FOOD MICROBIOLOGY: BIOFILMS
Diagram
Two ways of life for the same bacteria
Planktonic cells
Single cells floating free in liquid or food. They are what a plate count counts.
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Biofilm
The same cells stuck to a surface, held together by slime they make. The slime shields them from rinsing and cleaning.
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The risk
A biofilm keeps releasing cells into food, so a surface that looks clean can keep contaminating batch after batch.
Figure 1: Planktonic cells float free; biofilm cells stick to a surface in a slime layer. Drawn in our own style from the FTN Food Microbiology notes, Units 1 and 5, and from Romeo, Bacterial Biofilms.
Introduction
Biofilm and planktonic cells are not two kinds of bacteria: they are two ways of life for the same bacteria. Planktonic cells are single cells floating free in liquid or food, while biofilm cells are stuck to a surface and held together by slime they make. The difference matters because the slime shields biofilm cells from rinsing and cleaning, so a surface that looks clean can keep releasing cells into food, batch after batch.
The FTN Food Microbiology notes define a biofilm as a thin layer of microbes stuck to a surface and held together by slime they make, resisting rinsing, and Romeo, Bacterial Biofilms, opens with the same contrast: bacteria thrive predominantly in surface-attached, matrix-enclosed communities, as opposed to isolated planktonic cells. This article follows those two sources.
Quick comparison
Table 1: Biofilm compared with planktonic cells
| Aspect | Biofilm | Planktonic cells |
|---|---|---|
| Form | A thin layer of microbes stuck to a surface, held together by slime they make | Single, isolated cells floating free in liquid or food |
| Where found | On surfaces: pipe joints, cutting boards, cracks, corners and rough steel | Free in milk, water, juice and other liquids, and in food itself |
| Matrix | A slime layer of extracellular material that the cells make, which holds them together | No matrix: each cell is on its own |
| Resistance to cleaning | Resists rinsing and cleaning, because the slime shields the cells inside | Removed by rinsing and cleaning, because nothing shields them |
| Resistance to disinfectants | Shows resistance and persistence to antimicrobial agents during biofilm growth | Killed at the normal dose, because the cells are exposed |
| Detection | Missed by a test of the liquid alone: the cells stay on the surface until they release | Counted by plate counts of the liquid or food |
| Release into food | Keeps releasing cells into food over time, so one biofilm can contaminate batch after batch | Present in the batch at the time of testing, and no more arrive unless more cells enter |
| Example | A biofilm in a pipe joint keeps releasing cells into milk | Free cells in milk, counted by a total plate count |
| Control | Smooth, rounded, openable equipment with nowhere for food to hide, and cleaning that reaches every part | Chilling, heating and hygiene that keep free cells out of the food |
| Why it matters | A plant can pass its liquid tests and still contaminate food from a biofilm on a surface | The count in the food is the count that matters for the batch in hand |
Scientific explanation
Definition
A biofilm is a thin layer of microbes stuck to a surface and held together by slime they make, resisting rinsing: the food example in the FTN notes is a biofilm in a pipe joint that keeps releasing cells into milk. Planktonic cells are the same bacteria in their free form: single, isolated cells floating in liquid or food, with no slime layer and no surface to hold them.
Working principle
Bacteria thrive predominantly in surface-attached, matrix-enclosed, multicellular communities, as opposed to isolated planktonic cells, and the choice of lifestyle is not trivial: it involves major shifts in the use of genetic information and cellular energy, and it has profound consequences for bacterial physiology and survival. Growth within a biofilm can thwart antimicrobial action and complicate treatment, and the book’s chapters cover resistance and phenotypic persistence to antimicrobial agents during biofilm growth. In a food plant the same pattern holds: sharp corners, cracks and rough surfaces trap food and water where microbes grow into biofilms that resist cleaning, while smooth, rounded, openable equipment leaves nowhere for food to hide, so cleaning can reach every part. That is why rounded corners and smooth steel are part of food safety, not just design taste.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
The books used here name no single standard procedure for telling the two apart. The distinction is made from where the cells are found and how they behave: planktonic cells are counted in the liquid or food by the plate count methods of the FTN Food Microbiology notes, Unit 3, while a biofilm is found on a surface that stays contaminated after cleaning, and keeps releasing cells into the food. ATP bioluminescence, which measures the light given off when ATP from living cells meets a test chemical, gives a number in seconds and is used to check cleaning, as the FTN Food Safety and Security notes, Unit 7, describe.
Factors affecting readings
The surface decides whether a biofilm forms: sharp corners, cracks and rough surfaces trap food and water, and a biofilm grows where cleaning cannot reach. Time decides how thick it grows: a surface left uncleaned gives the cells time to make slime and settle. Food residues feed it, and a cleaning schedule that misses a joint or a dead leg leaves the biofilm in place. For planktonic cells, the temperature and the food itself decide growth, as for any free cell.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Biofilm: the reason equipment is built with smooth, rounded, openable surfaces, so cleaning can reach every part.
- Biofilm: the reason pipe joints, cutting boards and dead legs are watched, because a biofilm there keeps releasing cells into milk and other foods.
- Biofilm: the reason cleaning is checked with ATP bioluminescence, which gives a number in seconds after cleaning.
- Planktonic cells: the cells counted by a total plate count of milk, water or juice.
- Planktonic cells: the cells that chilling, heating and hygiene keep out of food.
- Planktonic cells: the form in which a laboratory test finds contamination in a batch.
Advantages and limitations
- Biofilm, advantage to the bacteria: the slime shields them from rinsing, cleaning and disinfectants, so they survive where free cells would die.
- Biofilm, limitation to the maker: it can be prevented by design, with smooth, rounded, openable equipment and cleaning that reaches every part.
- Planktonic cells, advantage to the analyst: they are counted directly by plate counts of the liquid or food.
- Planktonic cells, limitation as a risk: they are exposed, so rinsing, cleaning and disinfectants remove them at the normal dose.
FAQ
What is the main difference between biofilm and planktonic cells?
Biofilm cells are stuck to a surface in a slime layer they make, while planktonic cells are single cells floating free in liquid or food.
Are biofilm and planktonic cells different bacteria?
No: they are two ways of life for the same bacteria, and cells can move between the two forms.
Why is a biofilm hard to clean away?
The slime layer shields the cells inside from rinsing and cleaning, and sharp corners and cracks give it places to hide.
Can a plant pass its liquid tests and still contaminate food?
Yes: a biofilm on a surface keeps releasing cells into food, so the liquid can test clean while the surface keeps contaminating batch after batch.
How is cleaning checked after a biofilm is suspected?
By ATP bioluminescence, which measures the light given off when ATP from living cells meets a test chemical, and gives a number in seconds.
References and further reading
- FTN subject: Food Microbiology, Unit 1: Microorganisms in Food, Sources and Factors; Unit 5: Sanitation, Food Safety Management and Regulations.
- Tony Romeo (ed.), Bacterial Biofilms, Springer, 2010 (Google Books page read)
