The aim of this study was to characterize a broad collection of isolates of Listeria monocytogenes, of different serotypes, recovered in Portugal between 2003 and 2007 from foods (n=353) and from clinical cases of human
listeriosis (n=95), in terms of antimicrobial susceptibility. All the isolates were susceptible to
ampicillin, the preferred agent to treat
listeriosis. Resistances to
nitrofurantoin (n=99), to
ciprofloxacin (n=18), to
erythromycin (n=10), to
tetracycline (n=2), to
gentamicin (n=1) and to
rifampicin (n=1) were observed. One hundred (28.3%) and 20 (21.0%) food and clinical isolates, respectively, were resistant to at least one
antibiotic. Eight isolates (1.8%) were resistant to two or more antimicrobials of different classes, and all were collected from foods. Serogroup IVb included the highest percentage of isolates resistant to
erythromycin. The highest percentages of isolates resistant to
nitrofurantoin were of serogroup IVb and IIc. It was demonstrated that the incidence of
antibiotic-resistant isolates of L. monocytogenes, during the period 2003 to 2007, was low in Portugal but still higher than that observed in other countries. Given the increasing population at greater risk of
listeriosis, namely, the elderly, the high mortality rate of the
infection and the detection of resistant isolates, monitoring for antibiotic resistance in strains of L. monocytogenes on a large scale, and assessing the risk of
infection by these strains, is highly recommended.