New Year, New Prospects for Bioactive Metabolites in Fungi!

Marine fungi produce a wide range of bioactive secondary metabolites; an area of promising research, especially in medical circles, as many show antibacterial activity. This has recently piqued the interest of aquaculture organisations, as a large proportion of stock and hence profit is lost to bacterial diseases every year. With the global awareness of antibiotic resistance increasing, the overuse of antibiotics is now a more pressing issue to be solved. Vibrio species are particularly prolific pathogens worldwide, especially in shrimp culturing. More recently it has caused an increased mortality in these populations due to its role  in acute hepatopancreatic disease (AHPND) during early mortality syndrome (EMS). The AHPND-causing bacteria colonise the shrimp stomach cuticle, forming biofilms that release damaging toxins. Preventing the biofilm formation could therefore mitigate the negative effects of this pathogen. Consequently, in 2019, Soowannayan et al.investigated whether protection for the shrimp could be induced through reduction in biofilm formative ability in Vibrio, using bioactive isolates from marine fungi: a property previously shown in such compounds. This would reduce the risk of selection for resistant bacteria such as with antibiotics. 

7 different isolates of Vibrio parahaemolyticus and Vibrio harveyi were obtained from a variety of AHPND-affected, EMS-affected and unaffected shrimp pods in Thailand and continuously cultured. 39 known obligate marine fungal isolates were taken from a culture collection, originally sourced from decayed mangrove woods in intertidal zones. These were used to create a cell-free culture broth (CF-CB) containing secondary metabolites. This was first used to test for inhibition of Vibrio growth: CF-CB was added to Vibrio cultures and left to grow overnight. The optical density was compared between Vibrio cultures and negative controls and used to quantify the mean percentage of relative inhibition. The effect of the CF-CB on biofilm formation was assessed and quantified in a similar way. 

CF-CB from 25 isolates of marine fungi were capable of inhibiting biofilm formation in 7 different Vibrio isolates. The main Vibrio isolate of interest was one involved in AHPND, for which 11 fungal isolates inhibited biofilm formation, while mostly stimulating growth in the Vibrio. This is seen as a positive effect by the authors, who suggest that an increase in growth coupled with biofilm inhibition could drive selection towards reduced biofilm production and therefore reduced pathogenicity.  

The results of these assays were used to inform the choice of the four most successful inhibitory isolates; these were then used to supplement the feed of Panaeus vanname, a popular aquaculture species. The addition of fungal isolate to the food, in the form of CF-CB, was continued for 8 days, after which the specimens were immersed in a culture of V. parahaemolyticus for 24h. Only one isolate protected these shrimp from AHPND. The authors speculated that there was something in the test system that neutralised the activity of other isolate inhibitors. No mechanism for the effects has been investigated yet, but perhaps a mechanism that impacts quorum sensing is likely, as interruption of this has previously been reported to protect shrimp against vibriosis (Defoirdt et al., 2006). Further investigation into this would be valuable, as the implications of these results could have significant bearing on how we continue more sustainable aquaculture in the future, as well as potentially further informing how we treat pathogenic diseases in humans and other organisms. 

One interesting aspect mentioned previously in “Marine fungi – not so much fun for pathogens! Marine fungi show antibacterial activity for pathogens in fish aquaculture” is the influence of co-culture and competition on metabolite production and function (Özkaya et al. 2017). It would be interesting to compare the products and effects of co-culture of fungi and Vibrios in this study; this may initiate more active production of bacteria-interferring metabolites, or conversely stimulate an aggressive toxin-based response. Either outcome would be valuable to look into further.

Finally, given the communal nature of many saprotrophic assemblages, it follows that the secondary metabolites produced may be different in quantity and nature when fungi are grown in co-culture with many fungal strains, rather than mono-culture such as the ones in this study. This is yet another avenue which could illicit some interesting results and useful compounds. 

Reviewed paper:

Soowannayan, C., Teja, D.N.C., Yatip, P., Mazumder, F.Y., Krataitong, K., Unagul, P., Suetrong, S., Preedanon, S., Klaysuban, A., Sakayaroj, J. and Sangtiean, T., (2019). Vibrio biofilm inhibitors screened from marine fungi protect shrimp against acute hepatopancreatic necrosis disease (AHPND). Aquaculture, 499, pp.1-8.

References:

Defoirdt, T., Crab, R., Wood, T. K., Sorgeloos, P., Verstraete, W., Bossier P., (2006).

Quorum Sensing-Disrupting Brominated Furanones Protect the Gnotobiotic Brine Shrimp Artemia franciscanafrom Pathogenic Vibrio harveyi,Vibrio campbellii, and Vibrio parahaemolyticus Isolates.Applications of Environmental Microbiology,72 (9)6419-6423

from Marine fungi – not so much fun for pathogens! Marine fungi show antibacterial activity for pathogens in fish aquaculture:

Özkaya, F.C., Peker, Z., Camas, M., Sazak Camas, A. and Altunok, M., (2017). Marine Fungi Against Aquaculture Pathogens and Induction of the Activity via Co‐Culture. CLEAN–Soil, Air, Water, 45(8), p.1700238.