Does this have to do with scripting?
You can achieve this in both ways Grep Replace and/or Scripting.
Choosing Grep Replace will be easy and saves your time while you'd like to use this once/rarely
Otherwise, its better to choose scripting if you really need to do this for many times.
find : ^([^,]+),([^,]+),([^,]+),([^,]+),([^,]+),
change : $1,<b>$2</b>,$3,<b>$4</b>,<b>$5</b>,
(Since you cannot apply bold partially, add remark)
find : <b>([^,]+)</b>,
change : $1, (with bold style)
Now u can get your required output.
2. Scripting: Do the same or try some other way like looping through comma in your fav script.
Hope this will help.
Thanks guys, much appreciated. I'll go give it a try. Nested Style may well be a better options.