Space Microbiology: An Overview
Introduction
Microorganisms are natural constituents of the environment, existing in air, water, soil,
and biotic systems. Microorganisms readily adapt to changes in environmental variables
such as nutrient levels, temperature, oxygen concentration, atmospheric pressure,
weightlessness, and light intensity, and exhibit and variety of physiological and
morphological changes. Bacterial growth is defined as the “coordinated summation of a
complex array of processes including chemical synthesis, assembly, polymerization,
biosynthesis, fueling, and transport, the consequence of which is the production of new
cells”(1). It is possible that the conditions of space flight may alter one or more of these
processes.
Microorganisms can be introduced into the spacecraft through several avenues.
Crewmembers will be a primary source of microorganisms in confined space habitats (2),
as the healthy human body can host at least 50 microbial species (2). Biological
payloads, resupply vehicles, hardware and supplies will provide additional sources of
microorganisms. In the confined spacecraft environment, increases in interpersonal
exchange of potential pathogens may facilitate infectious conditions (3). Microbial
metabolism of metals, electronic components, fuels, or other spacecraft components may
result in production of volatile organic compounds (3).
The NASA Strategic Plan includes priorities for addressing crew health and
environmental design issues related to microorganisms through the Human Exploration
and Development of Space (HEDS) Program. Specific objectives focus on ensuring a
safe living and working environment for crewmembers, as well as design elements which
minimize the potential for microbial growth in and on the spacecraft. This effort has been
an integral part of the US space program since its inception, and has produced a
considerable body of scientific and empirical data. Studies of spaceflight effects on
microbial physiology, human physiology, and microbial ecology have generated many
answers, as well as many more questions, about the nature of the host-microbe interaction
in this unique environment. This research has included spaceflight investigations and
studies of specific parameters using ground-based analogs.
Microbial Physiology
The US and Soviet space programs have performed many studies of the effects of
spaceflight on microorganisms. Because all Earth-based microorganisms have evolved in
a 1g environment, the microgravity conditions of spaceflight may result in alterations to
cell structure and metabolic processes. The field of gravitational biology has not
generated sufficient data to adequately assess the effects of microgravity on intracellular,
intercellular, or extracellular processes (4). The mechanism by which microgravity
affects individual cells is unclear, perhaps by the action of gravity on the organism as a
whole or by direct action at the cellular level (4). Lack of a clear distinction may explain
some of the different descriptions of these effects.
Several studies of phage induction in bacteria during space flight have suggested an
increase in phage activity (5,6,7,8,9,10). The differences in degree of phage induction
among these studies may be attributed to differences in handling techniques, flight
conditions, test parameters, and evaluation methods. The effects of spaceflight on
microbial growth rates have also been studied. Although these studies have often
produced conflicting results, a general tendency toward increased cell growth, biomass
production, and growth rate has been observed (11,12,13,14). Other studies have
suggested spaceflight induced changes in microbial physiology such as altered antibiotic
resistance (15,16,17,18), changes in circadian rhythms (19), changes in metabolic
processes (20), and genetic alterations (20).
Human Physiology
The effects of spaceflight on human physiology have been the subject of constant study
since the first manned orbital flights. Development of better technology has resulted in
significant increases in spaceflight duration, which result in more pronounced
physiological effects in crewmembers. Effects on the cardiovascular, neurovestibular,
musculoskeletal, endocrine, and hematological systems are well documented (21). Many
changes in the human immune response have been observed (22, 23), which are of
particular significance to the ongoing efforts to prevent infectious disease among
crewmembers.
Because crewmembers will be the primary source of microorganisms, the risk of
infectious diseases may be affected by exchange of microbiota among crewmembers
within the close quarters of the spacecraft. This exchange was first demonstrated during
the Apollo program , when bacteriophage typing revealed that Staphylococcus aureus was
transferred from one crewmember to another during a mission (24). Such transfer was
subsequently demonstrated on additional Apollo flights (25, 26), Skylab missions (27,
28), and in the Apollo-Soyuz Test Project (29). These findings were further documented
in the Space Shuttle program using molecular techniques which provided a much more
sensitive means for strain typing. Transfer of Staphylococcus aureus and Candida
albicans among crewmembers was demonstrated using very precise DNA fingerprinting
techniques (30, 31). Further studies of the effects of spaceflight on infectious disease risk
are part of the current effort to prepare for the future of manned space exploration.
