text: how can you tell if yor specimen is contaminated or just bad technique? my mico lab specimens have been contaminated all semester, now that we are testing for an "unknown", i can't tell what i am doing.  i have gram+ and gram- cocci.  clusters it looks like.  anyhow, i talked to my professor, and she wouldn't even look at my sample, she said it was bad technique.  all of our samples all semester have been contaminated.  this has caused much confusion.  anything else i can do to figure out what kind of bacteria i have? i'm not sure what the difference between contamination and bad technique is. poor sterile technique could lead to contamination. however, maybe the bad technique is in the gram stain? if you destain for too long it may be possible for a gram+ to appear gram-. however, if you have some clusters that are clearly gram + and some that are clearly gram- and they also differ in morphology, it is likely that you have a mixed ("contaminated") sample. you can get a pure culture by streaking your contaminated mix for isolation, picking a single colony and then streaking that colony for isolation on a fresh plate. repeating this process a few times with a single colony should give you a pure culture. assuming your "unknown" is more numerous than the contaminants you should be able to separate it out and repeat your diagnostics tests (gram stain, catalase, hemolysis, etc). hope this helps.