Adding antibiotics to cell culture media is one of the most widespread practices in biomedical research, yet a growing body of evidence shows these drugs alter gene expression, metabolism, and cell behavior in ways that can quietly distort experimental results. Penicillin-streptomycin remains the go-to combination in labs worldwide, often added as a default rather than a deliberate choice. The consequences of that habit are more significant than many researchers realize, and the push to move away from routine antibiotic use in culture is gaining momentum.
Why Antibiotics Became Standard in Cell Culture
Cell culture work has always had a contamination problem. Bacteria, fungi, and mycoplasma can infiltrate cultures through airborne particles, contaminated reagents, or simple lapses in sterile technique. When researchers first began growing cells outside the body in the early twentieth century, keeping cultures alive long enough to study them was a constant battle. The introduction of antibiotics into culture media offered a practical safety net, and by the mid-twentieth century, supplementing media with penicillin-streptomycin or gentamicin (often both) had become standard practice in most labs.1Gülhane Medical Journal. The relevance of antibiotic supplements in mammalian cell cultures: Towards a paradigm shift This prophylactic approach was never really questioned for decades. Culture guidelines routinely recommended antibiotic-supplemented media, and the practice became so ingrained that many researchers treated it as a baseline rather than an active choice.2Scientific Reports. Use antibiotics in cell culture with caution: genome-wide identification of antibiotic-induced changes in gene expression and regulation
The logic made sense at the time. Losing a weeks-long experiment to a contamination event is costly, and antibiotics provided cheap insurance. But the assumption behind prophylactic use was that antibiotics were biologically inert toward the mammalian cells being cultured. That assumption has not held up well.
The Most Common Antibiotics Used and What They Do
The workhorse combination in cell culture is penicillin and streptomycin, usually sold as a pre-mixed solution and abbreviated Pen-Strep. Penicillin targets bacterial cell wall synthesis, which mammalian cells lack, so in theory it should have no direct effect on your cultured cells. Streptomycin blocks bacterial protein synthesis by binding to the bacterial ribosome. Gentamicin, another aminoglycoside like streptomycin, is the second most popular choice. Some labs use amphotericin B alongside antibiotics to cover fungal contamination as well.
For selection work in genetic engineering, a different class of antibiotics comes into play. Geneticin (G418), puromycin, hygromycin, and blasticidin are used not to prevent contamination but to kill cells that have not taken up a desired gene construct. These selection antibiotics are a core tool in molecular cloning, and each has a distinct mechanism of action that affects how cells survive the selection process.3Journal of Tropical Life Science. Effect of Combination of Different Antibiotics and Promoters for Expressing Recombinant Darbepoetin in Stable CHO K-1 Cell Line The distinction matters because selection antibiotics are meant to affect cells, while prophylactic antibiotics are supposedly just targeting bacteria. In practice, both categories alter cell biology.
How Prophylactic Antibiotics Change Cells
The most damaging revelation in recent cell culture research is that Pen-Strep does not leave mammalian cells untouched. A genome-wide study found that treating human cells with Pen-Strep at standard concentrations induced sweeping changes in gene expression and gene regulation. The affected pathways were not minor or isolated. They included broad shifts in the regulatory landscape of the cells, altering which genes were turned on or off and how actively their promoter regions were being used.4Scientific Reports. Use antibiotics in cell culture with caution: genome-wide identification of antibiotic-induced changes in gene expression and regulation In other words, the cells growing in antibiotic-supplemented media were not the same, at a molecular level, as cells growing without antibiotics.
Proteomic analysis has added detail to this picture. In liver-derived HepG2 cells, researchers identified 139 unique proteins whose abundance shifted when cells were exposed to Pen-Strep versus antibiotic-free conditions. Some proteins were consistently more abundant in antibiotic-treated cells, while others were consistently suppressed. Among the affected pathways were core metabolic programs and ribosomal machinery, which are about as fundamental to cell function as it gets.5American Chemical Society. Penicillin–Streptomycin Treatment Rewires Core Metabolic and Ribosomal Programs in HepG2 Cells
Gentamicin causes its own set of problems, particularly related to energy metabolism. In cultured breast cell lines, gentamicin at standard working concentrations significantly reduced the mitochondrial membrane potential of certain cell types. It also pushed cells toward aerobic glycolysis, a metabolic shift sometimes called the Warburg effect, by upregulating glucose transporters, glycolytic enzymes, and lactate dehydrogenase. In one mammary cell line, a glucose transporter gene was upregulated nearly nineteen-fold by gentamicin treatment.6PLOS ONE. The adverse effect of gentamicin on cell metabolism in three cultured mammary cell lines: “Are cell culture data skewed?” This is particularly concerning for cancer biology research, where metabolic reprogramming is itself a subject of study. If your antibiotic is pushing cells toward the same metabolic phenotype you are trying to investigate, your results could be deeply misleading.
