Probiotics in aquaculture: The need, principles and mechanisms of action and screening processes

Abstract

Aquaculture production of molluscs is worth US$11 billion per year and represents 65% of World mollusc product. A significant limitation to the industry is loss of stock through bacterial disease. Traditional methods to combat disease with antibiotics have been questioned and alternatives have been sought. The field of probiotics as well as the screening methods used to acquire probiotic strains for the alternative management of disease in aquaculture is discussed. This review provides a comprehensive summary of probiotics in aquaculture with special reference to mollusc culture.

Introduction

Forty percent of World aquatic product (including capture fisheries) derives from aquaculture, being valued at US$78 billion. Aquaculture produced molluscs account for 21% of total aquaculture product, and make up 65% of total mollusc product when capture fisheries are considered (FAO, 2007). Importance of aquaculture product is set to increase dramatically as a result of overfishing of the world's waters and an increasing demand for seafood. A significant issue affecting production is the loss of stock through disease. Diseases caused by Vibrio spp. and Aeromonas spp. are commonly implicated in episodes of mortality.

When faced with disease problems, the common response has been to turn to antimicrobial drugs (hereafter referred to as ADs). The livestock and aquaculture industries have experienced widespread use of ADs in their practices. While the use of such products has an obvious benefit to treat animals infected by bacterial disease, the use of ADs has been either prophylactic (preventative), or for growth enhancement (Van den Bogaard and Stobberingh, 2000). Certain ADs have been shown to positively influence growth of livestock and used widely (Acar et al., 2000, Witte, 2000, Wierup, 2001, Phillips et al., 2004). Given this, and the desire to prevent establishment of pathogenic bacteria, it is argued that ADs have been widely overused (Aarestrup, 1999, Schwarz et al., 2001). Schwarz et al. (2001) provided a good overview of AD use in animals and the potential hazards associated with this.

The use of ADs in agriculture and aquaculture has led to the emergence of antibiotic resistant bacteria (hereafter referred to as ARB) (Schwarz et al., 2001, Akinbowale et al., 2006). In aquaculture this was felt most dramatically in the shrimp industry where massive increases in production, overcrowding of animals and unchecked antibiotic usages led to the emergence of numerous ARB and production crashes in many Asian countries (Karunasagar et al., 1994, Moriarty, 1999). For example, production figures for shrimp in the Philippines dropped by 55% in 2 years; from 90,000 t to 41,000 t between 1995 and 1997. In fact, it has never recovered and, in 2002, a mere 37,000 t was produced. An industry previously worth US$760 million is now worth only $240 million (FAO, 2007). Similarly, Thai shrimp production dropped by 40% between 1994 and 1997 due to disease problems (Moriarty, 1999); bacterial pathogens and shrimp viruses. Within aquaculture, there are numerous reports of ARB of farm origin (Karunasagar et al., 1994, Son et al., 1997, Molina-Aja et al., 2002, Chelossi et al., 2003, Sahul Hameed et al., 2003, Alcaide et al., 2005).

However, the risk is not just the potential loss to the farmer. The emergence of ARB on aquaculture farms could pose a risk to human health. There are many reports illustrating the transferral of resistant genes between bacteria (Son et al., 1997, Aarestrup, 1999, Van den Bogaard and Stobberingh, 2000, Witte, 2000, Schwarz et al., 2001). This process means ARB originating from a shrimp farm could potentially transfer plasmids to bacteria involved in human health problems. This is an area of current debate. Studies point to a farm animal origin in certain ARB genes that have made their way into human bacteria (Van den Bogaard and Stobberingh, 2000, Witte, 2000, Schwarz et al., 2001). However, recent reports argue this phenomenon (Acar et al., 2000, Phillips et al., 2004). The argument is based on the view that, although ARB have arisen in animal husbandry through use of antimicrobials, there is insufficient data to show a linkage to resistant gene transferral to humans. They argue in favour of the beneficial role antibiotics play in farming, and caution against premature, unscientific decisions in the restriction of antibiotic usage.

Regardless of which argument represents the true situation, governments and organizations have introduced much tighter restrictions for antibiotic usage in animal production. The European Union (EU) initially put a ban on the use of avoparcin in 1997, and in 1999, included virginiamycin, spiramcin, tylosin and bacitracin as banned growth promoters in animal feed (Turnidge, 2004, Delsol et al., 2005). In 2005, the EU implemented a ban on the use of all non-therapeutic antimicrobials in animal production (Delsol et al., 2005).

The US has been less stringent. There was a proposal in 2000 to introduce a ban on the use of fluoroquinolone and there was concern also about the use of virginiamycin (Nawaz et al., 2001). More recently a bill called “Preservation of antibiotics for medical treatment act of 2005” was presented in the US congress. If passed this act would see a ban on the non-therapeutic use of any drug intended for human use, in the production of feed animals. This act would be enforced two years from the date of being passed (Martin, 2005).

Other countries which currently have less antibiotic control, such as many of the Asian countries, are likely to be pressured through foreign restrictions, via the export markets being tightly controlled for antibiotic-contaminated products. Despite chloramphenicol being banned in Thailand since 1999 as a result of worldwide concern over its use in animal production, trace levels are still detected in shrimp from Thailand, causing a temporary ban by the EU for Thai shrimp (Heckman, 2004). Chloramphenicol has also been detected in shrimp from Myanmar, India, Pakistan and Vietnam, highlighting the continuing misuse of ADs in Asian shrimp farming.

A leading example in the eradication of antibiotic use can be seen in the Norwegian salmon industry. After concern about the use of antibiotics in the late 1980s, there has been a 95% drop in usage from 50 tonnes to 1 tonne annually. During the same period, salmon production has increased 10-fold from about 5500 tonnes to 55,000 tonnes. Reasons for the turnaround have been attributed to the use of vaccines, better husbandry and selective breeding programs (Maroni, 2000).

There is a developing social attitude against unnecessary use of ADs and where possible, it is the move away from non-essential AD use that the responsible farmer now seeks. Given the threat that both ADs and bacterial pathogens pose to farmers, as well as in human health, alternatives are being sought. Probiotics is one field commanding considerable attention.