Lecture - 31 Reproduction biology and economic importance of Bacteria

Reproduction Biology of Bacteria:- 2 types -
1. Asexual reproduction
2. Sexual reproduction
1. Asexual reproduction:- It is characteristic of all bacteria. It take place by two methods: 
a. Vegetative reproduction
b. Sporulation
a. Vegetative reproduction:- It take place by the following methods -
i. Binary Fission
ii. Budding
iii. Cysts
iv. Segmentation
i. Binary Fission:- It is a process by which a single bacterial cell simply divides into two in half an hour time. The various events of binary fission are as follows:
- The nucleoid gradually become elongated in size and form dumbel-shaped structure.
- They still remain attached to the plasma membrane with the help of mesosome.
- The duplication of DNA and mesosome takes place and get separate from each other.
- The daughter mesosomes and nucleoids migrate towards the opposite poles.
- The plasma membrane invaginates at the center and the parent cell is divided into two
identical cells.
ii. Budding:- 
- In this case, a small protuberance, called bud, develops at one end of the cell.
- Genome replication follows, and one copy of the genome gets into the bud. 
- Then the bud enlarges, eventually become a daughter cell and finally gets separated from the parent cell. 
- It is comparatively rare process observed in few bacteria like Rhodopseudomonas, Hyphomicrobium,
Pedomicrobium, Hyphomonas etc. 
- Hyphomicrobeales, commonly called the budding bacteria, a branch strand of cell wall material may be initiated prior to the separation of a bud.
iii. 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 resistant structure somewhat similar to the endospore. It is called the cyst. 
- These are formed in certain species of Azobacter.
- Under suitable environment conditions the cyst germinate to produce the new bacterium.
iv. Segmentation:- 
- In this case the protoplast of the bacterium cell at some stage, divides to form very tiny body called gonidia. 
- The cell wall ruptured and the librated tiny gonidia grow into new bacterium cell under suitable conditions.
b. Sporulation:- Some bacteria produce non motile spores which are of the following types:
i. Conidia
ii. Oidiospores
iii. Sporangiospores
iv. Endospore
i. Conidia:- 
- Many filamentous bacteria (Eg.- Streptomyces) form chains of small, spherical sporelike conidia at the tips of the filaments. 
- A conidium develops by the formation of a transverse wall at the tip of the filament. 
- The filament bearing conidia are known as conidiophores. 
- After liberation each conidium gives rise to a new filamentous bacterium, provided conditions for germination are favourable.
ii. Oidiospores:- 
- In an another member Actinomyces the hypha instead of obstructing spore in succession at the free end, undergodes additional separation through its lnght to form numerous small reproductive units known as oidiospores. 
- Each oidiospores on germination produces, a filamentous bacterium.
iii. Sporangiospores:-
- In some branching bacteria sporangia like structure may developed at the end of certain hyphae. 
- The protoplast of the sporangia may divided to form tiny sporangiospores. 
- On libration of these spores germinate under suitable condition, each producing a filamentous  bacterium.
iv. Endospore:- 
- During the unfavorable condition, eubacteria have the ability to become endospores.
- In this state, the bacteria can tolerate exceedingly high and low temperatures, acidic and basic conditions, and large amounts of radiation.
- Endospores are extremely hard to kill. Surprisingly, they can be boiled for hours and still survive.
- Endospores can only be made by Gram positive bacteria.
- Within the endospore remains the bacterial DNA, but the cytoplasm has a decreased water
concentration.
- This is thought to help in protecting against high heat. 
- The bacteria will take on a tough coating composed of calcium and dipicolinic acid, creating a dense and impregnable barrier to stabilize the DNA within the cell. 
- DNA repair enzymes are also still active, aiding in the resistance of the endospore.
2. Sexual reproduction:- 3 methods -
a. Conjugation
b. Transformation
c. Transduction
a. Conjugation:- 
> It was first discovered in Escherichia coli by Lederberg and Tatum (1946). 
> Cell contact is required for this method.
> Bacteria showing conjugation are dimorphic, i.e., they have two types of cells, male (F+) or donor and female (F-) orrecipient.
> The male or donor cell possesses 1-4 sex pili on the surface and fertility factor (transfer factor, sex factor) in its plasmid. Fertility factor contains genes for producing sex pili and other characters needed for gene transfer. Sex pili are 1- 4 narrow protoplasmic outgrowths. 
> Both sex pili and fertility factor are absent in female or recipient cells.
> If these two types of cells happen to come nearer, a pilus of male cell establishes a protoplasmic
bridge or conjugation tube with the female cell. It takes 6-8 minutes. 
> Gene exchange can occur by two methods:
i. Sterile Male Method (F+ x F– → F– becomes F+):- The plasmid having fertility factor replicates.
