Immunization with Specific Polysaccharide Antigen Reduces Alterations in Corneal Proteoglycans During Experimental Slime-ProducingStaphylococcus epidermidisKeratitis

The staphylococcal exopolysaccharide PIA - Biosynthesis and role in biofilm formation, colonization, and infection

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PIA is a key extracellular matrix component in staphylococci and other bacteria.

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PIA is a cationic, partially deacetylated N-acetylglucosamine polymer.

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PIA has a major role in bacterial biofilms and biofilm-associated infection.

Exopolysaccharide is a key part of the extracellular matrix that contributes to important mechanisms of bacterial pathogenicity, most notably biofilm formation and immune evasion. In the human pathogens Staphylococcus aureus and S. epidermidis, as well as in many other staphylococcal species, the only exopolysaccharide is polysaccharide intercellular adhesin (PIA), a cationic, partially deacetylated homopolymer of N-acetylglucosamine, whose biosynthetic machinery is encoded in the ica locus. PIA production is strongly dependent on environmental conditions and controlled by many regulatory systems. PIA contributes significantly to staphylococcal biofilm formation and immune evasion mechanisms, such as resistance to antimicrobial peptides and ingestion and killing by phagocytes, and presence of the ica genes is associated with infectivity. Due to its role in pathogenesis, PIA has raised considerable interest as a potential vaccine component or target.

An extracellular Staphylococcus epidermidis polysaccharide: relation to Polysaccharide Intercellular Adhesin and its implication in phagocytosis

BACKGROUND

The skin commensal and opportunistic pathogen Staphylococcus epidermidis is a leading cause of hospital-acquired and biomaterial-associated infections. The polysaccharide intercellular adhesin (PIA), a homoglycan composed of β-1,6-linked N-acetylglucosamine residues, synthesized by enzymes encoded in icaADBC is a major functional factor in biofilm accumulation, promoting virulence in experimental biomaterial-associated S. epidermidis infection. Extracellular mucous layer extracts of S. epidermidis contain another major polysaccharide, referred to as 20-kDa polysaccharide (20-kDaPS), composed mainly out of glucose, N-acetylglucosamine, and being partially sulfated. 20-kDaPS antiserum prevents adhesion of S. epidermidis on endothelial cells and development of experimental keratitis in rabbits. Here we provide experimental evidence that 20-kDaPS and PIA represent distinct molecules and that 20-kDaPS is implicated in endocytosis of S. epidermidis bacterial cells by human monocyte-derived macrophages.

RESULTS

Analysis of 75 clinical coagulase-negative staphylococci from blood-cultures and central venous catheter tips indicated that 20-kDaPS is expressed exclusively in S. epidermidis but not in other coagulase-negative staphylococcal species. Tn917-insertion in various locations in icaADBC in mutants M10, M22, M23, and M24 of S. epidermidis 1457 are abolished for PIA synthesis, while 20-kDaPS expression appears unaltered as compared to wild-type strains using specific anti-PIA and anti-20-kDaPS antisera. While periodate oxidation and dispersin B treatments abolish immuno-reactivity and intercellular adhesive properties of PIA, no abrogative activity is exerted towards 20-kDaPS immunochemical reactivity following these treatments. PIA polysaccharide I-containing fractions eluting from Q-Sepharose were devoid of detectable 20-kDaPS using specific ELISA. Preincubation of non-20-kDaPS-producing clinical strain with increasing amounts of 20-kDaPS inhibits endocytosis by human macrophages, whereas, preincubation of 20-kDaPS-producing strain ATCC35983 with 20-kDaPS antiserum enhances bacterial endocytosis by human macrophages.

CONCLUSIONS

In conclusion, icaADBC is not involved in 20-kDaPS synthesis, while the chemical and chromatographic properties of PIA and 20-kDaPS are distinct. 20-kDaPS exhibits anti-phagocytic properties, whereas, 20-kDaPS antiserum may have a beneficial effect on combating infection by 20-kDaPS-producing S. epidermidis.

Active Immunization with Pneumolysin versus 23-Valent Polysaccharide Vaccine for Streptococcus pneumoniae Keratitis

PURPOSE

The purpose of this study was to determine whether active immunization against pneumolysin (PLY), or polysaccharide capsule, protects against the corneal damage associated with Streptococcus pneumoniae keratitis.

METHODS

New Zealand White rabbits were actively immunized with Freund's adjuvant mixed with pneumolysin toxoid (ψPLY), Pneumovax 23 (PPSV23; Merck, Whitehouse Station, NJ), or phosphate-buffered saline (PBS), before corneal infection with 10⁵ colony-forming units (CFU) of S. pneumoniae. Serotype-specific rabbit polyclonal antisera or mock antisera were passively administered to rabbits before either intravenous infection with 10¹¹ CFU S. pneumoniae or corneal infection with 10⁵ CFU of S. pneumoniae.

