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Microbes & Immunity Regulation of Staphylococcus aureus CP biosynthesis

CP compared to vancomycin-susceptible S. aureus (VSSA) 3. Biosynthesis of S. aureus capsule
controls. Correspondingly, expression of cap5E, a key gene
for CP biosynthesis, was significantly elevated in the VISA The genes encoding the enzymes responsible for capsule
strains SA137/93A, SA137/93G, and Mu50, compared to synthesis in S. aureus are clustered in a cap operon
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the VSSA strains SA1450/94, Reynolds, and Newman. comprising 16 genes, designated capA to capP. The
However, the mechanism by which the capsule contributes capsule serotypes are commonly used to distinguish gene
clusters from different cap operons, such as cap5A–cap5P
to antimicrobial resistance requires further investigation.
and cap8A–cap8P for CP5 and CP8, respectively. Amino
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As extracellular cell wall components composed of acid sequence comparisons revealed that 12 out of the 16
polysaccharide chains, CPs are good candidates for vaccine Cap proteins encoded CP5 and CP8 operons are highly
development. Most vaccine candidates have focused on conserved, exhibiting more than 97% sequence identity
CP5 and CP8. Berti et al. highlighted the critical role of (Figure 2). In contrast, the remaining four proteins – CapH,
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48
O-acetylation in the functional immune response. Both CP5 CapI, CapJ, and CapK – share <43% sequence identity and
and CP8 of S. aureus possess O-acetylation, but at distinct are considered serotype-specific enzymes, playing key
sites: C3 of L-FucNAc for CP5 and C4 of D-ManNAcA roles in determining CP5 and CP8 serotypes. 55
for CP8 (Figure 1B). Scully et al. demonstrated that The biosynthesis of S. aureus CPs is a complex and
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O-acetylation is essential for CPS-CRM197 conjugates to orderly process. Rausch et al. proposed a biosynthetic
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elicit effective antibody responses against S. aureus both in pathway for CP production in S. aureus (Figure 3).
vivo (mouse model) and in vitro. Although robust antibody Successful capsule formation requires the coordinated
responses were observed following primary vaccination, activity of all cap operon-encoded enzymes. The process
the StaphVAX-Nabi vaccine – comprising CP5 and CP8 begins with the formation of a tyrosine kinase complex
conjugated to Pseudomonas aeruginosa exotoxin A – failed by CapA and CapB. This complex regulates the activity of
in Phase III clinical trials. One possible explanation for several downstream enzymes through phosphorylation
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this failure is abnormal O-acylation modifications in the and controls the use of metabolic precursors for CP
vaccine antigen. A four-antigen S. aureus vaccine (CP5/ synthesis. CapC acts as a negative regulator by inhibiting
CP8/rmClfA/rMntC) demonstrated a good safety profile, CapB kinase activity, thereby reducing target protein
high tolerability, and strong immunogenicity in Phase I phosphorylation.
clinical testing. 52
UDP-D-N-acetylglucosamine (UDP-D-GlcNAc)
Chemical synthesis of a single repeating trisaccharide serves as a primary precursor for CP synthesis. CapD
often fails to generate sufficient antigenic activity. The and CapN enzymes catalyze its conversion into soluble
presence of rare monosaccharides, cis-glycosidic linkages, UDP-N-acetyl-D-fucosamine (UDP-D-FucNAc).
and O-acetylation poses significant challenges in the CapM then transfers the phosphosugar moiety of
synthetic production of CP5 and CP8 fragments. Østerlid UDP-D-FucNAc to the membrane-anchored lipid
et al. reported the stereoselective assembly of complex carrier undecaprenyl-phosphate (C P), generating
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CP8 fragments – including trimer, hexamer, nonamer, and the intermediate lipid I . In parallel, enzymes CapE,
cap
dodecamer units – and demonstrated through immunization CapF, and CapG convert UDP-D-GlcNAc to soluble
studies that a minimum of three repeating units is required UDP-N-acetyl-L-fucosamine (UDP-L-FucNAc). This
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to elicit an adequate immune response. Sorieul et al. is subsequently linked to lipid I by the transferase
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cap
developed a multiepitope vaccine by conjugating S. aureus CapL, forming lipid II . CapP and CapO catalyze the
cap
CP8 with a chimeric protein containing Hla (an S. aureus formation of UDP-N-acetyl-D-mannosaminuronic
cytotoxin) and PcrV (a P. aeruginosa cytotoxin). This vaccine acid (UDP-D-ManNAcA) from UDP-D-GlcNAc. This
successfully induced antibodies against all three antigens nucleotide-activated monosaccharide is added to lipid
and conferred functional protection, demonstrating its II by the transmembrane protein CapI, yielding
cap
potential as a proof-of-concept for multivalent vaccines the final capsule precursor lipid III . The resulting
cap
targeting polymicrobial infections. undecaprenyl-phosphate-linked trisaccharide is further
Collectively, CPs contribute to multiple aspects of modified by the acetyltransferase CapH, which catalyzes
S. aureus pathogenicity; however, the effect of CPs on the O-acetylation of L-FucNAc at the C3 position in
20,57
virulence may be strain-specific due to variability in CP5 strains. Finally, the completed and modified
CP expression among clinical isolates. Advances in precursor is translocated to the cell surface by CapK and
glycoconjugate vaccine development – leveraging bacterial CapJ, resulting in capsule formation.
carbohydrates as key antigens – hold promise for improved Understanding this putative biosynthetic pathway
strategies to control S. aureus infections. provides crucial insights into the regulatory mechanisms

Volume 2 Issue 4 (2025) 4 doi: 10.36922/mi.8392