Abstract

Five epidemiologically related urine isolates of Klebsiella pneumoniae (sensu lato), capsular serotype K21 may spontaneously lose the ability to synthesise K21 antigen in vitro and in vivo. Another four isolates of K. pneumoniae K21+, epidemiologically unrelated to the other five, did not exhibit this effect. Elimination of the ability to synthesise K21 antigen may be enhanced by treatment with acridine orange. Transfer of K21 antigen synthesis from K. pneumoniae to Escherichia coli K12 was observed in eight independent experiments. Elimination from or acquisition of the ability to synthesise K21 antigen is not accompanied by changes in the antibiotic sensitivity patterns or biochemical characteristics of bacteria.