The following article will guide you to find out how does bacteria reproduce.
Asexual reproduction is characteristic of all bacteria. Sexual reproduction was long thought to be absent but investigations with the help of electron microscope have clearly demonstrated the exchange of genetic material in some species of bacteria.
This is an essential feature of the sexual process and is denoted by the term genetic recombination.
A. Asexual Reproduction:
It takes place by two methods:
(1) Vegetative, and
(2) Spore formation.
1. Vegetative Reproduction in Bacteria:
The most common means of vegetative reproduction in bacteria is cell division or fission.
(a) Fission (Fig. 18.8 A-F):
Bacteria multiply by this method when there is sufficient amount of water, fair supply of nutrients and favourable temperature. Fission consists in the splitting of the mother cell into two or more daughter cells.
In bacteria the division of the mother cell into two daughter cells of approximately the same size is the rule. The process is thus called binary fission.
The bacterial cells of the bacillus and the spirillum forms typically divide in a transverse plane which is at right angles to the long axis of the cell. The coccal forms may divide in one or more than one plane.
Mazanec and Martinec studied cell division in Micrococcus cryophillus using electron microscope.
According to them, binary fission in this bacterium involves the following:
(i) Division of nuclear material (Fig. 18.8 A):
Reaching the maximum size which is fairly constant for the species, the bacterial cell elongates. This is followed by the division of the nuclear material by a process analogous to mitosis.
However, there is no spindle formation and resolution of chromatin material into chromosomes. The division of nucleus precedes the division of cytoplasm of the cell.
(ii) Septum formation:
The division of the nuclear matter is followed by the formation of a septum between the two halves of the nuclear material. The process starts with a ring-like invagination of the plasma membrane.
Concomitant with this is the thickening of the cell wall layer L2, which is electron dense and consists of mucopeptide, to form the septum initial (A) in the region of the plasma membrane sulcus.
Layer L1 which is electron transparent, bulges in front of the septum initial (B). The peripheral layers L3 and L4 of the cell wall are not involved in septum formation. They simply pass over this region or show a small median constriction at the early stages of cell division.
The continuous centripetal growth of the ring-like invagination of the plasma membrane, septum initial (L2) and layer L1 eventually form a complete partition (C-D), which divides the parent protoplast into two sister protoplasts.
The partition consists of a middle thin electron-dense mucopeptide septum enclosed on either side by the electron-transparent layer L1 and plasma membrane (E).
(iii) Splitting of the middle septum:
The middle mycopeptide septum thickens and finally splits centripetally into two separate thin electron-dense layers. The two sister cells are now completely separated but still enclosed within the common cell wall layers L3 and L4 as a single structural unit (E).
At this stage each sister protoplast has a continuous plasma membrane, electron- transparent layer L1 and electron opaque mucopeptide layer L2. The ends of the latter (Split septum) are still continuous with the common peripheral parent wall layers L3 and L4.
(iv) Separation of sister cells (F):
To achieve complete separation of the two sister cells which is the last step in binary fission, the small median constriction, that developed at the surface of the dividing cell at the early stage, gradually deepens during the process making fission between the daughter cells visible externally.
The growth of the sister cells in addition sets up turgor pressure which pulls on their adjacent walls in the region of the contact. Consequently the separation of the two sister cells starts at the periphery (E).
It slowly proceeds inwards towards the axis of the mother cell separating the two sister cells (F). The peripheral cell wall layers L3 and L4 which do not take part in the division of the cell, are formed de novo between the daughter cells before they actually separate.
Fission in bacteria often occurs as often as every 18-20 minutes under favourable conditions of temperature, food and moisture supply. Rarely the doubling period is as little as twelve and half minutes.
A cholera bacterium dividing at the rapid rate of 20 minutes will produce in 24 hours a progeny numbering 4,700,000,000,000,000,000,000 individuals. It would weigh about 2,000 tons.
Such a fantastic rate of multiplication, is only theoretically possible. Actually it does not happen so. The process of division slows down or may soon cease. The causes are space limitations, competition for oxygen in the case of aerobic bacteria, inadequacy or lack of food supply and production of substances unfavourable to growth.
