Biologist Volume 52 Number 2, May 2005

Scientists have long worried aboutthe adverse effects of spaceflight;indeed when Yuri Gagarin made hishistoric flight in 1961 on Vostok I, it waslimited to a single orbit around the Earthamidst fears that prolonged exposure tothe space environment might prove deadly.While these fears turned out to be exag-gerated, it is now evident that the spaceenvironment does produce several effectson both man and microbes which are ofconcern to space travellers.

The unique stresses encountered inspace include physical factors, such asexposure to markedly diminished gravityand unusually strong radiation, as well as

psychological stress caused by isolationand confinement to a restricted area.Sleep disruption, lack of appetite and con-sequent inadequate nutrition pose addi-tional problems. While all aspects areimportant, much interest has recentlycentred on the effects of decreased gravity.Since life on this planet evolved withgravity as a constant feature, there iscuriosity about how earthly life copes withthe entirely unprecedented experience ofreduced gravity (commonly referred to asmicrogravity).

The harmful effects of microgravity onhumans have been documented. Astronautscan lose up to 3% of bone density per

S V Lynch and A Matin

Stanford UniversitySchool of Medicine,USA

Title image. InternationalSpace Station. Centrifugeshave been transported tothe International SpaceStation that can generate1g gravity controls, makingit possible to study theeffects of radiation inde-pendent of gravitationaleffects.

Travails of microgravity:man and microbes in spaceSpace travel has been shown to have many and varied effects on the human beings that haveventured there. The effect of this environment on microbes is less well-known but investigationsare now pointing to important consequences for those of us who are still earthbound.

Biologist Volume 52 Number 2, May 2005

Travails of microgravity |IOB

month and are, as a result, prone toincreased incidence of fracture upon pro-longed space residence; also, the re-sorbedcalcium can result in kidney stones(Whitson et al). Muscle atrophy, resultingfrom lack of gravitational load duringmovement and minimised exercise in theconfines of a spacecraft, is an additionalproblem. Microgravity also reduces bloodproduction, resulting in diminished pump-ing by the heart. This, combined with con-comitant blood shift to the upper torsoresults in damage to heart muscle. Humanimmune response too is compromised inspace (Sonnenfeld and Shearer).While emo-tional stress contributes, studies carried outwith earth-based systems clearly establishthat reduced gravity is a major factor.

The identification of over one hundredstrains of bacteria and fungi from long-term manned missions leaves little doubtthat these organisms survive and propa-gate in microgravity. Much effort is madeto generate a germ-free environment forspace travellers. Spacecrafts are sanitisedby flushing with antimicrobial agents suchas ethylene oxide and methyl chloride;and astronauts are quarantined for severaldays prior to a mission. But these meas-ures only reduce rather than eliminate themicrobes, since astronauts themselves,like all humans, are a diverse reservoir ofmicrobial flora. These microbes includeintestinal bacteria (coliforms), oppor-tunistic pathogens like the skin-bornestaphylococci and streptococci, and latentviruses. Recent studies show that thesemicrobes are a cause for concern, especiallygiven the compromised immune responseof astronauts.

Microgravity effects on biological entitiescan also be exploited for human benefit,though. Mammalian cells, grown in micro-

gravity, form three-dimensional tissueaggregates that mimic human tissues moreclosely than traditional monolayer cultures;these provide a superior model system formedical research. Further, bacteria in orbit-ing satellites were found to produce certainantibiotics more efficiently, raising thepossibility that their productivity can beincreased in simulated microgravity sys-tems on earth.

Simulated microgravity on EarthUntil recently, investigations into theeffects of microgravity were confined toexperimental work on board space shuttlesand stations, though the logistical problemsinvolved greatly hindered progress.Although technological sophistication onspace stations and shuttles is increasing,there are severe limitations to the typesof procedures that can be performed.

Experimental equipment for on-boardinvestigations needs to be compact, light,and simple. Because of this, the constrainton cosmonaut time, and the inherent diffi-culty of working in microgravity, only rela-tively simple experimental manoeuvrescan be performed. Moreover, besides micro-gravity, other physical stresses exist inspace. In most of the experiments so farconducted in this environment therefore,it is difficult to ascertain which factorsaccounted for the observed effects.

