Dietary supplementation of yeast or yeast subcomponents (YYS) as commercial preparations of β‐glucan (MacroGard®; Biotec‐Mackzymal, Tromsø, Norway; and Betagard A®; Aqua‐In‐Tech, Inc., Seattle, WA, USA), mannan oligosaccharide (Bio‐Mos™ Aqua Grade; Alltech, Nicholasville, KY, USA), or whole‐cell Saccharomyces cerevisiae (Levucell SB20®; Lallemand Animal Nutrition, Milwaukee, WI, USA) at the manufacturer’s recommended levels was evaluated on the physiological performance of juvenile channel catfish, Ictalurus punctatus. Fish were fed YYS diets for 4 wk, followed by 2 wk of control diet. Fish were sampled at the end of each feeding period (4 and 6 wk) to measure hematological and immune parameters and growth and to determine the effects of dietary β‐glucan on resistance to Edwardsiella ictaluri infection and to low‐water stress (6 wk). Supplementation of YYS in diets did not affect growth performance, hematology, or immune function. Survival from E. ictaluri infection was from 5 to 17.5% higher in fish fed YYS diets than in the control group, but the increases were not significant. Some improvement in stress resistance was observed in YYS‐fed catfish after exposure to low‐water stress. Stress reduction in fish fed diets supplemented with yeast subcomponents has been reported previously, but thus far, no explanation has been proposed for this effect. The present study and the previously published research suggest that dietary YYS supplementation does not appear to improve resistance of channel catfish to E. ictaluri.

Aims: To evaluate the intraspecific diversity of the fish pathogen Flavobacterium columnare Methods and Results: Genetic variability among Fl. columnare isolates was characterized using restriction fragment length polymorphism analysis of the 16S rDNA gene, intergenic spacer region (ISR) sequencing, and amplified fragment length polymorphism (AFLP Ò ) fingerprinting. Thirty Fl. columnare cultures isolated from different fish species and geographical origins as well as reference strains were included in the study. Fifteen isolates belonged to genomovar I while eleven were ascribed to genomovar II. Analysis of the ISR sequence confirmed the genetic differences between both genomovars but revealed a higher diversity among genomovar I isolates. The maximum resolution was provided by AFLP Ò fingerprinting, as up to 22 AFLP profiles could be defined within the species. Conclusions: We confirmed the division of Fl. columnare isolates from cultured fish into different genogroups. We showed that both genomovars I and II are present in channel catfish from the US. We described a unique genetic group represented by four Fl. columnare isolates from tilapia in Brazil which appears to be related to both genomovars. We were able to further subdivide the species by analysing the ISR. Finally, the use of AFLP Ò allowed us to fingerprint the species at clone level without losing the higher genetic hierarchy of genomovar division. Significance and Impact of the Study: This paper reports on an extensive assessment of the use of molecular tools for the study of the epidemiology of the fish pathogen Fl. columnare.

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A specific and rapid PCR detection method for Flavobacterium columnare based on the 16S-23S rDNA intergenic spacer region (ISR) of the ribosomal RNA operon has been developed. The ISR of 30 F. columnare strains and other Flavobacterium species was amplified using universal primers and sequenced. Once F. columnare specific sequences within the ISR were recognized, specific PCR primers were designed against them (FCISRFL and FCISRR1). The primers were sensitive and able to detect as low as 7 colony forming units from pure culture by PCR. The new PCR detection method was applied to experimentally infected channel catfish. Two different experiments in which channel catfish fingerlings were infected by intramuscular injection or by immersion bath showed the advantage of the PCR method over standard culture techniques. F. columnare was detected by PCR in both tank water and catfish tissue samples with a higher frequency and in less time than standard microbiological methods. Furthermore, PCR detection confirmed that F. columnare can be transmitted horizontally indirectly through the water column without fish-to-fish contact. The newly developed PCR detection method for F. columnare was more sensitive and rapid than standard culture on bacteriological media for detection of F. columnare in channel catfish tissues and in tank water.