Bacteria do not multiply at a uniform rate by fission. In the beginning the rate of multiplication is very slow. After this short initial period the rate of multiplication increases rapidly and reaches a peak.
During the peak period the rate of increase remains constant for a time and then the rate of multiplication falls down rather gradually. Normally all bacteria reproduce by this vegetative method of fission.
This feature earned for them the name fission fungi. Hence, the older mycologists placed bacteria in the class Schizomycetes (Schizo, a Greek word meaning to split or cut, mycetes meaning fungi) coordinate with the other four classes of true fungi.
(b) Budding (Fig. 18.9):
Some bacteria multiply by budding. At the time of budding a small portion of the bacterial cell wall at or near one pole softens and thins. The cell protoplast in this region covered by the thin wall bulges out in the form of a small protuberance.
The protuberance gradually increases in size. It is termed the bud. Meanwhile the parent genetic material divides into two portions. One of these migrates into the bud.
The bud grows and may become separated by a constriction at the base as in Bifidibacterium. The bud cell subsequently grows to the normal size and may in turn produce a bud.
In one group of bacteria (Hyphomicrobeales) commonly called the budding bacteria, a branch strand of cell wall material may be initiated prior to the separation of a bud. Consequently many bud cells may be joined end to end by such thread-like secreted strands (Fig. 18.9).
(c) Cysts:
In certain bacteria the entire protoplast of the cell recedes from the cell wall and becomes rounded. A thick wall is then secreted around it to form a resistant structure somewhat similar to the endospore. It is called the cyst.
The cysts are formed in certain species of Azobacter. Under suitable environmental conditions the cyst germinates to produce a new bacterium cell.
(d) Segmentation:
Some other species of bacteria reproduce vegetatively by a process called segmentation. In this case the protoplast of the bacterium cell, at some stage, divides to form very tiny bodies called gonidia.
The cell wall ruptures and the liberated tiny gonidia grow into new bacterial cells under suitable conditions. To some bacteriologists vegetative reproduction by means other than binary fission seems doubtful.
2. Sporulation in Bacteria:
Some bacteria produce non-motile spores which are of the following types:
(a) Conidia (Fig. 18.10 A):
Streptomyces—an important member of a Actinomycetes which are now considered true gram-positive bacteria, has a filamentous branched thallus.
It cuts off tiny, oval or rounded, spore-like bodies, the conidia in chains at the tips of special aerial branches (conidiophores). Each conidium under suitable conditions germinates to give rise to new bacterial filament.
(b) Oidiospores (Fig. 18.10 B):
In an another member Actinomyces (order Actinomycetes) the hypha instead, of obstructing spores in succession at the free end, undergoes additional septation throughout its length to form numerous small reproductive units known as the oidiospores. Each oidiospore, on germination produces, a filamentous bacterium.
(c) Sporangiospores (Fig. 18.10 C):
In some branching bacteria sporangia-like structures may develop at the end of certain hyphae. The protoplast of the sporangium may divide to form tiny sporangiospores.
On liberation the sporangiospores germinate under suitable conditions, each producing a filamentous branching bacterium.
(d) Endospores:
These are highly resistant, physiologically dormant, single-celled structures formed usually inside a bacterium mother cell.
(i) Occurrence of Endospores:
All bacteria do not form endospores. Only a limited number of the larger rod-shaped forms (bacilli), which are all included in the family Bacillaceae, produce endospores. The common examples are species of aerobic genus Bacillus and anaerobic genus Clostridium.
None of the smaller bacilli, cocci with one exception (gram-positive Sporosarcina) and Spirilla produce endospores. The spore-forming species are generally gram positive.
Some bacteriologists, however, hold that a few spore producing species occur in the gram-negative genera Vibrio (Sporovibrio) and Spirilla.
(ii) Form and Position of Endospores (Fig. 18.11):
The endospores are usually formed singly. There are, however, instances in which two endospores are regularly formed in each bacterium cell. The bacterium cell with a mature endospore is called a sporangium.
The size, shape and position of the endospores within the sporangium varies from species to species.
