**3. Microbial sampling**

There is a great variety of airborne microorganism sampling systems, allowing us to select the most suitable one depending on our objectives [42]. On the other hand, no standardized protocols exist, which is a major pitfall when developing our objectives. This fact has led some authors to propose the creation of consortiums of interested parties for establishing standardized protocol reproducibility [20], as well as the need to establish global networks of aerobiological studies [11]. Two approaches are proposed: particles or cells can be collected passively or directly from the atmosphere. Passive media usually involves decanting [43] and collecting particles over snow [44] or through the collection of atmospheric water [45]. On the other hand, active methodologies entail three major approaches: filtration, impaction, and liquid impingement. All three approaches are very efficient when developing culture-dependent techniques. In contrast, culture-independent approaches produce some serious problems that make the work difficult: the high variability of the system and the low biomass mean that sampling campaigns are, in many cases, extremely inefficient [20]. Lastly, the use of airborne platforms is not very extended, but they represent a good opportunity to conduct a more direct study of the atmosphere [5, 19, 31].

#### **3.1 Filtration**

Filtration is a simple and cheap method that is often efficient. It involves pumping air through a filter where the mineral and biological particles are trapped. Filters of different materials and porosity are available made of cellulose, nylon, polycarbonate

**45**

*Microbial Ecology in the Atmosphere: The Last Extreme Environment*

or fiberglass, or quartz. Sizes used range from 0.2 to 8 μm, depending on the size of the particles to be captured and the capacity of the pump. In many cases, a PM10 filter can give better results when collecting smaller bacteria, as it allows greater airflow. Airflow filtration rates generally range between 300 and 1000 L/minute [4, 46]. Microorganisms trapped in the filter can be cultured, or the filters can be directly used for DNA extraction. In addition, filters are a very suitable support for microscopy, and

countless holders for filters are available (an example is shown in **Figure 2A**).

In impingement, particles are collected in a liquid matrix [20]. Normally a buffer is used such as phosphate buffer saline (PBS) that helps maintain the viability of the cells. One of the more widely used liquid impingers is BioSampler SKC (**Figure 2B**). In this case, the tangential movement of the particles inside the flow impinger retains the particles in the collecting liquid. The suspension obtained could be used for culturing or for molecular ecology assays [20]. One of the advantages of impingement collection is that it facilitates quantitative techniques such as

In this system, the particles generally impact into a petri dish with an enrichment medium. It is, possibly, the most efficient and most used method to conduct studies based on culture. Airflow impacting onto the plates is controlled by slots that allow the homogeneous distribution of the air. The system can be single stage or several stages in cascade, causing the particles to be distributed by size in the different petri dishes [20]. Some variants replace petri dishes with agarose filters or Vaseline strips, in order to carry out independent culture methodologies, but efficiency is very low. The original and more popular impactor is the Andersen cascade

Several studies explain and compare sampling methodologies in aerobiology, but most of them focus on the surface of the Earth (e.g., on top of mountains or buildings) or indoors [42, 49–54]. However, small studies have been conducted at higher altitudes or in open sea areas. The use of airborne platforms (balloons, aircraft, rockets, etc.) for aerobiology sampling would allow conducting a direct study of the microbial ecology of the atmosphere. Another advantage of airborne platforms is the possibility of studying the vertical distribution of airborne microbial communities. In addition, some aircraft allow us to develop studies in the upper troposphere or in the stratosphere. Unfortunately, atmospheric microbial collection instruments

Among the different airborne platforms, aircraft, due to their versatility and access, are particularly interesting. Some studies have been conducted, but not enough samples have been developed yet, and efficiency is still very low. As already mentioned, the efficiency of samplers in soil-level aerobiology faces a series of problems (low biomass, high variability of populations, lack of standardized protocols). In the case of airplanes, in addition to these intrinsic problems associated with atmospheric microbial ecology, other additional ones exist: (1) the high velocity of the aircraft in relation to the relative quiescent air mass. This makes it difficult to obtain an isokinetic sampler and, therefore, one that is sufficiently efficient that would allow us to obtain a correct quantification of the incoming air [55]; (2) the sampler

*DOI: http://dx.doi.org/10.5772/intechopen.81650*

flow cytometry or in situ hybridization [47].

have not been developed enough for airborne platforms.

**3.2 Impingement**

**3.3 Impaction**

impactor (**Figure 2C**) [48].

**3.4. Airborne platforms**

or fiberglass, or quartz. Sizes used range from 0.2 to 8 μm, depending on the size of the particles to be captured and the capacity of the pump. In many cases, a PM10 filter can give better results when collecting smaller bacteria, as it allows greater airflow. Airflow filtration rates generally range between 300 and 1000 L/minute [4, 46]. Microorganisms trapped in the filter can be cultured, or the filters can be directly used for DNA extraction. In addition, filters are a very suitable support for microscopy, and countless holders for filters are available (an example is shown in **Figure 2A**).
