Organic and conventional fruits and vegetables contain equivalent counts of Gram-negative bacteria expressing resistance to antibacterial agents.
The European Food Safety Authority (EFSA) recently attempted to ascertain to what extent food constitutes a vehicle for the acquisition by humans of antimicrobial resistant bacteria or resistance genes, to grade the risks of such acquisition, and to identify control options (EFSA, 2008). These problems have arisen at a time when many pathogenic Gram-negative bacteria (GNB), such as members of the family Enterobacteriaceae, have become resistant to most antibiotics (Levy and Marshall, 2004), resulting in difficult-to-treat infections that make it necessary to avoid further increases in resistance, because very few new antibiotics are becoming available (Shlaes, 2003). In this situation, better understanding of the paths via which resistance to antibiotics spreads might be crucial in helping to control such dissemination.
Intestinal commensal GNB are good recipients for the horizontal transfer of genes from environmentally resistant by transient inhabitants (Salyers et al., 2004) and can further disseminate resistance in pathogenic species (Alekshun and Levy, 2006). This resistance was found to decrease in subjects fed sterilized food (Corpet, 1988) but was frequent in vegetarians (Guinee et al., 1970; van den Braak et al., 1997). Exposure to resistant GNB has been traced to contaminated vegetables (Kapperud et al., 1995) as well as fruits, when either is eaten raw, with environmental bacteria acting as potential sources of resistance (Rossolini et al., 2008). Contamination and subsequent infections are indeed feared in patients with leukemia (Remington and Schimpff, 1981) and cystic fibrosis (Moore et al., 2001).
In France, most fruits and vegetables are conventionally produced, but organic products are also available and are attractive to consumers (Bio Agence, 2008). Manure from farms using antibiotics increases resistance in soil bacteria (Binh et al., 2007; Ghosh and LaPara, 2007; Heuer and Smalla, 2007), and this resistance may contaminate agricultural products. However, producers of organic fruit and vegetables must adhere to European rules (Anonymous, 2000): these forbid fertilization by chemical agents, but allow the use of manure from organic farming and sewage. Recently, specific types of genes which confer extended resistance on beta-lactams, such as the CTX-M gene family, have emerged worldwide in enterobacteria, causing infections in humans (Pitout and Laupland, 2008), and their sources have been traced to environmental bacterial species (Canton and Coque, 2006). In this study, the results for resistance to third generation cephalosporins indicated that environmental species were responsible for most of the colonization of fruits and vegetables, first, because most of the species isolated were indeed environmental, and second, because resistance was more frequent in the products grown in contact with the soil, and many soil bacteria indeed carry resistance genes (Dantas et al., 2008; Demaneche et al., 2008).
These results suggest that further investigations should be undertaken in this field.