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At least 523 records · Page 29Linked to original sources
PubMed2006

Endogenous driving and synchronization in cardiac and uterine virtual tissues: bifurcations and local coupling.

Cardiac and uterine muscle cells and tissue can be either autorhythmic or excitable. These behaviours exchange stability at bifurcations produced by changes in parameters, which if spatially localized can produce an ectopic pacemaking focus. The effects of these parameters on cell dynamics have been identified and quantified using continuation algorithms and by numerical solutions of virtual cells. The ability of a compact pacemaker to drive the surrounding excitable tissues depends on both the size of the pacemaker and the strength of electrotonic coupling between cells within, between, and outside the pacemaking region. We investigate an ectopic pacemaker surrounded by normal excitable tissue. Cell-cell coupling is simulated by the diffusion coefficient for voltage. For uniformly coupled tissues, the behaviour of the hybrid tissue can take one of the three forms: (i) the surrounding tissue electrotonically suppresses the pacemaker; (ii) depressed rate oscillatory activity in the pacemaker but no propagation; and (iii) pacemaker driving propagations into the excitable region. However, real tissues are heterogeneous with spatial changes in cell-cell coupling. In the gravid uterus during early pregnancy, cells are weakly coupled, with the cell-cell coupling increasing during late pregnancy, allowing synchronous contractions during labour. These effects are investigated for a caricature uterine tissue by allowing both excitability and diffusion coefficient to vary stochastically with space, and for cardiac tissues by spatial gradients in the diffusion coefficient.

Action Potentials↗
PubMed2000

Exercise thermoregulation in men after 1 and 24-hours of 6 degrees head-down tilt.

BACKGROUND: Exercise thermoregulation is dependent on heat loss by increased skin blood flow (convective and conductive heat loss) and through enhanced sweating (evaporative heat loss). Reduction of plasma volume (PV), increased plasma osmolality, physical deconditioning, and duration of exposure to simulated and actual microgravity reduces the ability to thermoregulate during exercise. HYPOTHESIS: We hypothesized that 24 h of head down tilt (HDT24) would alter thermoregulatory responses to a submaximal exercise test and result in a higher exercise rectal temperature (Tre) when compared with exercise Tre after 1 h of head down tilt (HDT1). METHODS: Seven men (31+/-SD 6 yr, peak oxygen uptake (VpO2peak) of 44+/-6 ml x kg(-1) x min(-1)) were studied during 70 min of supine cycling at 58+/-SE 1.5% VO2peak at 22.0 degrees C Tdb and 47% rh. RESULTS: Relative to pre-tilt sitting chair rest data, HDT1 resulted in a 6.1+/-0.9% increase and HDT24 in a 4.3+/-2.3% decrease in PV (delta = 10.4% between experiments, p<0.05) while plasma osmolality remained unchanged (NS). Pre-exercise Tre was elevated after HDT24 (36.71 degrees C +/-0.06 HDT1 vs. 36.93 degrees C+/-0.11 HDT24, p<0.05). The 70 min of exercise did not alter this relationship (p<0.05) with respective end exercise increases in Tre to 38.01 degrees C and 38.26 degrees C (degrees = 1.30 degrees C (HDT1) and 1.33 degrees C (HDT24)). While there were no pre-exercise differences in mean skin temperature (Tsk), a significant (p<0.05) time x treatment interaction occurred during exercise: after min 30 in HDT24 the Tsk leveled off at 31.1 degrees C, while it continued to increase reaching 31.5 degrees C at min 70 in HDT1. A similar response (NS) occurred in skin blood velocity. Neither local sweating rates nor changes in body weight during exercise of -1.63+/-0.24 kg (HDT1) or - 1.33+/-0.09 kg (HDT24) were different (NS) between experiments. CONCLUSION: While HDT24 resulted in elevated pre-exercise Tre, reduced PV, attenuation of Tsk and skin blood velocity during exercise, the absolute increase in exercise Tre was not altered. But if sweat rate and cutaneous vascular responses were similar at different core temperatures (unchanged thermoregulation), the Tre offset could have been caused by the HDT-induced hypovolemia.

Adult↗
PubMed2005

Modeling of stimulated hydrogel volume changes in photonic crystal Pb2+ sensing materials.

