Comparison of some physiological variables of four species of Cytophaga/Flexibacter-like bacteria (CFLB) and the pathogenesis and chemotherapy of diseases caused by some of these pathogens

The physiological requirements of Australian isolates of four species of Cytophaga!Flexibacter-like bacteria(CFLB), namely Cytophaga johnsonae (CJ), Cytophaga psychrophila (CP), Flexibacter columnaris (FC) and Ffexibacter maritimus (FM) were studied. Also, the pathogenesis and chemotherapy of some diseases caused by these bacteria were investigated. In vitro responses of the organisms to environmental conditions, including temperature, salinity and pH, showed that all species have psychrotrophic tendencies with CJ and CP growing at the lowest temperature, FC the highest and CP having the narrowest range of temperature for growth. FM preferred full salinity (seawater) for growth with no growth in the presence of NaCl alone, whereas' the other three strains preferred no salinity for growth. All species grew well over a similar pH range. In vivo assessment of susceptibility of a number of freshwater species of fish including barramundi (Lates calcarifer), goldfish (Carassius auratus), guppy (Poecilia reticulata) and rainbow trout ( Oncorhynchus mykiss) to infections by CJ and CP resulted in occurrence of infection by CJ only in barramundi. This occurred during bath exposure of fish to the organism when it was coupled with thermal stress and was achieved by rapidly reducing the maintenance temperature. No infection could be induced in the other species tested. Barramundi were more susceptible to FC infection than goldfish and the disease was more severe at higher water temperatures than at lower ones. Pathological features were similar in both species with acute necrosis of epithelial surfaces. Experimentally, FM induced infection in Atlantic salmon (Salmo salal), rainbow trout and greenback flounder (Rhombosolea tapirina) only by bath immersion at full or 15‚ÄövÑ‚àû salinity. Distribution of lesions, level of susceptibility and temporal pattern of infection were similar in both salmonids, and larger Atlantic salmon were more resistant to infection than smaller sizes. There was a great consistency in histopathological features of experimental infection in salmonids and in natural outbreaks in a number of captured species, including striped trumpeter (Latris lineata), flounder, yellow-eyed mullet (Aldrichetta forsten) and commercial salmonids. A remarkable lack of inflammatory response, consistent fragmentation and degeneration of the epithelium, with infiltration of amorphous protein-like materials and occasional intra-epithelial inflammatory cells, congestion and haemorrhage were also observed, with invading bacterial cells colonizing dense connective tissue and occasionally the underlying musculature. Scale loss, odema and low degree of inflammation in scale pockets were evident, but the remaining scales were intact. In vitro antimicrobial activity of skin mucus obtained from naive fish against these organisms gave variable results. In vitro and in viva efficacies of commonly-used chemotherapeutants were determined for these pathogens. Treatment of barramundi with oxolinic acid (OA) as a bath (50 ppm) or by mouth (10 mg/kg b w) resulted in serum levels above the minimum inhibitory concentration (MIG) for FC and produced significant clinical efficacy (P<0.05). Amoxycillin (AM) was found to produce adequate serum levels against FM, when used as a bath (200 ppm) or given orally (80 mg/kg b w) to Atlantic salmon and rainbow trout, respectively. At these dose rates this antibiotic was also clinically efficacious against this pathogen (P<0.05). Trimethoprim (TMP) produced more than adequate serum levels for the control of FM when given as a bath (50 ppm) or orally (10 mg/kg b w) to Atlantic salmon and rainbow trout, respectively. TMP was significantly more protective than AM when tested in viva (P<0.05). For CJ and CP the MIG values for OA and oxytetracycline were low, whereas that for TMP was high. MIG values indicated CP strains were more sensitive to AM and norfloxacin than was CJ.