Microbial Colonization of Space Stations
Long-term space station missions to date include Salyut, Almaz, Skylab, Mir, ISS (International Space Station), and Tiangong, with the latter two still ongoing. These expeditions demonstrate that space stations support a diverse microbial community. While most microbes are not harmful, routine microbial monitoring onboard space stations is still essential in preventing material biodeterioration and maintaining crew health. As currently identified, these microorganisms originate from Earth, with main source coming from crewmembers themselves and from materials manufactured on ground.  Environmental monitoring is important in identifying microbe and providing data for future studies in attempt to minimize microbial threat to crewmembers’ performance and spacecrafts’ health.
Microorganisms exist in the portable water, ventilating air, and on surfaces of space stations.  While these microorganisms share the same living space as the crewmembers, the crewmembers themselves also serve as incubators for various microbes.  Researchers have identified several strains of bacteria and fungi as opportunistic pathogen that may pose threats to crewmember. (refer to Key Microorganism section) Moreover, they have also identified the type of microbes responsible for biodegradation of structural materials.  (refer to Biodegradation section)
Microbial Diversity and Interactions
Other than in portable water and air, bacteria and fungi typically co-exist in biofilms on surfaces, including water pipe lines.  Biofilms provides suitable growth conditions, and intrinsically contribute to antibiotic and biocide resistance due to horizontal gene transfer of antibiotic resistance plasmid, and the fact that microbes positioned inside biofilms are harder to reach. One study shows that as much as 36 species of fungi and 58 species of bacteria can co-exist in a single biofilm. However, the composition varies drastically due to dynamic (changing) environmental conditions, available nutrient (surface materials), and also the thickness of biofilm. In addition, exchange of microflora carried by crewmembers travelling to and from the space craft also increases microbial diversity onboard space stations.
Space Microbial Processes
Biodegradation of the surfaces of spacecraft structural materials are results of colonization of bacteria and fungi.  While fungi are more abundant in early colonization and biofilm formation on polymeric materials, given sufficient humidity, subsequent microbes thrive in biofilms that forms at wide ranges of temperatures. Biodegradation of bacteria and fungi predominantly occur on these surfaces due to the availability of carbon in surface finishings, as well as other nutrients present in structural materials (polymeric-base and metal-based). In thin biofilms, oxic condition support aerobic biodegraders; in thicker biofilms, where oxygen diffusion is limited, anaerobic fermenters and metal reducers are come into play. Although these biodegradation of carbon (polymeric)-based and metal-based materials are due to similar hydrolysis reactions, the yielding products and degree of degradation are different depending on different microbes. 
According to various in vitro studies, the microgravitaional environment in space enhances microbial growth.  Several factors were examined in a closed, isolated environment on ground-based labs, and the results show a decreased lag phase and increased final yield compared to normal-gravity conditions.  This enhanced microbial growth also contribute to increase of antibiotic resistance.  In addition to the confined, microgravitaional condition, various unknown factors onboard also provide distinctive selective pressures that favor certain microbes that eventually become harder to eliminate.
Microbe Identification Techniques (Onboard Environmental Monitering)
Most antimicrobial and environmental monitoring implementations on ISS are results of studies done by previous expedition, Mir and Skylab. Before December 2006, isolation of microorganisms depended on the growth of environmental isolates on agar media onboard space stations and the subsequent ground-based analysis back on Earth.  However, this method restricts the number identifiable microorganisms since more than 95% of environmental microbes do not grow on conventional agar growth media. Furthermore, accuracy is limited since months often passed before returning to Earth for further analysis.
More recently, ISS launched Lab-On-A-Chip Application Development Portable Test System (LOCAD-PTS).  This new technology can be hand-held and allowed rapid microbial monitoring of bacteria and fungi since its implementation in December 2006. 
Dominant bacteria are Acidovorax spp., Afipia spp., Brevundimonas spp., Propionibacterium spp., Serratia spp.,  Sphingomonas spp., and Methylobacterium spp.,  and opportunistic human pathogens include Afipia spp., Brevundimonas spp., Propionibacterium spp., Serratia spp., and Sphingomonas spp. (rarely pathogenic to human).
All are opportunistic human pathogens.
