Curiosity is a rover. Rovers require cleanrooms for assembly for the reasons that you just mentioned, since they land. I don't think this is an issue with orbiters. For example, one of the reasons failing orbiters are of concern is that contamination can be an issue if they crash into the surface. See Japanese orbiter Nozomi, for example: http://www.japantimes.co.jp/news/2003/11/29/national/experts...

Using the expert rule, I can't imagine Indian engineers not being concerned about contamination. It is such an obvious risk that it is probably one of the first things they'd look for. I mean, there is 50 years of precedent available to anyone working in this field.

Edit: Also, they may have done their work in non-cleanroom environments and post that, they may have sterilized the orbiter.

Edit 2: Sorry for the multiple edits, but in spite of Hanlon's razor, I sense an underlying bias. Here's another NASA photo of a satellite assembly, and they are using ungloved hands: http://commons.wikimedia.org/wiki/File:Mars_Reconnaissance_O...

ESA Mercury orbiter: http://sci.esa.int/bepicolombo/50304-removing-the-clamp-band... (I think there's a guy leaning over in the bottom left corner of the pic and probably dripping snot on to the orbiter too.)