A considerable effort has therefore goneinto developing earth-based systems thatsimulate microgravity. Examples includedrop towers at various National Aero-nautics and Space Administration (NASA)Centers (Figure 1a) and aircraft whoseflight traces a parabola: their contents arethus exposed to short periods of simulatedmicrogravity conditions. Investigationsinto the effects of mechanical unloading

Figure 1. A. Schematic of a drop tower used to simulate short periods of microgravity by freefall. B. The hind limb suspensionmodel used to study the effect of gravitational unloading on rodents. The animal is suspended at a 30° angle from a guide wireacross the top of the cage (left hand cage) which simulates gravitational unloading similar to that experienced by cosmonauts.

IOB | Travails of microgravity

Biologist Volume 52 Number 2, May 2005

Figure 2A. RCCS system used to generate a simulated microgravity environment inground-based investigations (reproduced with permission, Synthecon Inc, Houston,Texas). B. Components of the HARV vessel. C. Differential rotation of HARV vesselsperpendicular to or parallel with the gravitational vector generates a 1g (control) orsimulated microgravity environment. Particles in control vessel are subject to gravita-tional forces; particles in simulated microgravity vessel remain in constant suspension.

experienced in microgravity are simulatedon earth using the head-out water immer-sion (HOWI) and head down bed-rest mod-els. These procedures induce fluid shifttowards the head, and abdominal organshift towards the chest, while minimisingthe gravity gradient on the cardiovascularsystem, paralleling the effects seen inspace. Another method is the hindlimbsuspension model, in which test animalsare suspended by their tail at a 30º anglefrom a guidewire, mechanically unloadingthe animal’s rear limbs of gravitationalload (Figure 1b). This method is commonlyused to examine the effects of simulatedmicrogravity on bones and muscles. Sinceonly the hind limbs are unloaded, the fore-limbs provide an internal control. The useof this system has revealed that suchunloading on Earth parallels the effects seenduring the first week of space residence.

To study changes occurring at the cellu-lar level in simulated microgravity, theNASA biotechnology group at JohnsonSpace Centre in Texas has developed avariety of cell culture methods. The mostcommonly used of these is the Rotary CellCulture System (RCCS), marketed bySynthecon (Texas, USA). This apparatusconsists of a motor, a cylindrical HighAspect to Ratio Vessel (HARV), and a plat-form on which the vessel is rotated (Figure2a). The HARV has separable front andback faces; the front face contains twosampling ports, and the back is provided

with a semi-permeable membrane for aer-ation (Figure 2b). The assembled vessel isfilled to capacity (zero headspace) withmedium and inoculum, and air bubblesare removed to eliminate turbulence andensure a low shear environment. It isthen attached to the platform and orientedso that it is either rotated about a verticalaxis perpendicular to the gravitationalvector, or a horizontal axis parallel to thisvector. Cells rotated in the former orienta-tion experience normal gravitational forcesand serve as a control (1g) environment.In the vessel rotated about a horizontalaxis, the liquid within moves as a singlebody of fluid in which the gravitationalvector is offset by hydrodynamic, centrifugaland Coriolis (circular movement) forcesresulting in maintenance of cells in a con-tinuous suspended orbit.

Rotation about the horizontal axis ran-domises gravitational vectors across thesurface of the cells and generates micro-gravity of about 10-2 g (Figure 2c; Hammondand Hammond). Mathematical modellinghas confirmed the existence of simulatedmicrogravity conditions in the horizontallyrotated HARV vessels for spherical objectsof the size and weight of mammalian andbacterial cells.

Microgravity and human physiologyNormal human immune systems rely onseveral types of specialised white bloodcells, or leukocytes. Among the most impor-tant of these are macrophages, whichengulf and destroy invading pathogens.Once inside, the pathogen is digested, itsproteins degraded to peptides and trans-ported to the surface of macrophages. Herethey appear as antigens with the assis-tance of surface proteins, widely presentin mammalian cells, called the humanleukocyte antigens (HLA). Specific recog-nition sites or epitopes on these peptidesare recognised by a receptor protein onanother type of leukocyte, the T-cell.Binding of the receptor protein on the sur-face of the T-cell to the foreign peptideresults in T-cell activation. Activated T-cells can directly destroy invading patho-gens; additionally they activate anothertype of leukocyte, the B-cells. The latterthen secrete special proteins called anti-bodies, which specifically identify the anti-gens that trigger their production.Antibodies facilitate the engulfment ofpathogens by the macrophages and neu-tralise the harmful toxins that thepathogens make inside the human body.B-cells are also responsible for immunememory and the antibodies they produce

Biologist Volume 52 Number 2, May 2005

Travails of microgravity |IOB

may be used in defence against futureinfection by the same pathogen (Figure 3).

