STEPTRACKERMICROBIOLOGY + IMMUNOLOGY | INFECTIONS BY SYSTEM | HIGH-YIELD REVIEW

Infections in the Compromised Host

Name the broken defense and the organisms follow — antibodies, complement, spleen, neutrophils, T cells, barriers or anatomy.

FOCUS ON
B cell / antibody defects → encapsulated bacteria (pneumococcus, Hib), enteroviruses, Giardia. Early complement (C3) → encapsulated bacteria; terminal (C5–C9) → Neisseria. Asplenia (sickle cell, splenectomy) → S pneumoniae, Hib, N meningitidis, Salmonella, Capnocytophaga, Babesia → vaccinate, early antibiotics. Neutropenia / neutrophil dysfunction → S aureus, gram-negatives incl. Pseudomonas, Candida, Aspergillus; CGD (no NADPH oxidase) → catalase-positive organisms (S aureus, Serratia, Burkholderia, Nocardia, Aspergillus). T-cell defects → viruses, fungi, intracellular bacteria, Pneumocystis. Transplant timeline: < 1 month nosocomial; 1–6 months CMV, opportunists; > 6 months community. Burns → Pseudomonas; CF → S aureus, Pseudomonas.
DEFECT
EXAMPLES
TYPICAL ORGANISMS
KEY POINT
Antibody deficiency
X-linked agammaglobulinemia, CVID, selective IgA deficiency
Encapsulated bacteria (sinopulmonary), enteroviral meningoencephalitis, Giardia
No live vaccines in XLA · IVIG
Early complement (C1–C4, C3)
C2 deficiency (lupus-like), C3 deficiency
Encapsulated bacteria, recurrent pyogenic infections
C3b opsonization lost
Terminal complement (C5–C9)
Inherited, eculizumab
Recurrent Neisseria
Meningococcal vaccines
Asplenia / hyposplenism
Splenectomy, sickle cell (autosplenectomy)
S pneumoniae, Hib, N meningitidis, Salmonella (osteomyelitis), Capnocytophaga (dog bite), Babesia
Howell-Jolly bodies · vaccines before elective splenectomy · standby antibiotics
Neutropenia
Chemotherapy, aplastic anemia, drugs
S aureus, Pseudomonas and enteric gram-negatives, Candida, Aspergillus, Mucor
Febrile neutropenia → cefepime/piperacillin-tazobactam now
Chronic granulomatous disease
NADPH oxidase defect (X-linked most)
Catalase ⁺: S aureus, Serratia, Burkholderia, Nocardia, Aspergillus
Abnormal DHR test · TMP-SMX + itraconazole prophylaxis, IFN-γ
Leukocyte adhesion deficiency
CD18 integrin defect
Skin and mucosal bacterial infections without pus
Delayed umbilical cord separation, very high WBC
T-cell deficiency
DiGeorge, SCID, HIV, steroids, calcineurin inhibitors
Viruses (CMV, VZV), Pneumocystis, Candida, Cryptococcus, mycobacteria, Listeria, Toxoplasma
No live vaccines
IL-12 / IFN-γ axis
Receptor mutations
Disseminated mycobacteria (incl. BCG), Salmonella
IFN-γ therapy
TNF-α inhibitors
Infliximab, adalimumab, etanercept
Reactivated TB, histoplasmosis, Listeria, HBV
Screen for latent TB and HBV first
Solid-organ transplant
< 1 month · 1–6 months · > 6 months
Surgical/nosocomial (MRSA, C diff, Candida) · CMV, EBV (PTLD), Pneumocystis, BK, Toxoplasma · community organisms
Prophylaxis: TMP-SMX, valganciclovir
Barrier breaks
Burns, IV catheters, Foley, ventilators, CF
Pseudomonas, S aureus, S epidermidis, Candida, Enterococcus
Remove devices
Diabetes
Neutrophil dysfunction, vascular disease, glycosuria
Mucor (DKA), malignant otitis externa (Pseudomonas), foot infections, emphysematous pyelonephritis, Candida
—
Iron overload
Hemochromatosis, deferoxamine
Yersinia, Vibrio vulnificus, Mucor, Listeria
Siderophilic organisms

Mechanism — why CGD patients get catalase-positive organisms

No NADPH oxidaseNo respiratory burst
→
Neutrophil relies on microbe's own H₂O₂
→
Catalase ⁺ organisms destroy their H₂O₂
→
Survive → granulomas, abscesses

Defect → organism quick map

DEFECT
ORGANISM TYPE
EXAMPLE
B cell
Encapsulated bacteria
Pneumococcus
T cell
Intracellular, viral, fungal
Pneumocystis, CMV
Neutrophil
Pyogenic bacteria, molds
S aureus, Aspergillus
Complement C5–C9
Neisseria
Meningococcus
Spleen
Encapsulated, Babesia
Pneumococcus

How it's tested

COMMON CONFUSIONRecurrent Neisseria (terminal complement) vs recurrent encapsulated sinopulmonary infections (antibody, early complement, spleen).
THE TRAPLive vaccines are contraindicated in severe T-cell and combined defects — but inactivated vaccines (including pneumococcal) are encouraged in asplenia.
YOUR APPROACHInfection pattern → which arm of immunity failed → test for it → prophylaxis and vaccines appropriate to that defect.

If you see… think…

Post-splenectomy patient with fulminant sepsis → S pneumoniae
Asplenic patient septic after a dog bite → Capnocytophaga
Boy with Serratia and Aspergillus abscesses → chronic granulomatous disease
Febrile neutropenia on chemotherapy → cover Pseudomonas now
Delayed cord separation and no pus → leukocyte adhesion deficiency
Transplant patient at 3 months with fever and leukopenia → CMV
Reactivated TB after starting infliximab → TNF-α blockade
Disseminated BCG after vaccination → SCID or IFN-γ/IL-12 defect
Burn patient with green wound exudate → Pseudomonas
Yersinia sepsis on deferoxamine → iron-loving organism