Page 69 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART I Basic Bacteriology
58
TABLE 8–3 Essential Host Defense Mechanisms Against Bacteria
Essential Host Defense Mechanism
Type of Bacteria or Toxin
Encapsulated pyogenic bacteria
Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococ-
Antibody-mediated opsonization
cus aureus, Neisseria meningitidis, Haemophilus influenzae, Pseu-
domonas aeruginosa
Antibody-mediated neutralization
botulinum
Mycobacterium tuberculosis, atypical mycobacteria, Legionella
Cell-mediated Exotoxins Corynebacterium diphtheriae, Clostridium tetani, Clostridium
Intracellular bacteria
pneumophila, Listeria monocytogenes
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FAILURE OF HOST DEFENSES
have Salmonella osteomyelitis, probably because the abnor-
PREDISPOSES TO INFECTIONS
mally shaped cells occlude the small capillaries in the bone.
The frequency or severity of infections is increased when
certain predisposing conditions exist. These predisposing
risk of osteomyelitis.
conditions fall into two main categories: patients are
Patients with certain congenital cardiac defects or rheu-
matic valvular damage are predisposed to endocarditis
immunocompromised or patients have foreign bodies such This traps the Salmonella within the bone, increasing the
caused by viridans streptococci. Neutrophils have difficulty
as indwelling catheters or prosthetic devices. Foreign bod-
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in penetrating the vegetations formed on the valves in
ies predispose because host defenses do not operate effi-
ciently in their presence. Table 8–4 describes the
endocarditis. Patients with an aortic aneurysm are prone to
predisposing conditions and the most common organisms
causing infections when these predisposing conditions are
Patients with reduced host defenses often have a muted
response to infection (e.g., a low-grade [or no] fever and a
present.
mild [or absent] inflammatory response). For this reason, a
Certain diseases and anatomic abnormalities also pre-
dispose to infections. For example, patients with diabetes
high index of suspicion regarding the presence of infection
often have S. aureus infections, perhaps for two reasons:
must be present when evaluating patients who are immu-
nocompromised, especially those who are intentionally
these patients have extensive atherosclerosis, which causes
relative anoxia to tissue, and they appear to have a defect in
immunosuppressed, such as transplant recipients.
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TABLE 8–4 Conditions That Predispose to Infections and the Organisms That Commonly Cause These Infections
Predisposing Condition
Organisms Commonly Causing Infection
Immunocompromised state
Pyogenic bacteria (e.g., Staphylococcus aureus, Streptococcus pneumoniae)
Low antibody
Pyogenic bacteria (e.g., S. aureus, S. pneumoniae)
Low complement (C3b)
Neisseria meningitidis
Low complement (C6,7,8,9)
Pyogenic bacteria (e.g., S. aureus, S. pneumoniae)
Low neutrophil number
Low neutrophil function as in CGD
S. aureus and Aspergillus fumigatus
Various bacteria (e.g., mycobacteria), various viruses (e.g., CMV), and various
Low CD4 cells as in AIDS
fungi (e.g., Candida)
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Presence of foreign bodies
Urinary catheters
Escherichia coli
Staphylococcus epidermidis, Candida albicans
Intravenous catheters
S. epidermidis, C. albicans
Prosthetic heart valves
S. epidermidis
Prosthetic joints
S. epidermidis, S. aureus, Salmonella enterica
Vascular grafts
AIDS = acquired immunodeficiency syndrome; CGD = chronic granulomatous disease; CMV = cytomegalovirus.

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