FOOD MICROBIOLOGY: COUNTING MICROBES
TOTAL PLATE COUNT
Counts most bacteria that can grow on a general medium. Tells how many microbes are growing, not which ones.
COLIFORM COUNT
Counts only the coliform group on a selective medium such as violet red bile agar. Signals faecal contamination and poor hygiene.
Figure 1: Total plate count measures the whole crowd, while coliform count measures one indicator group.
Introduction
Total plate count and coliform count answer different questions in a food lab. Total plate count tells how many microbes are growing, while coliform count tells whether hygiene has failed, by counting one indicator group.
Quick comparison
Quick comparison: Total Plate Count and Coliform Count
| Aspect | Total Plate Count | Coliform Count |
|---|---|---|
| What it counts | Most bacteria that can grow on a general medium | Only coliform bacteria |
| Medium | General medium such as plate count agar | Selective medium such as violet red bile agar |
| What it tells | How many microbes are growing | Whether faecal contamination or poor hygiene is present |
| What it does not tell | Which organisms are present | How many microbes there are in total |
| Units | CFU/g or CFU/ml | CFU/g or CFU/ml, or MPN per 100 ml in water |
| Incubation | Set temperature and time for the general population | Set temperature and time for coliforms, often 37 °C |
| Low-number samples | Misses small numbers in water | MPN or membrane filter methods count coliforms in water where numbers are low |
| Use in decisions | Shelf life, general quality, process control | Hygiene action: re-clean, hold, re-test, or reject |
| Limit | Counts only cells that grow on that medium at that temperature in that time | Other coliforms can come from soil and plants, not only from faeces, so E. coli is the stricter faecal signal |
Scientific explanation
Definition
The plate count is the workhorse of food microbiology. A measured amount of diluted sample is spread on, or mixed into, a growth medium in a petri dish, and the plate is incubated at a set temperature for a set time. Each living cell grows into a visible colony, and the colonies are counted and multiplied by the dilution to give colony forming units per gram or per millilitre (CFU/g or CFU/ml). A total viable count uses a general medium and counts most bacteria that can grow, while selective media pick out one group: violet red bile agar counts coliforms.
Working principle
Each colony comes from one colony forming unit in the volume plated. Counting colonies in the reliable range and multiplying by the dilution gives the count in the original food. The coliform count uses the same idea on a selective medium, so only coliforms grow, and the number signals hygiene failure rather than proving disease. Indicator organisms are microbes whose presence signals a hygiene failure: coliforms and E. coli in food or water point to faecal contamination and poor handling, and their count guides action even when no pathogen is found.
Apparatus
Petri dishes, general medium for total count, violet red bile agar for coliforms, pipettes, dilution blanks, incubator, and colony counter. For water, the most probable number (MPN) method uses tubes of lactose broth at several dilutions, and the membrane filter (MF) method filters a measured volume, often 100 ml, through a sterile membrane laid on a medium.
Measurement procedure
For a plate count, dilute the sample, spread or pour a measured volume, incubate at the set temperature for the set time, and count colonies in the 30 to 300 range. Below 30, chance plays a large part; above 300, colonies merge and hide each other, so the count reads low. For MPN, add sample to tubes of broth at several dilutions, count the tubes that turn cloudy or produce gas, and read the pattern against an MPN table. For the membrane filter, filter 100 ml of water, lay the filter on a medium, incubate, and count the colonies per 100 ml.
Factors affecting readings
Both methods count only cells that grow on that medium at that temperature in that time. Cells that are injured, slow growing, or need special conditions may not appear, so the count is always an estimate of the living, growing population, not a census of every cell present. A sampling plan also shapes the verdict: in a three-class plan with n = 5 samples, c = 2, m = 10,000 CFU/g and M = 100,000 CFU/g, the lot passes if no sample is above M and at most 2 samples fall between m and M.
Formulas and calculations
Plate count formula: CFU per gram or per ml = C divided by (V x d), where C is colonies counted, V is volume plated in ml, and d is the dilution factor. Worked example: a plate shows 120 colonies, 0.1 ml was plated, and the dilution was 10 to the power minus 3. Then CFU/g = 120 divided by (0.1 x 0.001) = 120 divided by 0.0001 = 1,200,000, or 1.2 x 10 to the power 6 CFU/g. A second plate from the next dilution should give a similar answer; if it does not, something went wrong in the dilution or the plating, and the test is repeated. Check: a plate with 45 colonies from 1 ml of a 10 to the power minus 4 dilution gives CFU/ml = 45 divided by (1 x 0.0001) = 450,000, or 4.5 x 10 to the power 5 CFU/ml, and 45 colonies sits inside the 30 to 300 range, so the count is reliable.
Industrial applications
- Total plate count: shelf life studies, general hygiene checks, raw milk and meat quality, and process control in dairies and meat plants.
- Coliform count: hygiene testing of milk, water, ice cream and ready-to-eat foods; drinking water testing by MPN and membrane filter.
- Both: release, hold, re-clean, re-test or reject decisions in food quality labs.
Advantages and limitations
Total plate count: advantages are a simple number for general quality and shelf life, and a wide view of the growing population; limitations are that it does not tell which organisms are present, and a high count can be harmless while a low count can still hide a strong spoiler.
Coliform count: advantages are a fast hygiene signal and a clear action trigger; limitations are that it misses non-coliform pathogens, and some coliforms come from soil and plants, not only from faeces.
FAQ
Why count coliforms if the total count is already high?
The total count tells how many microbes are growing, not which ones. The coliform count tells whether hygiene has failed and faecal contamination is likely, which the total count alone cannot show.
Why are plates with fewer than 30 or more than 300 colonies unreliable?
Below 30, chance dominates: one cell more or less changes the result a lot. Above 300, colonies merge and hide each other, so the count reads low.
When is MPN used instead of a plate count?
When numbers are low or the food cannot be plated easily, such as water testing. MPN is less precise than a plate count but it finds small numbers.
Why filter 100 ml of water instead of plating 1 ml?
Drinking water should carry very few coliforms, so 1 ml may hold none even when the water is contaminated. Filtering 100 ml gathers the cells from a large volume onto one filter.
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.
