C. sporogenes strain ATCC15579 was grown anaerobically at
37 C in tryptoneeyeast medium (TY). Spores of C. sporogenes were
prepared in Robertson's cooked meat broth (Southern Group Laboratories)
incubated at 37 C for a period of 10 days. Spores,
vegetative cells and debris, were harvested by filtration (20 mm)
and cleaned using discontinuous density gradient centrifugation
(Plowman and Peck, 2002). The pellets were washed six times in
50 ml chilled sterile water, with centrifugation at 2000 g
(10 min). After each centrifugation step the top layer of the pellet,
consisting predominantly of vegetative cells, was discarded and the
bottom layer resuspended. Pellets were finally resuspended in 2 ml
water, layered onto 10 ml 50% (v/v) aqueous solution of sodium/
meglumine diatrizoate (Urografin 370, Schering, Germany) and
centrifuged (6000 g, 4 C, 40 min). The top layers containing
debris, vegetative cells and germinated spores were removed,
while the pellet, consisting predominately of phase-bright spores,
was resuspended in 50 ml water. Pellets were then washed a
further six times in sterile water (2000 g, 4 C, 10 min), resuspended
in 2 ml water and stored at 2 C. Microscopic examination
confirmed that the suspensions consisted of >99% phase-bright
spores. Two distinct crops of spores were tested and gave identical
results. Prior to use, spore suspensions were enumerated using
a haemocytometer and adjusted to a final concentration of
~1 108 spores/ml