. Methods
2.1. Preparation of GBS-spiked samples
Streptococcus agalactiae (strain ATCC 12386) was cultured overnight
on Columbia Blood Agar (CBA) at 37 °C in a 10% CO2 atmosphere and resuspended
in sterile phosphate buffered saline (PBS) to an optical density
of ~1.5. Tenfold serial dilutions were prepared with the number of
colony forming units per ml (CFU·ml−1
) ascertained by the spread
plate method (Cruickshank). Triplicate aliquots (100 μl) of duplicate serial
dilutions were plated onto CBA plates. Following overnight incubation
at 37 °C in 10% CO2 atmosphere individual colonies were counted
and the mean CFU·ml−1 count determined. Aliquots (200 μl) of the
GBS suspension were stored at −20 °C until required. Each aliquot of
GBS cells underwent one freeze-thaw cycle only. Aliquots (2.8 ml) of
sterile PBS or whole EDTA-treated human blood were inoculated with
cell suspension (200 μl) containing either 6.3 × 104 cfu·μl
−1 (“high
spike” sample) or 63 cfu·μl
−1 (“low spike” sample) prior to DNA
extraction.