In form the endospores may be spherical or elliptical (oval) rearely cylindrical and may be central, sub-terminal or terminal in position in the sporangium. The endospore occupies one quarter to one half of the area of the cell.
The diameter of the endospore may be slightly less than, equal to or greater than that of the rest of the parent cell. The ovoid or spindle-shaped sporangium with a central enlarged endospore is known as Clostridium (C).
Plectridium is the term applied to the racquet-shaped sporangium with an enlarged terminal endospore (E). The mature endospore is a highly refractive body impervious to ordinary stains.
(iii) Structure of Endospore (Fig. 18.12):
The structure of endospore varies somewhat from species to species. There is a tiny core of protoplast consisting largely of chromatin or nuclear material (DNA) surrounded by dense cytoplasm.
The cytoplasm has low water content but the protein content may be as high as 90 percent. There is, in addition, the usual stabilizing agent dipicolinic acid. It contains fewer active enzymes and in small amounts.
The chromatin material lies near the surface (B) or centre of the spore cytoplasm (Fig. 18.13D). The protoplasmic mass is invested by a delicate spore wall which usually becomes the cell wall of the vegetative cell.
The contents of the endospore within the spore appear packed in a very orderly manner. Electron micrograph reveals a thick layer of relatively low density outside the spore wall. It is known as the cortex.
External to the cortex is the spore coat which is generally differentiated into layers. It consists largely of a protein which contains a disulphide rich component (Vinter, 1961) with physical structure resembling that of Keratins (Kadotal et al. 1965).
It is located in the outer layers of the spore coat. Presumably it plays an important role in the survival of spores in adverse environments. The spore surface in electron micrographs shows different surface patterns.
The surface may be smooth, wrinkled, ribbed or sculptured in geometric patterns. Sometimes the whole spore body is invested by an exosporium (Clostridium sporogenes). The size, form, location and surface patterns are characters important for classification.
(iv) Physiological characteristics of Endospores:
The endospores show astonishing tenacity of life being highly resistant to extremes of heat and cold, light, desiccation, disinfectants, toxic chemicals and other conditions which would readily kill a vegetative cell.
So resistant are some bacterial endospores that they can survive for several years, decades and probably for centuries. Endospores of Anthrax bacillus can withstand boiling water for an hour whereas those of other species survive prolonged boiling.
Endospores of one bacillus resist 100°C for over 20 hours and some can withstand pressure cooking at 120°C or remain uninjured for an hour or more in a hot oven. Even immersion in liquid air such as Helium (at- 269°C) for several hours has no adverse effect on them.
They can germinate readily after immersion. The endospores are thus the most resistant forms of life known. It is very difficult to kill them. Fortunately very few disease producing (pathogenic) bacteria produce endospores.
However, not all endospores are so resistant. Some are killed at temperatures slightly higher than those lethal to vegetative cells (60°C-70°C).
The remarkable resistance of endospores is attributed to:
(i) Low water content.
(ii) Low metabolic activity.
(iii) Protective nature and impermeability of relatively thick spore coat.
(iv) Presence of fewer active enzymes.
(v) Presence of unusual stabilizing agent dipicolinic acid.
(vi) Increased calcium content of endospore.
(vii) Physico-chemical nature of the spore protoplast.
(v) Factors Favouring Endospore Formation:
Until recently it was held that endospores are produced under adverse circumstances such as extremes of temperature, drought, starvation and accumulation of toxic substances in the surrounding medium.
The well-known fact that aerobic spore-forming bacilli do not form endospores in the absence of oxygen and anaerobic forms do not sporulate in the presence of oxygen makes it less likely that sporulation is entirely a reaction to unfavourable conditions.
Endospores, in fact, are produced by healthy bacterial cells under favourable conditions usually when multiplication has ceased. More frequently it takes place at a temperature favourable for vegetative growth.
According to Forbisher (1957), endospore formation appears to require some energy food. It may be glucose or manganese ions depending upon the species. Presence of iron and manganese in culture medium increases endospore formation.
Carpenter is of the opinion that endospore formation is inhibited by certain metabolic by-products such as straight chain organic (fatty) acids with 10- 14 carbon atoms. Knayse (1945) stated that drying does not favour sporulation to any appreciable extent.
