Lipopolysaccharides were isolated from Pseudomonas aeruginosa PAC 1 R and its lipopolysaccharide-defective mutants. Core fractions were obtained from them following mild acid hydrolysis, by gel chromatography using Bio-Gel P6. The core fraction from the parent strain contained glucose, rhamnose, galactosamine, heptose and alanine in the approximate molar ratios 3 : 1 : 1 : 1 : 1 while core fractions from the defective mutants lacked rhamnose and/or one, two, or three glucose units. Methylation analysis of the core fractions including identification of the partially methylated alditol acetates by combined gas-liquid chromatography and mass spectroscopy allowed the formulation of possible structures. These were confirmed by 3C nuclear magnetic resonance spectroscopy. Fractions consisting of core to which was attached unpolymerised 0-antigen were isolated from the parent strain and one of the mutants and analysed in the same way to identify the attachment point of the 0-antigen.The combined data leads to the formulation of the core oligosaccharide of PAClR and its mutants as follows:

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S U M M A R YMutants with defective lipopolysaccharides (LPSs) were isolated from Pseudomonas aeruginosa PACIR (Habs serogroup 3) by selection for resistance to aeruginocin from P . aeruginosa PI 6. Carbenicillin-sensitive mutants were isolated from P . aeruginosa PACI but not all had defective LPSs. Rough colonial morphology and resistance to bacteriophage 119X appeared to be independent of LPS composition.The LPSs from five mutants were analysed and compared with that of the parent strain. Separation of partially-degraded polysaccharides from LPS from PACI on Sephadex G75 yielded two different high molecular weight fractions and a phosphorylated low molecular weight fraction (L). The mutant LPSs lacked most or all of the high molecular weight fractions but retained some low molecular weight material. That from PACI and two of the mutants was separated by elution from Biogel P6 into two fractions. One, Lz, was the core polysaccharide while the other, LI, contained short antigenic side-chains attached to the core like the semi-rough (SR) LPSs of the Enterobacteriaceae. The two mutants which gave the LI fraction reacted with Habs 3 and PACI antisera as did the parent strain. The other three mutants were unreactive and their LPSs contained core components only. One appeared to have a complete core while the other two lacked rhamnose and rhamnose plus glucose respectively. Thus there may be four types of LPS in PACI : one contains unsubstituted core polysaccharide and yields L2 on acid hydrolysis, another has short antigenic side-chains of the SR type and yields the LI fraction, while the two high molecular weight fractions are derived from core polysaccharides with different side-chains.