Bacteria and Viruses — Short Questions

Bacterial cell wall
It is a rigid wall around the plasma membrane of bacterial cell. The major component of bacterial cell wall is a unique macromolecule, called peptidoglycan or murein. It is composed of long glycan (polysaccharide) chain, cross-linked with short peptide fragments. Its amount differs in different bacteria. Cell wall also contains lipids, which are linked to peptidoglycan.

Peptidoglycan layer of Bacterial cell wall
The composition of cell wall is quite different in Gram-positive and Gram-negative bacteria. The cell wall of Gram-positive bacteria contains thick layer of peptidoglycan and has less lipid content. While the cell wall of Gram-negative bacteria has a thin layer of peptidoglycan and more lipid content.

Mesosomes
In bacteria at some points, cell membrane invaginates and forms vesicles, tubules or lamellae in cytoplasm. These structures are involved in mesosomes. These are involved in DNA replication, cell division and also serve as respiratory centres.

Plasmids in genetic engineering
Plasmids serve as important vectors, in genetic engineering. They are used to carry selected genes to bacteria for cloning or for the synthesis of specific proteins. Scientists also insert human genes in bacteria and produce therapeutic proteins e.g., insulin, growth hormones or antibodies.

Sporulation
The process by which bacteria make endospores, is called sporulation. Endospore remains dormant unless favourable conditions return. Under favourable conditions, endospore germinates to give rise to a new vegetative cell.

Function of capsule
Some bacteria produce capsule outside their cell walls. It is a gelatinous layer and gives sticky characters to bacterial colonies. It provides protection from phagocytosis, prevent desiccation and adherence to surfaces.

Role of pili
Some bacteria have pili (singular: pilus). These are non-helical, filamentous appendages and are smaller and thinner than flagella.

• Pili are used for attachment of bacteria to various surfaces.
• They are also involved in the mating process (conjugation) between cells to exchange genetic material. Flagella enable bacteria to move to detect and to respond to chemical signals.

Plasmids
Some bacteria have circular, double-stranded extra chromosomal DNA molecules, called plasmids. They are self-replicating and can replicate before or after division. They contain genes that enable bacteria for resistance against unfavorable conditions (e.g., antibiotics).

Role of Endospores
Many bacteria can survive extended periods of harsh conditions by forming specialized "resting" cells, called endospores. Endospores are thick-walled and metabolically inactive (dormant). The process by which bacteria make endospores, is called sporulation. Endospore remains dormant unless favourable conditions return. Under favourable conditions, endospore germinates to give rise to a new vegetative cell.

Significance
The cell wall of Gram-negative bacteria has an outer membrane made of lipopolysaccharides and lipoproteins. The outer membrane makes Gram-negative bacteria resistant to many antibiotics. It contains a protein called porin, which acts like a pore for specific molecules.

Motility in Spirochete
Spirochaetes have a modified flagellum. It is known as axial filament. It is anchored at one end and runs length-wise in periplasmic space (between cell membrane and outer membrane). It consists of two sets of flagella-like fibrils anchored at the two poles of cell. It helps spirochaetes for flexing, swimming, creeping and spinning movements.

Twitching

• It is a type of movement in bacteria, that enable them to move over surfaces.
• It is mediated by pili, which bind to surrounding solid surface and retract. Thus, bacterial cell is pulled forward.

Gliding

• It is similar to twitching.
• In gliding, bacteria secrete slimy substance, which help them for smooth gliding over solid surfaces.

Brownian movement
Some bacteria (e.g., Streptococcus) that do not have flagella or pili, move due to the random and uncontrolled movements of the particles present in fluid which is called Brownian movement.

Swimming Motility Swimming refers to the movement of individual bacteria in liquid environment. They swim by using their flagella, most bacilli and spirilla move by means of flagella.

Swarming Motility The coordinated movement of a group / population of bacteria is called swarming.

Taxonomic status of bacteria
According to five kingdom classification system, proposed by Robert Whittaker all prokaryotes are included in a separate kingdom is kingdom Monera. But in the last decade, molecular studies have highlighted serious flaws in the five-kingdom classification system. Now, most biologists favour replacing it with a new system, called three-domain system, which is more aligned with the data gained from molecular studies. In this new system, bacteria are the prokaryotes classified in the domain of their own, i.e. the domain bacteria.

Periplasmic space
The space between peptidoglycan layer and cell membrane of bacteria is called periplasmic space.
The cell wall of Gram-negative bacteria has more periplasmic space than Gram positive.