Astronauts show diminished immuneresponse. This is predominantly due toalterations in the number and proportionof lymphocytes (a type of leukocyte) andtheir cytokine production (which facilitatesthe type of interaction between differentleukocytes described above), and depressionof dendritic cell (a kind of macrophage)function and T cell activation. Also, num-bers of monocytes, precursors of macro-phages, are decreased in astronauts.

This decrease can be explained by studiesin ground-based HARVs which demon-strated that monocyte cells when culturedin simulated microgravity activate theapoptotic response, effectively committingsuicide. Furthermore, when dendritic cellswere cultivated under simulated micro-gravity, they became less effective inengulfing fungal pathogens and exhibitedlowered expression of HLA and relatedproteins on their surface. Simulated micro-gravity also diminished the interactionbetween various types of leukocyteswhich is critical for an effective immuneresponse. These earth-based studies leavelittle doubt that microgravity has a directrole in impairing the human immuneresponse.

Studies employing the hindlimb suspen-sion model on earth have shown that sus-pended animals develop osteo-fragilityclosely resembling bone loss observed inastronauts and those suffering from chronicosteoporosis on Earth. In addition, muscleatrophy, a major problem for astronautswhich results in loss of muscle strengthand functionality, is also observed in hindlimbs of suspended animals. This earth-based model thus provides an excellentsystem to determine the mechanisticbasis of bone loss and muscle atrophy.Theseinsights should hasten the developmentof novel therapies for chronic osteoprosisand muscular atrophy diseases, such asWerdnig-Hoffmann Disease (spinal mus-cular atrophy).

Effects on latent virusesMany viral infections acquired duringchildhood, although cured, persist in aninactive state in asymptomatic humans.Examples include the causative agent ofchicken pox, Varicella Zoster; EpsteinBarr virus, which causes mononucleosis,and the virus responsible for coldsores,Herpes Simplex. There is increasing evi-dence that during space flight these latentinfections tend to re-activate causing illnessin space travellers.

According to a recent report, the load ofthe Varicella Zoster virus increased dra-matically in the saliva of eight astronautswho were studied prior to, during, andpost-space flight. This tendency towardsre-activation is undoubtedly due, in part, todiminished immune response. Emotionalfactors also play a role as evidenced bythe fact that changes occur in neuroen-docrine stress hormone concentrationsbefore and during space flight – this isknown to contribute to reactivation of cer-tain viral particles.

There is also emerging evidence thatmicrogravity itself has a role. Cultivationof human Rhinovirus under simulatedmicrogravity in the RCCS system wasfound to enhance replication, yielding moreviral particles than conventional rollerbottle culture. In addition, viral transmis-sion appeared to be as good or even betterin simulated microgravity than conven-tional culture, demonstrating a clear sim-ulated microgravity-induced enhancementof viral infectivity potential.

Effects on mammalian cell cultureMammalian cells grown in space on boardspace shuttles and on the MIR space stationform three-dimensional tissues whichmimic human tissues more closely thantraditional monolayer cultures. Becauseof the considerable medical potential ofthis finding, investigations have been con-ducted to determine if similar aggregatescan be generated in Earth-based systems.

Figure 3. Simplified schematic of immune response to invading pathogen. Interactionbetween peptides presented by HLA and T-cell receptor, results in T-cell activationwhich stimulates pathogen-specific antibody production by B cells.

IOB | Travails of microgravity

Biologist Volume 52 Number 2, May 2005

Figure 4. Mammalian cell lines cultured in low shear sim-ulated microgravity in the HARV system develop highly differ-entiated 3-dimmensional aggregates which model in vivotissue. The different cell lines are primary liver (A); smoothmuscle tissue (B); bone marrow (C); and prostate tissue(D). Images reproduced with permission from SyntheconInc. Houston, Texas, USA.

The results have been exciting. Cultivationin HARVs resulted in the generation ofhighly differentiated 3-D aggregates ofseveral mammalian cell types including,bone, cartilage, cardiac, neural, renal, andprostate cells (Figure 4). Many of theseHARV-generated tissues as well as carci-noma cell lines have proven excellentmodels in toxicology, vaccine development,chemotherapy and neurological disorderstudies. In addition, novel polymer scaf-folds have been developed to enhance 3-Dtissue formation in this environment inthe hope that organs generated in HARVsmay be used for improved transplant andtissue graft technologies.

