Fish aquaculture faces important losses as a result of bacterial resistance to antibiotics. Bacteriophages have proven a useful alternative therapy in other domains, but remain to be tested with fish. The interaction between Aeromonas salmonicida HER 1107, bacteriophage HER 110, and brook trout Salvelinus fontinalis was studied in 70‐L aquariums maintained at 9°C. Populations of A. salmonicida (108 colony‐forming units per milliliter) declined by six log units (base 10) in 3 d when bacteriophage HER 110 was introduced in a multiplicity of infection factor of 1. Concentrations of bacteriophages and bacteria in the open water of the aquariums were 2–3 log units lower than those in gravel interstitial water. However, the relative drop in bacterial populations with time was the same in both environments. Addition of the bacteriophage HER 110 delayed by 7 d the onset of furunculosis in brook trout. Further addition of A. salmonicida HER 1107 showed that bacteriophages remained in the aquariums. Mutants of the bacterium were isolated and used as targets for bacteriophage HER 110 and nine other phages. The tests revealed that more than one phage could infect A. salmonicida HER 1107 and that mutants resistant to bacteriophage HER 110 were sensitive to one or more phages. Bacteria resistant to bacteriophage HER 110 had a slower generation time than the original strain, and the success rate of replating in tryptic soy agar (TSA) was very low. More than 25% of the mutants seemed to revert to the original‐strain phenotype after a first replating in TSA. All mutants were sensitive to three or more phages. Finally, stock cultures of 109 plaque‐forming units per milliliter of bacteriophage HER 110 decreased by only one log unit in 80 d when held at 4°C in liquid brain–heart infusion broth culture medium. These results suggest that bacteriophage combinations could be successfully used in preventive programs on fish farms.

The bacterial biota of a methanol-fed denitrification reactor used to treat seawater at the Montreal Biodome were investigated using culture-dependent and molecular biology methods. The microbiota extracted from the reactor carriers were cultivated on three media. Three isolate types were recovered and their 16S ribosomal DNA (rDNA) genes were determined. The analysis showed that the isolate types were related to alpha-Proteobacteria. They are members of the Hyphomicrobium and Paracoccus genera and the Phyllobacteriaceae family. Uncultured bacteria were identified through a 16S rDNA library generated from total DNA extracted from the microbiota. Clones were screened for different restriction profiles and for different DGGE (denaturing gradient gel electrophoresis) migration profiles. More than 70% of clones have the same restriction profile, and the sequence of representative clones showed a relation with the Methylophaga members of the Piscirickettsia family (gamma-Proteobacteria). Sequences from other profiles were related to bacterial species involved in denitrification. The number of species in the denitrification reactor was estimated at 15. Bacterial colonization on newly added carriers in the denitrification reactor was monitored by PCR-DGGE. The DGGE migration profiles evolved during the first 5 weeks and then remained essentially unchanged. PCR-DGGE was also used to monitor the microbial profiles in various aquarium locations. As expected, bacterial populations differed from one location to another, except for the sand and trickling filters which presented similar DGGE migration profiles.

The Montreal Biodome operates a methanol-fed denitrification system that treats the water in its three million litre marine mesocosm. An unknown bacterium, named strain NL21T, was isolated from this system on TSA and R2A agar. The organism is a Gram-negative, rod-shaped (1×3 μm) facultative aerobe. Optimal growth conditions on R2A agar are 30–35 °C, pH 7–7·5 and 1 % (w/w) NaCl. Phylogenetic analysis of the 16S rDNA sequence reveals that strain NL21T forms a novel lineage in the family ‘Phyllobacteriaceae’ within the α2 subgroup of the Proteobacteria. The closest related genera are Aminobacter, Pseudaminobacter, Mesorhizobium and Defluvibacter. Major cellular fatty acids are C18 : 1 ω7c (75 %), C19 : 0 ω8c cyclopropane (9·4 %) and C18 : 0 (4·2 %). The DNA G+C content of strain NL21T (57 mol%) differs from those of all other described members of the ‘Phyllobacteriaceae’ (60–64 mol%). Strain NL21T reduces nitrate to nitrite, but does not reduce nitrite to nitrogen gas. Only a few sugars and amino acids can serve as carbon sources. Strain NL21T is able to grow without salt and tolerates up to 5 % NaCl. Phylogenetic analysis, as well as physiological and biochemical tests, showed that strain NL21T was different from all other members of the ‘Phyllobacteriaceae’ with validly published names. Strain NL21T therefore represents a novel genus, for which the name Nitratireductor aquibiodomus gen. nov., sp. nov. is proposed, with the type strain NL21T (=DSM 15645T=ATCC BAA-762T).

Fish species at risk (n = 29) and not at risk (n = 88) of extinction in the Great Lakes –St. Lawrence biozone were compared to determine if they could be distinguished by their life-history characteristics. A matrix of 51 variables representing 2 phylogenetic, 18 life-history, and 31 ecological variables was compiled from literature sources. Nonparametric analysis shows that 14 variables distinguish between species at risk and not at risk. Logistic regression indicates that seven factors are significant in defining the two groups: age at maturation, feeding habitat, fish feeding regime, feeding substrate, water flow over feeding area, breeding habitat, and breeding substrate. The resulting model is concordant at 97.1% (p = 0.0001, −2 log likelihood = 38.9). The model states that any fish species maturing at 18 years or more is at risk of extinction in the biozone. When applied to a set of fish species from the American Midwest using a decision level of p = 49%, the model had a reclassification success of 77.8%. The modeling technique could be useful to evaluate species at risk and to complement decision making with regard to vulnerability, especially in the case of wide-ranging species such as Atlantic sturgeon (Acipenser oxyrhynchus) and American eel (Anguilla rostrata).

Methylophaga spp. are methylotrophs commonly associated with marine environments and have been defined as strict aerobic methylotrophs. They have been shown previously to represent 50-70% of the bacterial population in the biofilm of the methanol-fed denitrification reactor treating a large seawater aquarium at the Montreal Biodome. It was therefore surprising to find such a high concentration of Methylophaga spp. in anoxic conditions. In this study, we showed by cultivationindependent and -dependent approaches that one Methylophaga strain present in the anoxic biofilm is involved in the denitrification process. DNA stable-isotope probing (SIP) experiments in which the biofilm was cultured under denitrifying conditions with 13 C-methanol have revealed the enrichment of one particular taxon. By screening a 16S ribosomal RNA gene library derived from a 13 C-DNA fraction of the SIP gradients, 62% of the library was composed of one sequence affiliated with the genus Methylophaga. One strain, named JAM1, representing this Methylophaga species was isolated. It grows aerobically but also under denitrifying conditions by reducing nitrate into nitrite. The nitrate-reducing activity was correlated with the presence and the expression of two highly divergent narG genes (narG1 and narG2). narG1 showed a high percentage of identity with the corresponding part of narG found in Thiobacillus denitrificans, which suggests a recent acquisition of narG in strain JAM1 by horizontal gene transfer. This study provides the first direct evidence of the adaptation of a Methylophaga species to an oxygen-limited environment.