Penicillin is an antibiotic that works by inhibiting cell wall growth in bacteria. There are two genera in bacteria which lack cell wall, they are Mycoplasmas and Spiroplasmas lack cell walls so their outer membrane is cell membrane that is why they are resistant to penicillin.

Cell membrane of bacteria
The cell membrane of bacteria does not have sterols (i.e. cholesterol) in its chemical makeup while eukaryotic cell membranes possess cholesterol in their lipid bilayer.

Cytoplasm of Bacteria
Bacterial cell lacks cytoskeleton, membrane-bounded organelle and nucleus in their cytoplasm. The only organelle they have is ribosome but these are smaller than eukaryotic ribosomes, while the cytoplasm of eukaryotes contain cytoskeleton, all membrane-bounded organelles and nucleus.

Habitat of Bacteria
The fossil record shows that prokaryotes i.e., archaea and bacteria were abundant 3.5 billion years ago. They evolved and remained all alone on Earth for the next 2 billion years. Today, prokaryotes (archaea and bacteria) are found wherever there is life. Bacteria are found in water, air, soil, food and in the bodies of animals and plants. They outnumber all eukaryotes. They can survive in extreme habitats.

Major groups of bacteria
Followings are the major groups of bacteria:
(i) Omnibacteria
(ii) Cyanobacteria
(iii) Mycoplasmas and Spiroplasmas
(iv) Spirochetes
(v) Pseudomonas
(vi) Actinomycetes
(vii) Nitrogen fixing bacteria
(viii) Chemosynthetic bacteria

Characteristics
These are rigid, rod-shaped, heterotrophic Gram-negative bacteria. Many important pathogens are included in this group. Most of these bacteria have flagella. They do not produce spores. They are usually aerobic. Escherichia coli and vibrios are examples of such bacteria.

Characteristics of mycoplasmas and Spiroplasmas

• These groups differ from all other bacteria in that they lack cell walls and that's why they are the smallest bacteria. As they lack cell walls, they are also resistant to penicillin and other antibiotics that work by inhibiting cell wall growth.
• Some mycoplasmas cause diseases in mammals e.g., certain types of pneumonia in humans.
• Spiroplasmas cause significant plant diseases e.g., the lethal yellowing disease of coconuts.

Cyanobacteria

• These are photosynthetic bacteria.
• They contain chlorophyll-a and accessory pigments like carotenoids, and blue and red phycobilins.
• Many cyanobacteria fix atmospheric nitrogen in their special cells called heterocysts. They are common in soil in the form of mats. The mats on the sediments in the sea are dominated by cyanobacteria.

N₂-Fixing bacteria

• Nitrogen fixing bacteria are found in soil and water and convert atmospheric nitrogen into nitrates.
• This group includes economically important bacteria.
• They are Gram-negative and most are flagellated.
• Azotobacter is a member of this group.

Spirochetes

• Spirochetes are long spirilla with Gram-negative cell walls.
• They may have 2 to more than 100 flagella.
• Treponema are important spirochetes.
• They cause syphilis (a fatal sexually transmitted disease).

Pseudomonas

• Pseudomonas are straight or curved Gram-negative rods with one or many flagella at one end.
• Some of them are autotrophic but many are plant pathogens. Some of them play role in denitrification.
• Pseudomonas aeruginosa occurs in soil, water and raw vegetables, it can cause serious infections in people with weak immunity.

Energy for survival
Chemosynthetic bacteria derive energy from the oxidation of inorganic compounds of nitrogen, sulphur and iron. They use this energy for the synthesis of their food. Nitrosomonas and Nitrobacter are included in this group. They oxidize nitrogen compounds (NH₃) to gain energy. The NH₃ is in turn converted to nitrite and nitrate. Thus, they play vital role in nitrogen cycle.

Following are the group of bacteria which are pathogenic as well as decomposers.
i. Omnibacteria
ii. Spirochetes
iii. Pseudomonas

Following are the group of bacteria which are Nitrogen fixers
i. Cyanobacteria
ii. Actinomycetes

(i) Leprosy is caused by Mycobacterium leprae.
(ii) Tuberculosis is caused by Mycobacterium tuberculosis.
(iii) Syphilis is caused by Treponema pallidum.
(iv) Lethal Yellowing disease of coconuts is caused by Spiroplasmas.