Earth-based studies on bacterial cells Given the detrimental effects of micro-gravity on the human immune system, itbecame especially important to determineif it also altered bacterial behaviour. Twobacteria have been extensively examined inthis respect, using the HARV bioreactors.The first, Salmonella typhimurium, causestyphoid-like disease in mice; the second isthe opportunistic pathogen Escherichiacoli. Cultivation of these bacteria in simu-lated microgravity even for a short periodof time (5-10 hours) increased their resist-ance to several lethal agents, such as highosmolarity or high acidity (Figure 5a;Lynch et al; Nickerson et al). Further, S.typhimurium culture in simulated micro-gravity killed mice more rapidly than itsnormal gravity-grown counterpart (Figure5b): LD50 (the oral lethal dose of bacteriarequired to kill 50% of mice) of the micro-gravity cultured bacteria was was one-

fifth that of cultures under control gravityconditions (Nickerson et al). Starvationconditions generally prevail in nature inwhich bacterial cells exist in a stationaryphase and space flight usually involvesprolonged exposure to microgravity.Therefore, protracted incubation of E. coliin simulated microgravity was examined,these cultures displayed a furtherincrease in resistance (Figure 5c), rein-forcing the conclusion that bacteria posereal danger to immuno-compromisedastronauts (Lynch et al).

Increased resistance in E. coli and S.typhimurium as well as increased virulencein the latter bacterium are highly sugges-tive of a more familiar and intensivelystudied phenomenon on earth, termed the‘general stress response’. Here, exposureof bacteria to a non-lethal dose of stress,such as heat-shock, hyperosmosis, or star-vation, makes them not only more resistantto the stress that was experienced, butalso to a large number of unrelated stresses.Thus, for example, starved cells not onlydevelop increased resistance to starvationitself, but also to stresses in general. Inthis respect, bacteria truly underscore therelevance of the adage: what does not kill,makes one stronger!

The reason for this increased compre-hensive resistance and virulence is thatall stresses induce in bacteria the synthesisof a common set of proteins, which areconcerned with preventing damage to,and promoting repair of, critical cellularmacromolecules such as proteins, DNAand lipids. Since exposure to differentstresses eventually leads to a similar out-come, namely macromolecular damage,this ‘core’ set of proteins (also called gen-eral stress proteins) makes bacteriahardier and more difficult to kill. Theresemblance of the simulated microgravityeffect to the general stress response sug-gested that microgravity acted as anotherform of stress akin to starvation, heatshock, etc, and prompted investigation ofwhether it too induced the core set ofgenes and proteins which is the hallmarkof other stresses. The results were a com-plete surprise.

Unlike the other stresses, short-termcultivation of E. coli or S. typhimurium insimulated microgravity conferred generalresistance without the induction of knownprotective genes. What accounts for theincreased resistance of microgravity-grown cells in the absence of this andother general-stress gene and proteininduction? The phenomenon hints at theexistence of a new biochemical paradigm

Biologist Volume 52 Number 2, May 2005

Travails of microgravity |IOB

of microbial resistance and virulence andis currently under intense investigation.

In addition, the enhanced virulence of S.typhimurium cells grown under simulatedmicrogravity conditions does not involveincreased expression of genes implicatedin virulence of this bacterium under normalgravity conditions. Among the 163 genesthat are differentially expressed in simu-lated microgravity-grown cultures, onlyone known virulence gene was identified.In fact, many of the genes known to beinvolved in virulence were expressed at alower level under simulated microgravityconditions. These included genes involvedin lipopolysaccharide (LPS) production.Consistent with this down regulation,microgravity-grown cells were seen topossess about half as much LPS as theirnormal gravity grown counterparts. Theseresults point to the intriguing possibilitythat low-shear, simulated microgravityconditions induce alternative pathogenictactics involving novel virulence functionspreviously uncharacterised in this andperhaps other bacteria (Nickerson et al).

Studies on the regulation of microgravity-conferred resistance in E. coli held furthersurprises. Bacteria respond to stressfulconditions by altering their gene expression,leading to the induction of the generalstress proteins. This change involves amodification of the enzyme RNA poly-merase, which is responsible for genetranscription and expression. This enzymeis made up of several protein subunits,one of which is a small, loosely-associatedprotein called the sigma (s) factor. AnRNA polymerase devoid of a s factor(called the ‘core’ enzyme) is incapable oftranscription, but becomes transcription-competent when converted to a ‘holoen-zyme’ upon association with a s factor.

