Bioaerosol
are considered ubiquitous constituents of the atmosphere, as a large number of
these particles are small-sized microorganisms. Estimates of biomass content in
the atmospheric particulate having an aerodynamic diameter of less than 2.5µm
(pm2.5) range from 3% to 11% by weight (Boreson et al., 2004; Peccia et al.,
2006), whereas other studies have estimated that up to 25% of the insuble
fractions of aerosols is made of biological materials (Peccia et al., 2006; Jones et al., 2004).
At remote sites representing background atmospheric
conditions, airborne bacteria and fungi cells have been found to reach
concentration of
Interestingly, bacteria and fungi have
be detected in various atmospheric layer such as the boundary layer (up to
1.5km altitude), the upper troposphere (up to 12km altitude) and even the
stratosphere at altitude of 20km above the sea surface (Griffin, 2004;
Wainwright, et al., 2003). In addition,
isolated cultures of the common mould, Penicillinm
notatum, have been collected at an altitude of 77km, and the bacteria Micrococcus albus and Mycobacterium luten at an altitude of
70km (Imshenetsky, 1978). Due to their small size, microbes can be transported
by upper air current over long distance within or between continents, and thus
are able to travel/deposited to the most distant areas of the world. In this
term, the movement of air masses serves as the primary mechanisms for the rapid
conveyance of microorganisms among widely dispersed habitats (Isard, et al., 2001; Prospero, et al., 2005).
More recently, desert dust storms have
been shown to be an important source of the most efficient transportation
mechanism of bioaerosols, enabling the spread of microbes for over 5000km away
from their sources (Prospero, et al.,
2005; Griffin, 2004; Kellogg et al.,
2006). The largest sources of dust to earth’s atmosphere are the Sahara and
Sahel regions of the North Africa and the Gobi, Taklamaka, and Badain Juram
desert of Asia (Kellogg et al., 2006).
The current estimate for the quality of arid soil that moves some distance in
earth’s atmosphere is 2 billion metric tons per year (Perkins et al., 2001), whereas 50% to 75% of
this quality is believed to originate from the Sahara and Sahel (Goudie et al., 2001; Moulin et al., 1997; Perry et al.,1997; Prospero, et
al., 2005). These region serve as sources of dust to earth’s atmosphere
throughout the year affecting the air quality in the Middle East, Europe, the Caribbean,
and the Americas (Goudie et al.,
2001; Moulin et al., 1997; Perry et al., 1997; Prospero, et al., 2005; Middleto et al., 2001).
In addition to inorganic particles, the
clouds of deserts dust can carry a sizeable innoculum of microorganisms and
microbiological materials (Griffin, et
al., 2007). Though, very few publication, have investigated desert
dust-associated microbes after long distance transport at both African and
Asian dust system (Kellogg et al.,
2006).
Different
analytical approached have been applied to answer different questions as the
focus of the previous investigations varied from allergens (Ho et al., 2005; Wu et al., 2004; Yeo, et al.,
2002), to a coral pathogens (Kuske, C.R 2006) and the phylogenetic characterization
of the dust-associated microbial community (Kellogg et al., 2006, Prospero, et
al., 2005, Griffin, et al., 2003;
Choi., et al., 1997;- Polymenakou et al., 2008). As a rough approximation,
Griffin and Kelloga (Griffin and Kelloga 2004) adopted a conservative estimate
of 104 bacteria per grain of soil and calculated that 1016
dust borne bacteria are moving around the atmosphere for every I million tons
of emitted soil particles (this estimate does not include the prevalent population
of fungi and viruses (Griffin and Kelloga 2004).
In
addition to physical and biological sources of airborne microorganism, human
activities such as sewage treatment, plants and animal rendering, fermentation
processes and agricultural activities, do emit microorganisms into the air
(Racer et al., 2001, Adhikari et al.,
2004; Gilum and Leventin, 2008). Several studies have identified human
activities such as talking, sneezing and coughing (Kalogeraskis et al., 2005), while other human
activities such as vehicular transportation, human movements, washing in homes
and business centres, flushing of toilets and sewages, sweeping of floors and
roadsides can generate bioaerosols indirectly (Kalogeraski et al., 2005; Chen and Hildermann, 2009).
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