A strain of (no. FAR4) has been isolated from a patient at frequent intervals over 18 months. This strain has previously been shown to lose resistance to several antibiotics (Lacey , 1973); during the last 6 months further changes have occurred in it

1. Lincomycin resistance appeared after the patient had been treated with clindamycin. This was due to mutation to constitutive resistance to both lincomycin and erythromycin at a locus adjacent to that determining “indu-cible” resistance to erythromycin. Both loci are probably carried by an extrachromosomal element.

2. Three types of variation in the PF plasmid were observed, Alteration in production of penicillinase from macroconstitutive to micro-constitutive, was not associated with any detectable change in plasmid size. This change gave the cell an advantage , because it was associated with a faster growth rate, and probably also , because the microconstitutive variants were isolated in increasing numbers after flucloxacillin therapy had ceased, Complete loss of determinants for penicillinase production and for resistance to cadmium ions from the PF plasmid was associated with a decrease in plasmid size of about 4 × 10 daltons. This change was also associated with increased growth rate Loss of resistance to fusidic acid from the PF plasmid was associated with a decrease in plasmid size of about 1 × 10 daltons.


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