There are many s factors in a bacterialcell, which associate or dissociate with RNApolymerase depending on environmentalconditions, giving rise to different RNApolymerase species. Because of s factorspecificity, each RNA polymerase holoen-zyme recognises different promotersequences found in front of the codingregion of genes, and thus a change in theholoenzyme species leads to altered geneexpression under, for example, stressfulconditions.

Stresses on earth evoke the generalstress response under normal gravity condi-tions due to an increase in the concentrationof a specific sigma factor, called ss, andthereby to an increased concentration ofthe RNA polymerase holoenzyme contain-ing this sigma factor. It is this holoenzyme

that is responsible for the expression ofgenes that confer general resistance undernormal gravity conditions. However, quanti-tative analysis of ss concentration in E. colicultured for five hours in simulated micro-gravity showed that the enhanced resist-ance observed under these conditions wasnot accompanied by increased ss concen-tration; on the contrary, such cells showeddecreased levels of this sigma factor com-pared to normal gravity-grown cells (Lynchet al). The use of mutant strains of S.typhimurium and E. coli, devoid of ss, alsoindicated the absence of a role for thissigma factor in simulated microgravity-conferred resistance observed in short-term exposed cultures: the mutant strainswere as efficient as their parent strains indeveloping resistance.

Figure 5. Panels A and C show increased resistance of SMG-grown E. coli cultures tohigh salt or acid stress following short-term (5-10 hours), or longer term culture (24 h),respectively. Panel B shows increased virulence of SMG-cultured S. typhimurium asevidenced by a shorter time to death of test animals infected with SMG-cultured bacteriacompared with normal gravity (NG) cultured cells. (A and C are reproduced from Lynchet al; and B from Nickerson et al with permission.)

% S

urvi

val

% S

urvi

val

% S

urvi

val

Time of Death (Days)

2.5 MNaCl

A

B

C

2.5 MNaCl pH 3.5

pH 3.5

100

90

80

70

60

50

40

30

20

10

0

100

90

80

70

60

50

40

30

20

10

0

100

80

60

40

20

00 5 10 15 20

IOB | Travails of microgravity

Biologist Volume 52 Number 2, May 2005

When these studies were extended to E.coli cells cultivated in simulated micro-gravity for a prolonged period, the resultsturned out to be even more surprising.These cells, which display super-resistance,possessed much higher ss concentrationscompared with the normal gravity-culturedcells in the corresponding growth phase.It appears, therefore, that two distinctmechanisms are responsible for the initialand ultimate increased resistance of E.coli in response to simulated microgravity.Moreover, the response governed by ss,reinforces the initial increase in resistanceleading to super-resistant cells.

In this respect, and in conferring resist-ance without the induction of the generalstress proteins, microgravity-conferredresistance represents a new paradigm.Studies have also been initiated to under-stand the molecular basis of these alter-ations in ss levels. Initial indications arethat microgravity may fundamentally alterthe mechanisms that regulate gene tran-scription, translation of messenger RNAand proteolysis.

The relevance of this paradigm extendsbeyond the realms of space travel. Thereare striking similarities between simulatedmicrogravity and certain low shear physi-ological niches on earth, such as brushborder epithelia of the gastrointestinal,respiratory and urogenital tracts. Thesesites are commonly encountered duringthe natural route of infection by pathogens.It is thus logical to suspect that the mech-anism for increased virulence and resist-ance utilised by bacteria in microgravitymay be analogous to those induced inpathogens encountering low shear environ-ments in earth-bound humans. Therefore,a comprehensive understanding of theseresponse mechanisms may provide infor-mation relevant to the space environmentand also to combating these infections onEarth.

While both the general stress responseand exposure to simulated microgravitylead to the generation of resistant bacterialcells, it is clear that they do so by differentmechanisms. This raises the question ofwhether synthesis of useful secondarymetabolites by bacteria, such as antibiotics,which under normal gravity is also trig-gered by stress, is affected by simulatedmicrogravity. Production of the antibioticnikkomycin by Streptomyces ansochro-mogenus improved following 15 days inorbit on board an unmanned satellite,raising hopes that simulated microgravitycould enhance the efficiency of secondarymetabolite production.

Studies in simulated microgravity have,however, given mixed results. Synthesis ofthe antibiotic gramicidin by Bacillus brevisis unaffected by culture in simulated micro-gravity, while that of b-lactam antibiotics(e.g. penicillin) and of rapamycin byStreptomyces species actually decreasedunder these conditions. In some cases,simulated microgravity cultivation shiftedthe site of product accumulation frominside the cell to outside. This effect facil-itates purification; further, it remains pos-sible that a systematic screening willidentify bacteria whose antibiotic produc-tion is enhanced under simulated micro-gravity conditions. Both phenomenawould be useful in industrial productionof valuable chemicals by bacteria.

