Cell culture contamination is a common and challenging problem in the laboratory. Once contamination occurs, it can affect cell growth and morphology, and may also alter cell function and characteristics, leading to unreliable experimental results or even complete loss of previous work. Does contamination always mean that all cells must be discarded? Which types of contamination may still be salvageable? How should the laboratory respond appropriately? In this issue of Cell Culture Academy, we explain how to respond to cell culture contamination from three perspectives: contamination identification, emergency response, and prevention.

Ⅰ. Rapid Identification of Contamination Types

When abnormal cell behavior is observed, first determine the type of contamination before deciding whether the cells should be salvaged or discarded.

1. Bacterial Contamination

Bacterial contamination is the most common type of contamination. It typically takes only 1–3 days from the initial contamination event to obvious proliferation.

Typical signs

Culture medium becomes cloudy rapidly.

The color of the culture medium changes noticeably, usually becoming yellow. This is caused by bacterial acid production, which lowers the pH and turns the phenol red indicator yellow.

Under a microscope, rapidly moving rod-shaped or spherical particles, or densely distributed stationary black dots, may be observed.

2. Fungal Contamination (Molds and Yeasts)

Fungal contamination generally develops more slowly, with obvious signs usually appearing after about one week.

Typical signs

Mold contamination: In the early stage, the culture medium may remain clear and transparent without obvious turbidity, making contamination difficult to detect. As contamination progresses, molds form visible colonies with a fuzzy, filamentous, or flocculent appearance.

Yeast contamination: Under a microscope, yeast appears as individual oval or spherical particles and may form smaller particles through budding.

3. Mycoplasma Contamination

Mycoplasma contamination is one of the most insidious forms of contamination in cell culture. It usually cannot be directly observed using a conventional light microscope and must be confirmed by mycoplasma testing.

Typical signs

The culture medium remains clear and does not develop an unusual odor.

Cell morphology deteriorates and cell attachment decreases.

Cell proliferation slows or cells may become detached and die.

Obvious black dots may appear between cells.

4. “Nanobacteria” Contamination

“Nanobacteria” contamination is a controversial type of contamination. At present, there is no clear definition of what the so-called “Nanobacteria” actually is.

Typical signs

The culture medium remains clear without turbidity.

Numerous black dots and cell debris appear around the cells.

Cell condition deteriorates and proliferation slows.

The black dots persist after medium replacement and may continue to spread with passaging.

5. Cross-Contamination

Although cell-to-cell cross-contamination is less common than other forms of microbial contamination, it is a well-documented and potentially serious problem in cell culture. Because it is difficult to accurately identify by eye or under a microscope, further confirmation is generally required using STR profiling.

Typical signs

Cell morphology differs from its previously established characteristics.

Functional assay results show significant deviations.

The culture medium is usually not turbid.

Ⅱ. Emergency Response After Contamination

Once contamination is detected, immediately isolate the affected cells and stop sharing culture medium, serum, PBS, trypsin, pipette tips, and other reagents or consumables. This helps prevent further spread of contamination.

1. Response by Contamination Type

Take appropriate measures according to the type of contamination:

For mild bacterial contamination, cells may be rinsed twice with 10× Penicillin-Streptomycin Solution or the antibiotic stock solution, followed by continued observation. For severe bacterial contamination, discard the cells after appropriate disinfection and decontamination.

Fungal contamination is difficult to eradicate, and fungal spores can readily become airborne and contaminate the laboratory environment. Once fungal contamination occurs, retaining the cells is not recommended; disinfect and decontaminate the affected area, then discard the cells.

If ells affected by mycoplasma or “Nanobacteria” contamination remain in acceptable condition, Anti-Mycoplasma Treatment Reagent or Anti-Nanobacteria Treatment Reagent may be added and the cells cultured for 2–3 passages while monitoring their condition. If the cells are severely compromised, disinfect and decontaminate the affected area, then discard them.

For cross-contamination, immediately stop using the affected cell line. All contaminated cultures should be discarded, and a new cell stock should be obtained from the original source or a reliable cell bank.

2. Investigate the Source of Contamination

Contamination usually originates from three areas: laboratory operations, reagents, and consumables. Each should be investigated systematically:

Operational factors: Check whether aseptic practices have been followed correctly. For example, was the mouth of a culture medium bottle left open for an extended period? Did a sleeve or arm pass over an open container during handling? Was the biosafety cabinet disinfected in advance before use? Was appropriate laboratory attire worn? Did a pipette tip contact a nonsterile surface during pipetting?

Regent factors: Transfer appropriate amounts of the culture medium and other reagents currently in use, such as serum, PBS, and trypsin, into clean containers separately. Incubate them at 37°C for 2–3 days and observe for turbidity, precipitation, or abnormal color changes. Examine them under a microscope for signs of microbial contamination to assess whether the reagents are contaminated and whether they can continue to be used.

Consumable factors: Check whether the packaging of culture flasks, centrifuge tubes, pipette tips, and other consumables is intact and within its expiration date, and confirm that appropriate sterilization or aseptic processing has been completed before use.

3. Environmental Disinfection and Decontamination

Once the source of contamination has been identified, thoroughly disinfect the relevant areas as soon as possible.

Wipe the surfaces of workstations, biosafety cabinets, and laboratory equipment with 75% alcohol.

For fumigation of the cell culture laboratory, use 10 mL/m² of 40% formaldehyde and 5 g/m² of potassium permanganate.

All culture flasks, serological pipettes, pipette tips, and other items that have come into contact with contaminated cells must be autoclaved before disposal in the medical waste container to prevent secondary spread.

Ⅲ. Prevent Recurrence at the Source

Compared with corrective measures after contamination occurs, standardized daily laboratory management is the most effective way to control cell culture contamination.

1. Standardize Personnel Management

Wear protective equipment such as a lab coat or protective gown and a face mask when entering the laboratory.

Minimize frequent entry and exit from the cell culture laboratory.

Strngthen personnel training and standardize aseptic operating procedures.

2. Strengthen Environmental Management

Disinfect the cell culture room with ultraviolet light before starting work each day and after finishing work. Periodically fumigate the laboratory as part of overall environmental decontamination.

The incubator should be disinfected and sterilized regularly.

Before use, turn on the UV lamp in the aseptic work area for at least 30 minutes. After the experiment, wipe the work surface with 75% alcohol and irradiate it with UV light for 30 minutes.

Commercial products such as Procell Room SafeGuard and Bath SafeGuard can be used for routine contamination prevention.

Laboratory waste must be autoclaved before disposal in the medical waste container.

3. Strictly Implement Cell Culture Laboratory Policies

Continuously improve the rules and procedures for aseptic laboratories and strengthen laboratory management. Visiting trainees, interns, and other temporary personnel should receive relevant cell culture operation training and instruction before entering the laboratory. They should master the necessary basic theory, experimental skills, and operating procedures, and perform experiments under the supervision of an instructor.

Cell culture contamination is an unavoidable challenge in cell culture work. Do not give up on every culture at the first sign of contamination, but do not take chances by trying to rescue severely compromised cultures either. Accurate identification of the contamination type, scientifically sound emergency response, and proactive prevention are the keys to minimizing contamination-related failures and protecting your experimental work.