Probiotic Modulation of the Gut Microbiota of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 185

8 Probiotic Modulation of the Gut

Microbiota of Fish

Daniel L. Merrifield 1 and Oliana Carnevali 2
1 School of Biological Sciences, Plymouth University, UK
2 Dipartimento di Scienze della Vita e dell'Ambiente, Università Politecnica delle
Marche, Ancona, Italy

ABSTRACT

A growing body of literature is available on the effects of probiotics on fish growth per-
formance, digestive function, immunity, disease resistance, fecundity, oxidative stress and
deformities but, perhaps surprisingly, many of these studies have not investigated the effect of
probiotic applications on the gastrointestinal (GI) microbiota. Despite the debate on the precise
definition of a 'probiotic' for aquaculture applications, the different definitions are consistent
in that host benefits are driven, in part at least, by modulation of the host microbiota. Too often
fish probiotic studies either lack a microbial investigation, or conduct the minimum assess-
ment: enumeration of probiotic levels. Given the complexity of the gut microbiome, and its
importance to the host, it is not sufficient to determine only the probiont levels and to attribute
host benefits solely to the direct presence of the probiotic, which is usually only present as
a minor component of the total microbial population. Changes in the indigenous populations
must be examined as these potential changes are also factors involved in driving host benefits.
This chapter summarizes the findings of studies which have addressed this topic and discusses
the data to support the claims of probiotic 'colonization'. From the literature it is clear that a
greater emphasis on understanding the effects of probiotics on the complex microecology of
the GI tract of fish is essential, and future studies must incorporate quantitative techniques to
determine microbial abundance as well as elucidating microbial functionality and activity.

8.1 INTRODUCTION

Despite the large variations in the definitions of the term 'probiotic' (as discussed elsewhere:
Merriield et al . 2010a; Chapter 7 ), the one consistency among the definitions is that the host
benefits are said to be driven, in part at least, by modulations in the microbiota of the host.