Salmonella typhi in vivo acquires resistance to both chloramphenicol and co-trimoxazole.

Salmonella typhi in the bowel of a patient with enteric fever treated with chloramphenicol and later co-trimoxazole acquired resistance to these drugs. Chloramphenicol and sulphonamide resistances were determined by one plasmid, of IncH, whose carriage also changed the Vi phage type. Trimethoprim resistance was determined by a transposon, Tn7, carried in another plasmid, of IncI. The same resistance genes were identified in a strain of klebsiella isolated from the patient.