FOOD MICROBIOLOGY: SAFETY AND QUALITY
PATHOGENIC MICROORGANISMS
Cause illness in people. Often grow with no change in smell, taste or appearance, so a dangerous food can look and taste normal.
SPOILAGE MICROORGANISMS
Damage quality: off flavours, slime, gas and colour change. The food becomes unpleasant, but spoilage alone does not always mean illness.
Figure 1: Pathogens threaten safety with no warning signs, while spoilage organisms threaten quality with visible and smellable signs.
Introduction
Pathogenic and spoilage microorganisms differ in what they do to people and to food. Pathogens cause illness, often with no warning signs, while spoilage organisms damage flavour, smell and texture, and the food becomes unpleasant to eat.
Quick comparison
Quick comparison: Pathogenic and Spoilage Microorganisms
| Aspect | Pathogenic Microorganisms | Spoilage Microorganisms |
|---|---|---|
| Effect on people | Cause foodborne illness: infection or intoxication | Usually do not cause illness; they damage quality |
| Effect on food | Often no change in smell, taste or appearance | Off flavours, slime, gas, sour smells, colour change, soft rot |
| Warning signs | Usually none: a dangerous food can look normal | Clear signs: sour milk, slimy meat, fizzy juice, fuzzy bread |
| Examples | Salmonella, Campylobacter, Listeria monocytogenes, Escherichia coli O157:H7, Clostridium botulinum, Staphylococcus aureus | Pseudomonas on chilled milk and meat, lactic acid bacteria in vacuum-packed meat, yeasts in juice, moulds on bread and fruit |
| Numbers that matter | Even low numbers can cause illness: E. coli O157:H7 has a low infectious dose | Spoilage usually shows when numbers reach about 1,000,000 to 10,000,000 cells per gram |
| What testing finds | Pathogen tests target one species or its toxin | Total counts and specific spoilage organism counts track quality loss |
| Control focus | Cooking, clean water, separation of raw and ready-to-eat food, cold chain | Cold storage, dry storage, packaging, acidity, and shelf life set from the spoiler’s growth |
| Shelf life link | Safety is set by pathogen growth and toxin formation | Shelf life is set by how long the specific spoilage organism takes to reach its spoilage level |
| Overlap | Some organisms do both jobs in different foods | Some spoilage organisms can also cause illness in weak or sensitive people |
Scientific explanation
Definition
Pathogenic microorganisms are microbes that cause disease in people who eat contaminated food. Spoilage microorganisms are microbes that change food so it loses quality: flavour, smell, texture and appearance. Food microbiologists also name the specific spoilage organisms: the particular organisms that actually cause the spoilage in a given food, rather than the whole microbial crowd.
Working principle
Pathogens cause illness by infection, by intoxication, or by toxicoinfection, and they often grow without changing the food, so spoilage signs warn about quality, not about safety. Spoilage organisms break down food parts: Pseudomonas species make off flavours and slime in chilled milk, lactic acid bacteria make sour smells and gas in vacuum-packed meat, yeasts ferment sugars in juice, and moulds digest bread and fruit. A high total count can be harmless if the count is made of organisms that do not spoil that food, and a low total count can still mean spoilage if the few organisms present are strong spoilers.
Apparatus
Pathogen testing uses selective and differential media, biochemical tests, and rapid tests such as PCR and ELISA. Spoilage testing uses total plate counts, yeast and mould counts, and sensory checks of smell, taste and appearance.
Measurement procedure
Counts use plate counts on general or selective media, reported as CFU/g or CFU/ml, with colonies counted in the 30 to 300 range. Pathogen tests report presence or absence in a set sample size, and toxin tests report the toxin found.
Factors affecting readings
Temperature, pH, water activity and packaging decide which group grows: cold storage favours Pseudomonas, vacuum packs favour lactic acid bacteria, and dry storage favours moulds. The food, the pack and the storage temperature together decide which organism is the specific spoiler.
Formulas and calculations
This topic has no standard formula. Spoilage levels are often around 1,000,000 to 10,000,000 cells per gram, and counts use the plate count formula, CFU/g = colonies counted divided by (volume plated in ml x dilution factor).
Industrial applications
- Pathogen control: pasteurization of milk and juice, cooking of minced meat through, HACCP plans, and pathogen testing of ready-to-eat foods.
- Spoilage control: cold chain for milk and meat, modified atmosphere packs for salads and meat, dry storage for grain, and shelf life studies for each product and pack.
- Both: quality assurance labs that run total counts, spoiler counts and pathogen tests side by side.
Advantages and limitations
Pathogenic: advantages for control are that cooking and hygiene break the chain, and targeted tests find them; limitations are that they give no warning signs, low numbers can still cause illness, and some, such as Listeria, grow in the fridge.
Spoilage: advantages are that their signs warn the eater and the maker, and shelf life can be set from their growth; limitations are that they cause food waste and lost sales, and a high count can hide the real spoiler if only the total is watched.
FAQ
If food smells bad, is it unsafe?
Not always. Spoilage signs warn about quality, not about safety. A food can smell fine and still carry pathogens, and a spoiled food may only be unpleasant, not dangerous.
What is a specific spoilage organism?
The particular microbe that actually causes spoilage in a given food and pack, such as Pseudomonas in chilled milk or moulds in bread, rather than the whole microbial crowd.
Can a spoilage organism also cause illness?
Some can, in some foods or in weak or sensitive people, but the usual split is clear: pathogens cause illness, spoilage organisms damage quality.
Why does shelf life follow the spoiler, not the total count?
Shelf life is set by how long the specific spoilage organism takes to reach its spoilage level, often around a million to ten million cells per gram, and that time depends on temperature, pH and packaging.
References and further reading
FTN subjects used: Food Microbiology (Units 1, 2, 3, 4, 5 and 8) and General Microbiology (Units 1, 2, 3, 4, 5, 6 and 10), from the notes at ~/workspace/food-tech-notes/notes/.
Books used: Adams, M. R. and Moss, M. O., Food Microbiology, Royal Society of Chemistry, 2007 (book on disk at food-tech-notes/books/Food-Microbiology.pdf; Google Books page: https://books.google.com/books?id=BZNKI088iooC). Ray, B. and Bhunia, A., Fundamental Food Microbiology, 5th edition, as cited in the FTN notes. Essentials of Industrial Microbiology (book on disk at food-tech-notes/books/Essentials-of-Industrial-Microbiology.pdf).
Google Books pages used: Food Microbiology by Adams and Moss, https://books.google.com/books?id=BZNKI088iooC (contents and chapter list read on 2026-11-11).
Related study notes: Food Microbiology covers microbial growth, foodborne pathogens, spoilage, testing methods and laboratory practice.
