contamination explanation needed - just interested in Microbiology is all (Jul/11/2007 )

I'm just a hi-school student, sorry if stupid question.

Why do they say to keep the cover over the petri plate when streaking to avoid dust, but it's okay to air dry a smear? Doesn't dust get onto that as well? I don't understand. Thanks

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The only stupid questions are the ones you don't ask...
I think there are two points here. First of all, the point of covering a petri dish is to avoid bacterial and fungal spores getting onto the surface.
Second point is more for clarification. What do you mean by a smear? A blood smear?

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I meant the kind of smear before a gram stain. Say I was given an unknown bacteria to id, so I'm looking to see if its positive or negative. So I made that smear and air dried it.
Isn't the slide getting contaminated by stuff in the air while its air drying? Thank you

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Strictly speaking, yes, the smear is getting contamination from anything airborne. But consider what you are doing: The smear will dry in say 10 minutes, then be fixed, stained etc etc. The doubling time for many bugs is about 20 minutes or so, and that's under really good conditions (37C, plenty of nutrients etc).
Any bug that happens to land on the slide (which will be totally swamped numerically by the cells that were applied in the first place) will probably not have completed even one doubling cycle. That's why you'll not see any other bugs, unless you have a mixed culture to start with.
I hope that's cleared things up.

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Oh, okay, that makes sense.... I've been wondering about that for a long time.

Thanks very much!

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