Jean C. Lee, PhD Associate Professor of Medicine
Harvard Medical School
Associate Microbiologist, Department
Most Staphylococcus aureus isolates are encapsulated, i.e., they produce uronic acid-containing, extracellular polysaccharides. Serotype 5 and 8 capsules predominate among clinical isolates, comprising ~22% and ~53%, respectively, of those tested. My laboratory investigates several aspects of S. aureus encapsulation.
The organization and function of the serotype 5 capsular polysaccharide (CP5) genes is a major interest of mine. We sequenced the S. aureus type 5 capsule gene region and showed that the capsule genes are clustered within a 17-kb fragment that encodes 16 open reading frames named cap5A through cap5P. Based on genetic homologies, we have proposed a CP5 biosynthetic pathway and confirmed the function of three of the genes by biochemical assays. We have shown that cap5H encodes an O-acetyltransferase enzyme that is responsible for O-acetylating ManNAcA. We have also demonstrated that cap5P and cap5O encode UDP-GlcNAc 2-epimerase and UDP-ManNAc dehydrogenase enzymes, respectively, that synthesize UDP-ManNAcA. We are currently characterizing the staphylococcal enzymes involved in synthesis of UDP-FucNAc and determining the molecular basis for lack of capsule expression by nontypeable S. aureus isolates.
The role of the S. aureus capsule in the pathogenesis of staphylococcal infections has been examined in a number of test systems. Serotype 5 and 8 strains of S. aureus resist opsonophagocytic killing by human polymorphonuclear leukocytes. In addition, CP5 enhanced virulence in mouse models of lethality, bacteremia, and septic arthritis. CP5 expression also promoted renal abscess formation, subcutaneous abscess formation, and long-term nasal colonization in mice. In contrast, capsule production attenuated staphylococcal virulence in a rat model of catheter-induced endocarditis.
We recently developed a mouse model of S. aureus nasal colonization and demonstrated that a capsule-defective mutant of S. aureus showed a reduced ability to persist in the nares of mice when compared to the parent strain. The colonization model will be useful for evaluating the role of capsule (and other virulence determinants) in persistence of colonization, as well as the ability of capsular antibodies to eradicate or prevent staphylococcal colonization.
Surface-associated capsular polysaccharide antigens have the potential to induce immunity to staphylococcal infections. We passively immunized rats with purified IgG induced by immunization with a bivalent vaccine composed of CP5 and CP8 conjugated to Pseudomonas exotoxin A. Animals given capsular antibodies were protected against staphylococcal endocarditis when challenged by the intraperitoneal route with either serotype 5 or serotype 8 strains of S. aureus. Quantitative cultures of the blood, kidneys, and aortic valve vegetations from the animals revealed that significantly fewer S. aureus cells were recovered from rats given capsule-specific IgG compared with rats given nonimmune IgG. Purified S. aureus CPs are zwitterionic molecules that induce intraabdominal abscess formation in animals when given via the intraperitoneal route with adjuvant. Rats prophylactically treated with CP8 by the subcutaneous route were protected against abscess formation induced by zwitterionic CPs or by viable S. aureus. Purified CP8 is also a potent activator of rat and human CD4+ T cells in vitro. The role of S. aureus CPs in the induction of abscesses at other body sites is currently under investigation.
Bhasin N, Albus A, Michon F, Livolsi PJ, Park JS, Lee JC. Identification of a gene essential of O-acetylation of the Staphylococcus aureus type 5 capsular polysaccharide. Mol Microbiol. 1998 Jan;27(1):9-21. [abstract]
Thakker M, Park JS, Carey V, Lee JC. Staphylococcus aureus serotype 5 capsular polysaccharide is anti-phagocytic and enhances bacterial virulence in a murine bacteremia model. Infect Immun. 1998 Nov;66(11):5183-9. [abstract]
Lee JC, Lee CY. Capsular polysaccharides of Staphylococcus aureus. In: Goldberg JB, editor. Genetics of bacterial polysaccharides. Boca Raton, Florida: CRC Press, Inc. 1999. p.185-205.
Kiser KB, Bhasin N, Deng L, Lee JC. Staphylococcus aureus cap5P encodes UDP-GlcNAc 2 epimerase with functional redundancy. J Bacteriol. 1999 Aug;181(16):4818-24. [abstract]
Kiser KB, Cantey-Kiser JM, Lee JC. Development and characterization of a Staphylococcus aureus nasal colonization model in mice. Infect Immun. 1999 Oct;67(10):5001-6. [abstract]
Tollersrud T, Kenny K, Reitz AJ, Lee JC. Genetic and serologic evaluation of capsule production by bovine mammary isolates of Staphylococcus aureus and other Staphylococcus spp. from Europe and the United States. J Clin Microbiol. 2000 Aug;38(8):2998-3003. [abstract]
Pohlmann-Dietze P, Ulrich M, Kiser KB, Doring G, Lee JC, Fournier JM, Botzenhart K, Wolz C. Adherence of Staphylococcus aureus to endothelial cells: influence of the capsular polysaccharide, the global regulator agr, and the bacterial growth phase. Infect Immun. 2000 Sep;68(9):4865-71. [abstract]
Portoles M, Kiser KB, Bhasin N,Chan KHN, Lee JC. Staphylococcus aureus Cap5O has UDP-ManNAc dehydrogenase activity and is essential for capsule expression. Infect Immun. 2001 Feb;69(2):917-23. [abstract]
Tzianabos AO, Wang JY, Lee JC. Structural rationale for the modulation of abscess formation by Staphylococcus aureus capsular polysaccharides. Proc Natl Acad Sci U S A. 2001 Jul 31;98(16):9365-70. Epub 2001 Jul 24. [abstract]