The Mitochondrial Problem
A unifying explanation for many of these effects comes from an uncomfortable evolutionary fact: mitochondria descended from bacteria. That means antibiotics designed to target bacterial structures sometimes hit mitochondria too. Tetracyclines are the most studied example. Several research groups have shown that tetracyclines disrupt mitochondrial protein production and physiology across a wide range of organisms, from roundworms and fruit flies to mice and human cell lines.7BioEssays. Antibiotic use and abuse: A threat to mitochondria and chloroplasts with impact on research, health, and environment Aminoglycosides like gentamicin and streptomycin also have mitochondrial toxicity because the mitochondrial ribosome shares structural features with the bacterial ribosome these drugs are designed to bind.
When mitochondria are impaired, the downstream effects cascade through the cell. Energy production shifts, reactive oxygen species levels change, and signaling pathways that depend on mitochondrial function get scrambled. For researchers studying anything related to cell metabolism, oxidative stress, or apoptosis, antibiotic-treated cells may be giving skewed readouts from the start.
Stem Cells and Primary Cultures Are Especially Vulnerable
The gene expression changes caused by Pen-Strep are not limited to metabolic genes. Chromatin analysis has revealed that Pen-Strep treatment alters the activity of regulatory regions associated with stem cell differentiation, cell cycle control, and response to reactive oxygen species.8Scientific Reports. Use antibiotics in cell culture with caution: genome-wide identification of antibiotic-induced changes in gene expression and regulation – Section: PenStrep induces differential enrichment of active promoter and enhancer regions marked by H3K27ac For anyone working with stem cells or trying to differentiate cells down a particular lineage, this is a serious confounder. The antibiotic may be nudging differentiation pathways in directions you did not intend and cannot easily detect without running antibiotic-free controls.
Primary cultures, meaning cells taken directly from tissue rather than from an immortalized cell line, tend to be more sensitive to antibiotic toxicity than established cell lines. Studies comparing primary human osteoblasts with osteoblast-derived cell lines found that the primary cells were more inhibited by high antibiotic concentrations, likely because of differences in how primary cells handle energy metabolism compared to cell lines that have been adapted to culture conditions over many passages.9Antimicrobial Agents and Chemotherapy. Influence on Mitochondria and Cytotoxicity of Different Antibiotics Administered in High Concentrations on Primary Human Osteoblasts and Cell Lines If your experimental system relies on primary cells to capture biology that is closer to what happens in living tissue, adding antibiotics may be undermining that very goal.
Selection Antibiotics Have Their Own Pitfalls
Antibiotics used for selecting stably transfected cells introduce a different set of complications. G418 and puromycin are essential tools for ensuring that only cells carrying a resistance gene survive, but the selection process itself can alter the cells you end up studying. In one well-characterized example, drug selection with G418 or puromycin altered expression of genes involved in metabolism, protein folding, and differentiation, and these changes occurred even in untransfected cells exposed to the drugs. Some of the affected transcripts increased by roughly 1.4- to 7-fold.10PubMed. Neomycin and puromycin affect gene expression in Giardia lamblia stable transfection The implication is that part of what researchers attribute to their transgene may actually be an artifact of the selection drug. Good experimental design accounts for this with appropriate controls, but the risk is real when controls are incomplete.
Antibiotic Stability in Culture Media
A practical detail that often gets overlooked is that antibiotics do not last forever once dissolved in culture media. Beta-lactam antibiotics, the family that includes penicillin and ampicillin, are particularly unstable at physiological temperature. Mecillinam, a beta-lactam used in some bacterial culture applications, has a half-life of only four to six hours in growth media at 37°C, with stability varying depending on pH and the concentration of metal ions in the medium.11PLoS ONE. Stability of β-lactam antibiotics in bacterial growth media Penicillin itself degrades over time in culture, meaning the effective concentration of antibiotic drops steadily between media changes.
This degradation creates two problems. First, cells may experience inconsistent antibiotic exposure over the course of an experiment, which makes the biological effects unpredictable. Second, a declining antibiotic concentration can create a window where bacteria are exposed to sub-inhibitory levels of drug, exactly the scenario that promotes the development of antibiotic-resistant organisms. Researchers who add antibiotics with the expectation that they are providing constant protection may actually be providing a selective pressure that favors resistant bacteria over time.
The Antimicrobial Resistance Concern
The connection between laboratory antibiotic use and antimicrobial resistance is not just theoretical. With repetitive antibiotic exposures, bacterial populations adapt, developing tolerance and eventually outright resistance. Research into this process has uncovered convergent evolution across different antibiotic classes, showing that certain resistance strategies emerge again and again.12PubMed Central. Laboratory Evolution of Antimicrobial Resistance in Bacteria to Develop Rational Treatment Strategies Within a lab setting, this means that contaminating organisms can become resistant to the very antibiotics being used to suppress them, eventually overwhelming a culture despite the drugs being present.