A copy of it gets transferred to the recipient cell through the conjugation tube. The recipient cell
also becomes donor. The phenomenon of reversibility of sex is called sexduction.
ii. Fertile Male Method (Hfr x F– →F– remains F–):- The F+ plasmid or fertility factor of the
donor cell gets integrated to bacterial chromosome or DNA. The attachable plasmid is known as
episome.
- Hfr (high frequency of recombination):- The donor cell having fertility factor integrated to its chromosome is called Hfr(high frequency of recombination), meta male or super male because it has a recombination frequency of 1000 times more as compared to normal F+.
- The integrated F+ factor breaks the bacterial chromosome at one end of its attachment. The bacterial chromosome now undergoes replication.
- A copy of the freed end of bacterial chromosome (end distal to F+ factor, also called zero end) passes into the recipient cell through the conjugation tube. Fertility factor is the last to do so.
- Generally whole of bacterial chromosome does not pass into recipient cell. F+ factor is very rarely transferred as conjugation is maintained for a brief period. Only a few genes are transferred, one in seven minutes, two in nine minutes, three in ten minutes, four in eleven minutes, etc.
- Merozygote:- Conjugation produces an incompletely diploid “zygote” known as merozygote or partial zygote. The new genes may replace the genes present in the recipient cells (those of the recipient cells disintegrate) or get added to them.
b. Transformation:-
> The phenomenon was discovered by Griffith in 1928. Its mechanism was worked out by Avery (1944).
> It is the absorption of DNA segment from the surrounding medium by a living bacterium. 
> A bacterium takes in DNA from its environment, often DNA that's been shed by other bacteria. If the DNA is in the form of a circular DNA called a plasmid, it can be copied in the receiving cell and passed on to its descendants.
> Receptivity for transformation is present for a brief period when the cells have reached the end
period of active growth. At this time they develop specific receptor sites in the wall. 
> Normally E. coli does not pick up foreign DNA but it can do so in the presence of calcium chloride.
c. Transduction:-
> Transduction was first discovered by Zinder and his teacher Lederberg (1952) in Salmonella typhimurium.
> Here genetic material is transferred by phage virus between two bacteria.
> Such a virus is never virulent. It passes over the gene of the previous host to the new host.
> Transducing viruses may carry the same genes (restricted transduction) or different genes
(generalized transduction) at different times.
>  It has two forms:
i. Generalized transduction:- 
- It occurs in lytic cycle of phage virus. 
- DNA of phages virus enter into E. coli bacteria. This DNA replicates and develops many new DNA and capsids.
- The DNA of bacteria is broken. Some pieces of DNA also enter into capsid of virus. 
- Bacteria burst and release new phage viruses. 
- Now this phage enters into recipient bacteria and transfer DNA of donor bacteria into the DNA of recipient bacteria.
- Bacterial endonucleases enzymes destroy the phage virus. 
- Now these bacteria incorporate genes of donor bacteria and replicates.
ii. Specialized transduction:- 
- It occurs in Lysogenic cycle of phage virus. 
- In this cycle viral DNA incorporate into bacterial DNA as prophage.
- It remains peacefully there. But sometime, it becomes lytic. It comes out of bacterial DNA. Some part of bacterial DNA remain attach with it.
- Viral DNA with a piece of bacterial DNA replicates and develops new capsids. Bacteria burst. Virus infects other bacteria and transfer genes of donor.
Economic importance of Bacteria:-
A. Useful activities
B. Harmful activities
A. Useful activities:-
1. Dairy products:- A few genera of bacteria are used in food preparation, directly or indirectly.
a. Formation of Curd:- Milk is converted into curd by bacterial action. The milk’s lactose is converted into lactic acid, which gives the characteristic sour taste of the curd. Lactic acid bacteria (LAB) like Lactobacillus are added to milk. Indian curd is prepared by inoculating milk with Lactobacillus acidophilus.
b. Yoghurt preparation:- It is produced by curdling milk with Streptococcus thermophilus and Lactobacillus bulgaricus.
c. Cheese production:- It starts with milk coagulated with lactic acid bacteria and the curd formed is filtered to separate the whey. The solid mass is then ripened with the growth of mould that develops flavour in it. Propionibacterium shermanii is used to make cheese.
2. In Industry:- A large number of products are obtained due to bacterial activity, directly proportional to the economic importance of Bacteria.
3. In Medicine:- Some bacteria have been exploited to produce antibiotics and vaccines. 
a. Antibiotics:-
b. Vaccines:- Several vaccines have been developed from either killed or attenuated (living but multiplying at low rates) bacteria. For example, tuberculosis vaccine, whooping cough vaccine, plague vaccine, DTP (Diphtheria, Tetanus, Pertussis) vaccine, pneumonia vaccine are all prepared with the help of bacteria.
