Resistance of Serratia marcescens to polymyxin B: a comparative investigation of two S-form lipopolysaccharides obtained from a sensitive and a resistant variant of strain 111

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Abstract
Lipopolysaccharide (LPS) of the polymyxin B (PmB)-sensitive variant of Serratia marcescens smooth-type strain 111 (senLPS) binds significantly more PmB than that of the resistant variant (resLPS), following mild acid hydrolysis this difference is abolished. The main compositional differences between these strains are: (i) the amount of 4-amino-4-deoxy-arabinose (Ara4N); and (ii) the substitution patterns of the phosphate groups. ResLPS contains three times more Ara4N than senLPS, mainly phosphodiesters, whereas senLPS contains mainly (di)-phosphomonoesters. Thus, the senLPS, and the bacterial surface, contain a higher negative charge density. Mild acid hydrolysis decreases the PmB-binding capacity of senLPS and increases that of resLPS. The resulting change in LPS composition was a reduction in the amount of phosphate (about 40%) and a loss of nearly all the Ara4N residues, from both LPS. Mild alkaline hydrolysis removes Ara4N from both LPS, and the hydrolysate contains Ara4N-phosphate, but no Ara4N-(1 →8)-Kdo was detected. The results suggest that the 4'-phosphate of lipid A mediates the binding of PmB, whereas its substitution by Ara4N prevents it.