Unit 6: Water and Food Microbiology - Subjective Questions

Water microbiology is the study of microorganisms present in natural, domestic, industrial, and drinking water, including their effects on public health and water quality.

Major sources of microbial contamination include:

• Domestic sewage: Contains fecal bacteria, viruses, protozoa, and helminth eggs.
• Agricultural runoff: Carries animal manure, soil microorganisms, and pathogens from livestock farms.
• Industrial effluents: May contain microorganisms along with organic chemicals and nutrients.
• Stormwater runoff: Washes contaminants from streets, open drains, and waste disposal areas into water bodies.
• Leaking septic tanks and sewer lines: Allow fecal organisms to enter groundwater.
• Decomposing vegetation and animals: Add saprophytic bacteria and fungi to surface water.

Microbial contamination is important because contaminated water can transmit diseases such as cholera, typhoid, dysentery, hepatitis, and gastroenteritis.

Bacterial pollutants are microorganisms introduced into water mainly through fecal matter, sewage, animal waste, and contaminated runoff. Important examples include:

• Escherichia coli: A major indicator of recent fecal pollution and possible presence of enteric pathogens.
• Salmonella species: Cause typhoid fever, paratyphoid fever, and gastroenteritis.
• Shigella species: Cause bacillary dysentery.
• Vibrio cholerae: Causes cholera, characterized by severe watery diarrhea and dehydration.
• Campylobacter species: Cause gastroenteritis and abdominal pain.
• Leptospira species: May enter water contaminated with urine from infected animals and cause leptospirosis.
• Clostridium perfringens: Indicates persistent or old fecal contamination because its spores survive for long periods.

The detection of pathogenic bacteria in water may be difficult because they can occur in low numbers. Therefore, indicator organisms are commonly tested to assess the sanitary quality of water.

Coliform bacteria are a group of Gram-negative, non-spore-forming, rod-shaped bacteria that can ferment lactose with acid and gas production within a specified incubation period. Important characteristics include:

• They are generally facultatively anaerobic.
• They ferment lactose with acid and gas production.
• They are commonly found in the intestinal tract of humans and warm-blooded animals, although some also occur naturally in soil and vegetation.
• Typical members include Escherichia coli, Enterobacter, Klebsiella, and Citrobacter.

Their importance is as follows:

• Total coliforms indicate the general sanitary quality of water.
• Fecal coliforms suggest contamination by fecal material.
• E. coli is considered the most reliable indicator of recent fecal pollution.
• Their presence indicates that pathogenic organisms may also be present.
• They are easier and less expensive to detect than most pathogens.

Coliforms do not always cause disease themselves, but their presence in treated drinking water indicates inadequate treatment or contamination after treatment.

Key distinction: Total coliforms provide a broad indication of sanitary quality, whereas fecal coliforms and E. coli provide stronger evidence of fecal contamination. Non-coliform indicators are useful when coliforms may have died or when more persistent or specific evidence of pollution is required.

The membrane filtration method is a quantitative technique for detecting coliform bacteria in water.

Procedure:

1. A measured volume of water is passed through a sterile membrane filter with pores small enough to retain bacteria.
2. The membrane is placed on a selective and differential culture medium.
3. The plate is incubated under suitable temperature and time conditions.
4. Characteristic colonies are counted and reported as colony-forming units per volume of water.

The result may be expressed as:

Advantages:

• Can process relatively large volumes of water.
• Provides a direct count of colonies.
• Is useful for treated drinking water with low bacterial numbers.
• Colonies can be subjected to confirmatory tests.

Limitations:

• Turbid water may clog the membrane.
• Injured bacteria may fail to grow.
• Non-coliform organisms may sometimes produce similar colonies.
• Proper aseptic technique is essential.

Sewage is the liquid waste discharged from homes, institutions, commercial establishments, industries, and sometimes stormwater systems. Its composition varies with the source, season, water consumption, and industrial activity.

The main components are:

• Water: Usually forms the largest proportion of sewage.
• Organic matter: Includes carbohydrates, proteins, fats, oils, urea, and other biodegradable substances.
• Inorganic matter: Includes sand, grit, salts, chlorides, phosphates, sulfates, and heavy metals.
• Suspended solids: Include fecal matter, food particles, paper, fibers, and other insoluble materials.
• Dissolved solids: Include soluble organic and inorganic substances.
• Microorganisms: Include bacteria, viruses, protozoa, fungi, algae, and helminth eggs.
• Nutrients: Nitrogen and phosphorus compounds may promote eutrophication.
• Toxic substances: Industrial sewage may contain pesticides, solvents, detergents, and metals.