4. In Agriculture:- Bacteria act as decomposers. They make the nutrient available for plants. Specific genera of bacteria are used as biocontrol agents in agriculture. Bacillus thuringiensis (Bt) yields protein-based toxins used to kill insects.
5. Nitrogen fixation and soil fertility:- Certain bacteria are helpful in the fixation of atmospheric nitrogen. Azotobacter and Clostridium are present in the soil and help in nitrogen fixation. Species of Rhizobium bacteria are present in the root nodules of leguminous plants, and they increase the soil’s nitrogen content by fixing up atmospheric nitrogen. The process is known as symbiotic nitrogen fixation.
6. Role in Nitrogen Cycling:- Nitrification is one of the most critical steps in the nitrogen cycle, performed by nitrifying bacteria. The denitrifying bacteria, like Pseudomonas denitrificans, Thiobacillus denitrificans, transform the nitrates to free atmospheric nitrogen.
7. Biogas Production:- Biogas is a standard domestic and industrial fuel, which contains (50-60%) methane, (30-40%) carbon dioxide, (0-3%) Sulphur compounds, and traces of other gases like hydrogen, CO, nitrogen, etc. In a biogas digester, cattle dung is used to obtain gas (gobar gas) in the following steps:
a. Hydrolysis:- It is the initial step that needs anaerobic bacteria like Clostridium, Pseudomonas, etc.
b. Acidogenesis:- It is the second step, in which the facultatively anaerobic, acidogenic bacteria and obligate anaerobic organisms help convert the simple organic material into acids like formic acid, acetic acid, etc.
c. Methanogenesis:- It is the last step, in which anaerobic Methanogenic bacteria like Methanobacterium, Methanococcus, etc., convert organic acids into Methane.
8. Sewage Treatment:- Sewage is agricultural and domestic waste products that pollute the water. The treatment to remove such waste is partially chemical, biological treatment.
a. Secondary treatment:- It is the biological treatment, which reduces the BOD significantly. Aerobic bacteria are used in this process.
b. Tertiary treatment:- It is done once there is a reduction of BOD in the settling tank. Mainly Methanogens grow anaerobically and produce biogas.
9. Formation of Curd with Bacteria:- Curd formation from milk is performed by mixing LAB (Lactic Acid Bacteria), like Lactobacillus, with warm milk. These bacteria convert the milk sugar (lactose) into lactic acid resulting in the formation of curd. Once the curd is made, it is transferred to a cooler place to reduce the further growth of bacteria. Otherwise, the bacteria would make it sour by excessive production of lactic acid.
B. Harmful activities:-
1. Spoilage of Foodstuff:- Cooked food, fruits, vegetables, butter, fish and meat are spoiled by bacteria, particularly during summer months, by causing putrefaction of food materials. Certain bacteria like Salmonella typhimurium and Clostridium botulinum cause a severe type of food poisoning when bacteria-contaminated food is consumed. Clostridium botulinum causes food poisoning, commonly known as botulism.
2. Reduction of Soil Fertility:-
Denitrification:- Certain species of anaerobic bacteria inhabit soils that are either waterlogged or have high organic matter content. These bacteria reduce soil fertility by depleting the soil’s nitrogen content. They break down nitrates present in the soil and release free nitrogen, which escapes into the air, thereby reducing soil fertility (denitrification).
3. Animal Diseases:- Bacteria cause tuberculosis in cattle, anthrax in sheep, cholera in chickens and pneumonia in horses, sheep and goats.
4. Human Diseases:- Many serious diseases are caused by bacteria in human beings. Some of these are as follows:
5. Plant Diseases:- Several plant diseases are induced by bacteria. They cause leaf spots, soft rots, vascular diseases, and bacterial galls. Xanthomonas citri causes citrus canker.
6. Bioweapons:- 
> A bioweapon or biological weapon is a device that carries and delivers a disease-causing biological agent (like bacteria, viruses, genetically altered organisms) or a toxin derived from it to the target organism. The biological agent or toxin is called a bioweapon agent. Biological agents are used against humans, crops and animals. Such use of bioweapons is termed BioWare or biological war. 
> Some of the bioweapons can cause anthrax, smallpox, plague and gastroenteritis. 
> Bacillus anthracis is the causative agent of anthrax, Vibrio cholera of gastroenteritis and Yersinia pestis of plague. 
> Botulinum toxin from Clostridium botulinum is used to cause fatal food poisoning. 
> Some of these agents have already been used. For example, anthrax bacterium was sent through letters in (2001.) 
> Bioweapons are low-cost weapons and cause far more casualties than conventional weapons.