Agriculture Reference
In-Depth Information
6.2 LACTIC ACID BACTERIA
LAB are Gram-positive, often non-motile, catalase- and oxidase-negative, non-sporing bacte-
ria that produce lactic acid as a major product of fermentative metabolism and are widespread
in nature from soil, water, plants, milk products and animals (Stanier et al . 1975). LAB are
able to tolerate bile and pancreatic enzymes (Cai etal . 1999; Ouwehand etal . 2002) and, there-
fore, they are often reported to be associated with the moist, nutrient-rich intestinal contents
of endothermic animals.
There are also numerous studies that have demonstrated the presence of LAB in fish (e.g.
Ringø and Gatesoupe 1998; Ringø 2004; Ringø etal . 2005; Balcázar etal . 2007a; Michel etal .
2007; Lauzon and Ringø 2012) and it is reported that dietary factors such as chromic oxide
(Ringø 1993a; 1993b; 1994), dietary form (Ringø and Strøm 1994), feed additives (Giannenas
et al . 2012), lipid levels (Ringø and Olsen 1999), lipid sources (Ringø et al . 2002a), polyun-
saturated fatty acids (Ringø 1993c; Ringø et al . 1998) and vegetable products (Heikkinen
et al . 2006; Ringø et al . 2006a; 2008; Mansfield et al . 2010; Merrifield et al . 2011a; Desai
et al . 2012) may affect the level, composition and/or diversity of LAB populations. Further-
more, factors such as stress (Olsen et al . 2008; Ringø et al. 2014), hierarchy formation (Ringø
etal . 1997), migration from freshwater and migration from seawater back to freshwater (Ringø
2000), seasonality (Hagi et al . 2004) and salinity (Ringø and Strøm 1994) also modulate LAB
populations in the GI tract of fish.
The key problem in estimating the proportion of LAB in the GI tract of fish is the
lack of quantitative analysis. Historically assessments of fish microbiota are dominated
by culture-dependent studies; these studies have used a general purpose medium, and as
a result LAB have not been routinely isolated in large numbers. This does not give an
accurate estimation of LAB levels as most LAB require complex nutrients (and in many cases
microaerobic conditions) and thus quantitative knowledge of LAB populations is somewhat
limited. Carnobacterium spp. do not have such specific nutrient requirements and thus are
routinely isolated from cold water fish on general purpose media when culture conditions
are correct (i.e. low temperature and long term incubation) (Table 6.1). Many LAB such
as Lactobacillus and Enterococcus have fastidious nutrient requirements and thus it is no
coincidence that these species in earlier studies have been less frequently isolated from the
gut of fish. However, when selective media such as de Man, Rogosa and Sharpe (MRS) are
incorporated into quantitative investigations, high LAB recovery (i.e.
10 5 CFU g −1 )from
the GI tract of fish has been reported (Cai et al . 1999; Hagi et al . 2004; Bucio et al . 2006;
Rawling et al . 2009; Kühlwein et al . 2013).
6.3 SALMONIDAE
While early pioneering studies suggested that the intestinal microbiota of salmonids were
mostly composed of Aeromonas species, members of the Enterobacteriaceae family, and did
not report LAB (e.g. Yoshimizu and Kimura 1976; Sakata 1990), subsequent studies provide
information that suggests that this may not be correct. Much of our understanding of the pres-
ence of LAB in the GI tract of salmonids is derived from culture-dependent studies, and studies
conducted over the last two decades reveal that the dominant viable LAB in the digestive tract
of salmonids belongs to the Carnobacterium genus (Table 6.1). The information available is
 
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