Is that normal the negative control showing signals? - (Apr/15/2014 )

You have just one melting curve peak, so looks like it is the same product that is amplified in your samples as in your negative controls, so I'd also go for the low level contaminant hypothesis. Presuming that the grey line at the amplification plot is your negative control, it has a Ct of 34, compared with a Ct of 8 for the samples I'd say it's OK. 

Hi xxcici,I was wondering if you managed to fix the problem. I am having the exact same problem, my amplification curves and corresponding melting curves look very much like the ones you posted. Were you able to determine if it a was a contamination issue at last?? 

You can try to reduce the number of the cycles, the primer dimer always appear late in the run ( CT > 35 ). 

Another solution is to try to run it on Gel ( PCR) 

The polymerase used in your PCR can have a big influence on the level of positivity of your negative control. Remember that enzymes are produced recombinantly by bacteria and that, although companies try their best to remove all host DNA from the polymerase, low levels are almost always present. Since the 16S is strongly conserved, the primers used for the 16S of your bacteria of interest are also amplifying the DNA from the polymerase producing bacteria. There are several publications that try to tackle this problem (treat Taq with DNase, use specific low-DNA containing mixes, ...) but I don't have the reference at hand for the moment.

Two simple solutions to your problem: use a bacterium-specific control gene or disregard the problem (if the contamination is minor)

A more difficult solution: optimise your PCR by trying out different polymerases