Effects of subminimal inhibitory concentrations of amoxicillin against Actinobacillus pleuropneumoniae.

The bactericidal effects of amoxicillin at below minimum inhibitory concentration (MIC) against Actinobacillus pleuropneumoniae NB001 were studied in vitro and in vivo. In vivo, the efficacy of amoxicillin on experimentally induced A. pleuropneumoniae infection in disease-free pigs was evaluated. Nine pigs were divided into three groups and all three groups were housed in the same room. Group I pigs were given long-acting amoxicillin injection 22 h prior to A. pleuropneumoniae challenge. Group II pigs were also A. pleuropneumoniae challenged but not given long-acting amoxicillin. Group III pigs were not treated. In vitro, A. pleuropneumoniae growth was suppressed in porcine blood with amoxicillin at below MIC. In vivo, clinical signs of disease were absent or mild in group I during 50 h post-challenge, and serum amoxicillin concentration was already less than MIC from 15 h post-challenge. Infected group II controls were severely affected by the infection, and mortality reached 100% within 50 h post-challenge. All non-treated pigs in group III became infected with NB001 from infected control pigs, and they displayed severe clinical signs of disease within 24 h post-challenge of groups I and II, and died within 50 h post-challenge of groups I and II.