Pathogens in the air

Pathogens in the air: Global aerial dispersion of microorganisms
Gemma Quílez

Lluís Casellas Escolà

David Abert Fernández

Introduction

Despite the atmosphere doesn’t have its own microbiota, it is the habitat and the
 transmission vector of many microorganisms, including pathogens.

These pathogens are called airborne pathogens and differ from the others for
using the air as their way to transport (to infect). We’ve written about this three
 types of airborne microorganisms: viral, bacterial and fungi.

This phenomenon is studied by the aerobiology, the branch of biology that studies
 microorganisms that are transported by the air.  To study aerial dispersion of 
pathogens, there are several characteristics to observe, for example the size they 
have, factors which may affect them or which metabolisms let them survive and how 
they adapt to the atmosphere.

Figure 1: Simple life cycle of microorganisms in the atmosphere

Therefore, in this blog, we will introduce classifications of the global pathogens, 
a small explanation of the different transmission vectors (emphasizing the aerial
 vector and what makes it especial),  we will talk in detail about the aerial propagation
 and the factors that take a part in it (size, until where they get and other 
possibilities) as well, quantification methods and  the resistance of the pathogens
 to the modern medicine.

Body Text

We can classify pathogens in several groups. The most common ones in the atmosphere are: virus, bacteria and fungus.

Bacteria

Bacteria are another airborne microbiota organisms that can be found from 10 to 10^7 individuals on each m^3 of air. Bacteria like Neisseria meningitidis are the most representative from a anthropological point of view because they cause exclusive humans illnesses, like meningitis.

The size distribution of pathogen bacteria is one of the most important factors that determines the inhalable degree, so this factor has an important effect on human health. Last studies have determined 3 grups of particles by his diameter: from 2.5 to 10 um, they are inhalable particles, smaller thant 2.5um are 'fine particles', and larger than 10um will be captured by vibrissa (nasal hair)  and they probably won't enter into a body, so they are less harmful to health.

Last researchs have shown that the concentration and proportion of sizes of airborne bacteria changes depending in many different factors, like the daytime or haze and non-haze days, as it’s shown in the Figure ‘2’.

Figure 2: A): Diameter variation of bacteria depending on daytime. B) Average concentration of bacteria CFU/m3 depending on haze-non-hazy days.

Bacteria have evolved with an inportant effect of this factor (size) and another ones, like relative humidity, temperature or wind. That's why some bacteria has a close correlation wuth concrete environment, like Bacillus or Pseudommonas, which are often detected in stores, or Staphylococcus and Sphingomonas, which are often detected in residences (Adams R.I, Miletto M., 2014).

Virus and archea

Virus and archea can be found in a proportion from 1 to 10% of every airborne microorganisms, but some of them own a strong pathogenecy.

This group is probably the one who has more effect on humans, and some of them are airborne and we all have suffered an illness from them, for example, the common cold, caused by rhinovirus. These virus are from 'picornavirus' family, which are the most common viral infectious agents, and they are transmited by aerosols of respiratory droplets and by contaminated surfaces. Their diameter is about 30nm and they prefer 33-35ºC temperatures, so they can proliferate in human nose.

The purpose of this classification is to study other groups like coronavirus or influenza virus, which are the RNA virus that cause some illness like influenza.

Fungi

Fungi are probably the most common microbes found in nature. As we know, a lot of fungis spread by airborne spores. The concentration of fungi in air is about 0-107UFC*m-3, with a lot of size diversity. Hyphal fragments and multicellular spores has a >10um diameter and unicellular spores have 1-10um diameter.

Figure 3: Fungi size range variation depending on A) Terrestrial fungi and year Season B) Indoor-Outdoor location C) Marine fungi and year season D) Haze and non-hazy days.

As is shown at the ‘3’ figure, fungal particles in the air has very different sizes depending on different conditions and factors, like haze days, indoor-outdoor or year’s season.

Influence factors

As we have mentioned above, there are several factors that can affect on atmosphere microorganisms. Most of them are meteorological parameters.

Temperature: this one is probably the most important factor for airborne pathogens, because every single microorganisms has its own optimal living temperature. In many reports (M. Almaguer, 2014), temperature had a significant correlation with microorganisms: in one hand, high temperatures were optimal for fungal growth, while low ones were a limiting factor. In the other hand, in other reports (J. Fröhlichnowoisky, 2016), for different microorganisms, cold air could facilitate the transportation of bacteria, temperature had a negative correlation to archea diversity, and when PM25 (inhalable particles)>300ppm, there was a negative correlation between airborne microorganisms and temperature, and that was probably because high temperatures could enhance toxic compounds and promote chemical reactions (M.A. Alghamdi)

Relative humidity: This factor has also a significative impact on airborne microorganisms concentration, diversity and composition. In some reports (F.L. Schaffer, 1976) microorganisms like Cladosporium, Epicoccum or Influenza virus A had a negative correlation with RH, but not every microorganisms do not behave in the same way. In the conclusion of many reports (Q.Zhen, 2017) factors had more impact on airborne bacterial communities than air pollutants) as RH increases particule weight and size, it will increase the deposition probability because suspended particles will absorb ambient moisture.

Wind speed (WS): This factor is really significative on microbial characteristics. High wind speed can bring more microbes into the surface of atmosphere and cause the suspension which will lead higher microbes concentrations.

Others: there are others factors that play a significant role in airborne microorganisms, for example, rain.

Usually is told that rain decreases airborne particles, but some papers show that it’s not true (D.W. Li,1995), because droplets cause a vibration that can facilitate the air suspension.

Another example is solar radiation. It usually has a negative relation with airborne microorganisms because it can cause negative conditions for growth and survival, and UV radiation may kill microbes.

Conclusions

As we’ve seen, there are a lot of factors that have significant effect on the distribution of airborne microorganisms,  which are very important to understand airborne pathogens behaviour. To understand this behaviour, we have to relate pathogens toxicity with aerobiology, which is the science that studies airborne microbes behaviour.

By one hand, we’ve learned about the importance of airborne microbial size, which is the factor that determines the inhalation of particles, which in the case of pathogens it may harm us. We also understood that airborne microbes may affect our human health by being carried by particle matters and water droplets in the air, which can be inhaled by us depending on the size.

On the other hand, if we want to understand life cycle of these microbes, we need a complex knowledge about the factors that can interact with its life cycle, like wind flows, meteorologic factors or chemical composition of atmosphere.

Bibliography

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A. Fernstrom, M. Goldblatt (2013). Aerobiology and Its Role in the Transmission of Infectious Diseases. Journal of Pathogens (Volume 2013). (Webpage: https://www.hindawi.com,  Article ID: 493960).

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M.A. Alghamdi, M. Shamy, M.A. Redal, M. Khoder, A.H. Awad, S. Iserougy (2014). Microorganisms associated particulate matter: a preliminary study. Sci. Total Environ., 479-480 , pp. 109-116

F.L. Schaffer, M.E. Soergel, D.C. Straube (1976). Survival of airborne influenza virus: effects of propagating host, relative humidity, and composition of spray fluids. Archives of virology, 51 , pp. 263-273

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D.W. Li, B. Kendrick (1995). A year-round study on functional relationships of airborne fungi with meteorological factors. International  Journal of Biometeorology., 39, pp. 74-80

Extra information:

Air sampling methods, information material: The Microbial World: Airborne Microorganisms: http://archive.bio.ed.ac.uk/jdeacon/microbes/airborne.htm