Antibiotic Resistance in Listeria spp

Due to the multiplicity of antibiotics, infections caused by drug-resistant bacteria did not represent a medical problem until the early 1980s. However, evolution of bacteria towards resistance has been considerably accelerated by the selective pressure exerted by overprescription of drugs in clinical settings and their heavy use as growth promoters for farm animals. Since bacteria have the remarkable ability to develop resistance to every antibiotic, we can anticipate that even bacterial species such as Listeria, which are still considered to be susceptible to almost all antibiotics, will evolve toward multiresistance. Listeria spp. are ubiquitous bacteria widely distributed in the environment (20, 41). Among the seven species of Listeria, only Listeria monocytogenes is commonly pathogenic for humans. It can cause serious infections such as meningitis or septicemia in newborns, immunocompromised patients, and the elderly or lead to abortion (16, 20, 27, 41, 43). Although human listeriosis occurs only sporadically (16, 41, 43) several outbreaks have been observed during the last two decades (20, 25, 43). It is now established that food-borne transmission constitutes the main route of acquisition of listeriosis (16, 34, 41, 43). Despite efficient antibiotic therapy, listeriosis represents a public health problem since it is fatal in up to 30% of cases (16, 26). In general, isolates of L. monocytogenes, as well as strains of other Listeria spp., are susceptible to a wide range of antibiotics except cephalosporins and fosfomycin (23, 24). The treatment of choice for listeriosis remains the administration of ampicillin or penicillin G combined with an aminoglycoside, classically gentamicin (4, 12, 18, 20, 26, 27, 29, 31, 32). The association of trimethoprim with a sulfonamide, such as sulfamethoxazole in co-trimoxazole, is considered to be a second-choice therapy. The most active agent in the combination seems to be trimethoprim, which is synergized by sulfamethoxazole (4, 26, 27, 29, 32, 43). Most isolates from clinical as well as food-borne and environmental sources are susceptible to the antibiotics active against gram-positive bacteria. The first L. monocytogenes strains resistant to antibiotics were reported in 1988 (35). The strains were resistant to >10 μg of tetracycline per ml. The first multiresistant strain of L. monocytogenes was isolated in France in 1988 (35). Since then, other strains of Listeria spp. isolated from food or the environment or in sporadic cases of human listeriosis resistant to one or several antibiotics have been described (2, 9, 14, 15, 18, 21, 30, 36, 38, 44). This review will focus on the recent emergence of antibiotic resistance in Listeria spp. by acquisition of three types of mobile genetic elements: self-transferable and mobilizable plasmids and conjugative transposons (2, 8, 9, 21, 35, 36).