Because sewage contains biodegradable organic matter and pathogens, untreated discharge reduces dissolved oxygen and creates serious risks to human health and aquatic ecosystems.

Sewage decomposition is mainly carried out by microorganisms that break down complex organic matter into simpler substances.

• Hydrolysis: Complex proteins, carbohydrates, and fats are converted into amino acids, sugars, and fatty acids.
• Acid formation: Fermentative bacteria convert these products into organic acids, alcohols, carbon dioxide, and hydrogen.
• Acetogenesis: Organic acids and alcohols are converted into acetate, hydrogen, and carbon dioxide.
• Methanogenesis: Under anaerobic conditions, methanogenic microorganisms convert acetate and hydrogen into methane and carbon dioxide.
• Nitrification: In the presence of oxygen, nitrifying bacteria convert ammonia into nitrite and then nitrate.
• Denitrification: Under anoxic conditions, bacteria reduce nitrate to nitrogen gas.
• Sulfur transformations: Sulfate may be reduced to hydrogen sulfide under anaerobic conditions, causing foul odors.

Aerobic decomposition generally produces less offensive odor and more stable end products, whereas anaerobic decomposition may produce methane, hydrogen sulfide, and other malodorous compounds.

Sewage treatment commonly consists of the following stages:

• Preliminary treatment: Screens and grit chambers remove rags, plastics, sand, and large inorganic particles.
• Primary treatment: Sedimentation removes settleable solids and floating materials.
• Secondary treatment: Microorganisms biologically oxidize dissolved and colloidal organic matter.
• Tertiary or advanced treatment: Removes nutrients, remaining solids, toxic substances, and specific pathogens.
• Disinfection: Chlorine, ultraviolet radiation, or ozone reduces disease-causing organisms.
• Sludge treatment: Sludge may be thickened, digested, dewatered, and safely disposed of or reused.

In secondary treatment, aerobic bacteria, protozoa, and other microorganisms form biological flocs. Bacteria consume organic matter, while protozoa feed on dispersed bacteria and help clarify the effluent. In activated sludge systems, aeration supplies oxygen and promotes microbial growth. In trickling filters, microorganisms grow as a biofilm on a solid support. The organic load is reduced and the treated water becomes more stable.

Both systems use microorganisms to degrade organic pollutants, but activated sludge uses suspended microbial communities, whereas trickling filters use attached microbial films.

Sewage may be disposed of or reused only after suitable treatment. Major methods include:

• Discharge into water bodies: Permitted only after adequate treatment to prevent oxygen depletion and pathogen transmission.
• Land application: Treated sewage or sludge may be applied to land as a nutrient source, provided pathogens and toxic chemicals are controlled.
• Septic tank systems: Used for individual houses or small communities; solids settle and anaerobic digestion occurs in the tank.
• Oxidation ponds: Shallow ponds use bacteria, algae, sunlight, and natural sedimentation to reduce organic matter and pathogens.
• Reuse of treated effluent: May be used for irrigation, industrial processes, or non-potable purposes after appropriate treatment.
• Sludge digestion and disposal: Sludge can be anaerobically digested, composted, incinerated, or placed in a sanitary landfill.

Precautions include:

• Treat sewage before discharge.
• Prevent contamination of drinking-water sources.
• Disinfect final effluent where necessary.
• Monitor microbial indicators and chemical oxygen demand.
• Protect workers from aerosols and infectious materials.
• Avoid land application on crops eaten raw unless strict safety standards are met.

Important groups of food microorganisms include:

• Yeasts: Used in bread, beer, wine, and some fermented foods. They may also spoil fruit juices, syrups, and acidic foods by producing gas and alcohol.
• Bacteria: Lactic acid bacteria are beneficial in yogurt, cheese, pickles, and fermented vegetables. Acetic acid bacteria produce vinegar. Other bacteria may cause food spoilage or disease.
• Molds: Used in the manufacture of some cheeses, soy products, and organic acids. They can also spoil bread, fruits, vegetables, and stored grains.
• Pathogenic bacteria: Salmonella, Staphylococcus aureus, Clostridium botulinum, Listeria monocytogenes, and pathogenic E. coli may cause foodborne illness.
• Viruses: Although they do not multiply in food, viruses such as norovirus and hepatitis A virus may be transmitted through contaminated food.
• Protozoa and helminths: May contaminate raw or inadequately washed foods.

Microorganisms can therefore be useful in food production, harmful through spoilage, or dangerous because of infection and toxin production.

