Unit 6: Water and Food Microbiology

BTY102 — Microbiology 10 min read

I. Orientation

Water and food microbiology examines microorganisms that indicate contamination, cause disease or spoilage, perform useful transformations, and respond to preservation methods. The central principle is that microbial growth depends on environmental conditions such as nutrients, moisture, temperature, pH, oxygen availability, and time; controlling these conditions determines whether microorganisms survive, multiply, or become inactive.

Microbial roles: Microorganisms may be pathogenic, indicator organisms, spoilage agents, or beneficial fermenters.
Indicator principle: An indicator organism signals possible contamination without necessarily causing the disease itself; coliform testing is the classic example.
Growth requirements: Bacteria generally multiply rapidly in nutrient-rich, moist foods, while molds tolerate drier and more acidic conditions.
Public-health basis: Water and food are important vehicles for fecal–oral transmission of organisms such as Escherichia coli, Salmonella, Shigella, Vibrio cholerae, and enteric viruses.
Control principle: Safe water and food require source protection, hygienic handling, treatment, preservation, and monitoring.

II. Water Microbiology — Microorganisms, contamination, and treatment

Water microbiology evaluates the biological quality of drinking water, recreational water, wastewater, and natural waters. Testing commonly combines indicator-organism counts, detection of specific pathogens, and assessment of treatment efficiency.

A. Bacterial pollutants of water

Bacterial pollutants enter water mainly through human sewage, animal manure, agricultural runoff, industrial discharge, and storm-water contamination. Their presence can indicate unsafe sanitary conditions or direct disease risk.

Fecal pathogens: E. coli may cause diarrheal disease; Salmonella Typhi causes typhoid fever; Shigella causes bacillary dysentery; and V. cholerae causes cholera.
Environmental pathogens: Pseudomonas aeruginosa can multiply in poorly maintained pools and hospital water systems, while Legionella pneumophila may grow in warm, aerosol-producing water systems.
Transmission route: Contaminated drinking water can carry organisms from feces to the mouth; inadequate chlorination and damaged distribution pipes increase this risk.
Pollution effects: Microbial decomposition consumes dissolved oxygen. A high biochemical oxygen demand (BOD) can produce oxygen depletion and harm aquatic life.
Detection approach: Direct pathogen testing is difficult because pathogens may be unevenly distributed or present in low numbers; indicator testing is therefore used for routine surveillance.

B. Coliforms

Coliforms are Gram-negative, non-spore-forming, rod-shaped bacteria that ferment lactose with acid and gas production within approximately 48 hours at defined test conditions. They are used primarily as indicators of sanitary quality.

Total coliforms: These include organisms from soil, vegetation, and intestinal environments. Their presence in treated drinking water suggests treatment failure, regrowth, or contamination.
Fecal coliforms: These are thermotolerant coliforms associated more closely with intestinal material; E. coli is the preferred specific indicator of recent fecal contamination.
Testing methods: The multiple-tube fermentation method estimates a most probable number (MPN), while membrane filtration counts colonies growing on selective media.
Interpretation: A positive coliform result does not prove that a pathogen is present, but it signals that water treatment or sanitary protection must be investigated.
Example: Detection of E. coli in a municipal sample indicates recent fecal contamination and requires corrective action, such as resampling, source inspection, and possible disinfection.

C. Non-coliforms

Non-coliform organisms are bacteria that do not meet the standard coliform definition but may still indicate pollution, cause disease, or interfere with water quality.

Enteric pathogens: Campylobacter jejuni, Yersinia enterocolitica, and Aeromonas species may be introduced through sewage, animals, or contaminated surface water.
Opportunistic bacteria: Pseudomonas, Acinetobacter, and some Mycobacterium species may survive in water systems and affect vulnerable individuals.
Spore-formers: Clostridium perfringens spores resist environmental stress and treatment; their detection can indicate persistent or historical fecal contamination.
Non-fecal indicators: Heterotrophic plate counts measure general viable bacteria and can reveal changes in distribution-system cleanliness, although they do not directly measure fecal pollution.
Limitation: Indicator organisms may survive differently from pathogens; therefore, no single test completely defines water safety.

D. Sewage composition

Sewage is wastewater containing domestic, industrial, and sometimes storm-water wastes. Its composition determines the treatment method and the environmental danger posed by discharge.

