The Effect of the Surface Mucus of Bluegills (Lepomis macrochirus) on Bacterial Activity

Abstract

The mucus obtained from the surface of bluegills (Lepomis macrochirus) can be utilized as a source of nutrients by the bacterial strains isolated from the mucus and the adjacent lake water. The isolates from both the mucus and the lake water grew better in 20-fold diluted mucus than in lake water. Almost all mucus isolates (10 of 12 strains) hardly grew at all in lake water, whereas half (6 of 12 strains) of the lake-water isolates were able to grow in lake water, according to the increase in optical density (O.D.) after culturing in mucus or lake water. The isolates grew faster in 20-fold diluted mucus than in lake water, with a greater enhancement of the growth rate for the mucus isolates than the lake-water isolates. The features of the mucus isolates, i.e., little growth in lake water and faster growth in mucus, were deduced to be responsible for the different bacterial flora of the fish mucus. The performance of 20-fold diluted mucus as a source of nutrients was evaluated by comparing it with nutrient broth (NB) medium and 100-fold diluted NB medium (DNB medium). In many strains, the amount of growth (maximum O.D.) was highest in NB, then mucus and lastly DNB, coinciding with the order in terms of the concentrations of glucose equivalent, i.e., NB, 20-fold diluted mucus and DNB. About 60% and 70% of the isolates grew faster in the 20-fold diluted mucus than in the NB and DNB mediums, respectively. The growth yield (increase in O.D. when the unit amount of glucose equivalent was consumed) was on average, ca. 70 times greater in the 20-fold diluted mucus than NB. The growth-rate constant in the mucus did not differ markedly among the isolates belonging to various phylogenetic groups. By contrast, in the NB medium, the isolates closely related to the Actinobacteria and α-Proteobacteria had smaller growth-rate constants and the isolates closely related to γ-Proteobacteria had greater growth-rate constants. The respiratory activity of the isolates was greatly enhanced by adding a small amount of mucus (1% as the final concentration), due to unknown factor(s). Thus, as a source of nutrients, the mucus seemed to be more effective, as well as being able to sustain the growth of various bacterial groups.