FOOD MICROBIOLOGY: DETECTION
Diagram
Two states of life, one blind spot in testing
Culturable bacteria
Alive and growing: they form colonies on standard plates, so a plate count finds them and counts them.
↓
VBNC bacteria
Alive but not growing: stress has pushed them into a dormant state where they cannot form colonies, so routine plates miss them.
↓
The risk
A plate that shows no growth does not mean no living bacteria: VBNC cells can wake up again when conditions turn friendly.
Figure 1: Culturable bacteria grow on plates; VBNC bacteria stay alive but do not grow on plates. Drawn in our own style from the opening chapters of Fratamico, Bhunia and Smith, Foodborne Pathogens: Microbiology and Molecular Biology, which cover viable but non-culturable organisms and molecular detection.
Introduction
Viable but non-culturable bacteria, written VBNC, and culturable bacteria are the same kinds of bacteria in two different states. Culturable bacteria are alive and growing, so they form colonies on standard plates and a plate count finds them. VBNC bacteria are alive but not growing: stress has pushed them into a dormant state in which they cannot form colonies, so routine plates miss them, even though the cells are alive, keep their membranes intact, and can wake up again. The difference is not life or death: it is whether the laboratory can grow them.
The FTN notes do not cover the VBNC state, so this article rests on the opening chapters of Fratamico, Bhunia and Smith, Foodborne Pathogens: Microbiology and Molecular Biology, which tackle viable but non-culturable organisms together with molecular detection, and on the FTN Food Microbiology notes, Unit 3, for plate counts. That chapter-level source is flagged here, as the brief asks.
Quick comparison
Table 1: VBNC bacteria compared with culturable bacteria
| Aspect | VBNC bacteria | Culturable bacteria |
|---|---|---|
| Alive | Yes: alive and metabolically active, with intact membranes | Yes: alive and growing |
| Grow on standard plates | No: they cannot form colonies on standard culture plates | Yes: they form colonies, which is what a plate count counts |
| Found by a plate count | No: plate counts miss them, so the count underestimates the living bacteria present | Yes: the plate count is the routine way to count them |
| How the state starts | Environmental stress: cold, starvation, drying, pH change, ultraviolet light, chlorine and disinfectants | Favourable conditions: food, warmth, water and time to grow |
| Cell changes | The cell changes size and its wall and membrane change, and nutrient transport and synthesis slow down | The cell grows and divides normally |
| Can they cause illness again | Yes: they keep their virulence, and they can resuscitate, recovering the ability to grow when conditions turn friendly | Yes: they grow and cause illness in the ordinary way |
| Resuscitation | Possible, within a window of time, when nutrients return, temperature changes, or other stimuli arrive | Not needed: they are already growing |
| Detection | Needs methods beyond plates, such as molecular tests and viability staining, which separate live cells from dead ones | Plate counts, and most probable number tests, which count colonies |
| Where they matter | Food and food processing environments, where stress from chilling, drying, salting, freezing or sanitisers pushes cells into the state | Any food where bacteria are growing and can be counted |
| Risk | A hidden risk: no growth on a plate does not mean no living bacteria, and the cells can wake up later | A visible risk: the count shows the load, and the batch is judged on it |
Scientific explanation
Definition
Viable but non-culturable bacteria are bacteria that are alive but cannot be grown on standard culture media: under environmental stress they enter a dormant state in which they keep their membranes intact, keep their metabolic activity, and keep their virulence, but they do not divide, so they form no colonies and routine tests miss them. Culturable bacteria are bacteria in the ordinary growing state: they divide on standard media, form colonies, and are counted by plate counts and most probable number tests.
