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The Gram-negative anaerobe Dichelobacter nodosus is the causative agent of footrot in sheep. Different strains of D. nodosus cause disease of differing severities, ranging from benign to virulent. Virulent strains have greater twitching motility and secrete proteases that are more thermostable than those secreted by benign strains. We have identified polynucleotide phosphorylase (PNPase) as a putative virulence regulator and have proposed that PNPase expression is modulated by the adjacent integration of genetic elements. In this study, we compared PNPase activity in three virulent and four benign strains of D. nodosus and found that PNPase activity is lower in virulent strains. We disrupted the pnpA gene in three benign D. nodosus strains and two virulent strains and showed that deletion of the S1 domain of PNPase reduced catalytic activity. In all but one case, deletion of the PNPase S1 domain had no effect on the thermostability of extracellular proteases. However, this deletion resulted in an increase in twitching motility in benign, but not in virulent strains. Reconstruction of the pnpA gene in two mutant benign strains reduced twitching motility to the parental level. These results support the hypothesis that PNPase is a virulence repressor in benign strains of D. nodosus.

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An experimental aquatic food chain system was tested to utilize eutrophic water for producing useful natural feed. Water flea (Daphnia magna) was the main focus in this food chain system, in which it was grown on algal biomass bloom in eutrophic water, and harvested following its function as fish feed. The eutrophic waters were originated from a small lake Situ Rawa Kalong and catfish ponds located in Bogor. Daphnia was grown in an aquarium filled with the eutrophic waters from the two localities and reared without artificial feed, with an initial density of 5 individuals/L. The experiment was conducted with three replications for 19 days. Sampling was carried out five times to observe the Daphnia development. The original chlorophyll content in the aquarium of catfish pond and Rawa Kalong water were 257.64 and 211.91 mg/m3, respectively. It was observed that Daphnia consumed algal biomass to support their growth, indicated by the increase of the chlorophyll content in the water was followed by the rise of the Daphnia density to reach a maximum of 245 individuals/L at day 15 for the catfish aquarium, and then overgrazing occurred which resulted in a decrease in the chlorophyll content which was observed from day 12. The density of Daphnia in Rawa Kalong water at day 19 was 152 individuals/L and kept increasing. In the experimentation of Daphnia as natural feed, Daphnia was grown in four concrete ponds and fed with fish pellets. The Daphnia biomass was harvested when it reached a productivity of 1.67 g/m2/day. The feed conversion ratio (FCR) of fish pellets into Daphnia biomass increased with the age of the culture, with the highest value of 0.46 at day 36. Five Bada fish (Rasbora maninjau) per aquarium were then grown for 60 days, fed daily with progressive amount of Daphnia from 1.5 to 4.0 g, and the growth was compared with the control which was fed with an increasing amount of fish pellet. Fish length and weight were observed every 20 days to calculate the growth rate, weight gain, and feed conversion ratio. The results showed that the growth rate of the fish ranged from 0.24–0.98% per day. Fish pellets stimulated a higher growth in the initial phase of growth, but decreased rapidly with the fish age, while Daphnia provided a more stable growth rate. Based on its lower FCR value, Daphnia is more efficient as fish feed. Thus, Daphnia proved to be a suitable natural food source for Rasbora cultivation. Overall, our results indicated that the experimental aquatic food chain system composing of Daphnia as the first level consumer grazing on algal biomass, followed by Rasbora as second-level consumer devouring of Daphnia not only is an efficient and flexible system for reducing the nutrient level in eutrophic waters but also can support a stable growth of cultured fish.

Water enrichment has become a national problem that needs to be resolved. This study examined the ability of water fleas Daphnia magna to grow and to control phytoplankton populations in the waters from eutrophic ponds Situ Cikaret and Situ Gunung Putri in Bogor Regency, West Java. Water samples from these water bodies were used to grow the daphnid populations in the styrofoam aquariums without artificial feeding with an initial density of 3.3 individuals/L. The experiment was carried out with three replications. Daphnid population was counted every 4–5 days to observe its growth and development. The chlorophyll content that reflects the phytoplankton biomass at the beginning of the study in the water of Situ Cikaret and Situ Gunung Putri were 52 and 22 μg/L, respectively. The water from fertile ponds allowed the development of phytoplankton populations and be utilized by daphnids to support their growth and reproduction. The results indicated that daphnids showed a preference for some phytoplankton genera more than other genera because not all phytoplankton populations were consumed. Following an increase in chlorophyll content, the daphnid density also increased to reach a maximum of 197 individuals/L in the water of Situ Cikaret and 169 individuals/L in the water of Situ Gunung Putri on day 15. The chlorophyll content decreased significantly towards the end of the experiment indicating the phytoplankton population is almost depleted. Hence, this study showed the potential of water flea Daphnia magna as bio-resources to utilize eutrophic pond waters and to control the phytoplankton population to improve water quality.