Yeasts are unicellular fungi that reproduce mainly by budding. They grow well in foods containing sugars and can tolerate acidic conditions and relatively low water activity.

Useful applications:

• Saccharomyces cerevisiae produces carbon dioxide that leavens bread.
• Yeasts ferment sugars to ethanol and carbon dioxide in beer and wine production.
• They are involved in the production of some traditional fermented foods.
• Some yeasts contribute desirable flavors and aromas during food maturation.
• Yeast biomass may be used as a source of protein, vitamins, and enzymes.

Spoilage activities:

• Yeasts ferment fruit juices, syrups, and soft drinks, producing gas, alcohol, and off-flavors.
• Film-forming yeasts may grow on the surface of acidic foods.
• Some yeasts cause swelling of packaged foods because of carbon dioxide production.
• They may tolerate preservatives and grow at low pH, where many bacteria cannot grow.

Control measures include pasteurization, refrigeration, reducing available moisture, using approved preservatives, and maintaining hygienic packaging.

Bacteria have both beneficial and harmful roles in food microbiology.

Beneficial roles:

• Lactic acid bacteria such as Lactobacillus, Lactococcus, and Streptococcus thermophilus produce yogurt, cheese, and fermented vegetables.
• Acetobacter oxidizes ethanol to acetic acid during vinegar production.
• Propionic acid bacteria contribute to flavor and eye formation in some cheeses.
• Certain bacteria produce vitamins, enzymes, and antimicrobial compounds.

Harmful roles:

• Spoilage bacteria produce slime, sour odors, discoloration, gas, and texture changes.
• Pseudomonas can spoil refrigerated meat, milk, and fish.
• Bacillus and Clostridium species may survive as spores and spoil heat-processed foods.
• Pathogens such as Salmonella, Listeria monocytogenes, Staphylococcus aureus, and pathogenic E. coli cause foodborne disease.
• Some bacteria produce toxins that remain active even after the bacteria are destroyed.

Temperature control, hygienic handling, adequate heat treatment, prevention of cross-contamination, and proper storage are essential for bacterial control.

Molds are multicellular filamentous fungi composed of hyphae that form a mycelium. They commonly grow on food surfaces, especially when oxygen is available.

Beneficial significance:

• Penicillium roqueforti and Penicillium camemberti are used in cheese ripening.
• Aspergillus oryzae is used in the production of soy sauce and other fermented foods.
• Molds produce enzymes, organic acids, antibiotics, and industrial metabolites.
• Controlled mold growth contributes characteristic color, flavor, and texture to selected foods.

Harmful significance:

• Molds spoil bread, fruits, vegetables, cereals, nuts, and stored foods.
• They cause discoloration, musty odors, softening, and loss of nutritional value.
• Some molds produce mycotoxins, such as aflatoxins, ochratoxin, and patulin.
• Mold toxins may remain in food even after visible mold is removed.

Control involves drying, refrigeration, proper ventilation, prevention of insect damage, hygienic storage, and removal or safe disposal of contaminated products.

Food preservation prevents or delays spoilage and foodborne disease by inhibiting microorganisms, destroying them, or preventing their access to food.

• Low temperature: Refrigeration slows microbial metabolism, while freezing prevents growth by converting water into ice.
• High temperature: Pasteurization reduces vegetative pathogens, and sterilization or commercial processing destroys more resistant microorganisms and spores.
• Drying: Removes available water and lowers water activity, limiting microbial multiplication.
• Salting and sugaring: Increase osmotic pressure and withdraw water from microbial cells.
• Acidification: Low pH inhibits many bacteria and may be achieved by adding acids or using fermentation.
• Chemical preservatives: Compounds such as benzoates, sorbates, nitrites, and sulfites inhibit selected microorganisms when used within legal limits.
• Irradiation: Damages microbial DNA and reduces viable microbial populations.
• Modified-atmosphere packaging: Alters oxygen and carbon dioxide concentrations to slow aerobic spoilage organisms.
• Aseptic packaging: Prevents recontamination after processing.

Several methods are often combined in a process called hurdle technology.

Milk and dairy products are highly nutritious and support the growth of many microorganisms. Common preservation methods include:

• Pasteurization: Heating milk sufficiently to destroy important vegetative pathogens while causing limited changes in flavor and nutrition. Common methods include batch and high-temperature short-time treatment.
• Ultra-high-temperature processing: Uses a higher temperature for a short time and is followed by aseptic packaging, giving extended shelf life.
• Refrigeration: Slows the growth of surviving bacteria, although some psychrotrophic organisms may still grow slowly.
• Fermentation: Lactic acid bacteria convert lactose to lactic acid, lowering pH and producing yogurt, cultured milk, and some cheeses.
• Cheese manufacture: Involves acidification, coagulation, salt addition, reduction of moisture, and often ripening.
• Drying: Produces milk powder with low water activity.
• Addition of permitted preservatives: May be used in selected dairy products under regulatory control.

