CBSE Questions for Class 12 Medical Biology Microbes In Human Welfare Quiz 8 - MCQExams.com

• The main (starter) cultures in yogurt are Lactobacillus bulgaricus and Streptococcus thermophilus. 
  
• The function of the starter cultures is to ferment the lactose (milk sugar) to produce lactic acid. 
• The increase in lactic acid decreases pH and causes the milk to clot, or form the soft gel that is characteristic of yogurt.
  
• The temperature is maintained at about $$45^o C (40^o - 46^o C$$ ) for four hours. 
• It has a flavor of lactic acid and acetaldehyde. 

• Methanogenic bacteria (bacteria that produce methane) are not found in activated sludge. Methanogenic bacteria are anaerobic bacteria.
• The microbes found in activated sludge are aerobic bacteria that grow rapidly and form flocs.
• Hence, Methanogenic bacteria are not found in activated sludge.

Correct Option: B

Explanation: B

• A symbiotic relationship is formed when two organisms are dependent on each other for survival. 
• $$\textit{Frankia}$$ is an example of Gram-positive symbiotic actinobacterium whereas $$\textit{Aulosira}$$ is an example of a free-living cyanobacteria. 

• $$Bacillus$$ $$thuringiensis$$  produce endotoxin proteins that kill certain insects.
• These proteins form crystalline deposits in bacterial spores and are called crystalline proteins or cry proteins. 
• The cry protein is not toxic to the bacterium because it exists in an inactive protoxin form, inside the gut.
• Due to the alkaline medium, the inactive protoxin is activated into toxin form that binds to the epithelial cells of the midgut and makes the surface porous.
• This causes swelling and lysis of the gut wall leading to the death of insects.

• In sewage secondary treatment, the bacterial flocs are a to sediment in a settling tank.
• The sediment is called activated sludge of which a part is used as inoculum in aeration tank and the remaining passed into the large tank called anaerobic sludge digester. 
• While the effluent is passed into the rivers and streams. 

• Antibiotics are substances produced by the natural metabolic processes of microorganisms as their natural defense against other microorganisms.
• Disinfectants are the chemical that is used to kill bacteria. 
• Antibiotics are safe bactericides that do not kill the patients and therefore, can be injected into patients. 
• Disinfectants are toxic chemicals (less efficient) that can kill the patients and can not be injected but are used as surface bactericides only.

• 2,4-Dichlorophenoxyacetic acid (2,4-D) is the third most commonly used herbicide all over the world.  The health hazards of 2,4-D to humans include that it may induce cytogenetic damage in human lymphocytes, irreversible eye damage, hepatotoxicity and nephrotoxicity. Many methods are proposed for treatment of chemical pollution, however, the safest and the cheapest one is the use of microorganisms for biodegradation of these pollutants. the most biologically active strains against 2,4-D proved to belong to the genera Pseudomonas, Achromobacter, Flavobacterium, Nocardia, Streptomyces and Aspergillus.
• 1,1,1 -trichloro-2,2-bis (p-chlorophenyl)ethane (DDT) is one of the best synthetic pesticides. It was used to control mosquitoes spreading malaria, typhus and other insect-borne diseases among both military and civilian populations. After the war, DDT was made available for use as an agricultural insecticide. It was used extensively for the control of grass grub {Costelytra zealandica White). It was necessary to introduce microbes to degrade or control DDT in soil. It was done so by using DDT-degrading strain of E. aerogenes.
• So all the above options are correct.

• Curdling of milk produces curd which occurs due to the presence of Lactobacillus that converts milk sugar to lactic acid at high temperatures.
• Curd is rich in minerals like calcium, phosphorus potassium, and zinc.
• It is also rich in protein.
  
• It is a good source of vitamins like riboflavin, vitamin $${B}_6$$ , vitamin  $${ B }_{ 12 }$$ , pantothenic acid, and vitamin A. 
• So, the correct answer is option D. 

• Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen.
•  The process is used for industrial or domestic purposes to manage waste and to produce fuels. 
• Hence, Wastewater treatment generates a large amount of sludge, which can be treated by anaerobic digesters.
• So, the correct answer is 'anaerobic digesters'.