It is generally held that adequate food supply favours vegetative growth but appears to delay spore formation. Experimentally it has been shown that cultivation in dilute media promotes sporulation in bacteria.
This lends support to the suggestion that exhaustion of nutrients is a factor in spore formation. Some bacteriologists hold that substances that interfere with amino acid metabolism inhibit sporulation.
The conclusion, therefore, is that endospore formation depends upon organism’s protein metabolism. Hardwick and Foster (19.52) envisaged that sporulation is due either to the conversion of vegetative proteins into spore proteins or formation of fresh spore proteins from the culture medium.
Substances in the culture medium which oppose conversion of vegetative proteins into spore proteins inhibit sporulation.
(vi) Development of Endospores (Fig. 18.13):
The development of spores in the two spore- producing genera of bacteria (bacilli and Clostridia) is in general similar.
The sequence of changes is distinguishable into the following seven stages:
Stage 1 Formation of an axial filament of nuclear material (A):
The nuclear matter of the bacterial cell by condensation become arranged in the form of an axial thread (A). It nearly extends from one pole to other.
Stage 2 Development of fore-spare (B):
A ring-like invagination of the plasma-membrane arises near one pole of the cell (A). It grows centripetally and finally fuses in the centre of the cell to form a septum which completely separates the fore-spore from the mother cell (B). A small amount of nuclear material is entrapped within the forespore.
Stage 3 Eugulfment of fore-spore (C-D):
This is brought about by the insinuation between the cell wall and fore-spore plasma membrane of mother cell. Cytoplasm is bounded and inner sides by new membrane (Mackay and Morris 1971).
The fore-spore embedded in the material cytoplasm secretes a cell wall around it. It is termed the spore wall (E).
Stage 4 (E):
It is characterised by the deposition of the cortical layer probably differentiated from the material cytoplasm.
Stage 5 (F):
The differentiation of multi-layered spore coat around the cortex represents this stage.
Stage 6:
It is characterized by the maturation of the spore inside the mother cell. It may extend over several hours during which the spore changes in size and form. The mother cell now called the sporangium often remains alive for a short time after maturation of endospore.
Stage 7 (G):
Eventually the cell contents not utilised in spore formation begin to disintegrate (G). Finally autolysis of the sporangium wall and remains of cytoplasm sets the endospore free. The lytic release of the mature spore represents stage 7.
The liberated endospores are dispersed by wind and thus occur everywhere. They may remain quiescent for months and years.
(vii) Biological Significance of Endospores:
The generally held view that endospore formation is a means of multiplication in bacteria is not tenable. Usually a single endospore is produced by each bacterial cell. The spore, in turn, germinates to give rise to one bacterium cell.
There is no increase in the progeny. Further, most of the species of bacteria which do not form endospores multiply perfectly without sporulation. Thus endospore formation is not a reproductive mechanism.
The other suggestion is that endospore formation is a means to tide over unfavourable conditions. The high resistance of endospores to heat, cold, drought and disinfectants tempted some scientists to put for this thesis.
The objection to this view is that setting in of such conditions in the environment does not induce endospore formation. Contrary to this endospore are formed under favourable conditions when there is adequate food supply and at a temperature suitable for vegetative growth.
Endospores are chiefly found in several days’ old, well matured cultures. Some bacteriologists, therefore, consider endospore formation to represent a mature stage of bacterial cell development!
The product of this mature stage which is the endospore, represents a dormant and resistant stage in which there is little metabolic activity and low rate of respiration.
However, it is alive but dormant. With its properties of low water content, low metabolism, relatively thick impervious wall, increased calcium content, physico-chemical nature of the protoplast, presence of dipicolinic acid and small amounts of active enzymes, the endospore functions as a resting structure and provides resistance to adverse environmental conditions typical of spores.
According to this view, therefore, endospore is evidently a resting stage in the life cycle of bacteria. It is a resting cell that remains viable for extremely long periods, Primary it enables the organism to withstand rigorous conditions of life and secondarily serves as a means of multiplication particularly when more than one endospores are formed per bacterial cell.
