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Bacteria as Multicellular Organisms
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korn of the University of Chicago have some species to form elaborate fruit they align themselves side by side and. shown that differentiation in Anabae ing bodies Movement is highly coordi either move off in the same direction. na involves a form of controlled ge nated when the population migrates or rub alongside each other before. netic engineering In the course of het over agar it displaces itself as an separating Daughter cells can be ob. erocyst differentiation a specific DNA intact unit When an individual cell served taking part in a similar routine. rearrangement occurs that results in moves a few microns beyond the edge following cell division These bacteria. the creation of a complete coding se it quickly pops back into place as literally keep in touch with each other. quence for one of the subunits of though drawn by an elastic thread Predator prey relationships exist in. nitrogenase Such a rearrangement oc Even those species of Myxobacteria the microbial world just as they do in. curs only in cells that are differentiat that do enter dormancy as single cell the world of larger organisms Sever. ing to form heterocysts Comparable spores are social throughout much of al predatory species of Myxobacteria. DNA rearrangements are involved in their life cycle When two cells of Myx feed by secreting enzymes that dis. the formation of specialized immune ococcus virescens meet for example solve the outer cell layer of other mi. system cells in vertebrates they go through a characteristic ritual croorganisms when the cells burst. In addition there are submicroscop,ic channels within each Anabaena fila. ment that connect the two kinds of,cells The transport of cellular prod. ucts fixed nitrogen to the photosyn,thetic cells and photosynthetic prod. ucts to the heterocysts takes place by,way of these channels In overall char. acter then it can be said that Anabae,na functions more like a multicellular.
organism than a unicellular one it,relies on division of labor among its. cells to carry out specialized and in,compatible chemical processes. ore spectacular examples of,multicellular behavior can be. found among the Myxobacte,ria the most morphologically complex. of all bacteria Their elaborate fruiting,bodies rival those of fungi and slime.
molds and have long been an object of,scientific curiosity Myxobacteria are. social creatures par excellence and,their intriguing almost psychedelic. patterns of aggregation and move,ment have been recorded in a fascinat. ing series of time lapse motion pic,tures produced by Hans Reichenbach. of the Society for Biotechnological Re,search in Braunschweig and his collab.
orators at the Institute for Scientific,Film LW F in Gbttingen. Unlike many bacteria which periodi,cally enter a dormant stage as individ. ual spores many Myxobacteria never,exist as single cells Instead they enter. dormancy in the form of a multicel,lular cyst that eventually germinates. and spawns a ready made popula,tion of thou sands of individuals Each.
cyst founds a new population as,the bacteria become more numerous. and dense a number of sophisticated,events speCific to multicellularity take. place Trails of extracellular slime are,secreted and serve as highways for the. directed movement of thousands of,cells rhythmic waves pulse through. the entire population streams of bac MULTICELLULAR FRUITING BODY of Chondromyces crocatus a species in the group. teria move to and from the center and Myxobacteria is magnified 268 diameters The structure consists of a central stalk. edges of a spreading colony and bac that branches to form specialized clusters of single cell spores When the clus. teria aggregate at speCific places with ters burst the spores disperse to form new colonies The micrograph was made by. in the colony to construct cysts or in Hans Reichenbach of the Society for Biotechnological Research in Braunschweig. SCIENTIFIC AMERICAN June 1988 83,1988 SCIENTIFIC AMERICAN INC.
ied the genetic basis of communica enzyme under study and the recombi. tion and movement in M xanthus By nant DNA is inserted into Pseudomo. searching for mutants that have lost nas then when the bacteria are grown. the ability to spread or to form fruit on agar containing those chemicals. ing bodies they have identified specif the amount and distribution of col. ic regions of the organism s DNA that or is a direct measure of the gene s. control for aggregation motility and expression, differentiation They found that motil When I plated my recombinant. ity in M xanthus is under the control strains of P putida on such indicator. of two different systems The A for agar I was astonished to find that. adventurous system enables individ every colony exhibited a characteristic. ual cells to move across the substrate flowerlike pattern of staining I repeat. the S for social system controls the ed the experiment with engineered E. movement of groups of cells If either coli and with strains of naturally pig. system is defective the cells are still mented bacteria such as Pseudomo. able to spread but they do so abnor nas cepacia Serratia marcescens and. mally If both systems are defective Chromobacterium violaceum Each. the cells cannot spread at all The two produced its own unique flowerlike. systems are surprisingly complex and pattern The fact that different strains. a great deal of speCific genetic infor and species produced distinctive colo. mation is required for each to be oper nies including species that were natu. ative A mutation affecting any one of rally colored and not subjected to ge. 23 different genetic loci will eliminate netic manipulation gave me reason to. the A motility system at least 10 dif believe colony growth in bacteria is a. ferent loci control the S system highly regulated process and is under. Kaiser and his colleagues have also some form of temporal control Subse. begun to unravel some of the ways M quent studies confirmed that hypoth. xanthus cells communicate with one esis It is now clear to me that colony. another They accomplished this by organization follows certain general. combining different mutant cells that rules which help to explain the exis. were defective for the same trait but tence of general patterns. in which the defect resulted from mu, tations at different loci They found he colonies tend to assume a. that when two motility defective mu circular configuration growing. ANABAENA a photosynthetic cyano tants are combined in the same petri outward by adding cells to the. bacterium forms filaments of cells in dish they regain motility as long as perimeter As a colony spreads across. freshwater ponds Most of the cells are, both mutants stay together Similarly the agar it is apparent that the pattern. photosynthetic but when nitrogen lev, if two sporulation defective mutants of growth consists of both concentric. els are low heterocysts develop The, are mixed in culture they will form and radial elements The concentric.