We modeled the stimulated hydrogel volume transitions of a material which binds Pb2+ and is used as a photonic crystal chemical sensing material. This material consists of a polymerized crystalline colloidal array (PCCA) hydrogel which contains a crown ether molecular recognition group. The PCCA is a polyacrylamide hydrogel which embeds a crystalline colloidal array (CCA) of monodisperse polystyrene spheres of approximately 100 nm. The array spacing is set to diffract light in the visible spectral region. Changes in the hydrogel volume induced by Pb2+ binding alter the array spacing and shift the diffracted wavelength. This system allows us to sensitively follow the hydrogel swelling behavior which results from the immobilization of the Pb2+ by the crown ether chelating groups. Binding of the Pb2+ immobilizes its counterions. This results in a Donnan potential, which results in an osmotic pressure which swells the hydrogel. We continue here our development of a predictive model for hydrogel swelling based on Flory's theory of gel swelling. We are qualitatively able to model the PCCA swelling but cannot correctly model the large responsivity observed at the lowest Pb2+ concentrations which give rise to the experimentally observed low detection limits for Pb2+. These PCCA materials enable stimulated hydrogel volume transitions to be studied.

Cations, Divalent↗
PubMed1996

Counting and classifying attractors in high dimensional dynamical systems.

Randomly connected Boolean networks have been used as mathematical models of neural, genetic, and immune systems. A key quantity of such networks is the number of basins of attraction in the state space. The number of basins of attraction changes as a function of the size of the network, its connectivity and its transition rules. In discrete networks, a simple count of the number of attractors does not reveal the combinatorial structure of the attractors. These points are illustrated in a reexamination of dynamics in a class of random Boolean networks considered previously by Kauffman. We also consider comparisons between dynamics in discrete networks and continuous analogues. A continuous analogue of a discrete network may have a different number of attractors for many different reasons. Some attractors in discrete networks may be associated with unstable dynamics, and several different attractors in a discrete network may be associated with a single attractor in the continuous case. Special problems in determining attractors in continuous systems arise when there is aperiodic dynamics associated with quasiperiodicity of deterministic chaos.

Animals↗
PubMed2005

An objective technique to measure the depth-of-focus in free space.

BACKGROUND: Determining the depth-of-focus (DOF) objectively in free space is important, as this provides information regarding the range of clear vision under more natural viewing conditions in which target blur, size, and proximal information, as well as other monocular depth cues, are present. METHODS: The DOF was assessed objectively in free space in young adults [n=20, ages 24.9+/-3.1years: myopes (n=14), emmetropes (n=5), hyperope (n=1)] by monitoring accommodation continuously using the commercially available Power Refractor II (PR II) under monocular viewing conditions. A high-contrast target ( approximately 83%) subtending a mean visual angle of 2.3 deg was placed on an optical bench at 25 cm (4 D) along the line-of-sight of the viewing right eye and was displaced slowly ( approximately 0.1-0.15 D/s) proximally and distally. This target provided blur, size, and proximal information, and other depth cues, as normally found in free space. The first consistent change in steady-state accommodative baseline level reflected one edge of the DOF. The mean dioptric difference between these distal and proximal endpoints was averaged, and this represented the total objective DOF. RESULTS: The group mean total objective DOF (n=20) was +/-0.61+/-0.09 D, with a range from +/-0.46 D to +/-0.81 D. Repeatability was excellent. CONCLUSION: A technique was developed and tested to measure the DOF in free space objectively. Further development of this technique will allow the assessment of blur perception under more complex natural viewing conditions simulating the everyday environment.

Accommodation, Ocular↗
PubMed2004

A dynamic model of how feature cues guide spatial attention.

We will describe a computational model of attention which explains the guidance of spatial attention by feedback within a distributed network. We hypothesize that feedback within the ventral pathway transfers the target template from prefrontal areas into intermediate areas like V4. The oculomotor circuit consisting of FEF, LIP and superior colliculus picks up this distributed activity and provides a continuous spatial reentry signal from premotor cells. In order to test this hypothesis, we simulate two experiments that require a response given a color cue. The first experiment indicates a parallel feature-based enhancement prior to any spatial selection. If two targets are behaviorally relevant, as in the second experiment, experimental findings indicate that subjects split their attention between two locations containing the searched feature. Our simulation results suggest that the split in attention between two foci is a transient effect occurring during competition. We predict that the time after cue presentation determines the state of this competition and ultimately the distribution of attention at different locations. In addition we provide simulation results to explain how reentrant processing through the oculomotor circuit might lead to variations of the time for target detection in visual search.

Attention↗
PubMed1990

Transport of fluid and macromolecules in tumors. II. Role of heterogeneous perfusion and lymphatics.

