Structural and Serological Characterization of Yet Another New O Antigen, O86, in Proteus mirabilis Clinical Strains

Abstract

Bacteria from the genus Proteus are facultative human pathogens, primarily attacking the urinary tract and wounds. A total of 85 O serogroups have been identified so far among these bacilli. P. mirabilis Bprz 86 was isolated from the fistula of a patient in Łódź, Poland. Enzyme-Linked Immunosorbent Assay (ELISA) and Western blotting studies involving the P. mirabilis Bprz 86 lipopolysaccharide (LPS) and the strain-specific rabbit antiserum indicated that the strain, which does not belong to any of the O1-O85 serogroups, shares a common epitope with Proteus O17 antigens and is identical to another clinical P. mirabilis strain, Sm 120, isolated from the urine of a patient in the area. The O-specific polysaccharide (O antigen) was obtained from P. mirabilis Bprz 86 LPS through mild acid degradation, and the six-constituent structure of its repeating unit was determined using chemical analyses and 1D and 2D ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy. It includes (R)-3-hydroxybutanoyl, which, along with fucosamine and glucose residues, forms a fragment also present in the O17 antigens. Based on the obtained serological and chemical data, the two studied P. mirabilis isolates were proposed as candidates for a new successive O serogroup in the genus Proteus, O86.

Keywords: (R)-3-hydroxybutanoic acid; O-specific polysaccharide; chemical structure; epitope; lipopolysaccharide; serotyping.

Figure 1

(a) Dienes test for the studied P. mirabilis Bprz 86 and Sm 120 isolates on nutrient agar plates. The border line (black arrow) is clearly visible between the swarms of the strains; (b) Western blot of P. mirabilis Bprz 86 lipopolysaccharide (LPS) (A) and P. mirabilis Sm 120 biomass (B) with P. mirabilis Bprz 86-specific serum.

Figure 2

Parts of a ¹H,¹³C Heteronuclear Single Quantum Coherence (HSQC) spectrum of the O polysaccharide (OPS) of the P. mirabilis Bprz 86 strain. The corresponding parts of the ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectra are shown along the horizontal and vertical axes, respectively. The numbers refer to H/C pairs in sugar residues denoted by letters as indicated in Table 3 and Figure 3a.

Figure 3

The O-repeating unit chemical structure of (a) the P. mirabilis Bprz 86 studied strain and (b) the representatives of the O17 serogroup [12]. The shared fragment forming the common serological epitope is marked in red.