RESISTANCE OF SERRATIA-MARCESCENS TO POLYMYXIN-B - A COMPARATIVE INVESTIGATION OF 2 S-FORM LIPOPOLYSACCHARIDES OBTAINED FROM A SENSITIVE AND A RESISTANT VARIANT OF STRAIN

Lipopolysaccharide (LPS) of the polymyxin B (PmB)-sensitive variant of Serratia marcescens smooth-type strain 111 (senLPS) binds significant ly more PmB than that of the resistant variant (resLPS), following mil d acid hydrolysis this difference is abolished. The main compositional differences between these strains are: (ij the amount of 4-amino-4-de oxy-arabinose (Ara4N); and (ii) the substitution patterns of the phosp hate groups. ResLPS contains three times more Ara4N than senLPS, mainl y phosphodiesters, whereas senLPS contains mainly (di)-phosphomonoeste rs. Thus, the senLPS, and the bacterial surface, contain a higher nega tive charge density. Mild acid hydrolysis decreases the PmB-binding ca pacity 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 alk aline hydrolysis removes Ara4N from both LPS, and the hydrolysate cont ains Ara4N-phosphate, but no Ara4N-(1-->8)-Kdo was detected. The resul ts suggest that the 4'-phosphate of lipid A mediates the binding of Pm B, whereas its substitution by Ara4N prevents it.