Working principle
The VBNC state is a survival tactic of non-spore-forming bacteria in front of environmental stress, and it was first described in 1982. Stressors include unsuitable temperature, high or low, high osmotic concentration, low oxygen, ultraviolet radiation, disinfection such as chlorine, changes in atmosphere during packaging, and food preservation processes such as salting, dehydration and freezing. On entering the state the cell changes: its size falls, and the composition of its wall and membrane, including proteins, fatty acids and peptidoglycan, changes. The cells differ from dead cells because their membranes are not damaged, they keep their DNA, they keep respiratory and metabolic activity, and they hold high levels of ATP, but they do not replicate, so they are not counted by conventional methods such as most probable number or plate count agar, which count colonies grown on a plate. Resuscitation is possible: the cells recover their ability to grow and return their physiology to normal when the environment turns favourable again, through more nutrients, a change in temperature, chemical stimuli, or co-culture with host cells, but only within a window of time.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
Routine culture methods only detect microorganisms able to grow under laboratory conditions, so they miss VBNC cells entirely. Detection needs methods beyond plates: fluorescence-based viability staining, molecular methods such as PMA-qPCR, which separates live cells from dead ones by membrane integrity, and flow cytometry, which counts membrane integrity and metabolic activity cell by cell. The FTN Food Microbiology notes, Unit 3, describe plate counts and molecular tests in general, and the Fratamico opening chapters cover molecular detection beside the VBNC state.
Factors affecting readings
The stress decides whether cells enter the state: cold, starvation, drying, pH change, ultraviolet light and disinfectants such as chlorine all push cells into it, and the same stresses at higher levels kill them instead. Time decides whether they can come back: resuscitation is possible only within a window, after which the cells cannot recover. The food process decides the risk: chilling, freezing, salting, drying and sanitising all apply VBNC stress, so a process that looks like it killed the bacteria may only have pushed them into the dormant state.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- VBNC bacteria: the reason a plate count can underestimate the living bacteria in a food that was chilled, dried, salted or sanitised.
- VBNC bacteria: the reason molecular tests such as PMA-qPCR are used where routine plates show no growth but living cells are suspected.
- VBNC bacteria: the reason E. coli O157:H7, Salmonella, Listeria, Campylobacter and Vibrio are watched even after disinfection, because all can enter the state.
- Culturable bacteria: the bacteria counted by routine plate counts and most probable number tests in food testing.
- Culturable bacteria: the bacteria that enrichment broth multiplies before an ELISA test, so the test can find them.
- Culturable bacteria: the bacteria that starter cultures are, because a starter must grow to ferment the food.
Advantages and limitations
- VBNC bacteria, advantage to the bacteria: the dormant state lets them survive stress that would otherwise kill them, and they can wake up later.
- VBNC bacteria, limitation to the maker: they can be found by molecular tests and viability staining, and the state only lasts within a window.
- Culturable bacteria, advantage to the analyst: they grow on plates, so routine tests find them and count them.
- Culturable bacteria, limitation as a risk: they are exposed to heat, drying and sanitisers, which kill them at the normal dose.
FAQ
What is the main difference between VBNC and culturable bacteria?
VBNC bacteria are alive but cannot form colonies on standard plates, while culturable bacteria are alive and growing, so plate counts find them.
Are VBNC bacteria dead?
No: their membranes are intact, they keep metabolic activity and ATP, and they can wake up again, which dead cells cannot do.
What pushes bacteria into the VBNC state?
Environmental stress: cold, starvation, drying, pH change, ultraviolet light, chlorine and disinfectants, and food preservation steps such as salting, dehydration and freezing.
Can VBNC bacteria cause illness?
Yes: they keep their virulence and can resuscitate when conditions turn friendly, so they are a hidden risk in food.
How are VBNC bacteria detected if plates miss them?
By methods beyond plates, such as viability staining, molecular tests like PMA-qPCR, and flow cytometry, which separate live cells from dead ones.
References and further reading
- Fratamico, Bhunia and Smith, Foodborne Pathogens: Microbiology and Molecular Biology, Caister Academic Press, 2005: opening chapters cover viable but non-culturable organisms and molecular detection. The FTN notes do not cover the VBNC state, so this article rests on that chapter-level source.
- Pina M. Fratamico, Arun K. Bhunia and James L. Smith (eds.), Foodborne Pathogens: Microbiology and Molecular Biology, Caister Academic Press, 2005 (Google Books page read)