We have recently developed a general theoretical framework for transvascular exchange and extravascular transport of fluid and macromolecules in tumors. The model was applied to a homogeneous, alymphatic tumor with no extravascular binding. For this simplified system, the interstitial pressure was found to be a major contributing factor to the heterogeneous distribution of macromolecules within solid tumors. A steep pressure gradient was predicted at the periphery of the tumor. Our recent experiments have verified these predicted profiles. The purpose of this investigation was to apply this theoretical framework to the more realistic case of a nonuniformly perfused tumor. The role of lymphatics for macromolecular transport was also studied using the model. The uptake and distribution of IgG and its fragment, Fab, were simulated. The novel result from this work is that necrosis does not reduce the central interstitial pressure in a tumor. Other results showed that (i) macromolecules do not penetrate a necrotic core at early times after injection; (ii) at longer time periods after a bolus injection (days for Fab, months for IgG in a tumor of radius approximately 1cm) a "reservoir" of material may be formed in the necrotic core; (iii) continuous infusion or repeated injections should maintain a higher interstitial concentration of macromolecules; and (iv) lymphatics, if present in a tumor, would rapidly remove material and result in much lower concentration levels. The model is also used to explain some previous experimental data in the literature on antibody distribution.

Animals↗
PubMed2007

An in vivo rat model simulating imaging of human kidney by diagnostic ultrasound with gas-body contrast agent.

One kidney of anesthetized rats was imaged by diagnostic ultrasound with contrast agent under conditions simulating both the geometry and the attenuation encountered during human perfusion imaging. Contrary to earlier predictions, glomerular capillary rupture with blood loss into Bowman's space and proximal tubules occurred in our clinically relevant model system. Quantitative analysis of histologic sections showed that 37 +/- 5% of the glomeruli at the center of the scan plane had blood cells in Bowman's space after imaging for 1 min with 1.8 MPa (mechanical index equivalent, MIe = 1.5) with a 1 s image trigger interval during IV injection of 10 microl/kg/min of Definity contrast agent (as recommended by the manufacturer). This percentage decreased rapidly with decreasing peak rarefactional pressure amplitude to an apparent threshold of 0.73 MPa (MIe = 0.6). The percentage of glomeruli with hemorrhage decreased in proportion to dose when reduced below the recommended value, but leveled-off at doses above it. The percentage of glomerular hemorrhage increased with increasing numbers of image exposures, with an initial rate of 1.1% per image. The glomerular hemorrhage also depended on the frame trigger interval with no hemorrhage evident for continuous imaging but a maximal effect for trigger intervals greater than about 1 s. These results indicated that there is a potential for clinical diagnostic ultrasound with contrast agent to induce glomerular hemorrhage.

Animals↗
PubMed2000

Reinforcement learning in continuous time and space.

This article presents a reinforcement learning framework for continuous-time dynamical systems without a priori discretization of time, state, and action. Based on the Hamilton-Jacobi-Bellman (HJB) equation for infinite-horizon, discounted reward problems, we derive algorithms for estimating value functions and improving policies with the use of function approximators. The process of value function estimation is formulated as the minimization of a continuous-time form of the temporal difference (TD) error. Update methods based on backward Euler approximation and exponential eligibility traces are derived, and their correspondences with the conventional residual gradient, TD(0), and TD(lambda) algorithms are shown. For policy improvement, two methods-a continuous actor-critic method and a value-gradient-based greedy policy-are formulated. As a special case of the latter, a nonlinear feedback control law using the value gradient and the model of the input gain is derived. The advantage updating, a model-free algorithm derived previously, is also formulated in the HJB-based framework. The performance of the proposed algorithms is first tested in a nonlinear control task of swinging a pendulum up with limited torque. It is shown in the simulations that (1) the task is accomplished by the continuous actor-critic method in a number of trials several times fewer than by the conventional discrete actor-critic method; (2) among the continuous policy update methods, the value-gradient-based policy with a known or learned dynamic model performs several times better than the actor-critic method; and (3) a value function update using exponential eligibility traces is more efficient and stable than that based on Euler approximation. The algorithms are then tested in a higher-dimensional task: cart-pole swing-up. This task is accomplished in several hundred trials using the value-gradient-based policy with a learned dynamic model.

Algorithms↗
PubMed2002

Flat medial-lateral conformity in total knee replacements does not minimize contact stresses.

