The channel catfish (Ictalurus punctatus) is a freshwater species native to the U.S. and northern Mexico, known for its rapid growth and disease resistance, making it ideal for aquaculture. However, bacterial infections are a challenge for the industry, particularly affecting I. punctatus and its hybrids. Iron supplementation is commonly used to prevent deficiencies and maintain health. Organic minerals, being more bioavailable, improve absorption and avoid negative dietary interactions. The dietary iron supplementation became a standard practice by the catfish industry to prevent idiopathic catfish anemia. This study aimed to evaluate the in vitro growth of bacteria in the blood and serum of fish fed with organic and inorganic iron diets.
For this study, 120 channel catfish, weighing ~250 g, were allocated to three aquariums of 200 L in a recirculating system with heating and aeration. The animals were fed for six weeks three experimental diets: 200 mg organic iron/kg (ORG) and 1,000 mg inorganic iron/kg (INORG), along with a control treatment containing a basal 70 mg inorganic iron/kg. Weekly, blood from four fish was collected, two fish for whole blood and two for serum. Each weekly collection was considered as a block in a manner that all tanks were exposed to the dietary treatments twice, totaling six blocks. Four bacteria of importance to the catfish industry were selected: Edwardsiella ictaluri (Ei), E. piscicida (Ep), Streptococcus dysgalactiae (Sd), and Aeromonas hydrophila (Ah). A standard for each bacterium was generated and diluted to ~1 × 104 CFU/mL in sterile phosphate buffer saline (PBS), with an optical density of 0.170 at 600 nm. The total blood and serum were subjected to a bacterial survival test, where 50 µL aliquots of each were pipetted into 96-well plates, incubated in triplicates with 3 µL of each bacterium suspension. The plates were incubated at 28°C with shaking at 150 rpm for 1 h. Afterward, six 10 µL aliquots of blood or serum were plated on TSA (Tryptic Soy Agar), incubated at 30°C. The Ah plates were evaluated after 24 h, and the others were evaluated after 48 h. The evaluation involved counting the number of colonies and comparing them to the negative control, which consisted of PBS, to estimate the bacterial survival percentage in each blood and plasma sample. The results were analyzed using one-way ANOVA and Tukey’s test (Statistica 7.1). No significant effects were found for bacterial survival; further studies are needed, considering a longer feeding period.