Sialic Acid-Containing Lipopolysaccharides of Salmonella O48 Strains—Potential Role in Camouflage and Susceptibility to the Bactericidal Effect of Normal Human Serum

Abstract

Sialic acid (N-acetylneuraminic acid, NeuAc) plays an essential role in protecting gram-negative bacteria against the bactericidal activity of serum and may contribute to the pathogenicity of bacteria by mimicking epitopes that resemble host tissue components (molecular mimicry). The role of sialic acid (NeuAc)-containing lipopolysaccharides (LPS) of Salmonella O48 strains in the complement activation of normal human serum (NHS) was investigated. NeuAc-containing lipooligosaccharides cause a downregulation of complement activation and may serve to camouflage the bacterial surface from the immunological response of the host. Serotype O48 Salmonella strains have the O-antigen structure containing NeuAc while its serovars differ in outer membrane protein composition. In this study, the mechanisms of complement activation responsible for killing Salmonella O48 serum-sensitive rods by NHS were established. Four of such mechanisms involving pathways, which are important in the bactericidal mechanism of complement activation, were distinguished: only the classical/lectin pathways, independent activation of the classical/lectin or alternative pathway, parallel activation of the classical/lectin and alternative pathways, and only the alternative pathway important in the bactericidal action of human serum. To further study the role of NeuAc, its content in bacterial cells was determined by gas-liquid chromatography-mass spectrometry in relation to 3-deoxy-D-manno-2-octulosonic acid (Kdo), an inherent constituent of LPS. The results indicate that neither the presence of sialic acid in LPS nor the length of the O-specific part of LPS containing NeuAc plays a decisive role in determining bacterial resistance to the bactericidal activity of complement and that the presence of sialic acid in the structure of LPS is not sufficient to block the activation of the alternative pathway of complement. We observed that for three strains with a very high NeuAc/Kdo ratio the alternative pathways were decisive in the bactericidal action of human serum. The results indicated that those strains are not capable of inhibiting the alternative pathway very effectively. As the pathogenicity of most Salmonella serotypes remains undefined, research into the interactions between these bacterial cells and host organisms is indispensable.

Searching for Outer Membrane Proteins Typical of Serum-Sensitive and Serum-Resistant Phenotypes of Salmonella

“…Long-term investigations carried out on Salmonella O48 shown that strains differ in the susceptibility to various sera, human or animal , 2010a, 2010b, 2009b, 2009cFutoma et al 2005). These authors suggest that the presense of sialylated LPSs do not play a decisive role in determining bacterial resistance to the bactericidal activity of serum, and that the presence of NeuAc in the LPS structure is not sufficient to block activation of the alternative pathway of complement in serum.…”

“…The above-mentioned authors , 2010a, 2010b, 2009b, 2009cFutoma et al 2005) and Sarowska and co-workers (2010) described the results of Salmonella susceptibility to human and bovine serum. Bactericidal activity of serum was determined as described previously (Doroszkiewicz, 1997).…”

“…In other experiments, aimed at analyzing the mechanism of serum complement activation, serovars, which were sensitive to the bactericidal action of serum, were used. Bugla-Ploskonska et al (2010a, 2010b, 2009b) examined serum in which AP or CP/LP pathways of C were inhibited. Authors established the following mechanisms of complement activation by Salmonella O48.…”

“…These data raise the question as to whether released OMPs, such as the 18-kDa OMP, could play a role in the pathogenesis of Gram-negative sepsis. References: Bugla-Ploskonska et al (2011, 2010a, 2010b, 2009b, 2009cFutoma et al (2005); Sarowska et al (2010) NHS -normal human serum; NCS -normal cord serum; NBS -normal bovine serum 1 OMPs characteristic for only serum-sensitive strains (not present in serum-resistant strains) or for only serum-resistant strains (not present in serum-sensitive strains) 2 NT -not tested 3 data presented only in this paper 4 there are any individual OMPs in this strain when compare to others 5 strains were regarded as resistant when their survival was above 100% in the highest concentrations of serum Table 2. The susceptibility of Salmonella strains to NHS, NCS, and NBS put together with OMPs…”

The mechanisms of complement activation in normal bovine serum and normal horse serum against Yersinia enterocolitica O:9 strains with different outer membrane proteins content

“…The numbers of CFU/mL from the bactericidal assays were converted to percentage survival values, where the starting inoculum at T 0 was set at 100%, and values for the survival rates were expressed as log10% CFU/mL (Bugla-Płoskońska et al 2010b, Skorek et al 2013. The data from each assay were compared by the analysis of ANOVA/MANOVA.…”