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Antimicrobial Agents and Chemotherapy, December 2005, p. 5075-5080, Vol. 49, No. 12
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.12.5075-5080.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

ß-Lactam Resistance in Staphylococcus aureus Cells That Do Not Require a Cell Wall for Integrity

University Hospital of North Durham, North Road, Durham DH1 5TW,¹ South Tyneside District Hospital, Harton Lane, South Shields NE34 0PL,² School of Biological and Biomedical Sciences, University of Durham, Durham DH1 3LE,³ Department of Clinical Sciences, University of Newcastle, Newcastle, United Kingdom⁴

Received 4 May 2005/ Returned for modification 13 July 2005/ Accepted 18 September 2005

Staphylococcus aureus ATCC 9144 cells with defective cell walls were generated on a medium with elevated osmolality in the presence of sublethal levels of penicillin G. On removal of antibiotic pressure, the cells exhibited stable penicillin and methicillin resistance. The resistance was homogeneous and its acquisition was enhanced following transient cell wall-defective growth. The resistant cells were mecA negative, ß-lactamase negative and did not contain any mutations in the coding regions of pbp genes. When penicillin was added back to resistant cells, they continued to grow and produced a diffuse cell wall that was resistant to the action by lysostaphin but was very sensitive to lysis with Triton X-100. These data indicate that the resistant cells are not dependent upon an intact cell wall for osmotic stability and they are able to switch readily to this mode of growth in the presence of penicillin G.

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