What do bacteria living in mechanical ventilation feed on ?

Submitted by malahchi t3_1118ssa in askscience

Legionella is a clade of bacteria that were discovered living in mechanical ventilation systems. Some countries currently forbid double-flow mechanical ventilation systems because of how much pathogenic microorganism can live in there.

My question is : what do they feed on ? Legionella are cultured in yeast extract agar. What are the sources of food and how can there be enough water in mechanical ventilation systems ?

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AdTechnical8967 t1_j8dvxuo wrote

There is a lot of stuff floating around in the air like human skin,pollen and fungal spores among many other things. Also, unless humidity is at 0%, there will be water in the air. Humidity is high in many ventilation systems because of the aerosols they use. All that stuff can stick to the walls of the ventilation system, creating biofilms where amoeba and other microbes can feed on.

Legionella lives in some of these microbes, absorbing the nutrients they get when they "eat". Legionela needs Iron to survive, so they can easilly get it from the corrotion ocurring from the "weathering" of the ventilation system.

Unless an environment is being constantly sterilized, microbes will eventually grow. They are everywhere, some are also floating in the air around you right now.

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ChimoEngr t1_j8dwptv wrote

Another thing about Legionella, the case that made it famous, was where it was growing in a cooling system. That involves significant changes in the air temperature, resulting in lots of condensation, making it real easy for the micro-organisms to get access to water.

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hausermaniac t1_j8dz6m0 wrote

Replying to AdTechnical8967 (#1,797,784)

FYI just to clarify some of the things you've said:

>All that stuff can stick to the walls of the ventilation system, creating biofilms where amoeba and other microbes can feed on

Biofilms are formed by bacteria, not the other way around. They do contribute to persistence and survival as many of the individual cells within a biofilm are not actively metabolizing, making them need fewer nutrients and making them very hard to kill

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>Legionella lives in some of these microbes

Legionella is a microbe (gram-negative bacterium), and it can live within other organisms (like amoebae) as an intracellular parasite, but it does not necessarily have to

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>Legionela needs Iron to survive, so they can easilly get it from the corrotion ocurring from the "weathering" of the ventilation system

Almost all bacteria need small amounts of iron (and other metals) to survive, but they don't need to be on a surface made of iron to acquire it. They can grow on surfaces made of many different types of materials

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supapoopascoopa t1_j8er07o wrote

There is a lot of water in mechanical ventilation circuits due to condensation from air exhalation from the lungs. In fact many ventilators add humidification to decrease this water loss, which further saturates the circuit.

Warm water will turn into an ecosystem if left undisturbed, especially when exposed to oropharyngeal secretions dripping around an endotracheal tube cuff.

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Onetap1 t1_j8et0de wrote

They don't live in mechanical ventilation. They live in tepid water which has nutrients. Anything with warm water is hazardous.

The first case, where it was identified, killed a lot of US Legionnaires (armed forces veterans) at a convention at a hotel in Philadelphia. The source was an evaporative cooling tower, which uses evaporating water to discard waste heat from the air conditioning systems.

The thing about cooling towers is that the water is recycled from a pond in the base of the tower. It collects insects, dust leaves, limescale, etc, nutrients for bacteria. And the water is warm. The bacteria was distributed on the water droplets leaving the tower.

https://en.wikipedia.org/wiki/Legionnaires%27_disease#Epidemiology

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HavanaWoody t1_j8fasef wrote

Replying to AdTechnical8967 (#1,797,784)

Frikkin skin cells and then the dogs too SOO many and then all the rest pollen The condensate on HVAC in high humid areas like Florida is a thriving jelly menagerie in an endless river of water from the air.

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BooopDead t1_j8fhub3 wrote

Replying to funkyonion (#1,801,403)

Usually shut down the system I imagine. Or reverse the flow and add disinfectants? I have experience inspecting them for mould growth but not any experience actually cleaning or maintaining systems

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moodragonx t1_j8fojsf wrote

Replying to [deleted] (#1,801,644)

This informative and seems to be a well informed response. On the other hand this sounds exactly like what ChatGPT3 would say when prompted with "How do you clean inaccessible ducts?" Which is maybe a compliment.

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navigationallyaided t1_j8fxv76 wrote

Replying to Onetap1 (#1,799,819)

Yep, the water used in the cooling tower is the perfect place for bugs to grow and many commercial HVAC systems don’t have HEPA/UV treatment for the air handlers. You can treat the cooling tower water with quats to suppress bacterial/viral growth. Also, the condensate that forms in the air handlers is also ideal for bugs to grow.

The movement in HVAC is to have separate air handlers and only one cooling tower.

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David_Warden t1_j8g9fpq wrote

Replying to supapoopascoopa (#1,799,699)

Condensation occurs where air meets a surface that is below the dewpoint temperature. This is mainly in winter or in warmer weather on cooling coils when mechanical cooling is needed.

Humidification is mainly used in hospitals and when the outdoor air contains very little moisture. (Winter or Desert). Hospitals have high ventilation rates, sick people and immune compromised people. Without humidification, the humidity in the occupied space can drop far below the optimum for health and comfort of around 50%

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Elfich47 t1_j8g9mon wrote

Which section of the mechanical ventilation system? The only places I could even conceive of legionella growing is in cooling towers that are not properly maintained, and immediately downtream of cooling coils where the condensation pans are plugged and there is standing water.

