Would clean dry hot air kill bacteria? - (Jun/13/2011 )

Hi there!

I'm having hard time trying to find out the factors influencing the growth of bacteria. I have to say my notions in microbiology are really poor (pre-university studies in engineering sciences), I'm just trying to understand the basic functioning of bacteria.
So basically I'd love to understand if blowing some clean dry hot air along a bacteria sample surface can make the bacteria reduce.
I understand that:
1) Physical properties of an air flow (e.g. speed) will physically take bacteria off the surface where they are;
2) Moisture helps promote bacteria growth, so obviously dry air, i.e. absence of moisture, prevents bacteria growth. But is it possible that dry air REDUCES bacteria presence?

Also, I'm not sure to understand the effect of the air temperature. Obviously, a low-temperature air would prevent bacteria growth, but a hot air makes sure it stays dry. Wouldn't that hot air promote bacteria growth?

Thanks a lot to help me with that,

jakeolson

Air flow is probably only going to be effective if the bacteria are free- ie not in a colony. When bacteria grow they often secrete sugars which they adhere to. Some bacteria also secrete biofilms (think scum in your shower). Thus blowing air will not be effective in these cases.

All bacteria have an optimal temperature at which they grow which is usually related to the environment they inhabit. For example, bacteria that colonize humans like temperature around 37 C.

Dry air will probably reduce growth, but may take some time to be effective and will also depend on the species. All life depends on water.

Also, hot air doesn't necessarily mean its dry air. Think the dessert vs. the jungle; both quite hot, but one more humid than the other!

It also depends on how hot and the type of bacteria? Some bacteria form spores that are quite resistant to heat and dessication. If the air is over about 200 deg C I would have thought this would probably be enough to kill most bacteria.

Wow, thanks a lot for those answers, they definitely precise some points I was totally unsure about.

Just to be sure I understood well, to be pretty certain that the air flow kill the bacteria, it needs to be REALLY dry and REALLY hot, right?

And what about a cold air flow? Let's say an air flow at 5 deg C, blowing on free bacteria that have a growth temperature between 15 and 45 degC? Would the cold temperature be enough to stop bacteria growth? Or would it be even better to also dry it?

Dry air sterilisation is normally done with 160°C for at least two hours. A humid environment such as in a autoclave makes bacteria more susceptible, here 120°C for at least 20 minutes is enough. I guess it's because of the mentioned spore development especially in dry conditions (not sure though, to long ago that I learnt it).
And cold temperatures of course let bacteria more and more develop and growth slower until it stops. Though there are many types that have no problem to survive this. Depends on species and rate of cooling.

All right, I think I get most of it. Looks like it depends mostly on the species of the bacteria concerned, that's a good starting point!

Thanks a lot to you,
jakeolson

cold air wouldn't kill bacteria but it would slow their growth I would have thought. but then with cold air you have the problem of condensation occurring in areas not directly under the air flow, which bacteria would love to sit and reproduce in, think legionnaires disease, famously found in air conditioning systems.

The most efficient way to get sterile air would be to pass it through a filter, the same way the air in category 1 and 2 hoods is sterilised.

The most effective killing method would not be hot dry air, but hot wet air, dry heat is less effective at killing than wet heat, the hot steam can penetrate the aqueous capsules and cell walls that bacteria surround themselves with much better than dry air, hence why autoclaving (around 125 degrees C steam at high pressure for at least 20-30 minutes to allow full penetration) is the preferred method for sterilisation in most circumstances.

And yes everything depends on the species of bacteria, name an environment, you will be able to find a bacteria somewhere in the world that can survive it somehow! Common bacteria are usually killed by autoclaving or standard disinfectant chemicals. In general spore producing bacteria can survive much worse environments than other bacteria