Characteristics of the Mucus Layer on the Surface of the Bluegill (Lepomis macrochirus) and the Bacterial Flora in the Mucus

Abstract

The layer of mucus on the surface of bluegills (Lepomis macrochirus) captured in Lake Biwa was characterized as 1) large enough to host microbes (ca. 76 mm thick), 2) a physically different environment from the surrounding lake water in viscosity and buffering capacity, and 3) chemically rich in organic substances, which may be utilized as nutrients. Based on DAPI staining and on the number of colonies formed respectively, it was found that ca. 10 3 times and 3 to 7 times the number of microbial cells were present in the mucus layer, as compared with the lake water. The bacterial flora of the mucus was greatly different from that of the lake water, according to a phylogenetic analysis. About 60% of the isolates from the mucus were Gram-positive. These Gram-positive isolates could be divided into two major groups. Each group consisted of strains sampled in one season, i.e., the strains sampled in July were closely related to the genus Staphylococcus, while the strains sampled in November were close to the genus Mycobacterium. In contrast, most isolates from the lake water were Gram-negative (72%); with all the strains closely related to band g-Proteobacteria sampled in July. With the exception of one strain, the Gram-positive isolates from the lake water (6 strains) were all sampled in November. Almost all of the isolates from the mucus could metabolize glucose, whereas only half of the isolates from the lake water could do the same.

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

“…On the other hand, these mucus strains would hardly be able to grow in sufficiently large numbers in the lake water, as described above, so it might be hard for them to maintain a stable population in the lake water. The faster growth-rate constant of mucus strains in the 20-fold diluted mucus and their insufficient growth in the lake water may result in a characteristic bacterial flora in the mucus which is different from that in the lake water, as revealed in our previous study 6) .…”

“…The fish (bluegills, or Lepomis macrochirus) used in this study were sampled as previously reported 6) , at the site (34°58'30'' N, 135°54'30'' E) where the river Seta flows out from Lake Biwa (the largest lake in Japan). The fish were brought back to our laboratory within several hours.…”

“…The isolates were obtained from NB or DNB agar (1.5 wt%) medium onto which diluted mucus samples or diluted lake water samples had been poured and incubated at 20°C for 30 days, as reported 6) . The NB medium contained (per liter), 10 g of polypepton (Nihon Seiyaku), 10 g of bonito extract (Wako Pure Chemicals), and 5 g of NaCl (pH 7.2).…”

“…After the incubation period, 60 strains (36 from the mucus and 24 from the lake water) were randomly isolated. After the construction of phylogenetic trees using 16S rRNA gene sequences of the isolates as reported 6) , representative strains were selected from each phylogenetic group, i.e., the CFB (Cytophaga-Flavobacterium-Bacteroides) group, a-, b-and g-Proteobacteria, the Firmicutes and the Actinobacteria.…”

“…Previously 6) , we clarified the physical and chemical properties of the mucus on the surface of bluegills (Lepomis macrochirus). We also revealed that the bacterial flora in the layer differed greatly from that in the surrounding lake water.…”