Survivability of microorganisms in the air

Airborne microorganisms survive in the atmosphere based on intrinsic adaptive features and the prevailing environmental conditions. The residence times for each group of organisms vary and may depend on whether they exist as spores or vegetative cells, and whether they are metabolically active or not. Bacterial vegetative cells, endospores, and fungal spores are the most abundant microbial aerosols in the air. Generally, the survival of microorganisms in the air is affected by a number of physical and chemical factors including relative humidity, temperatures, solar radiation and ions, microbial intrinsic factors, pollutants, pressure changes, and seasonal changes.

In the indoor air, microbial survival is further affected by reduced air mobility, protection from UV, sunlight, and other radiations and controlled ventilation. Most of the microorganisms, especially the bacteria found in the indoors environment, are human-derived, while the fungal spores mostly come from outdoors and moisture or water-damaged indoor materials such as woods, paints, floorboards, and pipes. Human habitation and behavior, including hygiene, are important modulators of indoor air composition and concentration. Thus, the microbiome of the built environment (MBE) is imprinted with the footprints of the inhabitant.

In the outdoor environment, natural sources such as soil, waters, and dead or living animals and plants play a significant role. Anthropogenic activities related to agriculture, industrial manufacture, waste management, sporting, and leisure are also some of the important sources and modulation of aerosolization and transport of microorganisms. Fungal spores are usually more abundant and resilient than bacterial cells. The viability and virulence of viruses are reduced by exposure to sunlight. Viruses such as SARS-CoV-2 with fragile outer envelopes easily get desiccated and lose viability. This has been suggested as the reason why indoor spread of COVID-19 is considered more important than outdoor.

Source: European Molecular Biology Laboratory (EMBL)

Aerosolized microorganisms may remain airborne for only a short period or for days depending on the size of the aerosols and composition before deposition. Thermal turbulence mostly accounts for vertical movement reaching at times several kilometers. Horizontal movement in the upper lithosphere and lower stratosphere leads to spread across regions of the world such as from North Africa to Europe, East Africa Asia, North Africa to the Amazon, etc. Airborne microorganisms have been shown to be important in meteorology, particularly as related to cloud nucleation and precipitation. This is further driven in recent times by climate change, which has resulted in increased atmospheric circulation. It has also been shown that quite differently than initially thought, microorganisms in the air are not exclusively metabolically dormant, but rather that some are adapted to utilize sparingly available nutrients as oligotrophs and the low water activity that characterize the environment.

The extreme conditions in the upper troposphere and in the stratosphere generate concerns about the viability of microorganisms. This is because at high altitude, the thinner atmosphere permits penetration of higher intensity ultraviolet rays such that for every 100 m increase in altitude, there is about 10%–20% increase in UV intensity. However, following the pioneering works of Imshenetsky and his colleagues (1977, 1978, 1979), reports now abound of organisms in these zones, especially in recent times as a result of more efficient sampling and advent of culture independent assays. These organisms are adapted to resist the harsh environment including ultraviolet radiation as a result of possession of pigmentation and other features and are of great meteorological importance.

In the outermost reaches of the atmosphere boundary with space and even in space itself, the conditions are even more inclement. Notwithstanding interest in exploration and venturing into space and long-term residence-abode spacecrafts, the international space station (ISS) and future extraterrestrial planetary residency have fired research into this area. Some organisms grown in space such as Deinococcus radiodurans have been shown to exhibit tolerance related to changes in metabolism and molecular properties. The fact that microorganisms have been shown to grow better, with active response to stressors and increased tolerance to antimicrobial agents, and concern for outbreaks among astronauts in space has opened up a new vista in the area of pathogenicity in space.

One question that needs an answer is whether the presence of water and aerosolized organic nutrients can translate into availability of metabolically active microbes in the atmosphere. It is most probable that as the conditions in the atmosphere become harsher, so will the diversity and population density of organisms keep reducing. The troposphere is closer to the oceans, soils, and plants and definitely contains more airborne microbes than the stratosphere with reduced temperature, oxygen, and humidity.

Matthew Olusoji Ilori and Oluwafemi Sunday Obayori (2023). Introduction to Aeromicrobiology. Developments in Applied Microbiology and Biotechnology Pages 1-16.

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