Isolation of round-cell mutants of Salmonella typhimurium

Round-cell mutants of Salmonella typhimurium affected in genes divD, envB, and rodA (or pbpA) were obtained, without specific selection, in strains hypersensitive to overproduction of the histidine enzymes. All those types of round-cell mutations promoted cell division, suppressing the filaments formed by histidine-derepressed strains in media containing 2% dextrose. Only envB and rodA or pbpA mutants were found in round-cell strains isolated by resistance to mecillinam. divD mutants did not appear because, in spite of their definite round-cell morphology, they remain sensitive to mecillinam.