Microbes in Human Welfare

Lactobacillus (curd), Saccharomyces cerevisiae (bread/dough), and Propionibacterium sharmanii (Swiss cheese) are household product microbes. Aspergillus niger (industrial citric acid) and Trichoderma polysporum (cyclosporin A) are used in industrial/pharmaceutical biotechnology.

$Clostridium\ butylicum$ – butyric acid $Saccharomyces\ cerevisiae$ – ethanol $Trichoderma\ polysporum$ – cyclosporin-A (immunosuppressant) $Streptococcus$ sp. – streptokinase (clot buster) Correct match: A-III, B-I, C-IV, D-II.

Retting is the process of separating the fibres of jute (and flax, hemp) from the stem.
The stems are submerged in water, where bacteria decompose the pectin of the middle lamella that binds the fibres to the surrounding tissues.
This fermentation is carried out by butyric acid bacteria such as Clostridium butyricum (and related Clostridium species).
After retting, the fibres can be easily separated by beating and washing.
Helicobacter pylori causes stomach ulcers, Streptococcus lactis is used in curdling milk, and methanogens produce methane.
So the microbes used for retting of jute are butyric acid bacteria.

The sediment in the settling tank after secondary treatment is called activated sludge. A small part of it is pumped back into the aeration tank as inoculum; the remaining major part is pumped into large tanks called anaerobic sludge digesters.

Streptococcus is a source of the clot buster streptokinase (A-II); Trichoderma polysporum is a source of the immunosuppressant Cyclosporin A (B-III); Monascus purpureus is a source of statins (blood cholesterol-lowering agents) (C-IV); Aspergillus niger is a source of citric acid (D-I).

Antibiotics are chemicals produced by microorganisms that can inhibit the growth of, or destroy, other microorganisms, so Statement I is correct. Antibodies (immunoglobulins) are produced in the body in response to pathogens and help eliminate microbes, so Statement II is also correct.

Cyclosporin A, an immunosuppressive drug commonly used in organ transplant patients, is produced by the fungus Trichoderma polysporum.

Aspergillus niger, a fungus, is used industrially for the commercial production of citric acid.

Dragonflies help get rid of Aphids and mosquitoes (a-ii); Bacillus thuringiensis is a biocontrol agent of lepidopteran plant pests (b-iv); Glomus absorbs phosphorus from soil (c-v); Baculoviruses have narrow spectrum insecticidal applications (d-iii).

In secondary (biological) treatment, the primary effluent is passed into large aeration tanks where it is constantly agitated and air is pumped in; aerobic microbes grow into flocs and consume the organic matter, reducing the BOD.
The effluent then goes to a settling tank where the flocs sediment as activated sludge.
The major part of this sludge is pumped into anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi of the sludge and produce biogas ($CH_4$, $H_2S$ and $CO_2$).
So the sewage is processed first by aerobic and then by anaerobic bacteria.
Domestic sewage is 99.9% water with only about 0.1% impurities, so it does not have very high amounts of suspended solids and salts.
Its high BOD is due to biodegradable organic matter, not because it contains both types of bacteria, and aeration is essential because sewage is poor in dissolved oxygen.
Hence the correct statement is that it is processed by aerobic and then anaerobic bacteria in the secondary treatment in STPs.

In secondary treatment, aerobic microbes grow in the aeration tanks as flocs (masses of bacteria associated with fungal filaments). When the effluent is passed into a settling tank these flocs sediment; this sediment is called activated sludge, so it is rich in aerobic bacteria. This statement is correct.
Biogas is produced by anaerobic bacteria (methanogens) acting on cellulosic material such as cattle dung.
$Methanobacterium$ is an anaerobic bacterium, found in anaerobic sludge and in the rumen of cattle.
Biogas is a mixture of gases, mainly methane with $CO_2$ and some $H_2$ and $H_2S$; it is not pure methane.
Hence the correct statement is the one about activated sludge.

Bioethanol is produced by the fermentation of sugars by yeast; so the raw material must be rich in sugar or in starch that can be converted to sugar.
$Zea\ mays$ (maize) grains are rich in starch, which is hydrolysed to glucose and fermented to ethanol; maize and sugarcane are the major sources of bioethanol.
$Jatropha$ and $Pongamia$ have oil-rich seeds; their oil is used to make biodiesel, not bioethanol.
$Brassica$ (mustard, rapeseed) is also an oilseed crop.
Hence $Zea\ mays$ would be selected for the production of bioethanol.

Modern (biological) detergents contain enzymes such as proteases, lipases and amylases to remove protein, oily and starchy stains from clothes.
A detergent solution is strongly alkaline, with a pH of about 9 to 11.
Ordinary enzymes would be denatured at this pH, so the enzymes used must remain stable and active under alkaline conditions.
Such enzymes are obtained from alkaliphiles, micro-organisms that grow best at high pH, for example alkaliphilic species of Bacillus that give alkaline protease (subtilisin).
Acidophiles and thermoacidophiles yield enzymes that work at acidic pH, and enzymes of thermophiles are heat stable but are not specially suited to alkaline conditions.
So modern detergents contain enzyme preparations of alkaliphiles.

In a biogas plant, anaerobic bacteria break down organic waste in stages: hydrolysis of complex polymers, acid formation, and finally methane formation by methanogens.
Carbohydrates such as cellulose and hemicellulose, and also proteins and lipids, are hydrolysed and fermented by the bacteria.
Lignin is a complex, highly cross-linked aromatic polymer found in wood. It is broken down mainly by aerobic fungi (white rot fungi) and is hardly degraded under anaerobic conditions.
So lignin is left undegraded and remains in the spent slurry.