The potential for wear in UHMWPE components for total knee replacements can be reduced by decreasing the stresses and strains arising from tibial-femoral contact. The conformity of the articular surfaces has a large effect on the resultant stresses, and components that achieve flat medial-lateral contact have been assumed to produce the lowest stresses due to their perfect conformity. We computed the stresses arising from curved and flat contact on a half-space using two-dimensional, plane strain elasticity solutions and finite element analyses to compare the performance of curved and flat indenters. These indenters were represented by a polynomial so the profiles could be continuously varied from curved to flat. Curved contact resulted in maximum stresses at the center of contact, while flat contact produced maximum stresses at the edge of contact. In addition, three contemporary tibial configurations (flat-on-flat, curved-on-flat, and curved-on-curved geometries) were analyzed using the finite element method with nonlinear material properties. The maximum contact stress, von Mises stress, and von Mises strain were lowest for the curved-on-curved model. The other configurations resulted in higher contact stresses, von Mises stresses, and von Mises strains. The perfect conformity arising from flat contact did not reduce the contact stresses in the UHMWPE component. The tensile stresses, however, were lowest for the flat-on-flat geometry compared with the other two configurations. Relating these distinct differences could prove useful in interpretation of data from simulator and retrieval studies.

Finite Element Analysis↗
PubMed1998

Dynamic On-line Clustering and State Extraction: An Approach to Symbolic Learning.

Although recurrent neural nets have been moderately successful in learning to emulate finite-state machines (FSMs), the continuous internal state dynamics of a neural net are not well matched to the discrete behavior of an FSM. We describe an architecture, called DOLCE, that allows discrete states to evolve in a net as learning progresses. DOLCE consists of a standard recurrent neural net trained by gradient descent and an adaptive clustering technique that quantizes the state space. We describe two implementations of DOLCE. The first implementation, called DOLCE(u), uses an adaptive clustering scheme in an unsupervised mode to determine both the number of clusters and the partitioning of the state space as learning progresses. The second model, DOLCE(s), uses a Gaussian Mixture Model in a supervised learning framework to infer the states of an FSM. DOLCE(s) is based on the assumption that a finite set of discrete internal states is required for the task, and that the actual network state belongs to this set but has been corrupted by noise due to inaccuracy in the weights. DOLCE(s) learns to recover the discrete state with maximum a posteriori probability from the noisy state. Simulations show that both implementations of DOLCE lead to a significant improvement in generalization performance over earlier neural net approaches to FSM induction. The idea of adaptive quantization is not just applicable to DOLCE but can be applied to other domains as well.

Journal Article↗
PubMed2002

Diffusion of epicenters of earthquake aftershocks, Omori's law, and generalized continuous-time random walk models.

The epidemic-type aftershock sequence (ETAS) model is a simple stochastic process modeling seismicity, based on the two best-established empirical laws, the Omori law (power-law decay approximately 1/t(1+theta) of seismicity after an earthquake) and Gutenberg-Richter law (power-law distribution of earthquake energies). In order to describe also the space distribution of seismicity, we use in addition a power-law distribution approximately 1/r(1+mu) of distances between triggered and triggering earthquakes. The ETAS model has been studied for the last two decades to model real seismicity catalogs and to obtain short-term probabilistic forecasts. Here, we present a mapping between the ETAS model and a class of CTRW (continuous time random walk) models, based on the identification of their corresponding master equations. This mapping allows us to use the wealth of results previously obtained on anomalous diffusion of CTRW. After translating into the relevant variable for the ETAS model, we provide a classification of the different regimes of diffusion of seismic activity triggered by a mainshock. Specifically, we derive the relation between the average distance between aftershocks and the mainshock as a function of the time from the mainshock and of the joint probability distribution of the times and locations of the aftershocks. The different regimes are fully characterized by the two exponents theta and mu. Our predictions are checked by careful numerical simulations. We stress the distinction between the "bare" Omori law describing the seismic rate activated directly by a mainshock and the "renormalized" Omori law taking into account all possible cascades from mainshocks to aftershocks of aftershock of aftershock, and so on. In particular, we predict that seismic diffusion or subdiffusion occurs and should be observable only when the observed Omori exponent is less than 1, because this signals the operation of the renormalization of the bare Omori law, also at the origin of seismic diffusion in the ETAS model. We present predictions and insights provided by the ETAS to CTRW mapping which suggest different ways for studying seismic catalogs. Finally, we discuss the present evidence for our predicted subdiffusion of seismicity triggered by a main shock, stressing the caveats and limitations of previous empirical works.

Journal Article↗
PubMed2004

Stability analysis of social foraging swarms.