Physical components: Sewage contains about 99% water, with the remainder consisting of suspended solids, dissolved solids, colloids, and floating materials such as grease.
Organic matter: Carbohydrates, proteins, fats, urea, and fecal material provide nutrients for decomposer microorganisms and contribute to BOD.
Inorganic matter: Nitrogen, phosphorus, chlorides, sulfates, detergents, heavy metals, and mineral particles may be present; excess nitrogen and phosphorus promote eutrophication.
Microbial content: Sewage may contain coliforms, enteric pathogens, helminth eggs, protozoan cysts, viruses, and antibiotic-resistant organisms.
Key measurements: BOD measures oxygen consumed by biodegradable material, while chemical oxygen demand (COD) estimates oxygen needed to chemically oxidize organic substances. High values indicate a heavy pollution load.
Solids classification: Settleable solids sink during sedimentation; suspended solids remain dispersed; dissolved substances pass through ordinary settling tanks.

E. Sewage disposal

Sewage disposal aims to remove solids, reduce organic load, destroy pathogens, and prevent contamination of soil and receiving waters. Modern practice favors treatment before discharge or reuse.

Preliminary treatment: Screens remove rags and large debris, while grit chambers remove sand and gravel that could damage equipment.
Primary treatment: Sedimentation separates settleable solids and produces primary sludge; floating grease is skimmed from the surface.
Secondary treatment: Aerobic microbial communities oxidize dissolved and colloidal organic matter. Activated sludge uses aeration tanks followed by secondary settling; trickling filters use microbial biofilms on a support medium.
Tertiary treatment: Filtration, nutrient removal, activated carbon, membrane processes, or advanced oxidation improve effluent quality when discharge standards require them.
Disinfection: Chlorine, ultraviolet radiation, or ozone reduces microbial pathogens. Chlorination is effective but may form disinfection by-products if organic matter remains.
Sludge handling: Anaerobic digestion reduces sludge volume and produces methane-rich biogas; stabilized sludge may be processed for safe disposal or restricted agricultural use.
Natural systems: Oxidation ponds and constructed wetlands use sunlight, sedimentation, microbial activity, and plant uptake, but require substantial land and careful monitoring.

III. Food Microbiology — Food safety, spoilage, and useful microbial activity

Food microbiology studies microorganisms associated with raw materials, processing environments, finished foods, and human health. Microbial effects depend strongly on water activity, acidity, temperature, oxygen, preservatives, and packaging.

A. Important microorganisms in food microbiology

The important groups include pathogens, spoilage organisms, and organisms deliberately used in production. Their significance depends on the food and its storage conditions.

Foodborne pathogens: Salmonella, Staphylococcus aureus, Clostridium botulinum, Listeria monocytogenes, pathogenic E. coli, and Bacillus cereus can cause intoxication or infection.
Spoilage bacteria: Pseudomonas causes slime and off-odors in chilled meat and milk; lactic acid bacteria may sour dairy products; and proteolytic bacteria break down proteins.
Useful organisms: Lactobacillus, Streptococcus thermophilus, Saccharomyces cerevisiae, and selected molds produce acids, alcohol, gases, enzymes, and flavor compounds.
Food factors: Low pH inhibits many pathogens, reduced water activity limits growth, and refrigeration slows multiplication but does not sterilize food.
Safety distinction: A spoiled food may show odor or texture changes, whereas a contaminated food may appear normal while containing toxin-producing organisms.

B. Yeasts

Yeasts are unicellular fungi that reproduce mainly by budding and are especially important in sugary, acidic, or moderately salty foods.

Fermentation: S. cerevisiae converts glucose into ethanol and carbon dioxide:

TEXT

C6H12O6 → 2 C2H5OH + 2 CO2 + energy

Here, C6H12O6 is glucose, C2H5OH is ethanol, and CO2 is carbon dioxide.

Useful applications: Yeast raises bread through CO2 production and produces ethanol in beer, wine, and bioethanol manufacture.
Spoilage activity: Zygosaccharomyces species tolerate high sugar concentrations and can spoil jams, syrups, fruit concentrates, and soft drinks.
Growth traits: Yeasts generally tolerate acidity better than many bacteria and can grow at lower water activity, making them important in concentrated foods.
Control: Heat treatment, hygienic filling, preservatives such as sorbates, refrigeration, and reduced oxygen exposure help control yeast spoilage.

