Antibiotic resistance determinants and genetic analysis of Salmonella enterica isolated from food in Morocco
Introduction
Non-typhoidal Salmonella (NTS) are one of the most frequent causes of foodborne diseases worldwide, with severe public health consequences (Rabsch et al., 2001). It is estimated that every year 94 million persons contract NTS infections, with about 155,000 deaths (http://www.who.int/drugresistance/documents/surveillancereport/en/).
Although uncomplicated NTS gastrointestinal infections do not require antimicrobial treatment, in most severe cases (e.g., acute diarrhea, high fever and systemic infection) an appropriate antimicrobial therapy is needed (Gordon, 2008, Morpeth et al., 2009, Reddy et al., 2010), and the drugs of choice are fluoroquinolones for adults and third-generation cephalosporins for children (White et al., 2001). The selective pressure due to the misuse of antibiotics in humans and domestic livestock has led to the emergence of antibiotic resistance in zoonotic bacteria, and multidrug-resistant (MDR) Salmonella are increasingly isolated from different food products worldwide (Ben Aissa and Al-Gallas, 2008, Bouchrif et al., 2009b, Threlfall et al., 2000). Multiple resistance genes, incorporated into integrons or transposons, can be easily exchanged among Salmonella and other bacteria through genetic mobile elements, causing the rapid dissemination of MDR strains (Guerra et al., 2000, Michael et al., 2006). A well-known example is the Salmonella Genomic Island 1 (SGI1), related to the wide spread of MDR S. Typhimurium DT104 epidemic strains and described in different Salmonella serovars (Boyd et al., 2001, Levings and Lightfoot, 2005). Conjugative plasmids are also actively involved in the transmission of resistance genes, as reported for the IncI1 plasmids associated to the dissemination of β-lactamase genes among Salmonella from food of animal origin (Cloeckaert et al., 2007, García-Fernández et al., 2008). Infections caused by multidrug-resistant NTS are more acute and invasive, and therapeutic failures due to antimicrobial resistance increase the risk of death, especially in developing countries where alternative treatments are often unavailable (http://www.who.int/drugresistance/documents/surveillancereport/en/).
Information and surveillance on antimicrobial resistance are extremely limited in Africa. As in a previous study we evidenced the presence of antibiotic-resistant Salmonella in different food products collected in Morocco (Bouchrif et al., 2009b), in the present work we investigated the molecular basis of antimicrobial resistance in these isolates by the search of integrons, resistance genes and SGI1. Further, we analyzed the genetic relatedness of the strains by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST) and plasmid profiling.
Section snippets
Bacterial strains
Ninety-four Salmonella enterica strains, isolated from food samples in Morocco, were included in this work for genotypic analysis. Isolates belonged to 13 different serovars including Infantis (n = 24), Mbandaka (n = 15), Bredeney (n = 13), Blockley (n = 11), Typhimurium (n = 8), Indiana (n = 6), Hadar (n = 4), Anatum (n = 3), Enteritidis (n = 3), Altona (n = 2), Senftenberg (n = 2), Kentucky (n = 2), Cerro (n = 1)] twenty-six isolates displayed resistance to at least one of the following antimicrobial agents:
PFGE and epidemiological analysis
The 94 Salmonella enterica isolates were typed by PFGE into 32 XbaI macrorestriction profiles (pulsotypes). Among these, 13 PFGE serovar types (X1 to X13) were identified. Strains of serovar Hadar, Kentucky, Senftenberg and Cerro exhibited only one pulsotype, whereas the remaining serovars showed up to five different pulsotypes, clustering into nine groups corresponding to the serovars (Fig. 1). The highest diversity of pulsotypes within the groups was observed in serovar Mbandaka (69%), the
Discussion
The ability to distinguish strains or phylogenetic lineages of a bacterial pathogen is required for addressing many questions in food microbiology and epidemiology, and this is very true for Salmonella strains responsible to foodborne diseases. Unfortunately, data of the epidemiology and of the resistance mechanisms of foodborne Salmonella in Morocco are extremely limited (Ammari et al., 2009a, Ammari et al., 2009b, Bouchrif et al., 2008, Bouchrif et al., 2009a). The results of our research
Conclusions
This study increases the knowledge of the epidemiology and molecular basis of antibiotic-resistance of foodborne Salmonella in Morocco, evidencing the presence in beef meat, mollusk and poultry of MDR strains potentially dangerous in case of human infections. Results obtained provide a basis for further studies, essential for a more extensive understanding of the Salmonella diffusion along the food chain in Morocco.
Conflict of interest statement
The authors declare no conflicts of interest.
Acknowledgments
This work was partially supported by a grant from Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR), PRIN prot 2007 W52X9M. BP is supported by Sardinian Region [POR-FSE 2007–2013-Obiettivo Competitività regionale e occupazione, asse IV capitale umano, linea di attività 1.3.1].
We thank Ida Luzzi from the Istituto Superiore di Sanità (ISS) of Rome, Italy, for supplying Salmonella Braenderup strain H9812, and Massimo Deligios from the Department of Biomedical Sciences, University
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