The present study was conducted to examine the morphology
and antigenicity of Photobacterium damselae
subsp. piscicida by culturing the bacterium in vivo in the
peritoneal cavity of sea bass (Dicentrarchus labrax) within
dialysis bags with either a low molecular weight (LMW)
cut-off of 25 kDa or a high molecular weight (HMW)
cut-off of 300 kDa. Differences were observed in the
growth rate between the bacteria cultured in vivo or in
vitro. Bacteria cultured in vivo were smaller and produced
a capsular layer, which was more prominent in bacteria
cultured in the HMW bag. Antigenicity was examined by
Western blot analysis using sera from sea bass injected
with live Ph. d. subsp. piscicida. The sera recognised bands
at 45 and 20 kDa in bacteria cultured in vivo in the LMW
bag. Bacteria cultured in vivo in the HMW bag did not express
the 45 kDa band when whole cell extracts were examined,
although the antigen was present in their extracellular
products. In addition, these bacteria had a
band at 18 kDa rather than 20 kDa. Differences in glycoprotein
were also evident between bacteria cultured in vitro
and in vivo. Bacteria cultured in vitro in LMW and
HMW bags displayed a single 26 kDa band. Bacteria cultured
in the LMW bag in vivo displayed bands at 26 and
27 kDa, while bacteria cultured in vivo in the HMW bag
possessed only the 27 kDa band. These bands may represent
sialic acid. The significance of the changes observed
in the bacterium's structure and antigenicity when cultured
in vivo is discussed.