RESULTS

After active immunization, clinical scores of corneas of the rabbits immunized with ψPLY and Freund's adjuvant were significantly lower than scores of the rabbits that were mock immunized with PBS and Freund's adjuvant or with PPSV23 and Freund's adjuvant at 48 hours after infection (P ≤ 0.0010), whereas rabbits immunized with PPSV23 and Freund's adjuvant failed to show differences in clinical scores compared with those in mock-immunized rabbits (P = 1.00) at 24 and 48 hours after infection. Antisera from rabbits actively immunized with PPSV23 and Freund's adjuvant were nonopsonizing. Bacterial loads recovered from infected corneas were higher for the ψPLY- and PPSV23-immunized rabbits after infection with WU2, when compared with the mock-immunized rabbits (P ≤ 0.007). Conversely, after infection with K1443, the ψPLY-immunized rabbits had lower bacterial loads than the control rabbits (P = 0.0008). Quantitation of IgG, IgA, and IgM in the sera of ψPLY-immunized rabbits showed high concentrations of PLY-specific IgG. Furthermore, anti-PLY IgG purified from ψPLY-immunized rabbits neutralized the cytolytic effects of PLY on human corneal epithelial cells. Passive administration of serotype-specific antisera capable of opsonizing and killing S. pneumoniae protected against pneumococcal bacteremia (P ≤ 0.05), but not against keratitis (P ≥ 0.476).

CONCLUSIONS

Active immunization with pneumococcal capsular polysaccharide and Freund's adjuvant fails to produce opsonizing antibodies, and passive administration of serotype specific opsonizing antibodies offers no protection against pneumococcal keratitis in the rabbit, whereas active immunization with the conserved protein virulence factor PLY and Freund's adjuvant is able to reduce corneal inflammation associated with pneumococcal keratitis, but has variable effects on bacterial loads in the cornea.

Protection from Streptococcus pneumoniae keratitis by passive immunization with pneumolysin antiserum

PURPOSE

To determine whether passive immunization with pneumolysin antiserum can reduce corneal damage associated with pneumococcal keratitis.

METHODS

New Zealand White rabbits were intrastromally injected with Streptococcus pneumoniae and then passively immunized with control serum, antiserum against heat-inactivated pneumolysin (HI-PLY), or antiserum against cytotoxin-negative pneumolysin (psiPLY). Slit lamp examinations (SLEs) were performed at 24, 36, and 48 hours after infection. An additional four corneas from rabbits passively immunized with antiserum against psiPLY were examined up to 14 days after infection. Colony forming units (CFUs) were quantitated from corneas extracted at 20 and 48 hours after infection. Histopathology of rabbit eyes was performed at 48 hours after infection.

RESULTS

SLE scores at 36 and 48 hours after infection were significantly lower in rabbits passively immunized with HI-PLY antiserum than in control rabbits (P < or = 0.043). SLE scores at 24, 36, and 48 hours after infection were significantly lower in rabbits passively immunized with psiPLY antiserum than in control rabbits (P < or = 0.010). The corneas of passively immunized rabbits that were examined up to 14 days after infection exhibited a sequential decrease in keratitis, with an SLE score average of 2.000 +/- 1.586 at 14 days. CFUs recovered from infected corneas were not significantly different between each experimental group and the respective control group at 20 or 48 hours after infection (P > or = 0.335). Histologic sections showed more corneal edema and polymorphonuclear leukocyte (PMN) infiltration in control rabbits compared with passively immunized rabbits.

CONCLUSIONS

HI-PLY and psiPLY both elicit antibodies that provide passive protection against S. pneumoniae keratitis.

Infectious keratitis

PURPOSE OF REVIEW

Infectious keratitis is a medical emergency. Improper management can lead to marked loss of vision. This review identifies recent trends in the study of infectious keratitis.

RECENT FINDINGS

A multicountry outbreak of Fusarium keratitis emphasizes that contact lens wear is a major risk factor for infectious keratitis. Acanthamoeba and fungal keratitis are the most expensive forms of infectious keratitis to treat. Noninvasive methods and molecular techniques have improved diagnosis of infectious keratitis. Fortified topical antibiotics and fluoroquinolones are still the mainstay of bacterial keratitis therapy. Voriconazole and new routes of administration of conventional antifungals appear promising for fungal keratitis. Antivirals and amelioration of host inflammatory response are promising for viral keratitis; the host response is also crucial in pathogenesis of Pseudomonas aeruginosa keratitis. Trauma-induced bacterial and fungal keratitis and contact lens-associated keratitis are preventable entities.

SUMMARY

Improved modalities of diagnosis and treatment have improved the outcome of infectious keratitis, but therapy of acanthamoebal, fungal and P. aeruginosa keratitis is still a challenge. Effective strategies must neutralize potential risk factors and counter host response overactivity without impairing killing of infecting microorganisms. Trauma-induced bacterial and fungal keratitis can be prevented.