You would be surprised to know that we use microbes or products derived from them every day. A common example is the production of curd from milk. Micro-organisms such as Lactobacillus and others commonly called lactic acid bacteria (LAB) grow in milk and convert it to curd. During growth, the LAB produce acids that coagulate and partially digest the milk proteins. A small amount of curd added to the fresh milk as inoculum or starter contains millions of LAB, which at suitable temperatures multiply, thus converting milk to curd, which also improves its nutritional quality by increasing vitamin B12. In our stomach too, the LAB play a very beneficial role in checking disease-causing microbes. The dough, which is used for making foods such as dosa and idli, is also fermented by bacteria. The puffed-up appearance of dough is due to the production of CO2 gas. Can you tell which metabolic pathway is taking place resulting in the formation of CO2? Where do you think the bacteria for these fermentations come from? Similarly, the dough, which is used for making bread, is fermented using baker’s yeast (Saccharomyces cerevisiae). A number of traditional drinks and foods are also made by fermentation by the microbes. ‘Toddy,’ a traditional drink of some parts of southern India, is made by fermenting sap from palms. Microbes are also used to ferment fish, soyabean and bamboo shoots to make foods. Cheese is one of the oldest food items in which microbes were used. Different varieties of cheese are known by their characteristic texture, flavour and taste, the specificity coming from the microbes used. For example, the large holes in ‘Swiss cheese’ are due to the production of a large amount of CO2 by a bacterium named Propionibacterium sharmanii. The ‘Roquefort cheese’ is ripened by growing a specific fungi on them, which gives them a particular flavour.

Even in industry, microbes are used to synthesise a number of products valuable to human beings. Beverages and antibiotics are some examples. Production on an industrial scale requires growing microbes in very large vessels called fermentors (Figure 8.4). Microbes especially yeasts have been used from time immemorial for the production of beverages like wine, beer, whisky, brandy or rum. For this purpose, the same yeast Saccharomyces cerevisiae used for bread-making is used for fermenting malted cereals and fruit juices, to produce ethanol. Depending on the type of the raw material used for fermentation and the type of processing (with or without distillation) different types of alcoholic drinks are obtained. Wine and beer are produced without distillation whereas whisky, brandy and rum are produced by distillation of the fermented broth. Antibiotics produced by microbes are regarded as one of the most significant discoveries of the twentieth century and have greatly contributed towards the welfare of human society.

We know that large quantities of waste water are generated every day in cities and towns. A major component of this waste water is human excreta. This municipal waste-water is also called sewage. It contains large amounts of organic matter and microbes, many of which are pathogenic. Before disposal, sewage is treated in sewage treatment plants (STPs) to make it less polluting. Treatment of waste water is done by the heterotrophic microbes naturally present in the sewage. This treatment is carried out in two stages: Primary treatment involves physical removal of particles from the sewage through filtration and sedimentation. The supernatant forms the effluent. The primary effluent is passed into large aeration tanks where it is agitated mechanically and air is pumped into it, allowing useful aerobic microbes to grow. These microbes consume major parts of the organic matter in the effluent, reducing the BOD (biochemical oxygen demand). Once the BOD of sewage is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. The remaining sludge is pumped into larger tanks called anaerobic sludge digesters, where other bacteria digest the sludge, producing methane and other gases.

Biogas is a mixture of gases (containing predominantly methane) produced by microbial activity and which may be used as fuel. Certain bacteria, which grow anaerobically on cellulosic material, produce large amounts of methane. These bacteria are collectively called methanogens, such as Methanobacterium. Dung can be used for generation of biogas, commonly called gobar gas. The biogas plant consists of a concrete tank in which bio-wastes are collected; a floating cover is placed over the slurry, which keeps rising as the gas is produced in the tank due to microbial activity.

Biocontrol refers to the use of biological methods for controlling plant diseases and pests. In agriculture, there is a method of controlling pests that relies on natural predation rather than introduced chemicals. An example of microbial biocontrol agents that can be introduced to control pests is the bacterium Bacillus thuringiensis (Bt). These are available in sachets as dried spores mixed with water and sprayed onto vulnerable plants. Bacterial diseases will kill the caterpillars, leaving other insects unharmed. Moreover, scientists have introduced B. thuringiensis toxin genes into plants, resulting in crops that are resistant to pests.

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. Rhizobium bacteria form symbiotic associations with leguminous plants, fixing atmospheric nitrogen into organic forms. Other bacteria like Azospirillum and Azotobacter can fix nitrogen while free-living in the soil. Fungi that form mycorrhizal relationships with plants absorb phosphorus and help supply it to the plants, while benefiting from the organic nutrients the plants provide.