270
Type IV Pilin
Type IV pili are regarded as important virulent determinants among Gram-negative
bacterial pathogens, participating as adhesins. A four gene cluster, tapABCD , from
a virulent Aer. salmonicida has been found to encode proteins with homology to
those necessary for biogenesis of type IV pili (Masada et al. 2002 ). TapA , which was
regarded as ubiquitous among Aer. salmonicida isolates, encoded a protein with
homology to type IV pilin subunits in common with other Gram-negative bacterial
pathogens, e.g. Aer. hydrophila and V. vulnifi cus. A mutant Aer. salmonicida defective in tap A was less pathogenic to rainbow trout following i.p. injection. Tap B is
part of the ABC-transporter family with nucleotide-binding regions; Tap C homologues are cytoplasmic membrane proteins that exert a role in the anchoring and/or
assembly of pili; Tap D has homology with type IV prepilin leader peptidases
(Masada et al. 2002 ). Tap D was capable of restoring type IV pilin assembly and
type II extracellular protein secretion (albeit in Ps. aeruginosa ) and was presumed
to have a similar function in Aer. salmonicida (Masada et al. 2002 ).
Outer Membrane Proteins
In many Gram-negative bacterial pathogens, OMPs exert an important role in virulence. A proteomic analysis of Aer. salmonicida identifi ed 76 unique proteins
including the dominant S-layer Vap protein, >10 porins, phosphoglycerate kinase,
enolase and receptors involved in nutrient acquisition (Ebanks et al. 2005 ).
Capsules
Capsular polysaccharides have been found to develop around cells of Aer. salmonicida in the presence of glucose, phosphate, magnesium chloride and/or trace elements. Production of this material was improved in the presence of yeast extract
(Bonet et al. 1993 ). Interestingly, a striking difference in cells grown in vitro or in
vivo refl ected the presence or absence of capsules. Using intraperitoneal chambers,
it was observed that Aer. salmonicida produced capsules with virulence functions
(Garduño and Kay 1995 ). Adherence to fi sh cell cultures was slightly higher in
cultures of Aer. salmonicida grown in conditions to promote capsule formation.
Also, invasion of fi sh cells was more pronounced in the capsulated cells (Merino
et al. 1996 ). Another role for the capsule concerns resistance to complement. Thus,
it was recorded that when grown under conditions promoting capsule development,
Aer. salmonicida were partially resistant to complement (Merino et al. 1997 ).
5 Aeromonadaceae Representative (Aeromonas salmonicida)
Type IV Pilin
Type IV pili are regarded as important virulent determinants among Gram-negative
bacterial pathogens, participating as adhesins. A four gene cluster, tapABCD , from
a virulent Aer. salmonicida has been found to encode proteins with homology to
those necessary for biogenesis of type IV pili (Masada et al. 2002 ). TapA , which was
regarded as ubiquitous among Aer. salmonicida isolates, encoded a protein with
homology to type IV pilin subunits in common with other Gram-negative bacterial
pathogens, e.g. Aer. hydrophila and V. vulnifi cus. A mutant Aer. salmonicida defective in tap A was less pathogenic to rainbow trout following i.p. injection. Tap B is
part of the ABC-transporter family with nucleotide-binding regions; Tap C homologues are cytoplasmic membrane proteins that exert a role in the anchoring and/or
assembly of pili; Tap D has homology with type IV prepilin leader peptidases
(Masada et al. 2002 ). Tap D was capable of restoring type IV pilin assembly and
type II extracellular protein secretion (albeit in Ps. aeruginosa ) and was presumed
to have a similar function in Aer. salmonicida (Masada et al. 2002 ).
Outer Membrane Proteins
In many Gram-negative bacterial pathogens, OMPs exert an important role in virulence. A proteomic analysis of Aer. salmonicida identifi ed 76 unique proteins
including the dominant S-layer Vap protein, >10 porins, phosphoglycerate kinase,
enolase and receptors involved in nutrient acquisition (Ebanks et al. 2005 ).
Capsules
Capsular polysaccharides have been found to develop around cells of Aer. salmonicida in the presence of glucose, phosphate, magnesium chloride and/or trace elements. Production of this material was improved in the presence of yeast extract
(Bonet et al. 1993 ). Interestingly, a striking difference in cells grown in vitro or in
vivo refl ected the presence or absence of capsules. Using intraperitoneal chambers,
it was observed that Aer. salmonicida produced capsules with virulence functions
(Garduño and Kay 1995 ). Adherence to fi sh cell cultures was slightly higher in
cultures of Aer. salmonicida grown in conditions to promote capsule formation.
Also, invasion of fi sh cells was more pronounced in the capsulated cells (Merino
et al. 1996 ). Another role for the capsule concerns resistance to complement. Thus,
it was recorded that when grown under conditions promoting capsule development,
Aer. salmonicida were partially resistant to complement (Merino et al. 1997 ).
5 Aeromonadaceae Representative (Aeromonas salmonicida)