In vivo pathogenicity and resistance to phagocytosis of Streptococcus equi strains with different levels of capsule expression.

This study compares the pathogenicity and resistance to phagocytosis of different strains of Streptococcus equi—NCTC 9682 (non-encapsulated), Hidaka /95/2 (medium capsule), and CF32 (large capsule)—and reveals how capsule size affects disease progression in horses.

Introduction

• The research revolves around understanding the pathogenicity—that is, the capacity to cause disease—of different strains of bacteria Streptococcus equi, which is primarily responsible for a condition known as strangles in horses.
• The strains under investigation were categorized according to their capsule sizes: non-encapsulated, medium, and large capsule. The capsule on a bacterium can offer protection from the immune system and is a major factor in bacterial virulence.

Methodology and Results

• The researchers compared these strains in terms of their resistance to phagocytosis, a process by which cells of the immune system (like neutrophils) engulf and destroy bacteria.
• They discovered that the non-encapsulated strain NCTC 9682 had no resistance to phagocytosis regardless of the presence of opsonin (a substance that enhances phagocytosis), while the encapsulated strains (Hidaka /95/2 and CF32) showed nearly complete resistance.
• All three strains produced similar amounts of M-like proteins, cytotoxin, and mitogen after three hours of incubation, but with different yields of bacteria at 2.0, 3.1, and 5.3 mg/g wet cells for NCTC 9682, Hidaka /95/2, and CF32, respectively.

Clinical Observations

• From a clinical viewpoint, it was seen that while NCTC 9682 infected the guttural pouch (an air sac in the equine respiratory tract) and led to seroconversion (the period during which antibodies develop and become detectable), it did not produce clinical disease.
• On the contrary, the encapsulated strains (Hidaka /95/2 and CF32) caused typical strangles with guttural pouch invasion in all infected horses. This indicates that the encapsulated strains were more virulent.

Conclusion

• Ultimately, this research indicates that the presence and size of the capsule in Streptococcus equi strains can significantly influence their resistance to phagocytosis and their potential to cause disease.
• This finding may contribute to the development of more effective strategies in treating and preventing strangles in horses.