Emergence of resistance to chloramphenicol in salmonella.

Surveillance of antibiotic resistance of salmonella isolates from humans in the north-eastern United States has detected resistance to chloramphenicol on eight occasions, three of which were hospital outbreaks. Resistance was developed during treatment in five instances, and in another resistance this situation was suspected. On two other occasions chloramphenicol-resistant Salmonella appear to have been imported from outside the country. Common features of the cases were multiple antibiotic resistance, transfer of R-factor, simultaneous presence of different patterns of resistance, similar patterns of antibiotic resistance in other intestinal bacteria, and apparent in vivo and in vitro loss of resistance to chloramphenicol with time.