C. Bacteria

Bacteria are the most diverse food microorganisms, including beneficial fermenters, spoilage organisms, and major pathogens. Their growth rate is strongly influenced by temperature, pH, oxygen, and available nutrients.

Lactic acid bacteria: Lactobacillus, Lactococcus, and Leuconostoc convert sugars into lactic acid, lowering pH in yogurt, cheese, pickles, and fermented vegetables.
Pathogenic bacteria: C. botulinum produces botulinum neurotoxin under anaerobic conditions in improperly processed low-acid canned foods; pressure canning is required to destroy resistant spores.
Toxin-mediated illness: S. aureus can produce heat-stable enterotoxins in foods held improperly, while B. cereus may cause emetic or diarrheal disease, especially in cooked rice.
Spoilage: Proteolysis produces ammonia, amines, and sulfur compounds; lipolysis produces rancid flavors. These reactions are common in meat, fish, milk, and eggs.
Control measures: Pasteurization, refrigeration below the growth optimum, acidification, salt or sugar addition, and prevention of cross-contamination reduce bacterial hazards.

D. Molds

Molds are filamentous fungi composed of hyphae that form a visible mycelium. They commonly grow on exposed food surfaces and tolerate conditions that inhibit many bacteria.

Food spoilage: Rhizopus causes soft rot of fruits and bread; Penicillium grows on citrus, bread, and cheese; and Aspergillus may spoil grains and nuts.
Useful molds: Selected Penicillium species ripen blue and soft cheeses, while Aspergillus oryzae is used in soy sauce and miso production.
Mycotoxins: Aspergillus flavus can produce aflatoxins in improperly stored maize, peanuts, and tree nuts. These toxins are chemically stable and require prevention rather than reliance on cooking.
Growth conditions: Molds require oxygen and often grow at low pH and reduced water activity, so surface exposure and damp storage are important risks.
Control: Drying, moisture-proof packaging, temperature control, removal of visibly moldy portions when appropriate, and quality testing limit mold hazards.

E. Preservation of various types of foods

Food preservation prevents or delays microbial growth, toxin formation, enzymatic deterioration, and nutrient loss. Different foods require different combinations of barriers.

Milk and liquid foods: Pasteurization reduces vegetative pathogens; refrigeration slows survivors. Ultra-high-temperature processing gives commercially sterile, shelf-stable milk when aseptically packaged.
Meat and fish: Chilling or freezing slows bacteria, while salting, curing, smoking, drying, vacuum packaging, or modified-atmosphere packaging extends shelf life. Anaerobic packaging requires control of C. botulinum.
Fruits and vegetables: Refrigeration, drying, canning, acidification, irradiation, and approved chemical preservatives are used. Acid foods can often be processed at 100°C, whereas low-acid foods require pressure processing.
Grains and nuts: Drying to low water activity and protection from insects and moisture reduce bacterial and mold growth; storage humidity is a major control factor.
Principle of combined hurdles: Salt, acidity, refrigeration, preservatives, and packaging can be combined so that each barrier is moderate but their total effect prevents growth.

F. Fermented foods

Fermentation is the controlled conversion of food components by microorganisms, producing acids, alcohol, gases, and flavor compounds while often improving preservation and digestibility.

Dairy products: In yogurt, S. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus convert lactose to lactic acid, causing casein coagulation and a pH near 4.5.
Bread and alcoholic beverages: S. cerevisiae produces CO2 in dough and ethanol in beer and wine; yeast strain, sugar concentration, temperature, and oxygen availability influence the result.
Vegetable fermentations: Lactic acid bacteria convert vegetable sugars to acid in sauerkraut and pickles, lowering pH and suppressing many pathogens.
Legume and cereal products: Soy sauce, miso, tempeh, and some traditional beverages use molds, yeasts, and bacteria in sequential or mixed cultures.
Safety requirements: Fermentation must use suitable starter cultures, clean equipment, controlled salt or acidity, and adequate time; uncontrolled fermentation may permit pathogens or toxin producers to grow.