Antibiotics

• Tetracycline
• Chloramphenicol
• Erythromycin
• Neomycin

Nitrifying Bacteria Nitrosomonas
Nitrobacter
Azotobacter

Denitrifying Bacteria Pseudomonas

Makers of useful products
Many bacteria e.g., Lactobacillus in combination with yeasts and molds, have been used for thousands of years in the preparation of fermented foods such as cheese, pickles, soy sauce, vinegar, wine and yogurt. In pharmaceutical and agrochemical industry, bacteria are most important in the production of important chemicals. Some bacteria are used for the production of antibiotics. Commercial preparation of animals' skin for making leather goods, involves the use of bacteria.

Environmental cleaners
Many bacteria can degrade organic compounds very easily. Such bacteria have been used for the removal or degradation of pollutants (bioremediation) from environment.

• For example, bacteria are used to decompose city sewage into harmless products.
• Some bacteria can digest the hydrocarbons present in petroleum. These bacteria are used to clean up oil spills.
• Bacteria are also used for the bioremediation of industrial toxic wastes.

Biopesticides
Bacteria are used in the place of pesticides in biological pest control. This commonly involves Bacillus thuringiensis, a Gram-positive, soil dwelling bacterium. These biopesticides are environmental friendly and have little or no effect on humans, wildlife, pollinators and most other beneficial insects.

Quality of bacteria
Bacteria can grow quickly and scientists can manipulate with them very easily e.g., Escherichia coli. Due to these reasons, bacteria are used in the fields of molecular biology, genetics and biochemistry.

• Scientists make mutations in bacterial DNA and examine the changes in characteristics.
• In this way, they determine the function of genes and enzymes in bacteria.
• This knowledge is then applied to study the same genes and enzymes in more complex organisms. Scientists also insert human genes in bacteria and produce therapeutic proteins e.g., insulin, growth hormones, or antibodies.

Normal Flora

• The mixture of organisms regularly found at any anatomical site is referred to as the normal flora.
• In a healthy animal, the internal tissues, i.e., blood, brain, muscle, etc., are normally free of microorganisms. On the other hand, the surface tissues, e.g., skin and mucous membranes, are constantly in contact with environment and are colonized by certain microbial species.
• The normal flora of humans consists of bacteria, few fungi, protists, and some methanogenic archaea. Bacteria are the most numerous and obvious microbial components of normal flora.

Benefits of Bacterial Flora of Humans
The associations between humans and their normal flora are mutualistic. In human body, normal flora gets nutrients, a stable environment and constant temperature, protection, and transport. Similarly, body also gets many benefits from normal bacteria; for example;
i. Synthesis of vitamins (Vit.K, Vit.B₁₂)
ii. Prevent colonization by pathogens
iii. Inhibit or kill pathogens
iv. Stimulates the production of cross-reactive antibodies

Cross reactive antibodies
The normal flora behaves as antigens; they induce immunological response. Low levels of antibodies produced against the normal flora are known to cross-react with certain pathogens, and thereby prevent infection or invasion.

1. Synthesis of vitamins: Bacteria in alimentary canal produce vitamins. These excrete vitamins which are in excess of their needs. From alimentary canal, these vitamins are absorbed and distributed in body. For example, enteric bacteria secrete Vitamin K and Vitamin B₁₂, and lactic acid bacteria produce certain B-vitamins.
2. Prevent colonization by pathogens: The bacteria of normal flora compete with pathogens for attachment sites and nutrients. So, pathogens have less chance of entering body tissues.
3. Inhibit or kill pathogens: The intestinal bacteria produce a variety of substances, which inhibit or kill pathogen bacteria.

Following are the differences between Capsid and Capsomeres.

Capsid

• In viruses the core is surrounded by a protein coat, called capsid.
• It gives definite shape to virus.
• Central core and capsid are collectively called as nucleocapsid.

Capsomeres

• Capsid is made up of protein subunits called as capsomeres.
• The number and kind of capsomeres is characteristic of a particular virus.

Examples
i) Herpes virus (causes cold sores, chickenpox etc.) contains 162 capsomeres in its capsid.
ii) Adenovirus (causes common cold) contains 252 capsomeres in its capsid.

Naked viruses
In some animal viruses, nucleocapsid is covered by another membrane called envelope. It is a lipid-rich membrane and is derived from host cell. Non enveloped viruses are known as naked-viruses.

General appearance
There is a great diversity in the general appearance of viruses.

