Investigating the Ability of Edwardsiella ictaluri and Flavobacterium covae to Persist within Commercial Catfish Pond Sediments under Laboratory Conditions

Abstract

Two prevalent bacterial diseases in catfish aquaculture are enteric septicemia of catfish and columnaris disease caused by Edwardsiella ictaluri and Flavobacterium covae, respectively. Chronic and recurring outbreaks of these bacterial pathogens result in significant economic losses for producers annually. Determining if these pathogens can persist within sediments of commercial ponds is paramount. Experimental persistence trials (PT) were conducted to evaluate the persistence of E. ictaluri and F. covae in pond sediments. Twelve test chambers containing 120 g of sterilized sediment from four commercial catfish ponds were inoculated with either E. ictaluri (S97-773) or F. covae (ALG-00-530) and filled with 8 L of disinfected water. At 1, 2, 4-, 6-, 8-, and 15-days post-inoculation, 1 g of sediment was removed, and colony-forming units (CFU) were enumerated on selective media using 6 × 6 drop plate methods. E. ictaluri population peaked on Day 3 at 6.4 ± 0.5 log₁₀ CFU g^-1. Correlation analysis revealed no correlation between the sediment physicochemical parameters and E. ictaluri log₁₀ CFU g^-1. However, no viable F. covae colonies were recovered after two PT attempts. Future studies to improve understanding of E. ictaluri pathogenesis and persistence, and potential F. covae persistence in pond bottom sediments are needed.

Keywords: bacterial diseases; catfish; commercial aquaculture; environmental adaptations; soil microbiology.

Figure 1

Persistence of Edwardsiella ictaluri S97-773 population (log₁₀ CFU g⁻¹) in sediment samples collected from 12 study chambers (2 farms × 2 ponds/farm × 3 replicate tanks per pond). Within each box plot, the horizontal line indicates the median, symbols indicate the mean and error bars around the symbol represent the standard error of the mean. Box plots with different lowercase letters are significantly different at p < 0.05.

Figure 2

Comparison of Edwardsiella ictaluri population (log₁₀ CFU g⁻¹) in sediment samples collected from two farms (2 ponds/farm × 3 replicate tanks per pond). Within each box plot, the horizontal line indicates the median, symbols indicate the mean and error bars around the symbol represent the standard error of the mean.

Figure 3

Relationship between Edwardsiella ictaluri population in sediment (CFU g⁻¹: (A–C); log₁₀ CFU g⁻¹: (D–F)) and time (Days) using a smoothing spline (SS) model and 95% confidence intervals (green shadow). (A,D) represent all samples; (B,E) represent farm A; (C,F) represent farm B. Estimates of SS model descriptors are summarized in Table 1.

Figure 4

Unique bacterial colonies were visually identified on selective EIM during the EI_PT. All colonies expressing different sizes, morphologies, and colors were accounted for. All blue lines next to each distinct colony represent 1000 μm. Sampling days of first appearance and identities of colony types A (A), B (B), C (C), D (D), E (E), F (F), G (G), H (H), I (I), and J (J) are listed in Table 2.

Figure 5

Gel electrophoresis image with visualized products of conventional polymerase chain reaction using ESCF and ESCR primers specific to Edwardsiella ictaluri. Bacterial isolates (arranged in the order in lanes 1–19); 1, positive control (S97-773); 2–4, colony type A (Days 0, 7, and 14); 5–7, colony type B (Days 5, 7, and 14); 8–9, colony type C (Days 1 and 7); 10–11, colony type D (Days 5 and 7); 12–13, colony type E (Days 5 and 7); 14, colony type F (Day 14); 15, colony type G (Day 7); 16, colony type H (Day 14); 17, colony type I (Day 14); 18–19, colony type J (Days 5 and 7); 20, no template, negative control; M = 50 bp DNA ladder.