Dominant bacteria are Staphylococcus spp.,  Pseudomonas aeruginosa, Ochrobactrum anthropi, Alcaligenes denitrificans, Xanthomonas maltophila, and Vibrio harveyi, while dominant fungi are Aspergillus spp. And Cladosporium spp. Opportunistic human pathogens included Staphylococcus spp., Pseudomonas aeruginosa, Ochrobactrum anthropi, Alcaligenes denitrificans (rarely pathogenic to human), Xanthomonas maltophila, Aspergillus spp., and Cladosporium spp. (rarely pathogenic to human). Out of these, the dominant species in biofilm include Pseudomonas aeruginosa, Ochrobactrum anthropi, Alcaligenes denitrificans, Xanthomonas maltophila, and Vibrio harveyi.
 van Tongeren, S. P., Krooneman, J., Raangs, G. C., Welling, G. W. and Harmsen, H. J. M. “Microbial detection and monitoring in advanced life support systems like the International Space Station.” Microgravity Science and Technology, 2007, DOI: 10.1007/BF02911866
 Gu, J. “Microbial colonization of polymeric materials for space applications and mechanisms of biodeterioration: A review.” International Biodeterioration & Biodegradation, 2007, http://dx.doi.org/10.1016/j.ibiod.2006.08.010
 Castro, V. A., Thrasher, A. N., Healy, M., Ott C. M., and Pierson, D. L. “Microbial Characterization during the Early Habitation of the International Space Station.” Microbial Ecology, 2004, DOI: 10.1007/s00248-003-1030-y
 Pierson, D. L. “Microbial Contamination of Spacecraft.” Journal of the American Society for Gravitational and Space Biology, 2001, cited October 2012. http://gravitationalandspacebiology.org/index.php/journal/article/view/261
 Ilyin, V. K. “Microbiological status of cosmonauts during orbital spaceflights on Salyut and Mir orbital stations.” Acta Astronautica, 2005, http://dx.doi.org/10.1016/j.actaastro.2005.01.009
 Novikova, N., De Boever, P., Poddubko, S., Deshavaya, E., Polokarpov, N., Rakova, N., Coninx, I. and Mergeay, M. “Survey of environmental biocontamination on board the International Space Station.” Research in Microbiology, 2006, http://dx.doi.org/10.1016/j.resmic.2005.07.010
 Vesper, S. J., Wong, W., Kuo, C. M. and Pierson, D. L. “Mold species in dust from the International Space Station identified and quantified by mold-specific quantitative PCR.” Research in Microbiology, 2008, http://dx.doi.org/10.1016/j.resmic.2008.06.001
 NASA: International Space Station, cited October 2012. http://www.nasa.gov/mission_pages/station/main/index.html
 Robinson, J. A., Thumm, T. L. and Thomas, D. A. “NASA utilization of the International Space Station and the Vision for Space Exploration.” Acta Astronautica, 2007, http://dx.doi.org/10.1016/j.actaastro.2007.01.019
 Barbosa, R.C. “China launches TianGong-1 to mark next human space flight milestone.” NASASpaceFlight.com, 2011, cited October 2012. http://www.nasaspaceflight.com/2011/09/china-major-human-space-flight-milestone-tiangong-1s-launch/
 Gunter, D., Flores, G., Effinger, M., Maule, J., Wainwright, N., Steele, A., Damon, M., Wells, M., Williams, S., Morris, H. and Monaco, L. “Rapid Monitoring of Bacteria and Fungi aboard the International Space Station (ISS)” NASA Technical Reports Server (NTRS), 2009, http://naca.larc.nasa.gov/search.jsp?R=20090017684&qs=N%3D4294950110%2B4294848119%2B4294301177
 Klaus, D. M. and Howard, H. N. “Antibiotic efficacy and microbial virulence during space flight.” Trends in Biotechnology, 2006, http://dx.doi.org/10.1016/j.tibtech.2006.01.008
 Gu, J., Roman, M., Esselman, T. and Mitchell, R. “The role of microbial biofilms in deterioration of spacestation candidate materials.” International Biodeterioration & Biodegradation, 1998, http://dx.doi.org/10.1016/S0964-8305(98)80005-X
 La Duc, M. T., Sumner, R., Pierson, D., Venkat, P. and Venkateswaran, K. “Evidence of pathogenic microbes in the International Space Station drinking water: reason for concern?” UK Pubmed Central, 2004, cited on October 2012. http://ukpmc.ac.uk/abstract/MED/15880908/reload=0;jsessionid=40fVP5QP57u3BxVyDAfi.4
 La Duc, M. “103rd General Meeting of the American Society for Microbiology.” American Society for Microbiology, 2003, cited October 2012. http://www.asm.org/index.php?option=com_content&view=article&id=4205&title=How+Do+I+Get+My+First+Position+as+a+Microbiologist%3F+&Itemid=302
 Klintworth, R. and Reher, H. J. “Biological induced corrosion of materials II: New test methods and experiences from Mir station.” Acta Astronautica, 1999, http://dx.doi.org/10.1016/S0094-5765(99)00069-7
 “STS-135 Shuttle Mission Imagery.” NASA, cited November 2012. http://spaceflight.nasa.gov/gallery/images/shuttle/sts-135/html/s135e011814.html
 “Astronaut Gloves Tested for Biological Contamination.” SPACE.com, cited November 2012. http://www.space.com/6486-astronaut-gloves-tested-biological-contamination.html