In this article we specify an M-member "individual-based" continuous time swarm model with individuals that move in an n-dimensional space according to an attractant/repellent or a nutrient profile. The motion of each individual is determined by three factors: i) attraction to the other individuals on long distances; ii) repulsion from the other individuals on short distances; and iii) attraction to the more favorable regions (or repulsion from the unfavorable regions) of the attractant/repellent profile. The emergent behavior of the swarm motion is the result of a balance between inter-individual interactions and the simultaneous interactions of the swarm members with their environment. We study the stability properties of the collective behavior of the swarm for different profiles and provide conditions for collective convergence to more favorable regions of the profile.

Animal Migration↗
PubMed1996

Effect of tracheal gas insufflation on demand valve triggering and total work during continuous positive airway pressure ventilation.

Tracheal gas insufflation (TGI) improves CO2 clearance and may reduce work of breathing by lowering the required minute ventilation (VE). However, TGI might also impair the ability to trigger the ventilator, because to lower external circuit pressures, inspiratory effort must outstrip catheter flow rate (Vc) and overcome the dynamic hyperinflation caused by TGI. We studied these effects using a two-chamber lung model of the respiratory muscles (RM) and lungs (L). The RM-chamber was ventilated using a sinusoidal flow pattern with a tidal volume (VT) of 0.5 L at various peak inspiratory flow rates (Vpk) to simulate differences in effort intensity. The L-chamber was connected to a 60-L/min continuous flow circuit with a 10 cm H2O positive end-expiratory pressure valve and to 3 different ventilatory demand valve circuits, each set at continuous positive airway pressure (CPAP) of 10 cm H2O. We used continuous TGI at 0, 2.5, 5, 10, and 15 L/min. The work of triggering (W-trig) increased with increasing Vc and decreased with increasing Vpk. The L-ventilator failed to trigger when Vc was 15 L/min and Vpk was 20 L/min. At a fixed VE, the effect of TGI on total mechanical inspiratory work (W-tot) was relatively small and varied among the different CPAP systems used. We conclude that weak patients may fail to open the demand valve of the CPAP system during TGI at high catheter flow rates. The net effect of TGI on the effort made by ventilated patients would depend not only on the interactions between TGI and the ventilator, but also on the efficiency of TGI in decreasing dead-space and lowering the VE requirement.

Humans↗
PubMed2005

Model selection in binary trait locus mapping.

Quantitative trait locus (QTL) mapping methodology for continuous normally distributed traits is the subject of much attention in the literature. Binary trait locus (BTL) mapping in experimental populations has received much less attention. A binary trait by definition has only two possible values, and the penetrance parameter is restricted to values between zero and one. Due to this restriction, the infinitesimal model appears to come into play even when only a few loci are involved, making selection of an appropriate genetic model in BTL mapping challenging. We present a probability model for an arbitrary number of BTL and demonstrate that, given adequate sample sizes, the power for detecting loci is high under a wide range of genetic models, including most epistatic models. A novel model selection strategy based upon the underlying genetic map is employed for choosing the genetic model. We propose selecting the "best" marker from each linkage group, regardless of significance. This reduces the model space so that an efficient search for epistatic loci can be conducted without invoking stepwise model selection. This procedure can identify unlinked epistatic BTL, demonstrated by our simulations and the reanalysis of Oncorhynchus mykiss experimental data.

Animals↗
PubMed2005

Perilymph sampling from the cochlear apex: a reliable method to obtain higher purity perilymph samples from scala tympani.

Measurements of drug levels in the fluids of the inner ear are required to establish kinetic parameters and to determine the influence of specific local delivery protocols. For most substances, this requires cochlear fluids samples to be obtained for analysis. When auditory function is of primary interest, the drug level in the perilymph of scala tympani (ST) is most relevant, since drug in this scala has ready access to the auditory sensory cells. In many prior studies, ST perilymph samples have been obtained from the basal turn, either by aspiration through the round window membrane (RWM) or through an opening in the bony wall. A number of studies have demonstrated that such samples are likely to be contaminated with cerebrospinal fluid (CSF). CSF enters the basal turn of ST through the cochlear aqueduct when the bony capsule is perforated or when fluid is aspirated. The degree of sample contamination has, however, not been widely appreciated. Recent studies have shown that perilymph samples taken through the round window membrane are highly contaminated with CSF, with samples greater than 2microL in volume containing more CSF than perilymph. In spite of this knowledge, many groups continue to sample from the base of the cochlea, as it is a well-established method. We have developed an alternative, technically simple method to increase the proportion of ST perilymph in a fluid sample. The sample is taken from the apex of the cochlea, a site that is distant from the cochlear aqueduct. A previous problem with sampling through a perforation in the bone was that the native perilymph rapidly leaked out driven by CSF pressure and was lost to the middle ear space. We therefore developed a procedure to collect all the fluid that emerged from the perforated apex after perforation. We evaluated the method using a marker ion trimethylphenylammonium (TMPA). TMPA was applied to the perilymph of guinea pigs either by RW irrigation or by microinjection into the apical turn. The TMPA concentration of the fluid sample was compared with that measured in perilymph prior to taking the sample using a TMPA-selective microelectrode sealed into ST. Data were interpreted with a finite element model of the cochlear fluids that was used to simulate each aspect of the experiment. The correction of sample concentration back to the perilymph concentration prior to sampling can be performed based on the known ST volume (4.7microL in the guinea pig) and the sample volume. A more precise correction requires some knowledge of the profile of drug distribution along the cochlear prior to sampling. This method of sampling from the apex is technically simple and provides a larger sample volume with a greater proportion of perilymph compared to sampling through the RW.