Microbial Ecology
Understanding the behavior of microorganisms in closed environments is essential to
development of systems for long duration manned spaceflight. Terrestrial systems having
comparable properties have been extensively studied and used as a model for spaceflight
systems. Several studies of microbial load aboard submarines have been conducted by
the US Navy. In one study of bacterial sedimentation rates and microbial load in the air
were examined. The sedimentation rate decreased during the study period, and there was
no change in the microbial load between the control and test periods (32). The
predominant microorganisms in the air during both periods were Micrococci and
Streptococci. Clinical data on the submarine crew indicated that these organisms were
carried by a significant number of subjects in the throat (32). This finding supports the
longstanding assertion that crewmembers will be the major source of microbial loading in
the spacecraft environment.
In another study, Neisseria meningitidis was isolated from 26% of crewmembers and a
21% increase in resistance to the antibiotic sulfadiazine was observed during the patrol
period (33). No resistant strains were isolated initially, and this drug was not
administered during the patrol period. It was suggested that the closed environment and
interactions among the crew promoted selection of resistant strains (34). A number of
studies have also examined fungal growth aboard submarines (35). Although
Penicillium, Aspergillus, and yeast species were recovered, their relative quantities
changed significantly during the patrol period. Penicillium was initially the predominant
organism, but was superseded by yeasts after two weeks at sea (35). Yeasts were most
abundant on surfaces, while Penicillium predominated in the air samples. Despite the
presence of these organisms, there was little evidence of infections or allergies among
crewmembers (35).
The Soviets have conducted many experiments using “airtight environments” to model
spaceflight conditions. These studies have indicated changes in microbial populations
including increases in antibiotic resistance (36) and shifts in relative quantities of
commensal microbiota in human subjects (37). The presence of molds in these
environments was consistently found despite extensive use of disinfectants (38).
The microbial ecology of the spacecraft has also been extensively studied during the US
manned spaceflight programs (39-43). The microbial populations in the spacecraft
environment were well characterized during the Apollo mission series. In general, the
numbers of aerobic microorganisms were found to increase, while the diversity of species
and numbers of anaerobes decreased (40). Decreases in the numbers of fungal isolates
were observed in the Apollo missions, as well as during the Skylab program (40).
Although humans are the chief contributors to the microbial populations aboard
spacecraft, there are other sources of microbial loading. During the assembly and testing
associated with development of planetary testing requirements, it was reported that about
25 percent of the microorganisms found on the Viking lander capsules, orbiters, and
shrouds were soil bacteria (44) The remaining 75 percent were classified as indigenous
human microbiota. Some of these organisms survived even the terminal heat treatment of
the spacecraft. The Explorer 33 spacecraft was found to have a microbial burden of 2.6 x
105 CFU at launch (45). Similarly, the Apollo 10 and 11 command modules were
contaminated by 2.7 x 104 CFU per square foot (46). Approximately 95% of the
organisms recovered in each case were indigenous human microbiota (47). Other sources
of microorganisms in spacecraft include experimental animals and plants, supplies, foods,
and water.
Conclusion
The presence of microorganisms and their potential effects has been a source of study
since the advent of manned spaceflight. The unique aspects of the spaceflight
environment, particularly the microgravity condition, have been shown to effect microbial
and human physiology in many ways. Changes in microbial growth rates, survival,
phage-induction, and antibiotic susceptibility have been observed. Effects of spaceflight
on human physiology in such areas as cardiac, musculoskeletal, neurological, and
immune function have been well documented. The unique microbial ecology of the
spacecraft environment has also been the focus of many studies. Formation of an
indigenous population within the closed environment of the spacecraft has been
demonstrated.
As we prepare for the longer duration spaceflights necessary to enter the era of manned
planetary exploration, it is critical that we develop a better understanding of the changes
that may be induced in the host-microbe relationship in the unique environment of
spaceflight. Development of countermeasures to undesirable microbial interactions with
the spacecraft and crewmembers is an important part of current research efforts. The
study of microbial impacts on humans and spacecraft will continue to be a vital part of
manned space exploration.
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