Increased overall antibiotic susceptibility in Staphylococcus aureus femAB null mutants

Abstract

The staphylococcal pentaglycine side chain of the peptidoglycan is reduced to one glycine in femAB null mutants. This is associated with increased susceptibility to methicillin and to a whole range of unrelated antibiotics as well. Genetic evidence suggests that femAB null mutants are only viable because of a compensatory mutation in an unlinked site.

FIG. 1

Genetic restriction map of the femAB region. The bottom line shows the EcoRV fragments recognized by the 10.5-kb PstI probe indicated above by the heavy bar. The fragments are numbered according to their size as they light up in the BB255 and BB270 digests shown in Fig. 2. The approximate insertion sites of the 5.2-kb-long Tn551 inserts Ω2000, Ω2006, and Ω8 in that part of the chromosome are shown by triangles. The deletion in AS145 is shown by a black box, which contained a substitution of a tetK cassette of 1.8 kb, as shown by an inverted triangle. Tn551 has no EcoRV site, whereas the tetK cassette has an internal EcoRV site.

FIG. 2

Restriction fragment size polymorphism due to genetic manipulations. EcoRV restriction digests of the different strains were probed with the 10.5-kb PstI fragment covering femAB (Fig. 1). Lanes a and b, strains BB255 and BB270, respectively, both showing the wild-type pattern as numbered in Fig. 1 (bottom line). Lane c, BB815, a femB mutant with the Ω2006::Tn551 in fragment 6, which has moved up by 5.2 kb. Lane d, strain AS145, which has lost fragments 6 and 4 by the deletion but has instead two new fusion fragments containing parts of the tetK cassette (14). Lane e, strain K14, which is essentially the same as AS145 with the insert Ω8::Tn551 in fragment 5, which has moved up by 5.2 kb to the top of the lane. Lane f, strain BB1305, which is a backcross of femAB into AS145 by cotransduction of the insert Ω2000::Tn551 in fragment 1, and in which the wild-type restriction pattern around femAB is restored, and fragment 1 is moved up by 5.2 kb. Lane g, strain UK17, with a point mutation in femA that does not appear on the gel. Lane h, strain 1308, femAB⁺ backcross with insert Ω2000::Tn551, the same as strain BB1305. Lane i, strain BB413, the donor of Ω8::Tn551. Lane j, strain BB291, the donor of Ω2000::Tn551.