ConclusionMicrogravity profoundly affects criticallife processes. From the perspective ofspace travel, the combination of immuno-compromised astronauts and potential forincreased bacterial aggressiveness is partic-ularly problematic and needs to be exam-ined and understood in molecular detail.The advent of the RCCS system permittingground-based investigations is allowingscientists to begin to shed light on thesephenomena and it is already clear that theirmechanistic basis is contrary to the intu-itive expectations based on previouslyestablished earth-based models of bacterialresistance and virulence.

These and other ground-based systemsare proving indispensable in providingrelevant information on the response ofman and microbes to microgravity as wellas microbial mechanisms of colonizationof physiologically relevant niches onEarth. The validity of these systems hasbeen confirmed by comparative experimen-tation on board space shuttles and stationsdemonstrating that ground-based modelsystems mimic several of the aspects of thespace environment. Increasing sophistica-tion of equipment on space ships and sta-tions is beginning to complement thesesystems. Such equipment includes shieldsto protect biological material from exposureto radiation. Thus, the effect of microgravityalone can be examined in space itself.Similarly, centrifuges have been trans-ported to the International Space Stationthat can generate 1g gravity controls, mak-ing it possible to study the effects of radi-ation independent of gravitational effects.The novel paradigms that have begun toemerge from these studies promise toreveal new vistas in biology that enhanceour understanding as well as provide new

Biologist Volume 52 Number 2, May 2005

Travails of microgravity |IOB

measures for improving well-being of bothspace- and earth-bound humans.

AcknowledgementThis research from this laboratory wassupported by NASA Grant NNA04CC51G.Susan Lynch was supported, in part, by aDeans Fellowship from Stanford University,School of Medicine.

ReferencesHammond T G and Hammond J M (2001) Optimised

suspension culture: the rotating wall vessel. Am.J. Physiol. Renal Physiol 281(1): F12-25.

Lynch S V, Brodie E L and Matin A C Role and reg-ulation of ss in low shear simulated microgravity-conferred general resistance in Escherichia coli. J.Bacteriology (In Press).

Nickerson C A, Ott C M, Wilson J W, Ramamurthy R,LeBlanc C L, Honer zu Bentrup K, Hammond Tand Pierson D L (2003) Low-shear modeled micro-gravity: a global environmental regulatory signalaffecting bacterial gene expression, physiology, andpathogenesis. J. Microbiol. Methods 54(1): 1-11.

Sonnenfeld G and Shearer W T (2002) Immune func-tion during space flight. Nutrition 18(10): 899-903.

Whitson P A, Pietrzyk R A and Sams C F (1999)Space flight and the risk of renal stones. J GravitPhysiol 6(1): P87-8.

Further readingCann C (1997) Adaptations of the skeletal system.

Introduction to space life sciences. 135:168.Luo A, Gao C, Song Y, Tan H and Liu Z (1998)

Biological responses of a Streptomyces strain pro-ducing-Nikkomycin to space flight. Space Med.Med. Eng 11:411-4.

Mehta S K, Cohrs R J, Forghani B, Zerbe G, GildenD H, Pierson D L (2004) Stress-induced subclini-cal reactivation of varicella zoster virus in astro-nauts. J. Med. Virol 72:174-9.

Milsread J R, Simske S J and Bateman T A (2004)Spaceflight and hindlimb suspension disuse mod-els in mice. Biomed. Sci. Instrum 40:105-110.

Simske S J, Broz J and Luttges M W (1995) Effect ofsuspension on mouse bone microhardness.Journal of Material Science: Materials inMedicine. 6:486-491.

Zerwekh J E (2002) Nutrition and renal stone dis-ease in space. Nutrition 18(10): 857-63.

Dr A Matin is a full professor on the faculty of Stanford Universityin California. He has been a Fulbright scholar and is an elected fel-low of the American Academy of Microbiology. Tel: + 1-650-725-4745. Email: a.matin@stanford.eduSusan Lynch is a postdoctoral associate in Professor Matin’s lab-oratory working on the the physiology and molecular life processesunder microgravity conditions. She has authored several scholarlyarticles, has served as a reviewer for scientific journals such asAdvances in Space Research and has been a recipient of StanfordMedical School Deans fellowship.