Catfish aquaculture is big business in the US. Big business. Total sales of these large, freshwater fish were worth over $340 million in 2012, with channel catfish (Ictalurus punctatus) accounting for more than 80% of US aquaculture business.
Controlling disease-causing bacteria is important in any type of farming, and catfish husbandry is no different. One of the most important pathogens of commercially-raised fish is Edwardsiella ictaluri, a gram-negative bacterium that can cause fatal systemic disease. New research, published in the July issue of Microbiology, highlights a new way to tackle the disease.
Channel catfish have a well developed innate immune system, producing and secreting huge numbers of cationic antimicrobial peptides (CAMP, for short). These short, positively-charged molecules are the fish’s main defence against bacterial disease.
However, CAMP defences are useless against E. ictaluri, as this species of bacteria is extremely resistant to their action. Until recently, no one knew why… New research, led by Dr Javier Santander at Arizona State University and published in Microbiology, explains how the bugs survive and may offer the opportunity for a new, more efficient vaccine to be developed against the disease.
The key to E. ictaluri’s resistance lies in the makeup of its lipopolysaccharides (LPS): large molecules of fats and sugars that protrude out from the surface of gram-negative bacteria. In many other species of bacteria LPS have been shown to be important in both virulence and in resistance to antimicrobial peptides.
Santander and colleagues looked at the genes involved in the production of E. ictaluri’s LPS, and found two of interest: ugd and gne. The protein encoded by the ugd gene adds a positively-charged molecule to LPS during their production. This charge matches the positive charge found in the antimicrobial peptides and helps to repel them like a magnet, protecting the bacteria. Deleting the gene stops the LPS being made correctly, rendering the bacterial cells sensitive to the antimicrobial peptides. Deleting this gene has other effects, preventing the E. ictaluri from producing flagellae and making them clump together.
Deleting the gne gene only affects the production of the outer edge of the LPS, leaving the part of the molecule attached to the bacterium intact. Other than losing part of the LPS, these mutant bacteria behaved fairly normally.
Both of these mutants (and others), were used to immunise catfish to test their potential as vaccine candidates. This involved inoculating hundreds of juvenile fish, known as ‘fingerlings’ (because they’re about the length of your finger), either by immersing them in water containing the E. ictaluri, or anaesthetising them and putting the bacteria into their mouths.
The ugd mutant turned out to be too weak to be considered a viable vaccine choice, while the gne mutant struck a good balance between attenuation (being unable to cause disease), and motility – making it an ideal vaccine candidate.
While Dr Santander and colleagues may have identified E. ictaluri’s method of resistance against antimicrobial peptides, the team are hoping to further understand what it is about the structure of the LPS molecule that confers resistance.
Dr Santander is excited about the results: ‘We have here, in my opinion, a wonderful vaccine candidate. In the lab setting this candidate had 80-90 per cent protection rate, higher than the currently available commercial vaccine, which is 60-70 per cent.’
Santander J, Martin T, Loh A, Pohlenz C, Gatlin DM 3rd, & Curtiss R 3rd (2013). Mechanisms of intrinsic resistance to antimicrobial peptides of Edwardsiella ictaluri and its influence on fish gut inflammation and virulence. Microbiology 159, 1471-86 PMID: 23676433