• The animal and plant viruses may be polyhedron (having many sides) or helical.
• The bacterial viruses (bacteriophages) may be cubic, icosahedral (having 20 faces), helical, or complex (polyhedral head and rod-shaped tail).

Role of bacteriophages
Bacteriophages are used as carriers in genetic engineering. The gene of interest is inserted into the DNA of bacteriophage, which carries it to the target bacterial cell. When virus incorporates its DNA into bacterial chromosome, the gene of interest also becomes a part of bacterial DNA. Such transgenic bacteria (transgenic: whose genome has DNA of some other organism) can be grown to get copies of the gene of interest and to get the required protein.

[diagram showing the labeled structure of a bacteriophage with Head, Capsid, Core (DNA), Neck, Sheath, Tail core, Tail, Tail fiber, End plate, and Pin]

Tail of bacteriophage
The tail of a bacteriophage is also made of inner core and outer sheath, both of which are made of different proteins. A neck attaches sheath with head and an end plate is present on the other side of sheath. Six tail fibres are attached with end plate.

Function
They help the phage to attach with bacterial wall. These structures are also made of proteins.

Retroviruses
Retroviruses contain RNA and capsids. The most distinguishing character of retroviruses is the presence of a specific enzyme, reverse transcriptase. This enzyme catalyses the process of reverse transcription in which a single stranded RNA is reversely transcribed into a strand of DNA. The enzyme then uses DNA strand to complete a double helix of DNA.

Entry of HIV
Experts have concluded that HIV originated in the jungles of Africa among wild chimps. Evidence suggests that a form of this virus entered human species and became HIV by way of monkey bites or ingesting monkey meat and brains.

[diagram showing the labeled structure of HIV with Glycoprotein, Viral envelope, Capsid, RNA, and Reverse Transcriptase]

Discovery of AIDS
The disease was first reported in 1981 and the patients were found homosexual. Later on, AIDS was discovered in non-homosexual patients too who had received blood or blood products from other AIDS patients. In 1984, it was discovered that the agent causing AIDS was a virus. In 1986, the AIDS causing virus was given the name Human Immunodeficiency Virus (HIV). It is a host specific virus. It can multiply in monkeys but do not cause AIDS in them.

See Q2. of theory.

See Q5. of theory.

See Q6. of theory.

See Q4. of theory.

See Q8. of theory.

See Q9. of theory.

See Q10. of theory.

See Q12. and Q13. of theory.

Ribosomes in bacteria
Bacteria lack membrane bounded organelles, because they lack sub-cellular membrane they have only one organelle which is not bounded by any membrane, that is ribosome (the sole organelle of prokaryotes). The prime function of ribosomes is same as in eukaryotes that is protein synthesis. Although they are smaller (70s) than eukaryotic ribosomes (80s).

Generation of energy
Bacteria although lack mitochondria, but they still obtain energy through cellular respiration, especially involving electron transport chain and ATP synthase, which are located in their plasma membranes (eukaryotes have ETC in mitochondrial membrane).

Twitching motility
Twitching motility also called crawling motility is used by bacteria to move over surfaces. It is provided and mediated by pili, which bind to surrounding solid surface and retract. Thus, bacterial cell is pull forward in a crawling or twitching fashion.

Prevention is better than cure
(i) Preventive ageing are helpful in reducing the risk of spreading harm and disease to others e.g. isolation in case of contagious diseases like mumps, measles, COVID-19 etc.
(ii) It helps to avoid the consequences which can not be fully reversed e.g. chronic diseases.
(iii) It ensures long term well being like regular screening tests helps in the early detection of lethal diseases like cancer, emphysema, and increases survival chances.
(iv) Reduces physical, emotional or financial distress and is also cheaper than cure, which are now a days very expensive.

Prevalence of AIDS
AIDS is a sexually transmitted disease and have many other modes of transmission (like unscreened blood transfusion, body fluids, infected surgical instruments), and still today it is incurable, and its prevelence is significantly influenced by the social and cultural values of a country, following socio-cultural factors can contribute for AIDS prevelence.
(i) Lack of education and poor awareness about its contraction.
(ii) Religious oppositions.
(iii) Traditional healing (spiritual false) delaying in HIV treatment.
(iv) Weak health care systems i.e. lack of testing and treatment.
(v) Polygamy (in case of infected males).
(vi) Lack of circumcision (as male circumcision reduces female-to-male HIV transmission).
(vii) High risk group protections (drug user).