heterocysts which are capable of ni, normal fruiting bodies and sporulate elements are rings that encircle the. trogen fixation but not photosynthesis, are larger than the photosynthetic cells In some instances such mixed cell colony the radial elements or sectors. and have special storage granules for complementation is now known to be look like slices of pie Each sector. nitrogen rich compounds The Nomar mediated by the production of extra consists of outwardly growing proge. ski micrograph was made by the author cellular substances in other cases it ny descended from a common ances. the cells are enlarged 1 625 diameters may result either from direct cell to tor Some grow better than others and. cell contact or from the physical pres expand whereas other sectors merely. ence of two complementary cell types hold their own or even disappear as. the myxobacteria absorb their con the colony gets bigger. tents One species Myxococcus xan y own studies of multicellular. thus has evolved a specialized meth behavior in bacteria began as. od of prey capture in response to its is often the case as the result. aquatic environment In water it can of a chance observation A little over. not release its digestive enzymes be five years ago I was experimenting. cause they would immediately be di with a genetic engineering tool de. luted as would the prey s nutrients signed by Malcolm Casdaban of. Jeffrey c Burnham and his colleagues the University of Chicago and his stu. at the Medical College of Ohio have dents in order to study enzyme ex. shown that instead M xanthus con pression in Pseudomonas The tech. structs spherical colonies containing nique enabled me to join the genes for. millions of bacteria The colonies sur certain Pseudomonas putida enzymes. round suitable prey organisms trap with the DNA sequence from Esche. ping them in pockets on the surface of richia coli that encodes the enzyme. the sphere where both the digestive beta galactosidase The advantage of. enzymes and the prey contents can be beta galactosidase as a genetic engi. effectively corralled neering tool is that it causes certain FORMATION OF FRUITING BODY in the. Over the past decade A Dale Kaiser chemicals to turn color when they are Myxobacteria follows a characteristic se. of the Stanford University Medical exposed to it If the beta galactosidase quence of steps as is shown from the. School and his colleagues have stud sequence is linked to the gene for the far left in these scanning electron micro. 84 SCIENTIFIC AMERICAN June 1988,1988 SCIENTIFIC AMERICAN INC. PREDATORY SPHERES are formed by millions of individual eventually digested within pockets on the sphere s surface. cells of Myxococcus xanthus as a means of capturing prey in an The spheres are enlarged 440 diameters in this scanning. aquatic environment Microscopic prey such as the cyanobac electron micrograph made by Jeffrey C Burnham Susan A. terium Phormidium luridum inset stick to the colony and are Collart and Barbara W Highison of the Medical College of Ohio. In many cases it is possible to select N Patrick Higgins of the University of The concentric elements in a colony. individual cells from different sectors Alabama Medical School and I found are less familiar than the sectors and. grow them in culture and show that that differences in DNA between dis hence more puzzling The cells in a. the cells in one sectorial culture have tinct sectors and concentric zones can concentric zone or ring share a com. heritable properties differing from sometimes be visualized directly by mon property such as the level of. those of the cells in another sectorial picking colonies up on filter paper expression of beta galactosidase ac. culture often cells from different sec extracting their DNA in situ and then tivity but are not related by common. tors can be distinguished on the basis applying radioactive probes to detect ancestry they are directly related to. of differences in their DNA Recently specific sequences bacteria in the preceding and succeed. graphs tracing the morphogenesis of Stigmatella aurantiaca from the Indiana University campus the stalk is elaborately. At first cells form aggregation centers As the centers accu branched far right The S aurantiaca fruiting body is en. mulate bacteria they bubble upward until the vertical stalk larged 450 diameters Micrographs were made by Gabriela M. is complete In some species such as this unidentified one Vasquez Frank Qualls and David White of Indiana University. SCIENTIFIC AMERICAN June 1988 85,1988 SCIENTIFIC AMERICAN INC. ing zones not to one another If the,organization of the colony into dis.
tinct concentric zones cannot be ex,plained by heredity how might it be. explained Some system must exist,that bestows common properties on. bacteria within a ring and distinguish,es them from bacteria in other rings. One set of clues to the origins of,concentric patterns lies in the differ. ent ways the sectorial and concentric,elements interact with one another.
Photographs of colonies often show,that concentric rings persist through. sectors that grow faster than the rest,of the colony The resulting pat. tern contains rings that are stretched,outward The stretching shows that. the rings are formed not at specif,ic positions on the agar that is at. particular distances from the center,but at a specific time in the course.
FLOWERUKE COLONIES can be produced by streaking bacterial cultures over an agar. plate Each colony when it is not crowded assumes a pattern characteristic of its of colony development This sug. strain Chromobacterium violaceum top left naturally produces the pigment viola gests bacteria have biological clocks. cein and forms purple colonies Serratia marcescens top right synthesizes prodigio that enable them in some way to pro. sin it forms bright red colonies once thought to be drops of blood Pseudomonas gram cellular differentiation at spe. cepacia bottom left is yellow and has a unique surface texture the result of cell ag cific times during development The. gregation at the surface An Escherichia coli colony bottom right carries genetically rhythmic pulsations of spreading nyx. engineered DNA sequences encoding the enzyme beta galactosidase where the en obacterial colonies also reflect the. zyme is expressed the colony turns blue The colonies are at various magnifications operation of a clocklike mechanism. Bacteria as Multicellular Organisms all bacteria lead multicellular lives Examples of na functions more like a multicellular organism than a unicellular

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