Xavier Rubires

Emerging pathogens: Aeromonas spp.

Aeromonas spp. are Gram-negative rods of the family Vibrionaceae. They are normal water inhabitants and are part of the regular flora of poiquilotherm and homeotherm animals. They can be isolated from many foodstuffs (green vegetables, raw milk, ice cream, meat and seafood). Mesophilic Aeromonas spp. have been classified following the AeroKey II system (Altwegg et al., 1990; Carnahan et al., 1991). The major human diseases caused by Aeromonas spp. can be classified in two major groups: septicemia (mainly by strains of A. veronii subsp. sobria and A. hydrophila), and gastroenteritis (any mesophilic Aeromonas spp. but principally A. hydrophila and A. veronii). Most epidemiological studies have shown Aeromonas spp. in stools to be more often associated with diarrhea than with the carrier state; an association with the consumption of untreated water was also conspicuous. Acute self-limited diarrhea is more frequent in young children, in older patients chronic enterocolitis may also be observed. Fever, vomiting, and fecal leukocytes or erythrocytes (colitis) may be present (Janda, 1991). The main putative virulence factors are: exotoxins, endotoxin (LPS), presence of S-layers, fimbriae or adhesins and the capacity to form capsules.

Molecular characterization of a 17-kDa outer-membrane protein from Klebsiella pneumoniae

A cosmid-based genomic library of Klebsiella pneumoniae 52145 (O1:K2) was introduced into Escherichia coli, and clones were screened for the bacteriocin 28b resistance phenotype. One clone was found which conferred partial resistance to bacteriocin 28b. By using Tn5tac1 insertions, it was shown that this phenotype was due to the expression, in E. coli, of an outer-membrane protein (OMP) with an apparent molecular mass of 17 kDa (OmpK17). The DNA region defined by insertion mutagenesis was sequenced and found to contain an ORF of 510 bp. The deduced amino acid sequence has 170 residues with a theoretical molecular mass of 18.4 kDa. The protein contains an N-terminal signal sequence of 24 amino acid residues. When compared with other enterobacterial OMPs, OmpK17 most closely resembles members of a family of small OMPs of Enterobacteriaceae the known functions of which appear to be related to virulence. Immunoblotting experiments showed that OmpK17 is also present in various K. pneumoniae strains belonging to different O and K serotypes.

The role of the capsular polysaccharide of Aeromonas hydrophila serogroup O:34 in the adherence to and invasion of fish cell lines

The ability of Aeromonas hydrophila serogroup O:34 strains grown under different conditions (capsulated and non-capsulated) to adhere to and invade two fish cell lines was compared. The level of adherence was slightly higher when the strains were grown under conditions promoting capsule formation than when the same strains were grown under conditions which did not promote capsule formation. However, the most significant difference among the wild-type strains grown under conditions promoting capsule formation was the ability to invade the fish cell lines, which was significantly higher than when the same strains were grown under conditions which did not promote capsule formation. Isogenic unencapsulated mutants grown under conditions promoting capsule formation showed a lower ability to invade the fish cell lines than the parental capsulated strains. From these results, we concluded that the capsular polysaccharide is an important factor in intracellular invasion.

Mesophilic Aeromonas strains from different serogroups: the influence of growth temperature and osmolarity on lipopolysaccharide and virulence.

Growth of mesophilic Aeromonas sp. strains from serogroups O:13, O:33 and O:44 at different temperatures and osmolarity resulted in changes in the lipopolysaccharide (LPS) and virulence of the strains tested, as we had previously reported for strains from serogroup O:34. The effect of osmolarity could be observed when the cells grew at 37 degrees C but not at 20 degrees C. Purified LPS from cells cultivated at 20 degrees C (high or low osmolarity) or at 37 degrees C at high osmolarity was smooth, whereas the LPS extracted from the cells cultivated on low osmolarity was rough. The smooth strains were resistant to the bactericidal activity of non-immune serum, while the rough strains were sensitive and showed better adhesion to Hep-2 cells than the rough strains. Furthermore, the smooth strains were more virulent for fish and mice than the rough strains. For mesophilic Aeromonas sp. strains from serogroups O:1 to O:44, these changes were not observed, except for serogroups O:13, O:33, O:34 and O:44.