Assessing the cultured bacterial flora of Nematodes Panagrellus redivivus and Radopholus similis as live feed for larval fish

Using live organisms as feed is essential for successful larval breeding in aquaculture, especially during early developmental stages when digestive capabilities are at their initial stages. While rotifers and Artemia are commonly used, alternative organisms, such as free-living nematodes, have gained interest due to their ease of culture, small size, and low production costs. It is well known that live feeds harbour dense microbial communities. Some of these may include opportunistic or pathogenic bacteria, which can pose risks to larval health. This study aimed to assess the cultured bacterial flora associated with two nematode species, Panagrellus redivivus (microworms) and Radopholus similis (bananaworms), which are commonly considered to be live feed alternatives in aquaculture. Both nematode species were cultured in an oatmeal-yeast-water medium under identical conditions. Total viable counts (TVC) were determined from disinfected nematode samples, and bacterial isolates were identified to the genus level through morphological, biochemical, and phenotypic assays, complemented by genus and limited species-level verification using the VITEK 2 Compact identification system. Both species primarily had Gram-negative bacteria (75%), with Aeromonas and Pseudomonas being the most common types found. Additional isolates included Enterobacter, Citrobacter, Proteus, Staphylococcus, and Vagococcus. The range of nematodes compares favourably to high-load cultures of traditional live feeds. Species-level characterisation suggested the presence of potential fish or human pathogens, including Klebsiella pneumoniae ssp pneumoniae, Serratia rubidaea and Enterococcus faecalis. The findings suggest that while nematodes offer a promising alternative to traditional live feeds, their associated bacterial communities include potential pathogens that require careful consideration. Further studies focusing on bacterial virulence, larval interaction, and microbial management strategies (such as light treatment) are essential to ensure the safe application of nematodes in larval rearing systems.