There is also an environmental dimension. Antibiotic resistance genes are powerful and convenient markers for genetic engineering work, which means they are present in enormous quantities in research labs worldwide. When biological waste containing these resistance genes is disposed of improperly, or when standard disposal methods fail to fully degrade the DNA, those genes can potentially enter the environment and transfer to pathogenic bacteria. Incomplete degradation of both the antibiotics themselves and the resistance genes may even create selective pressure hotspots in waste streams that promote the uptake and spread of resistance.13Cell Reports Methods. Disposal methods of cell cultures containing recombinant DNA are insufficient to destroy antibiotic-resistance genes The improper discarding of laboratory waste has been identified as a contributor to antibiotic contamination of drinking water sources.14Advances in Environmental and Engineering Research. Environmental and Human Health Impact of Antibiotics Waste Mismanagement: A Review
This broader resistance picture adds weight to the argument that routine prophylactic antibiotic use in culture is not just a data-quality issue for individual labs but a collective responsibility for the research community.
When Antibiotics Are Still Justified
None of this means antibiotics should never be used in cell culture. There are clear situations where they remain appropriate or necessary. Selection antibiotics are fundamentally required for establishing stable cell lines carrying a transgene. Short-term antibiotic treatment to rescue a valuable, irreplaceable culture from a contamination event is reasonable. And in some settings, like teaching labs where sterile technique is still being learned, antibiotics serve as a practical buffer against the high contamination risk that comes with inexperienced hands.
The key shift is from prophylactic use, adding antibiotics by default to every flask, toward targeted use, adding them only when you have a specific reason. For experiments where data quality matters, growing cells in antibiotic-free media removes an entire category of confounders. Researchers who make this switch often find that their contamination rate does not increase substantially, provided they maintain rigorous aseptic technique. The antibiotics were masking sloppy handling more often than they were preventing genuine contamination events that good practice would not have caught.
Mycoplasma and the Limits of Antibiotics
One of the most common and insidious contaminants in cell culture, mycoplasma, is not even effectively controlled by Pen-Strep or gentamicin. Mycoplasma species lack a cell wall entirely, so penicillin is useless against them. These tiny organisms can grow to high titers in culture without causing visible turbidity or pH change, meaning they can persist undetected for months. They alter cell growth rates, gene expression, and immune responses, which makes them a serious experimental variable.
Preventing mycoplasma contamination requires awareness of contamination sources and reliable detection methods, not routine antibiotic supplementation.15PubMed Central. Prevention and detection of Mycoplasma contamination in cell culture Specialized anti-mycoplasma agents exist for treatment purposes, but they are not typically included in standard Pen-Strep cocktails. The fact that the most common prophylactic antibiotics fail against the most common contaminant further weakens the case for routine use. Labs that rely on Pen-Strep as contamination insurance may have a false sense of security about mycoplasma, which ironically can thrive in the absence of competing bacteria that the antibiotics have killed off.
Emerging Alternatives to Conventional Antibiotics
The push away from conventional antibiotics in culture has spurred interest in alternative antimicrobial approaches. Antimicrobial peptides are one promising avenue. In work related to cultivated meat production, where cell culture contamination control is a practical necessity but antibiotic residues in the final product are unacceptable, researchers have tested peptides like nisin and lactoferricin B. Both effectively inhibited bacterial growth for at least three days at working concentrations without showing cytotoxicity toward muscle stem cells or impairing their ability to differentiate into mature muscle tissue.16Elsevier / Food Control. Antimicrobial peptides can substitute antibiotics to control bacterial growth in cultivated meat production without compromising myogenic potential
These peptides work through mechanisms distinct from conventional antibiotics, typically disrupting bacterial membranes rather than targeting specific molecular machinery. Because they do not share mechanisms with the antibiotics used in medicine, they also carry less risk of contributing to clinically relevant resistance. Antimicrobial peptides are not yet widely adopted in standard research cell culture, but they represent one piece of a broader effort to find contamination control methods that do not compromise cell biology or create resistance hazards.
Other strategies being explored include improved media formulations with antimicrobial properties, more rigorous environmental controls in culture facilities, and automated monitoring systems that detect contamination early enough to intervene before it spreads. None of these are silver bullets, but together they point toward a future where antibiotic prophylaxis is the exception rather than the default.
What This Means for Published Research
The accumulated evidence on antibiotic effects in culture raises an uncomfortable question about the existing literature. Decades of cell biology, cancer biology, and drug screening data were generated in antibiotic-supplemented media, often without antibiotic-free controls. If Pen-Strep changes the expression of hundreds of genes and gentamicin shifts cells toward glycolytic metabolism, then published results from antibiotic-treated cultures may reflect drug artifacts layered on top of genuine biology. The scale of the problem is difficult to estimate because most papers do not report whether antibiotics were present in the culture media, and those that do rarely test whether the antibiotics affected the outcome being measured.
This does not mean all published work done with antibiotics is wrong. Many findings are robust enough to survive the noise introduced by antibiotic exposure. But for experiments measuring subtle differences in gene expression, metabolic flux, drug sensitivity, or differentiation efficiency, antibiotic-induced changes could be sitting right in the range of the effect being studied. Researchers revisiting older findings or building on published work would do well to check whether the original experiments used antibiotic-supplemented media and consider whether that matters for interpretation.