After splenectomy the body loses its main filter for opsonised, capsule-bearing bacteria, so the patient becomes prone to overwhelming post-splenectomy infection (OPSI). The encapsulated trio to remember is pneumococcus, Hib and meningococcus, but the dominant pathogen is Streptococcus pneumoniae, accounting for the large majority of fatal OPSI episodes.
The mechanism: the marginal zone B cells and macrophages of the spleen clear poorly opsonised encapsulated organisms and generate type-specific antibody. Without the spleen, pneumococci proliferate unchecked and can cause fulminant sepsis within hours.
This is precisely why guidelines mandate vaccination against S. pneumoniae, H. influenzae b and N. meningitidis at least two weeks before an elective splenectomy, along with consideration of long-term antibiotic prophylaxis.
The other listed organisms ($Staphylococcus\ aureus$, $E.\ coli$, $Pseudomonas$) are not the characteristic OPSI agents and are not selectively dependent on splenic clearance.
\[\boxed{\text{Streptococcus pneumoniae} - \text{commonest cause of post-splenectomy sepsis}}\]