During growth in magnesium (Mg)-deficient mineral media, cells synthesise large amounts of H1 outer-membrane protein and are resistant to polymyxins and EDTA. It has been suggested that H1 protein replaces Mg as an outer-membrane-stabilising component in Mg -deprived cells, thereby removing the EDTA target and blocking an adsorption site for polymyxins. Induction of H1 protein synthesis also occurred in cells grown in Antibiotic No. 3 Broth (Ab3B), although this medium is not Mg -deficient. Generally, significant induction of H1 protein did not occur in cultures grown in other complex media such as Proteose Peptone and Nutrient Broth, which contained less Mg than Ab3B, nor in Isosensitest Broth or Mueller Hinton Broth, which contained higher Mg concentrations. H1-protein-induced cells from Ab3B cultures, unlike those from Mg -deficient mineral-broth culture, remained fully sensitive to polymyxin B and, with one exception, to EDTA. It is concluded that induction of H1 protein does not itself confer resistance to polymyxin B, and has no more than a minor role in EDTA resistance. Other cell-wall changes, such as phospholipid modifications and the absence of Mg, probably account for the resistance of Mg -deprived cells.


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