Staphylococcal hemolysins and leukotoxicity: Which hemolysin produced by Staphylococcus lacks leukocidal (leukocyte-killing) activity?

Introduction / Context:
Staphylococcus aureus elaborates several cytolysins (α, β, γ, δ) and leukocidins that contribute to tissue injury and immune evasion. Understanding their targets helps interpret pathogenic mechanisms and lab observations such as hemolysis patterns.

Given Data / Assumptions:

• Leukocidal activity refers specifically to toxins that kill or strongly damage leukocytes.
• Hemolysins differ in substrate specificity and mechanisms (pore forming vs enzymatic).
• Panton–Valentine leukocidin (PVL) is a separate bicomponent leukotoxin (not among the four hemolysins listed), but γ hemolysin is related and leukotoxic.

Concept / Approach:
α hemolysin is a pore-forming toxin with broad cytotoxicity, including effects on leukocytes and platelets. γ hemolysin forms bicomponent pores and exhibits leukocidal activity. δ hemolysin is a surfactant-like cytolysin with broad, albeit weaker, cell membrane activity. β hemolysin (sphingomyelinase C) is a hot–cold hemolysin acting enzymatically on membrane lipids and is not characteristically leukocidal.

Step-by-Step Solution:
List hemolysins and their key properties. Associate leukocidal effects with α and γ (and PVL, though not listed). Recognize β hemolysin as sphingomyelinase lacking specific leukocyte-killing activity. Select “β haemolysin.”

Verification / Alternative check:
Hot–cold hemolysis due to β hemolysin on sheep blood agar exemplifies its enzymatic nature rather than leukocyte-specific cytotoxicity.

Why Other Options Are Wrong:

• α haemolysin: pore-forming, leukotoxic among other effects.
• γ haemolysin: bicomponent leukocidal toxin.
• δ haemolysin: broad surfactant cytolysin; while not strongly leukocidal, it does exhibit general cytotoxicity; β is the recognized non-leukocidal hemolysin.

Common Pitfalls:
Confusing PVL (a distinct leukocidin) with γ hemolysin; both are leukotoxic, but β hemolysin is the enzymatic hemolysin without primary leukocidal action.

Final Answer:
β haemolysin