Characterization of Ultrasmall Chryseobacterium Strains FM1 and FM2 Isolated from Xenopus laevis Skin

Two strains of ultrasmall gram-negative bacteria (USGNB), FM1 and FM2, were isolated from the skin of the smooth clawed frog Xenopus laevis. The cytological, physiological, biochemical, and genotypic characteristics of the isolates were studied. Based on the sequencing of their 16S rRNA genes and on their phenotypic properties, the isolates were assigned to the genus Chryseobacterium. The cells were extremely small, with cell volumes of ~0.06 and ~0.015 µm3 for developing cultures of strains FM1 and FM2, respectively. Since the USGNB cells were firmly attached to the skin surface and could not be removed by repeated washing with water, these bacteria may be classified as epibionts. Adhesive properties of the fimbria-like appendages revealed in strains FM1 and FM2 by electron microscopy could probably contribute to tight binding of USGNB cells to the skin. Localization of ultrasmall gram-negative bacteria on skin surface of the frogs may indicate their action as a protective bacterial filter; skin surface of Xenopus laevis is thus characterized for the first time as a specific habitat of ultrasmall Chryseobacterium strains. Isolation and characterization of two ultrasmall Chryseobacterium strains, FM1 and FM2, improves our understanding of diversity of the cellular structural and functional characteristics and of the ecological niches of this bacterial genus.