Importance of fish microbiota

External mucus secretion cannot be studied independently

to the general health status of fish. Indeed, fish

health is strongly dependent on the internal microbiota

which helps in nutrients uptake and protect fish from

pathogenic invasion (Ringo et al., 2000; Makridis et al.,

2005). Adhesion capacity and growth on or in intestinal or

external mucus has been demonstrated in vitro for fish

pathogens like V. anguillarum and A. hydrophila (Garcia

et al., 1997; Krovacek et al., 1987) and for candidate

probiotics such as Carnobacterium strain K1 (Jo¨born et al.,

1997) and several isolates inhibitory to V. anguillarum

(Olsson et al., 1992). In one of these studies, the aim was to

measure the in vitro capacity of the strains to adhere to and

grow in turbot intestinal mucus in order to investigate

their potential to colonize farmed turbot as a means of

protecting the host from infection by V. anguillarum

(Olsson et al., 1992). The intestinal isolates generally

adhered much better to a film of turbot intestinal mucus,

skin mucus, and bovine serum albumin than did V.

anguillarum, indicating that they could compete effectively

with the pathogen for adhesion sites on the mucosal

intestinal surface (Verschuere et al., 2000). Mechanisms

that may be implicated include the production of antimicrobial

substances such as organic acids or bacteriocins,

competition for nutrients or adhesion receptors, inhibition

of virulence gene expression, and enhancement of the

immune response (Balcazar et al., 2007).

 Inhibitory compounds present in fish mucus

Depending on several factors, such as the fish species,

health, environment, mucus origin and microbiota colonizing

the mucus, many types of interaction could be

implicated to prevent pathogen infection. It is important to

take into account that, additionally to the immune system,

fish have a bacterial community that can inhibit the

adhesion or the development of pathogens. Effectively,

some bacteria produce inhibitory compounds, particularly

in the digestive tract, and may be responsible for

controlling the colonization of potential pathogens in fish

(Ringo et al., 2000; Makridis et al., 2005). For example a

Vibrio sp. recovered from the intestine of a spotnape

ponyfish (Leiognathus nuchalis) in Japanese coastal waters

inhibited the causal agent of pasteurellosis/pseudotuberculosis,

i.e. P. damselae subsp. piscicida. Specifically, the

inhibitory compound was heat-labile and proteinaceous,

with a molecular mass of