The Gut Microbiota of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 83

Lactic acid bacteria (LAB) constitute an interesting bacterial group that has been
investigated in several animal hosts. It has also been reported that LAB are present in fish
intestines (Ringø and Gatesoupe 1998; Ringø et al. 2000; Ringø 2004; Ringø et al. 2005;
Vazquez et al. 2005; Gatesoupe 2008; Navarrete et al. 2010a; Lauzon and Ringø 2012;
Chapter 6 ). A number of endogenous LAB strains have been tested for their probiotic
potential (Merrifield et al. 2010). The presence of LAB and their relevance to their aquatic
animal hosts is further discussed in Chapter 6 .
4.1.2 Viruses
The digestive tract of fish is also an environment available for bacteriophages and the micro-
bial composition, particularly bacterial, might be influenced by the effect of bacteriophages.
Lysis by bacteriophages has been claimed as an important selective pressure participating in
the complex regulation of bacterial populations in the microbiota of animals. The estimated
1200 viral genotypes in human faeces suggest that phage attack is a powerful shaper of the
gut's microbial landscape (Backhed et al. 2005). The number of free bacteriophage particles
in coastal seawater may exceed 10 8 particles per ml (Berg et al . 1989) and bacterial mortality
due to viral lysis has been estimated as 30% to 60%. Bacteriophages have been described in
the majority of bacterial genera, among which are various pathogenic bacteria such as Vibrio
spp. and Aeromonas salmonicida (Hansen and Olafsen 1999). Recently, Bastías et al. (2010)
isolated bacteriophages from digestive tracts of fish commonly found in Mexico and Chile
and after characterization they showed that some bacteriophages might correspond to a cos-
mopolitan phage group widely spread in separated geographical locations. The determination
of the types, numbers and importance of phages in the fish GI tract is a topic which should be
explored comprehensively in future studies.
4.1.3 Yeast
To our knowledge, the first study isolating yeast from fish - topsmelt ( Atherinopis affinis lit-
toralis ) and Pacific jack mackerel ( Trachurus symmetricus ) - was demonstrated by van Uden
and Castelo Branco (1963). Since then yeasts have been identified as part of the normal micro-
biota of fish; sometimes high population densities are observed in healthy fish, but the data are
variable in terms of colony counts and taxonomical diversity (Gatesoupe 2007). However, the
literature on the role of yeast in fish health and nutrition is scarce. Yeasts are widely distributed
in several natural environments including freshwater and seawater. Marine yeast participate in
several ecological processes in the sea, especially in estuarine and near-shore environments,
such as decomposition of plant substrates, nutrient recycling, biodegradation of oil and recal-
citrant compounds, and as part of the microbiota of marine animals (Kutty and Philip 2008).
This is due in part to the fact that yeast have an extraordinary metabolic potential available
for exploitation (Kutty and Philip 2008; Song et al. 2010). Notably, the vast majority of this
potential has yet to be discovered. Several compounds that are produced by yeast have huge
biological value as reagents, cell proteins, vitamins, pigments, immunostimulants and enzymes
(Chi etal. 2009) and thus the presence of yeast in the GI tract of fish is likely to be of importance
to the host.
The yeast load in the fish gut is variable and can fluctuate from non-detectable levels to up to
10 7 CFU g -1 of intestinal content (Gatesoupe 2007). It is important to note that yeast cells can
be a hundred times bigger than bacterial cells, which may explain the fact that the introduction