Electron microscopic evidence that expression of capsular polysaccharide by Photobacterium damselae subsp. piscicida...

The expression of capsular polysaccharide (CPS) by the fish pathogen Photobacterium damselae subsp. piscicida was analysed in the virulent strain DI 21 in relation to the growth phase and presence or absence of available iron in the culture medium. Bacterial cells were processed for electron microscopy by a procedure that improves visualisation of the capsule through stabilisation with polycationic ferritin, and electron micrographs of ultrathin sections were scanned with an acquired computerised image analyser to measure capsular area. Cells grown under iron-limited conditions always had a significantly lower amount of capsular material on their surfaces than iron-supplemented cells, even when cells from different culture phases were compared. Irrespective of the presence or absence of iron in the culture medium the amount of CPS decreased with the age of the culture, i.e., from early log phase to late log phase to stationary phase. The in vivo significance of this regulatory role of iron remains to be investigated.