(viii) Germination of Endospore (Fig. 18.14):
The highly resistant endospores germinate immediately (within a few hours) when placed in suitable environment conditions. The conditions which favour germination are the presence of moisture, food supply (nutrients), suitable temperature and oxygen tension.
The addition of certain germinating agents such as an amino acid, glucose, and adenosine to the culture medium accelerates the process in the case of aerobic bacilli. Certain chemical compounds and heavy metals such as copper, zinc, nickel, and cobalt retard germination.
The Spore coat imbibes water and softens. The intake of salts and nutrients causes swelling of the spore contents which lose their refractility. Subsequently the softened spore coat or coats rupture at the equator (A, Bacillus maycoides) or at one end (Clostridia, B).
In the latter case the new growing cell emerges at the pole of the endospore. Usually the spore wall becomes the cell wall of the young vegetative cell.
In the case of equatorial splitting, the remnants of the cracked spore coat appear as caps at each pole of the emerging vegetative cell (Fig. 18.14A). Eventually these fall off.
The first step after the initiation of germination but prior to the initiation of the germination outgrowth is the initiation of germination messenger RNA which is completely absent in the dormant state of the endospore.
During germination the endospores lose about 25 to 30 percent of their dry weight. Heat shocking within the range of 80 C to 85 C for a few (8-10) minutes promotes germination in the endospores of bacilli.
B. Genetic Recombination (Sexual Reproduction):
Sexual reproduction has long been thought to be absent in Bacteria. Experiments with the help of electron microscope, however, have demonstrated the exchange of genetic material followed by genetic recombination in some species of bacteria.
This is an essential feature of a sexual process. Bacteria are able to exchange genetic material by exotic means as well as by a straightforward conjugation process which closely resembles sexual reproduction in other organisms.
There are two principal method of bringing about genetic changes by exotic means in bacteria. In both these there is transfer of DNA from one organism to another without the two ever coming into contact but the technique employed is dissimilar. These two phenomena are transformation and transduction.
(i) Transformation:
The first transformation experiment was carried out with Pneumococcus which causes pneumonia. The bacterium is oval in shape and is surrounded by a capsule.
On the basis of the nature of capsule polysaccharides, the bacteriologists have grouped pneumococci into several types. Occasionally a Pneumococcus cell loses the capacity to secrete a capsule around it.
This inability is transmitted to all its progeny. In this way a clone of non-capsulated bacterial cells is produced. These clones of pneumococci have lost their virulence and do not cause disease.
On a solid medium these non-capsulated, non-pathogenic pneumococci form colonies which appear rough. The capsulated or encapsulated pneumococci, on the other hand, form colonies which appear smooth.
It has become customary with the bacteriologists to apply terms ‘rough’ and ‘smooth’ to pneumococci without or with capsules. The former are denoted as R type and the latter S type pneumococci.
A British bacteriologist Griffith in 1928, injected mice simultaneously with a mixture of R type (rough, non-pathogenic) and heat killed S type (smooth, pathogenic) bacteria.
Neither of these would have been harmful to the mice. On the contrary the mixture killed many animals. The animals that died were found to be infected with capsulated (smooth) pneumococci of S type.
The result was astonishing because S type organisms which were injected had been killed by heat. It was obvious from this experiment that association with the dead smooth type bacteria had in some mysterious way restored virulence of living, rough R type bacteria.
On examination, the bacteria from the dead mice were found to possess capsules and formed smooth colonies. It is evident that the association transformed living R type bacteria into S type bacteria.
The fact that the progeny of the latter were all S type showed that it was not due to their having surrounded themselves with capsules of the dead ones. In all similar experiments, the acquired property was that of the dead organism.
In other words the dead, smooth bacteria transformed the living rough bacteria into their own capsular type.
The substance in dead, capsulated bacteria responsible for the transformation has subsequently been isolated and is found to be deoxyribonucleic acid (DNA).
It has also been found that transformation is not dependent on the presence of dead bacteria. It could equally take place with cell-free extracts from smooth bacteria.
The cells of the transformation strain are said to be competent cells. The DNA portion, in which resides the whole transforming ability, is called the transforming principle.
The initial steps in transformation are adsorption and penetration of the transforming principle (exogenous DNA) into living bacterial cell which is assumed to possess areas in the wall.
