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We present here the biochemical and genetic characterization and antimicrobial susceptibility of 58 isolates of Aerococcus viridans isolated in pure culture from different clinical specimens of normally sterile body sites of pigs. A. viridans isolates were commonly susceptible to ␤-lactam antimicrobials and exhibited a great genetic heterogeneity as determined by pulsed-field gel electrophoresis typing. The results indicate that A. viridans might be included in the list of possible etiological agents causing disease in pigs.The genus Aerococcus was initially described including a single species, Aerococcus viridans. Five new species of Aerococcus have been further identified: Aerococcus urinae, Aerococcus sanguinicola, Aerococcus christensenii, Aerococcus urinaeequi, and Aerococcus urinaehominis (6; http://www.bacterio .net). The clinical significance of these species has been clearly established for A. urinae and A. viridans. A. urinae is a rarely reported human pathogen that has been identified as responsible for urinary tract infections, septicemia, sepsis, endocarditis, or lymphadenitis (22)(23)(24)32). A. viridans has been associated with different human infections such as endocarditis, urinary tract infections, arthritis, or meningitis (7,10,13,20). Moreover, this species is a pathogen for crustaceans, causing gaffkemia in marine lobsters (1, 26), and it has been associated with septicemia in sea turtles (27). A. viridans has also been isolated from the milk of cows with subclinical mastitis (5) and has been associated with septicemia in immunodeficient mice (3); there have been no reports, as far as we know, of the isolation of this microorganism from other animal species. In the present study we report the isolation, biochemical and genetic characterization, and antimicrobial susceptibility of 58 isolates of A. viridans from different clinical specimens of pigs.According to the source of isolation, 30 isolates were obtained from the joints of pigs with arthritis, 14 isolates were obtained from the brains of pigs with meningitis, and 14 isolates were obtained from the lungs of pigs with pneumonia. All isolates were recovered in pure culture from individual animal samples in moderate to high numbers, which is indicative of their clinical significance and therefore likely to be associated with bacterial infection. Details of the clinical background of the A. viridans isolates included in the study are given in Table 1. Isolation of A. viridans isolates from clinical specimens was carried out on Columbia agar containing 5% of defibrinated sheep blood (bioMérieux España, s.a.) after incubation at 37°C for 24 h under aerobic and anaerobic conditions. All isolates were gram-positive catalase-negative cocci arranged in single cells, in pairs, in tetrads, or in small groups. The organisms were facultatively anaerobic and produced unpigmented, alpha-hemolytic, circular, colonies Ͻ1 mm in diameter after 24 h of incubation at 37°C on blood agar. Biochemical identification of A. viridans isolates was carried out by ...