The HeLa cell line, derived in 1951 from a cervical cancer biopsy taken from Henrietta Lacks, marked a profound turning point in biomedical science as the first immortal human cell line cultured outside the body. These cells demonstrated an unprecedented ability to divide and multiply indefinitely in a laboratory setting, making them an invaluable tool for research into viruses, cancer, and genetics. This robust nature, however, also introduced a major problem: cell line contamination, where one fast-growing cell type inadvertently infiltrates and overgrows another culture. The HeLa cells’ unique biological advantages made them the most notorious contaminant in the history of cell culture, leading to the misidentification of thousands of cell lines and the invalidation of countless research findings.
Why HeLa Cells Are Uniquely Aggressive
The cellular characteristics that made HeLa cells so valuable to researchers enabled their contamination of other cultures. Normal human cells have a finite lifespan, dividing only about 50 times before they stop replicating, a process known as senescence. HeLa cells, however, are immortal, bypassing this natural limit due to several genetic alterations.
The high activity of the enzyme telomerase is a significant factor. Telomerase works to rebuild the protective caps on the ends of chromosomes, called telomeres. These caps normally shorten with every cell division. By maintaining the length of their telomeres, HeLa cells prevent the DNA damage that signals a cell to stop dividing.
The original tumor was linked to an infection with the Human Papillomavirus (HPV). The HPV genome inserted itself into Ms. Lacks’ DNA near an oncogene, which is a gene that can cause cancer when activated. This accidental insertion scrambled the genetic information in a way that profoundly activated the oncogene, driving the cells to divide at a rapid rate. This hyper-proliferation, combined with a highly unstable genome and an abnormal number of chromosomes (aneuploidy), allowed a single HeLa cell to consistently outcompete any slower-growing cell line it encountered in a shared culture environment.
How Contamination Spread Across Laboratories
The worldwide distribution of this cell line, coupled with poor laboratory practices common at the time, allowed the contamination to spread. In the 1950s and 1960s, cell culture techniques were nascent, and scientists often shared cell lines freely without authentication protocols. The lack of awareness regarding the potential for cross-contamination meant that basic aseptic techniques were often insufficient to contain such a robust cell line.
The cells spread through aerosols produced by pipetting, on unsterilized shared equipment, or through cross-transfer of cells on gloved hands or microscopic droplets. Because HeLa cells grow so quickly, even a single cell introduced into a new culture medium would rapidly take over the flask, replacing the original, slower-growing cells. Researchers would continue their experiments, believing they were studying a liver or breast cancer line, when in reality they were studying HeLa cells.
The true scope of the problem became apparent in the late 1960s and 1970s through the work of geneticist Stanley Gartler, who used isoenzyme analysis to demonstrate that many cell lines shared a specific enzyme variant found only in HeLa. Subsequent testing confirmed that a substantial fraction of human cell lines in use—estimates range from 10% to over 36%—were actually contaminated with HeLa. This revelation exposed decades of research built on misidentified cells, leading to irreproducible results and a significant loss of scientific credibility.
Identifying and Eliminating Contaminated Cultures
The contamination crisis necessitated the development of methods for cell line authentication. The primary scientific solution implemented today is Short Tandem Repeat (STR) profiling, a DNA fingerprinting technique. STRs are short, repeating sequences of DNA that are highly variable among individuals, providing a unique genetic signature for every human cell line.
Scientists amplify a core set of these DNA markers to generate a numerical profile. This profile is then compared against a reference database to confirm the cell line’s identity or to detect the presence of a contaminant like HeLa. This technique is sensitive enough to detect contamination, even when the contaminant makes up as little as 2 to 5% of the culture.
Beyond genetic testing, modern laboratories rely on strict aseptic technique to prevent recurrence. This includes:
- Dedicating media and reagents to a single cell line.
- Working with only one cell line at a time inside the biosafety cabinet.
- Disinfecting all surfaces and equipment with 70% alcohol.
Obtaining cells from accredited cell repositories that provide authentication certificates is also a standard practice. Regular STR testing before experiments and publication ensures that the results produced are based on the cell line the researcher intends to study.