Animals↗
PubMed2003

Spectral intermode coupling in a model of isotropic turbulence.

We investigate the nonlinear coupling between the so-called explicit modes, identified with wave numbers k such that 0< or =k< or =k(c), and implicit modes, defined such that k(c)< or =k< or =k(max). Here k(c) is an arbitrarily chosen cutoff wave number and k(max) is the ultraviolet cutoff as determined by viscous damping. The stresses arising from the nonlinearity in the Navier-Stokes equations are categorized as "implicit-implicit" (or "Reynolds") and "explicit-implicit" (or "cross"). These arise from dynamic coupling between different regions of wave number space. Their respective effects on momentum, kinetic energy, and energy flux are assessed. The analysis is based on a model system comprising the Navier-Stokes equations and the Edwards-Fokker-Planck energy equation [S. F. Edwards, J. Fluid Mech. 18, 239 (1964)] which is known to retain all the symmetries of homogeneous, isotropic turbulence. The Reynolds stress is found to be responsible for long-range energy transfers. It can be represented by an effective viscosity and is mainly determined by dynamical friction. The cross term is more complicated, involving both diffusive and frictional effects. For long-range coupling it can be expressed as a modification of the effective viscosity, while for short-range coupling it may be modeled on the assumption that implicit scales are slaved to explicit scales. Thus, both the random and coherent aspects of intermode coupling in turbulent flows are relevant in the cross term. The imposition of a continuity requirement on energy transfer leads to a new parametrization that represents the effect of absent modes in a truncated spectral simulation, and takes into account the phase-coupling (coherent) effects, as well as the usual viscositylike (random) effects.

Journal Article↗
PubMed1964

Biological contamination of Mars. I. Survival of terrestrial microorganisms in simulated Martian environments.

It has been postulated that the accidental introduction of terrestrial microorganisms to other planets during the course of space exploration might impede or bias the detection of organic matter and possible indigenous organisms, and thereby confuse subsequent studies of extraterrestrial life. To assess the likelihood of biological contamination of Mars, we have applied the principle of natural selection on a laboratory scale. Terrestrial microorganisms were collected from a variety of environments, including regions of high alkalinity, low mean daily temperature, and low annual rainfall. The air-dried soils were then subjected to a simulated Martian environment involving 12-hour freeze-thaw cycles from about -60 degrees C to about +20 degrees C; atmospheres of 95 per cent nitrogen, 5 percent carbon dioxide and low moisture content: < or = 0.1 atm pressure; and a total ultraviolet dose at 2537 angstrom of 10(9) erg cm-2. In some experiments, organic supplements were provided. Survivors were scored on supplemented agar. Preliminary results indicate a wide variety of survivors, even when no organic supplements were introduced. Survivors included obligate and facultative anaerobic spore-formers and non-spore-forming facultative anaerobic bacteria. Diurnal freezing and thawing was continued for six months. There was no significant loss of viability after the first freeze-thaw cycle. An extensive literature survey shows that survival of terrestrial microorganisms under individual simulated Martian conditions has been known for decades. The present investigation shows the absence of pronounced synergistic effects inhibiting survival. The probable existence of organic matter and moisture on Mars, at least in restricted locales and times, makes it especially likely that terrestrial microorganisms can also reproduce on Mars. The demonstration that all samples of terrestrial soil tested contain a population of microorganisms which survive in simulated Martian environments strongly underscores the need for scrupulous sterilization of all spacecraft intended for Mars landing.

Bacillus subtilis↗