A new study by the University of Veterinary Medicine, Vienna (Vetmeduni) revealed that refrigerators host complex microbial communities that can pose a more significant food safety risk than previously understood, and that refrigerator cleaning is possibly more important than temperature control in managing these communities.

Filling Knowledge Gaps About Microbiological Profiles of Refrigerators

Global regulatory agencies recommend operating household refrigerators at temperatures below 4–5 °C to limit the growth of pathogenic microorganisms. While refrigerator temperature profiles in households have been well-studied, to date, an integrative analysis of the entire microbial community within these appliances has been lacking. Researchers at Vetmeduni aimed to fill this gap.

To the researchers’ knowledge, their study is the first to use high-resolution shotgun metagenomics to investigate the microbial communities on refrigerator surfaces in private households. Analysis of swabs from 45 domestic refrigerators not only identified bacteria and fungi down to the species level, but also quantified microbial loads and characterized antimicrobial resistance (AMR) genes.

The project was led by Evelyne Selberherr, Ph.D., D.V.M., Tenured Associate Professor of Microbiome Research in the Center for Food Sciences at Vetmeduni. The findings were published in the journal LWT.

Poor Hygiene More Influential on Microbial Diversity than Temperature Control 

A survey of the temperatures of the home refrigerators reaffirmed an already understood issue. Only 38 percent of the refrigerators examined were below the recommended threshold of 5 °C. Additionally, 24 percent had temperatures exceeding 7 °C, representing what is considered a “worst-case scenario” by the food industry and researchers.

However, temperature alone did not show a significant impact on microbial diversity. Refrigerators with lower temperatures were not automatically microbiologically "cleaner." Instead, the most important factor was determined to be the cleaning frequency, and more specifically, the time since the last cleaning.

The findings suggested that hygiene practices have a more lasting impact than short-term temperature differences, which is an aspect that is largely overlooked in consumer recommendations.

“Our findings suggest that simple measures, such as regular cleaning, have a significant impact on microbiological safety,” said Dr. Selberherr. “These findings are relevant not only for private households, but also for sensitive areas such as hospital kitchens, care facilities, and communal catering. Especially in places where particularly vulnerable groups of people are cared for, the microbiological significance of refrigerators should be more strongly integrated into prevention strategies."

Microbial Risks from Poor Refrigerator Hygiene 

Refrigerators that had not been cleaned for extended periods showed significantly higher microbial loads, lower species diversity, and the dominance of certain highly adaptable microorganisms. This structure is typical of mature, stable microbial communities, such as those found in biofilms. Such communities are more resistant to disturbances and can serve as reservoirs for harmful pathogens.

The composition of refrigerator microbiomes reflected daily life. Cold-tolerant and food-associated bacteria such as Acinetobacter, Pseudomonas, Psychrobacter, and Brochothrix were dominant, complemented by microorganisms from fermented foods and human skin flora.

Concerningly, in 60 percent of the refrigerators examined, potentially pathogenic bacteria were detected. The most common was Bacillus cereus, followed by Staphylococcus aureus. Although Listeria monocytogenes was detected only sporadically, its presence underscores that the home is not a risk-free endpoint in the farm-to-fork continuum.

The high taxonomic resolution provided by metagenomic analysis revealed that, even within dominant genera, there is a great diversity of species, indicating constant new entries into refrigerator microbial communities through food, hands, and surface contact. These findings indicate that cross-contamination, such as through the direct placement of unpackaged food or hand contact, plays a central role.

The Refrigerator as a Reservoir for Antibiotic Resistance

Another concerning finding was the presence of AMR genes in numerous refrigerators, particularly against beta-lactams, tetracyclines, and aminoglycosides. Notably, there was a clear correlation between the age of the refrigerator and the burden of resistance genes. Older appliances showed significantly more robust resistomes, likely due to the long-term accumulation of stable biofilms.

Based on their findings, the researchers stressed the importance of considering private refrigerators in the One Health framework; the refrigerator should not be viewed as a passive storage space but as an active interface for humans, food, and microorganisms.