Indigenous Lactic Acid Bacteria in Fish and Crustaceans - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 135

et al . (2006) and Mansfield et al . (2010) reported that C. maltaromaticum was a dominant
component of the gut contents of laboratory reared rainbow trout. According to clone library
analyses, Mansfield et al . (2010) reported that C. maltaromaticum represented 55%-97.2% of
the total bacterial community of the distal gut contents of rainbow trout.
In contrast to the aforementioned studies, a number of studies have assessed the gut micro-
biota of salmonids and have not retrieved carnobacteria as components of the indigenous gut
microbiota (e.g. Heikkinen et al . 2006; Moffit and Mobin 2006; Dimitroglou et al . 2009;
Merrifield et al . 2009a; 2009b; Navarrete et al . 2010; 2012). It is difficult to draw conclu-
sions as to what levels of Carnobacterium spp. are typically present in the gut of salmonids
from the somewhat contradictory findings of the present literature - and the different and not
always appropriate (to isolate carnobacteria) methodology used can compromise our conclu-
sions. Studies are required to determine carnobacteria levels in the gut of salmonids in order
to better understand their importance as components of the normal salmonid microbiota.
Despite this, it is clear that carnobacterial populations are of potential importance to the
host fish as Carnobacterium isolates have been extensively tested both in vitro , to ascertain
their impacts on transient pathogenic bacteria, and ex vivo and in vivo as probiotics. Kim and
Austin (2006a) extensively characterized two probiotic carnobacteria isolates derived from the
intestine of rainbow trout: C. maltaromaticum (B26) and C. divergens (B33). Both carnobac-
teria grew between 0 and 37 ∘ C in 0-10% NaCl at pH 5-10, were plasmid free, were resistant
to a broad spectrum of antibiotics, and inhibited in vitro growth of Aeromonas salmonicida ,
Aeromonashydrophila , Streptococcusiniae and Vibrioanguillarum . These characteristics sug-
gest that such carnobacteria species in the intestines of fish may play an important defensive
role against pathogenic organisms. Indeed, Kim and Austin (2006a) used these strains as pro-
biotics and demonstrated that transplanting high levels of C.maltaromaticum and C.divergens
to the intestines of trout via dietary applications stimulated non-specific immunity. However,
as the application led to carnobacterial levels of 90% of culturable levels, which is generally

far higher than natural carnobacterial levels reported, it is difficult to ascertain their degree
of importance in mediating normal immune function and mucosal tolerance. However, it has
been reported from invitro studies that incubation of C.maltaromaticum and C.divergens with
(undefined) gut cells did not induce any effects on IL-1β, IL-8, TGF-β and TNF-α mRNA lev-
els (Kim and Austin 2006b). However, future exvivo and invivo studies will yield more precise
information on the importance of carnobacterial-epithelial interactions in fish.
Several studies suggest that the carnobacteria populations within the gut of fish are affected
by dietary factors. For example, Ringø et al . (2006a) observed that the presence of dietary krill
meal reduced culturable epithelial associated C. maltaromaticum populations in the hindgut
of Atlantic salmon to non-detectable levels, down from log 3.23 CFU g −1 in the control fish.
Mansfield et al . (2010) reported that C. maltaromaticum represented 55% of the total bacterial
community of the distal gut contents of rainbow trout fed a fishmeal based diet, and when
the dietary protein source was changed to casein or soybean meal the abundance increased
to 87.8% and 97.2%, respectively. Indigenous C. maltaromaticum populations in the gut of
Atlantic salmon have also been reported to be sensitive to oxytetracycline (Bakke-McKellep
et al . 2007). Carnobacteria composition also appears to be affected by dietary carbohydrates
(Ringø and Olsen 1999; Ringø et al . 2006b). Arctic charr fed high levels of dietary carbohy-
drates displayed different adherent culturable carnobacteria profiles in the intestine, such as
C. mobile in the small intestine and C. maltaromaticum in the distal intestine; however, fish
fed low levels of dietary carbohydrates displayed only C. divergens in both intestinal regions.

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