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JoeFelice t1_j8h8sn4 wrote

Replying to Onetap1 (#1,799,819)

Did the bacteria enter the hotel's ventilation system? Aren't they sealed off from each other? I'm picturing my home cooling unit and an invasive element would have to penetrate the metal coil to get into the house, and repeat that to access the air.

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Pizza_Low t1_j8hcr3p wrote

What I think most the posts right now are missing that condensation itself isn’t enough to sustain life. Moisture is only one part. There is a lot of dust in the air that gets sucked into the vents. Usually hvac systems blend a mix of inside and outside air. Indoor air has a lot of things like skin cells, fibers from clothing and carpets. Outdoor air has dust air pollution pollen and other stuff.

Commercial hvac systems in theory have filters, but they aren’t perfect filters, and roof mounted systems are notorious for having old dirty filters, missing or not properly placed so they intake a lot of unfiltered air.

All that stuff settles in the vents, add moisture and it’s a perfect combination for bacteria to grow. Especially in poorly maintained systems they might have other mechanical issues, such as condenser coils that don’t run a defrost cycle long enough or often enough to prevent the formation of ice or excess condensation build up. Burnt out uv lamps in some commercial hvac systems.

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MidnightSun77 t1_j8hcsmc wrote

Replying to Onetap1 (#1,799,819)

And finding out that Legionella was the main cause was also very lucky as the Hotel had started to clean the ventilation shafts before a proper investigation of them had started.

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oundhakar t1_j8hnw5r wrote

Replying to JoeFelice (#1,805,367)

+1 to this. The cooling tower ought to be in the secondary circuit - a refrigerant is used in the primary circuit to cool the air in the evaporator - the inside unit of the air conditioner, and condensed back to a liquid in the condenser, with the heat being removed by the secondary circuit water. The water gets warmed up and then cooled down again in the cooling tower.

Was the party out on the terrace near the cooling towers where the legionnaires were exposed to the cooling tower spray carried by the wind?

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Onetap1 t1_j8hqn3v wrote

Replying to Elfich47 (#1,803,229)

The cooling towers weren't 'properly maintained' by modern standards because the hazard wasn't recognized.

Cooling towers mostly vanished from AC systems in subsequent decades because the maintenance & chemical water treatment regimes are now so onerous and expensive. Air cooled condensers are more usual now.

Similarly, spray humidifiers were replaced with steam humidifiers, etc..

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Onetap1 t1_j8hqv1t wrote

Replying to JoeFelice (#1,805,367)

They probably inhaled contaminated water droplets outside.

There was an outbreak in Cumbria, UK, 15 years or more back and it was random people passing through the vicinity who were infected.

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Onetap1 t1_j8hrv1m wrote

The post below was something I posted on a UK motoring forum 20 years ago, so I've just copied and pasted it.

https://www.honestjohn.co.uk/forum/post/index.htm?t=6781

I also mentioned car/truck windscreen washer bottles as a legionella risk, which was 8 years before the UK's HSE issued a warning about using screen wash to kill the bacteria.

https://www.bbc.co.uk/news/10293519

"The wet cooling towers are the problem, as stated by many others.

The heat removed by the cooling process is rejected at the condenser, where the refrigerant vapour is condensed into a liquid and it's latent heat of condensation is released.

In wet AC systems the condensers are cooled by water. The water is cooled at the cooling tower and recirculated. The cooling tower ponds tend to collect all the particles in the air, limescale, leaves, insects, etc, and are often at lukewarm temperatures, providing ideal conditions for the growth of many organisms. The water is intentionally sprayed into the air to cause some of it to evaporate, creating an aerosol of water droplets containing any organisms that have been cultivated in the water.

The legionella bacteria is present in water everywhere and will multiply if given favourable conditions.

The risk of infection is much greater for the elderly or infirm, so the disease has affected many hospitals, in the past. The risk wasn?t recognized until a group of elderly ex-servicemen, attending an American Legion Convention were felled by a hotel cooling tower in Philadelphia in 1976. The disease had existed prior to that, but had been regarded as yet another strain of pneumonia.

The (UK) HSE's Code of Practice on Legionnaires' disease, L8, states that a foreseeable risk exists in;

a) water systems incorporating a cooling tower;

b) water systems incorporating an evaporative condenser;

c) hot and cold water systems;

d) other systems containing water at which is likely to exceed 20 degC and which may release a spray or aerosol."

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NibblersNosh t1_j8ib354 wrote

It was my understanding that most Legionella that lives within such environments are actually inside of free-living amoebas, like Acanthamoeba. These cells can phagocytose other bacteria for sustenance, and can ingest nutrients from their surroundings as well. They are much tougher than the more fastidious Legionella bacteria.

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bringmeadamnjuicebox t1_j8j6w9f wrote

Ventilators often run with a heated humidification system. The circuit will collect enough water that it has to be drained. Patients are coughing all sorts of gross crud into the tubing. The circuit also gets broken from time to time, as stuff gets soiled or clogged, parts need to get changed out. So there's often opportunity for bugs to get in there.

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