How to Tell Fungal and Bacterial Contamination Apart in Tissue Culture Vessels
Fungal and bacterial contamination is one of the most important problems in plant tissue culture. Once microorganisms enter a culture vessel, they can compete with plant tissues for sugars and nutrients, alter the culture medium, suppress growth, and eventually cause the entire vessel to fail.
Visual inspection is useful for preliminary screening, but it should not be used to identify a microorganism with certainty. Bacteria, filamentous fungi, and yeasts can sometimes produce very similar colony appearances, especially during the early stages of contamination.
For practical laboratory work, the key is to recognize common visual patterns while understanding their limitations.
What Bacterial Contamination Usually Looks Like
The signs most commonly associated with bacterial contamination include:
Cloudy or hazy culture medium
A thin whitish halo around the plant base or roots
Milky-looking deposits
Slimy or slippery material on the agar surface
Localized cream or yellow colonies
Sediment around the explant
In the earliest stage, contamination may be extremely subtle. A vessel may show only a faint translucent or whitish ring around the cut surface of the explant.
Research on sweet potato collections maintained by the International Potato Center has shown that some internal bacteria can persist in apparently normal cultures without producing obvious contamination symptoms.
For this reason, vessels should be examined against the light and rotated so that the plant base, roots, agar surface, and lower portion of the vessel can all be inspected.
Looking only at the leaves is not enough.
Bacteria Do Not Always Produce the Same Appearance
As bacterial populations increase, their appearance may become more obvious.
The medium may become:
Creamy white
Yellowish
Patchy and cloudy
Slimy around the stem base
Covered with small wet-looking colonies
Associated with white sediment
Research involving the aquatic plant Hygrophila polysperma has reported bacterial contamination ranging from milky-white films and yellow colonies to slimy droplets and dry white deposits.
This is an important reminder that bacterial contamination does not always appear as uniformly cloudy white agar.
A plantlet may still look green and healthy while a small amount of mucus-like material develops around the roots. Within a few days, that area may expand and plant growth may begin to slow.
What Filamentous Fungal Contamination Usually Looks Like
Filamentous fungi are often easier to recognize visually.
Typical signs include:
A small initial spot
Cotton-like growth
Spiderweb-like filaments
Fuzzy or hairy colonies
Circular spreading growth across the agar surface
Many fungal colonies begin as white growth. As they mature and produce spores, they may become green, gray, brown, or black.
For example, if a small white spot appears on the agar and expands within two days into a fuzzy circular colony, fungal contamination should be strongly suspected.
Such vessels should be isolated from the production area immediately.
Colony Color Alone Cannot Identify the Fungus
Color should not be used to identify a fungal species.
A green colony should not automatically be labeled Penicillium, and a black colony should not automatically be called Aspergillus.
Many fungi can produce similar colors, and colony appearance changes with:
Colony age
Culture medium
Temperature
Light
Humidity
Sporulation stage
Yeasts add another layer of complexity. Although yeasts are fungi, they may not produce obvious fuzzy mycelium. Instead, they can form smooth cream-colored colonies or make the medium cloudy in a way that resembles bacterial contamination.
Accurate identification requires additional testing, such as culture isolation, microscopy, and molecular confirmation. ITS sequencing is commonly used for fungal identification.
Browning Is Not Automatically Contamination
Another important distinction is that brown or blackened plant tissue does not always indicate microbial contamination.
Some plants release phenolic compounds after cutting. These compounds oxidize and turn the surrounding medium brown, sometimes resembling weak tea.
This phenomenon is known as phenolic browning.
It is more likely to be plant-derived rather than microbial if:
There is no slime
There is no cloudy halo
No fungal filaments are visible
The brown area does not expand like a colony
New green shoots can still develop
For example, a freshly sterilized explant may turn the agar around its cut base brown within 24 hours while remaining alive and later producing a healthy shoot.
That pattern is very different from contamination that progressively expands through the vessel.
Hidden or Endophytic Contamination Is Especially Difficult
One of the most challenging problems in commercial tissue culture is latent or endophytic contamination.
Microorganisms may already exist inside the mother plant and therefore survive surface sterilization.
A culture may appear completely clean during establishment and even during the first multiplication cycle, only to develop bacterial haze or other symptoms after later subcultures.
Research examining more than 2,000 sweet potato accessions detected internal bacteria in approximately 10% of the samples.
This means that contamination appearing after several passages does not necessarily prove that the most recent operator introduced the microorganism.
The investigation should also consider:
The original mother plant
The source culture
Earlier passages
The explant line
Previous contamination history
Traceability is essential when diagnosing delayed contamination.
Where to Look When Inspecting a Culture Vessel
Effective inspection should include more than the leaves and shoots.
The operator should examine:
The base of the plant
Cut surfaces
The root zone
The agar surface
The sides of the agar
The bottom of the vessel
Condensation patterns
Any unusual sediment or films
A useful production classification is:
Clean — no visible contamination or suspicious symptoms.
Suspected — faint haze, unusual halo, sediment, or other changes that require isolation and observation.
Confirmed contaminated — obvious fungal mycelium, strong bacterial slime, expanding colonies, or clearly deteriorating contaminated medium.
Suspected vessels should be quarantined rather than immediately returned to clean production stock.
Vessels containing obvious fungal contamination should not be opened in the normal subculture area because airborne spores can spread to equipment and other cultures.
A Simple Way to Remember the Difference
For preliminary visual screening, a useful rule is:
Bacteria: haze – cloudiness – milky film – slime – halo around the plant base
Filamentous fungi: spot – filament – fuzz – cotton-like growth – circular spreading colony
This rule is useful for routine screening, but it is not absolute.
Some bacteria form discrete colonies rather than cloudiness. Some yeasts resemble bacteria. Some fungi remain compact before producing visible mycelium. And some cultures may contain latent microorganisms without any obvious visual symptoms.
How to Confirm the Cause
When accurate identification is required, visual inspection should be followed by appropriate laboratory methods.
These may include:
Isolation on microbiological media
Microscopic examination
Biochemical or physiological testing
16S rDNA analysis for bacteria
ITS sequencing for fungi
For commercial production, these results should be linked to the batch code, mother-plant code, passage number, and date when contamination was first detected.
This makes it possible to distinguish between contamination introduced during recent handling and microorganisms that may have originated from the donor plant or an earlier culture generation.
Visual Inspection Is a Screening Tool, Not a Final Diagnosis
The easiest practical distinction is that bacterial contamination often produces cloudiness, halos, milky deposits, and slime, while filamentous fungi more often produce spots, visible threads, fuzzy growth, and spreading circular colonies.
However, visual appearance alone cannot provide reliable species identification.
The most effective production system combines careful daily inspection, quarantine of suspicious cultures, traceability, microbiological testing when necessary, and molecular confirmation for high-value or recurring contamination problems.
In tissue culture, identifying contamination correctly is not only about saving one vessel. It is about understanding where the problem entered the production system and preventing the same contamination from spreading through an entire culture line.
✨ channel for ordering ✨
Facebook Fanpage : ไทยทิชชู – ต้นไม้เพาะเนื้อเยื่อ ( Inbox 📩)
TikTok Shop : https://www.tiktok.com/@thaitissueshop
Shopee : https://shopee.co.th/thaitissue
🌱Other Contacts🌱
☎️ : 06-4475-7495 , 08-8629-4513
Line OA : https://lin.ee/UQFnpoN
Website : https://www.thaitissues.com/
Comments