After penetration the subsequent steps consist in, (i) the incorporation of the transforming principle (DNA) into the genetic material (chromosome) of the recipient cell, (ii) replication of the modified chromosome with new information, and (iii) formation of a transformed cell population.
From the account given above it is clear that transformation may be defined as the transmission of heritable property from one cell to another by way of free, naked DNA without the recipient and donor cells ever coming into contact. The genetic change in transformation is permanent.
(ii) Transduction:
It is a widespread phenomenon which differs from transformation in the mode of genetic transfer. In transduction, genetic transfer is carried out by the agency of a bacterial virus known as the bacteriophage.
Only temperate as opposed to virulent bacteriophages can act as agents of this transaction. Transduction thus is a genetic transfer mediated by a bacteriophage.
The phenomenon of transduction has been studied in Escherichia coli with the phage vector P1 and Salmonella typhimurium with its vector P2. The bacterial cell infected with a temperate phage is termed Lysogenic.
Zinder and Lederberg (1952) reported the transfer of several characters between strains of Salmonella. They cultivated a lysogenic mutant of S. typhimurium which required tryptophan with another strain which required histidine but was susceptible to the same bacteriophage.
They found that some cells of the lysogenic organism acquired one of the characteristics of second organism. They required neither tryptophan nor histidine. As the phage multiplies within its host cells, the fragments of the host DNA are incorporated in the newly formed phage particles.
The host cell disintegrates and the phage particles are released. The host cell disintegrates and the phage particles are released. The released phage particles, which carry a small amount of the host DNA, may infect the cells of the susceptible strain.
In this way the host DNA is transferred to susceptible cell (new host) and incorporated into genome of the recipient. This is transduction.
Lederberg and Tatum (1944) furnished a clear experimental evidence of genetic recombination mechanism which closely resembled sexual reproduction in other organism.
It is termed bacterial conjugation. On the basis of this startling discovery Lederberg and Tatum were awarded a Nobel prize in 1959.
(iii) Bacterial conjugation (Fig. 18.15):
It has been studied in colon bacillus (Escherichia coli). There are two mating types of cells in E. coli. Both are haploid. One of the these is the donor or fertile cell (F+) and the other recipient or receptor cell (F -).
The cells of the opposite types come together in pairs. The donor cell has a special kind of pili known as the sex pili which help to attach it to the cell wall of the recipient cell.
A conjugation bridge or tube develops between the two conjugant at the point of contact. Through it the genetic material (DNA strand) of the donor cell enters the recipient cell in a linear series.
It appears that during the conjugation process (Fig. 18.15) the ring-shaped DNA strand breaks at a definite point and becomes a filament (B). Without this the transfer of genetic material cannot take place.
However, it still remains double stranded. The genes ABCDEFGHI are arranged on it in a linear series. Wolman and Jacob (1956) experimentally demonstrated that the first gene A of the donor cell moves into the recipient cell at about seven minutes.
This is followed by gene B at 9 minutes, gene C at 10 minutes, D at eleven minutes, gene E at 18 minutes and F at 25 minutes (D). The conjugants stay joined for a short time so that only a portion of the DNA strand of the donor cell could get transferred to the recipient cell through the conjugation bridge (D).
The recipient ex-conjugant or zygote is thus partly or incompletely diploid. It possesses a complete recipient “chromosome” (DNA) but only a part of a donor “chromosome” (D).
When the progeny of zygote cell are grown it is found that the cell is a recombinant. It grows under conditions that will support neither of the parents. This partly diploid state is maintained only for a short period.
The zygote cell (recombinant) shortly discards the fragment of the donor ‘chromosome’ and reverts to the haploid state. Thus, bacterial conjugation differs from true sexual reproduction in two respects, absence of meiosis and formation of partly diploid zygote.
After transfer of genetic material the donor cell must perish. Mandlelstam and Mcquillen (1968) suggested that only one strand of the donor cell DNA enters the recipient cell.
As it does, synthesis of complementary Strand takes place. At the same time a new DNA strand is synthesised on the donor DNA to replace the strand which has been removed.
