator of a healthy, functioning state, rather than a sign of infection.
Defining Routine Respiratory Flora
The term “routine respiratory flora” describes the indigenous organisms that have established a stable presence within the upper airways. These microbes are considered routine because they are expected to be found in healthy people. The term “flora” is a traditional way to refer to the collection of microorganisms, a concept now often replaced by “microbiota” or “microbiome” in scientific circles. These organisms live in a relationship known as commensalism, meaning they benefit from the host, such as by having a stable environment and nutrients, while the host remains largely unaffected. This relationship can also be symbiotic, offering mutual benefit. The routine flora are distinct from “pathogens,” which are foreign or aggressive agents capable of causing disease.
Where the Flora Resides
The respiratory system is typically divided into two main regions, which differ significantly in their microbial inhabitants. The upper respiratory tract (URT), including the nose, mouth, and pharynx, is a warm, moist, and nutrient-rich environment highly hospitable to a diverse and abundant community of microorganisms. This is where the majority of the routine respiratory flora is consistently found. In contrast, the lower respiratory tract (LRT)—composed of the trachea, bronchi, and lungs—is generally considered to be a much more hostile environment for microbes. The LRT is constantly being cleansed by the mucociliary escalator, which sweeps mucus and trapped particles upward, preventing microbes from settling and colonizing. Specialized immune cells, such as alveolar macrophages, also patrol the lung tissue, actively consuming any microbes that manage to bypass the upper airway defenses.
The Protective Role of Respiratory Flora
The routine flora provides significant benefits to the host by actively discouraging the establishment of harmful, disease-causing organisms. This protective mechanism is known as competitive exclusion, where the established flora occupy all available physical niches and consume limited nutrients. By taking up space and resources, they prevent outside pathogens from gaining a foothold and multiplying to dangerous levels. The routine microbes also engage in direct conflict with invaders by producing specific compounds, such as bacteriocins and hydrogen peroxide, that are toxic to competing bacterial species. Furthermore, the constant presence of the normal flora helps regulate the local immune system, maintaining a balanced immune response ready to respond quickly and effectively to actual threats.
When Routine Flora Becomes a Problem
The organisms normally considered routine flora can cause disease, but only under specific, changed circumstances, a situation known as an opportunistic infection. One primary scenario involves physical displacement, where the microbes are moved from their normal location in the upper airway to the typically sterile lower respiratory tract. For example, if a person aspirates upper respiratory secretions into the lungs, the routine bacteria can overwhelm the lower airway’s defenses and cause a serious infection like aspiration pneumonia. The other main factor is a compromise in the host’s defense mechanisms, which allows the routine organisms to become pathogenic. When the immune system is weakened—due to underlying illnesses, the use of immunosuppressive drugs, or hospitalization—the balance that keeps the flora in check can be disrupted, allowing the organisms to proliferate unchecked and cause disease.