Hygienic milking, clean equipment, prevention of post-pasteurization contamination, and maintenance of the cold chain are essential for safety.

Meat and fish:

• Refrigeration and freezing slow or stop microbial growth.
• Drying and salting reduce water activity.
• Curing uses salt, nitrite, and sometimes smoke to inhibit microorganisms and develop flavor.
• Smoking provides drying, antimicrobial compounds, and surface protection.
• Canning uses heat processing and hermetic sealing.
• Vacuum or modified-atmosphere packaging reduces oxygen-dependent spoilage, but temperature control remains necessary.

Fruits:

• Refrigeration slows yeasts, molds, and some bacteria.
• Drying, concentration, and sugaring reduce water activity.
• Pasteurization is used for juices and fruit beverages.
• Acidification and approved preservatives inhibit microbial growth.
• Canning and aseptic packaging provide longer storage.

Vegetables:

• Blanching reduces enzymes and surface microorganisms before freezing or canning.
• Freezing preserves quality when preceded by proper blanching.
• Pickling and lactic acid fermentation lower pH.
• Drying and controlled-atmosphere storage reduce spoilage.
• Canning requires adequate heat treatment to control resistant organisms.

The selected method depends on moisture, acidity, texture, target shelf life, and the expected spoilage organisms.

Fermentation is a controlled biochemical process in which microorganisms convert food components, especially carbohydrates, into acids, alcohols, gases, or other products. It improves preservation, flavor, texture, digestibility, and sometimes nutritional value.

General stages include:

1. Selection of raw material: A suitable carbohydrate-rich or nutrient-rich substrate is chosen.
2. Preparation: The material may be washed, crushed, cooked, diluted, or supplemented.
3. Heat treatment or sanitation: Undesired microorganisms are reduced where appropriate.
4. Starter culture addition: A selected yeast, bacterium, or mold is introduced.
5. Controlled incubation: Temperature, pH, oxygen, moisture, and time are controlled.
6. Metabolic conversion: Microorganisms produce acids, alcohol, carbon dioxide, enzymes, or flavor compounds.
7. Termination: Fermentation is stopped by cooling, heating, drying, or filtration.
8. Packaging and storage: The product is protected from contamination and stored under suitable conditions.

Examples include yogurt, cheese, bread, vinegar, wine, beer, pickles, and fermented cereal products.

Yogurt is produced by the lactic acid fermentation of milk. The principal starter organisms are Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus.

Production steps:

• Milk is standardized and commonly heated to improve texture and reduce unwanted microorganisms.
• It is cooled to the inoculation temperature.
• The starter culture is added under hygienic conditions.
• Incubation allows the bacteria to ferment lactose into lactic acid.
• The pH decreases to approximately the acidic range, causing casein coagulation and gel formation.
• The product is cooled to slow further acid production.
• Flavorings or fruit preparations may be added, followed by hygienic packaging.

Importance of fermentation:

• Lactic acid inhibits many spoilage and pathogenic bacteria.
• The product develops a characteristic acidic flavor and smooth texture.
• Partial lactose breakdown may improve digestibility.
• Some fermented dairy products contain beneficial live cultures.

Refrigeration is necessary because surviving microorganisms can continue producing acid and may cause excessive sourness.

Yeasts, particularly Saccharomyces cerevisiae, carry out alcoholic fermentation under conditions with limited oxygen.

Bread production:

• Flour, water, yeast, and other ingredients are mixed to form dough.
• Yeast enzymes break down fermentable sugars.
• The yeast produces carbon dioxide and ethanol.
• Carbon dioxide becomes trapped in the gluten network, causing the dough to rise.
• Baking kills the yeast and evaporates most of the ethanol while setting the bread structure.

Alcoholic beverage production:

• A sugar-containing raw material such as fruit juice, malted grain, or a prepared sugar solution is selected.
• The material is prepared and inoculated with a suitable yeast.
• Yeast converts sugars mainly according to the overall reaction:

• The fermented liquid is clarified, matured, filtered, or distilled depending on the product.
• Packaging and storage protect the beverage from contamination and oxidation.

Temperature, sugar concentration, pH, oxygen availability, and yeast strain influence the final product.