‘Frozen’ Microbial Ecology

This past Saturday, my wife picked ‘Frozen’ for movie night (no, we don’t have kids). In the movie, one of the main characters was born with the power to freeze her surroundings, which she struggles to control. By the time, ‘Let It Go’ was playing, I couldn’t help but wonder how the repeated freezing of the castle was impacting the indoor microbiology, which led to Google Scholar (I am a terrible movie watcher) to investigate what we know about the microbial ecology of refrigerators and freezers.

The amount of cold storage we use is staggering (at least to me) — the USDA reports that the US has ~4 billion cubic feet of refrigerated storage. Searching for information on the microbial ecology of refrigerators is difficult – imagine how many studies use some formation of the word ‘refrigerated’ or ‘frozen’ when referring to sample storage and processing.  However, The microbiology of psychrophiles (cold-loving microorganisms) dates back to at least 1902, when Schmidt-Nelson coined the term ‘psychrophile’. Psychrophiles include members of the genera Pseudomonas, Psychrobacter, Staphylococcus, Photobacterium, and Halomonas, among others. The biology behind these microbes is fascinating (anti-freeze proteins!) and they are widely distributed in the environment. Almost all psychrophile ecology research that I was able to identify investigated the impact on food storage and spoilage — however I can’t help but wonder about the microbial ecology of the refrigerated spaces themselves. I’m starting the search in the back of the grad student lunchroom fridge.


2 thoughts on “‘Frozen’ Microbial Ecology

  1. Now I will never be able to watch Frozen without thinking of this. Three weeks ago we rented the movie on a Friday night. Then we watched it again on Saturday and Sunday. And then all through Spring break we watched parts of it almost every day. Just a few minutes ago we were singing the snowman in summer song … But now I will always think of frozen microbes. Speaking of which – what would happen to those microbes if they went in the sun with Olaf?

  2. Kyle,

    Thanks for your post. I think you are a good parent for watching this children’s movie, so don’t be so hard on yourself for being stimulated to do some related, parallel research on line. Your comments on refrigeration are important in relationship to the indoor microbiome

    Refrigeration is a very big, mostly invisible industry, (not withstanding the abundance of freezers in modern American homes and the convenience of frozen food and frozen meals). What occurs on the cooling coils is cold temperature and condensed water vapor. That is how air conditioning works. It is a unique environment in buildings with air-conditioning (e.g., refrigerated air), and the cooled air circulated by the ventilation system all passes by these coils to be chilled. The condensate builds up and drips into what are called the “drip pans.” Visits to many modern office buildings result in typical findings of a microbial slime growing in these drip pans.

    It has been found that air-conditioned (read, ‘refrigerated’ air) buildings have higher prevalence rates of so-called sick building syndrome (also — and more correctly – known as “building related symptoms”) than non-air-conditioned buildings, with or without mechanical ventilation systems. This was the finding of one of the earliest published studies of SBS (Finnegan et al, Br Med J (Clin Res Ed). Dec 8, 1984; 289(6458): 1573–1575) and these findings have been supported by later, more comprehensive studies and literature reviews (e.g., “The available evidence suggests, but does not prove, that better hygiene of air handling systems, and better HVAC
    commissioning, operational checks, training of operators, and maintenance may be particularly important for reducing the risks of SBS symptoms, Indoor Air 2002; 12: 98–112).

    Dick Menzies and his co-workers found lower illness absence among workers in an office building while the cooling coils of the air conditioning system were irradiated with germicidal UV than when they were not.

    Objectives—The indoor environment of
    modern office buildings represents a new
    ecosystem that has been created totally by humans. Bacteria and fungi may contaminate this indoor environment, including the ventilation systems themselves, which in turn may result in adverse health effects. The objectives of this study were to
    test whether installation and operation of germicidal ultraviolet (GUV) lights in central ventilation systems would be feasible, without adverse effects, undetected by building occupants, and effective in eliminating microbial contamination.
    Methods—GUV lights were installed in the
    ventilation systems serving three floors of
    an office building, and were turned on and
    off during a total of four alternating 3 week
    blocks. Workers reported their environmental
    satisfaction, symptoms, as well as
    sickness absence, without knowledge of whether GUV lights were on or off. The indoor environment was measured in detail including airborne and surface bacteria and fungi.
    Results—Airborne bacteria and fungi were not significantly different whether GUV lights were on or off, but were virtually eliminated from the surfaces of the ventilation system after 3 weeks of operation
    of GUV light. Of the other environmental variables measured, only total airborne particulates were significantly different under the two experimental
    conditions—higher with GUV lights on
    than off. Of 113 eligible workers, 104 (87%)
    participated; their environmental satisfaction
    ratings were not different whether GUV lights were on or off. Headache, difficulty concentrating, and eye irritation occurred less often with GUV lights on
    whereas skin rash or irritation was more common. Overall, the average number of work related symptoms reported was 1.1 with GUV lights off compared with 0.9 with GUV lights on.
    Conclusion—Installation and operation of
    GUV lights in central heating, ventilation
    and air conditioning systems of office
    buildings is feasible, cannot be detected by
    workers, and does not seem to result in
    any adverse effects.”
    (Occup Environ Med 1999;56:397–402)

    In a later, follow-up study, they found a clear effect of UVGI.

    Background: Workers in modern office buildings frequently have unexplained work-related symptoms or combinations of symptoms. We assessed whether ultraviolet germicidal irradiation (UVGI) of drip pans and cooling coils within ventilation systems of office buildings would reduce microbial contamination, and thus occupants’ work-related symptoms.
    Methods: We undertook a double blind, multiple crossover trial of 771 participants. In office buildings in Montreal, Canada, UVGI was alternately off for 12 weeks, then turned on for 4 weeks. We did this three times with UVGI on and three times with it off, for 48 consecutive weeks. Primary outcomes of self-reported work-related symptoms, and secondary outcomes of endotoxin and viable microbial concentrations in air and on surfaces, and other environmental covariates were measured six times.
    Findings: Operation of UVGI resulted in 99% (95% CI 67–100) reduction of microbial and endotoxin concentrations on irradiated surfaces within the ventilation systems. 771 participants appeared to remain masked, and reported no adverse effects. On the basis of within-person estimates, use of UVGI was associated with significantly fewer work-related symptoms overall (adjusted odds ratio 0·8 [95% CI 0·7–0·99]), as well as respiratory (0·6 [0·4–0·9]) and mucosal (0·7 [0·6–0·9]) symptoms than was non-use. Reduction of work-related mucosal symptoms was greatest among atopic workers (0·6 [0·5–0·8]), and never-smokers (0·7 [0·5–0·9]). With UVGI on, never-smokers also had large reduction of work-related respiratory (0·4 [0·2–0·9]), and musculoskeletal symptoms (0·5 [0·3–0·9]).
    Interpretation: Installation of UGVI in most North American offices could resolve work-related symptoms in about 4 million employees, caused by microbial contamination of heating, ventilation, and air-conditioning systems. The cost of UVGI installation could in the long run prove cost-effective compared with the yearly losses from absence because of building-related illness.”
    (Lancet 2003; 362: 1785–91)

Leave a Reply

Kyle Bibby

Kyle Bibby is an assistant professor at the University of Pittsburgh.