When microbiologist Hannah Gavin started a faculty position at Providence College two years ago, her lab was assigned to a new building. This meant having a say in what type of equipment she could put into it and how to build out her lab space. “Gas lines and open flames were a part of that conversation,” she said, recalling some pushback from a building code and fire code point of view about whether they were necessary. “I essentially said, ‘Absolutely, this is standard in the field of microbiology. Let me go find the literature to demonstrate it,’” she said. “And then I couldn't. I did search after search.”
Gavin, who studies bacteria and bacteriophages in human milk, decided to test the matter. The results, recently published in Microbiology Spectrum, showed that Bunsen burners are not as universally helpful at preventing microbial contamination in experiments as some might believe, potentially changing the way researchers think about aseptic technique.1
To investigate this question about Bunsen burners’ effectiveness, Gavin and her team used a settle plate method. This is an air sampling technique that captures airborne microbes onto an open agar plate sitting on a lab bench for a predetermined amount of time so that any contaminating bacteria can settle on the plate. When Gavin’s team tried the method, they found that they didn’t capture any bacteria after several minutes.
So, they decided to intentionally inoculate the air in the lab with a ubiquitous environmental bacterium. The team did this in two ways. In one, they sprayed bacteria from spray bottles to mimic wet particles, such as those from talking or coughing or from water splashing from sinks. In the other, they soaked a towel in a bacterial culture, let it dry, and then shook it out to mimic bacteria from rustling clothing. For each experiment, researchers stood either in front of a lit Bunsen burner or an unlit one and opened their agar plates to see if they captured any bacteria.
Study authors (Kate DeVincent, Ariana Carretta, and Hannah Gavin, left to right) examine bacterial colony forming units on plates from their Bunsen burner experiments.
Justin James Muir
By introducing microbes into the air, the researchers now found that they picked up a large amount of contamination. “We were trying to represent a span, a range of possible conditions that might be experienced in different laboratories,” said Gavin. “We know that different things transiently can increase airborne contamination and contribute to that.”
Jordan Peccia, an environmental engineer at Yale University who was not involved in the study, mentioned that introducing microbes into the air and seeing whether a Bunsen burner had an impact “seems like it might just be easy to test” but is actually “very hard work to do.” He added, “They took on a hard problem and were able to execute it.” Bryan Heit, an immunologist from Western University who was also not a part of this study, said that Gavin's team “set up a great experiment.”
What the researchers found was that on average, plates next to lit Bunsen burners didn’t have less contamination on them compared to plates next to an unlit burner. Pre-lighting the Bunsen burners before opening the plates also didn’t reduce the number of bacterial colonies on the plate.
Despite this, the researchers did find modest effects depending on how much contamination was in the air. When the rate of contamination from dry particles landing on the control plates exceeded a specific amount—83 colony forming units per minute—Bunsen burners helped reduce the contamination from dry particles. But below this number, having the Bunsen burners on was actually detrimental. The researchers hypothesized that at lower levels of contaminants in the air, burners could generate currents that could pull contaminated air toward the plates.
While the team aimed to generate contaminating particles of different sizes and numbers using their spray bottle and rustling towel techniques, they didn’t measure the resulting particle sizes directly, so it’s not completely clear where their particles fell on the range of particles normally found in laboratory environments. “I think their data is solid here that larger particles are not stopped by a Bunsen burner, but those aren’t the particles that we’re typically thinking about being a concern when we're working with a Bunsen burner,” said Heit.
Still, the results of these experiments echo those of a study from decades ago that showed that flaming the top of a test tube doesn’t reduce contamination.2 “There’s so much myth and lore around a lot of this, and I can understand why it develops,” said Gavin. “Something works once, and you say, ‘Let's not change anything about what we just did.’”
Peccia believes that this paper is going to have a “significant impact” on aseptic technique practices. “If this paper doesn’t completely catalyze health and safety offices to say, ‘You don't need to do this,’ it’s going to catalyze more studies to really show definitively whether this is helpful,” he added.
Now that Gavin’s lab is up and running, she does not have gas lines or Bunsen burners, preferring a biosafety cabinet for experiments that are more sensitive and using a HEPA filter and infrared sterilizers for inoculating loops. “Our baseline level [of contamination] in my lab is also very low, so I don't think it’s supported,” she added.
From this experience, Gavin has one piece of advice for scientists: “Question your methods,” she said. “So many of us have methods that were taught that become habitual, and I think it’s worth fact-checking them.”
- DeVincent K, et al. As a component of aseptic technique, Bunsen burners do not universally decrease deposition of microbe-containing particles. Microbiol. Spectrum. 2026;0:e01028-26.
- Brunker T, Fernāndez B. Effect of flaming cotton-plugged tubes upon the contamination of media during culture transfers. Appl. Environ. Microbiol. 1972:23.
