The primer pairs were designed to have a
similar thermal melting temperature (Tm), to have minimal
base pairing interactions with other primers in the reaction,
and to yield products that differed from each other by
approximately 100 bp, which could be resolved by agarose
gel electrophoresis. The Tm estimations for the primers
shown in Table 2 were calculated based on nearestneighbor
thermodynamic parameters by the Primer Designer
5a software. The specificity of each of the 18 primers
was initially confirmed using BLASTN (nucleotide database
using a nucleotide query; http://www.ncbi.nlm.nih.
gov/blast/Blast.cgi) and the GenBank sequence database.1
The PCR reaction mixtures contained 18 primers at a
concentration of 0.5 mmol each, 0.2 mmol deoxyribonucleotide
triphosphate mix,b 13 AmpliTaq Gold reaction
buffer II,c 5 mmol MgCl2, and 2.5 units of AmpliTaq Gold
polymerasec in a final volume of 20 ml. For convenience,
master mixes for 50 to 100 tubes containing all the
components of the PCR reaction except the Taq polymerase
and template DNA were prepared in advance. This
master mix was divided into tubes and dehydrated using a
Speed Vac concentrator.d Polymerase chain reaction tubes
prepared this way were stored at 220uC and were stable for
at least 1 year. Crude bacterial DNA template for PCR was
prepared by picking a single colony into 50 ml sterile
distilled H2O and heating at 95–100uC for 10 min.
Template DNA (19.5 ml) was used to reconstitute a
previously prepared PCR tube containing dehydrated
PCR reaction mix, and 2.5 units of AmpliTaq Goldc was
added. Polymerase chain reaction assays were done using a
Perkin-Elmer 2400 thermocycler with the following amplification
conditions: an initial denaturation at 94uC for
10 min, followed by 30 cycles of denaturation for 30 sec at
94uC, annealing at 55uC for 45 sec, and extension for
1.5 min at 72uC. The extension time was increased 3 sec
each cycle, and the final extension was 10 min at 72uC