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Veterinary Microbiology 127 (2008) 191–195 www.elsevier.com/locate/vetmic

Short communication

The fish pathogen Renibacterium salmoninarum: Growth in a microaerophilic atmosphere
¨ Varpu Hirvela-Koski *
Finnish Food Safety Authority (Evira), Oulu Research Unit, PO Box 517, 90101 Oulu, Finland Received 19 March 2007; received in revised form 2 August 2007; accepted 6 August 2007

AbstractRenibacterium salmoninarum is the etiologic agent of bacterial kidney disease (BKD) occurring worldwide in salmonid fish. This bacterium has previously been regarded as a strict aerobic species. However, in this study it is shown that R. salmoninarum grows well in microaerophilic atmosphere, the colony size being larger and the colonies being more mucoid than in aerobic conditions. Microaerophiliccultivation might be one possibility to increase the sensitivity of the cultivation method for the detection of this slowly growing pathogen. # 2007 Elsevier B.V. All rights reserved.
Keywords: Renibacterium salmoninarum; Microaerophilic growth; Salmonid fish

1. Introduction Renibacterium salmoninarum is the etiologic agent of bacterial kidney disease (BKD) in salmonid fish, a chronic, systemicinfection characterized by granulomatous lesions in the kidney and other organs (Fryer and Sanders, 1981; Fryer and Lannan, 1993). This bacterium is widespread in nearly all countries where salmonid fish are cultured and it also affects wild fish populations (Fryer and Sanders, 1981; Klontz, 1983). It was characterized in terms of the G + C% content of the DNA, the cell wall sugar composition andthe amino acid composition of the peptidoglycan cell wall
* Tel.: +358 8 20 7724902; fax: +358 8 20 7724915. E-mail address: varpu.hirvela-koski@evira.fi.

layer and was recognised as forming a single species belonging to a new genus, Renibacterium (Sanders and Fryer, 1980). R. salmoninarum is a Gram-positive short rod occurring frequently in pairs. It is aerobic, catalase positive, cytochromeoxidase negative, non-acid fast and non-motile, does not produce acid from sugars and does not form endospores. All strains require cysteine for growth. Optimal growth occurs at 15–18 8C, and growth is very slow at 5 or 22 8C and non-existent at 37 8C. Growth on all media is slow, often requiring several weeks of incubation (Sanders and Fryer, 1980; Benediktsdottir et al., 1991). R. salmoninarumstrains isolated from different regions form a very homogeneous group in terms of both their morphological, biochemical and physiological (Goodfellow et al.,

0378-1135/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.vetmic.2007.08.011

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¨ V. Hirvela-Koski / Veterinary Microbiology 127 (2008) 191–195

1985; Bruno and Munro, 1986) and also their antigenicproperties (Bullock et al., 1974; Getchell et al., 1985; Daly and Stevenson, 1990). The bacterium is a highly conserved genospieces, and studies on isolates from different parts of the world indicate relatively low genetic diversity (Starliper, 1996; Grayson et al., 1999; Rhodes et al., 2000). R. salmoninarum has been described as an aerobic bacterial species (Sanders and Fryer, 1980). In BKD thereare often large granulomatous lesions in the internal organs of the diseased fish, pointing to an ability to survive and proliferate in circumstances involving a decrease in oxygen concentration. In this study we tested the growth of R. salmoninarum under aerobic, microaerophilic and anaerobic conditions.

2. Materials and methods 2.1. Trial 1 Three strains of R. salmoninarum were used: type strainof R. salmoninarum (ATCC 33209) and two strains, which had been isolated from rainbow trout in northern Finland in the years 1997 and 2002 through the fish health control programme, using the Icelandic cultivation method (Benediktsdottir et al., 1991). All the strains had been kept frozen at À80 8C prior to the experiment. The bacterial population obtained on KDM2 agar (Evelyn, 1977) after 14...