Related Experiment Videos
Phagocytic killing of encapsulated and microencapsulated Staphylococcus aureus by human polymorphonuclear leukocytes
1Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts.
Abstract:
Phagocytosis by human polymorphonuclear leukocytes (PMNs) is an important host defense against infections caused by Staphylococcus aureus. Using an in vitro assay, we compared the opsonic requirements for phagocytic killing of prototype strains of encapsulated (type 1) and microencapsulated (type 5 and type 8) S. aureus by human PMNs. More than 85% of broth-grown, logarithmic-phase type 5 and 8 S. aureus organisms were killed by PMNs incubated with fresh normal human, rabbit, or guinea pig serum with complement activity. Under similar conditions, the highly encapsulated type 1 strain was not killed. Both encapsulated and microencapsulated strains were opsonized for phagocytosis by heat-inactivated serum raised in rabbits to killed bacteria. Opsonization by homologous serum was required for phagocytosis of the type 1 strain. In contrast, microencapsulated type 5 and 8 S. aureus organisms were killed by heat-inactivated rabbit serum raised to type 5, type 8, or nonencapsulated isolates; this result suggested that antibodies to the capsule or to cell wall components other than the capsule could opsonize these organisms for phagocytosis. The specificity of the assay was confirmed with capsule type 5-specific monoclonal antibodies, which were opsonic only for the type 5 S. aureus isolate. These studies indicate that, unlike the highly encapsulated type 1 strain, broth-grown microencapsulated S. aureus strains do not resist opsonophagocytic killing in vitro by normal serum.
Insights
Human polymorphonuclear leukocytes (PMNs) effectively kill microencapsulated Staphylococcus aureus strains in normal serum. Highly encapsulated strains resist this killing, requiring specific opsonization for phagocytosis.
Area of Science:
- Immunology
- Microbiology
- Host-pathogen interactions
Background:
- Phagocytosis by human polymorphonuclear leukocytes (PMNs) is crucial for defense against Staphylococcus aureus infections.
- Staphylococcus aureus exhibits diverse encapsulation, influencing its interaction with the host immune system.
Purpose of the Study:
- To compare the opsonization requirements for phagocytic killing of encapsulated and microencapsulated Staphylococcus aureus strains by human PMNs.
- To elucidate the role of complement and specific antibodies in the opsonophagocytic clearance of different Staphylococcus aureus serotypes.
Main Methods:
- In vitro phagocytosis assay using human PMNs and various strains of Staphylococcus aureus (type 1, 5, and 8).
- Assessment of bacterial killing in the presence of normal serum (with complement) and heat-inactivated serum (without complement).
- Evaluation of opsonization using homologous and heterologous antisera, including monoclonal antibodies.
Main Results:
- Microencapsulated Staphylococcus aureus (type 5 and 8) were efficiently killed by PMNs in normal serum (>85%).
- Highly encapsulated Staphylococcus aureus (type 1) resisted killing by PMNs in normal serum.
- Both encapsulated and microencapsulated strains required opsonization for phagocytosis, with type 1 needing homologous serum, while type 5 and 8 could be opsonized by various sera and antibodies.
Conclusions:
- Microencapsulated Staphylococcus aureus strains are susceptible to opsonophagocytic killing by normal serum.
- The degree of capsule formation in Staphylococcus aureus significantly impacts its resistance to innate immune clearance mechanisms.
- Antibodies targeting capsular or cell wall components are essential for effective opsonization and phagocytosis of certain Staphylococcus aureus strains.