Antibiotic Resistance in Salmonella: A Risk for Tropical Aquaculture

Abstract

Salmonelas are rod-shaped, non-spore-forming Gram-negative facultative anaerobes measuring 0.7-1.5 by 2-5 µm. With the exception of the serovars Gallinarum and Pullorum, salmonelas are motile organisms. They are classified according to morphology and staining pattern and are divided into serotypes and serovars based on their reaction to somatic (O) and flagellar (H) antigens (Bremer et al., 2003). According to Kumar et al.(2003), the genus Salmonella has over 2,000 serovars. Two of these―Saintpaul and Newport―have been isolated from seafood (Ponce et al., 2008). The prevalence of specific Salmonella serovars is related to food type. Thus, the serovars Weltevreden and Rissen are predominant in seafoods, as shown by Kumar et al. (2009) in a study on the distribution and phenotypical characterization of Salmonella serovars isolated from samples of fish, crustaceans and mollusks from India. High incidences of Salmonella in seafoods have been reported worldwide (Kumar et al., 2010; Asai et al., 2008) in association with outbreaks of fever, nausea, vomiting and diarrhea (Ling et al., 2002). Since Salmonella inhabits the intestinal tract of warm-blooded animals, its presence in aquaculture livestock is most likely due to the introduction of fecal bacteria into culture ponds (Koonse et al., 2005). In fact, in a study on Salmonella in shrimp, Shabarinath et al. (2007) concluded this pathogen is generally found in rivers and marine/estuarine sediments exposed to fecal contamination. The quality of aquaculture products may be compromised by exposure to pathogens and biological or chemical contaminants. The latter include chemical agents commonly used in aquaculture, such as veterinary antibiotics, antiseptics and anesthetics. Few antibiotics have been adapted to or developed specifically for use in aquatic organisms. Thus, in Europe several classes of antibiotics may be used in aquaculture, including sulfonamides, quinolones, macrolides, tetracyclines and emamectin. This, however, poses a considerable risk of release of antimicrobials into the environment and eventually of the development of resistance in pathogenic bacteria (Fauconneau, 2002). The second half of the 20th century saw two major events in the epidemiology of salmonellosis: the appearance of human infections caused by food-borne S. enteritidis and by Salmonella strains with multiple resistance (Velge et al., 2005). In fact, Angulo et al. (2000) suggested that the factors determining resistance to multiple antibiotics in strains of S. Typhimurium DT104 may first have developed in bacteria in the aquaculture environment, possibly as the result of the regular use of antibiotics in fodder. The present study is a review of the literature on resistant Salmonella strains in aquaculture and an assessment of the risk this represents for human health. In addition, information was collected on the incidence of resistant Salmonella strains isolated from shrimp farm environments in Northeastern Brazil.