Editor,—The paper by Therese et al 1 raises several issues which require clarification. The contamination of Taq polymerase by bacterial DNA is now well established in the published press. Taq DNA polymerase is known to be contaminated with low levels of bacterial DNA not originating from either Thermus aquaticus orEscherichia coli and is easily amplified using universal bacterial primers based on ribosomal gene sequences.2-4 Although this level of contamination is insufficient to give a detectable amplification product after just one round of polymerase chain reaction (PCR), it is easily detected following nested amplification. The specific Taq used in the study (AmpliTaq DNA polymerase) is well known for being unsuitable for bacterial PCR using pan-bacterial primers such that the company itself (Perkin-Elmer) has more recently introduced a “low DNA” Taq (Amplitaq LD) in order to reduce the size of the problem. The reduced level of contamination in this Taq is still sufficient to yield positive “negative controls” after two rounds of PCR with eubacterial primers. Therefore, before first round amplification, it is of paramount importance to pretreat the Taq polymerase with restriction enzymes (unpublished observations), and to include the first round negative control as a test sample in the nested PCR reaction. The levels of DNA contamination are easily detectable at the sensitivity (40 fg) for the second round PCR reported by this group of authors and neither in the text nor in the figures is there any mention of a first round negative control as a test sample in the nested PCR reaction. The results submitted by Professor Madhavan’s group reflect PCR in the absence of adequate negative controls and are, therefore, meaningless.

It is also well known that 22–43% of anterior chamber cultures are positive immediately after cataract surgery in patients that subsequently do …