Page 502 - Robbins Basic Pathology by Vinay Kumar, Abul K. Abbas, Jon C. Aster
P. 502
488 C H A P T E R 12 Lung congestion can be seen, with proteinaceous fluid, scattered
neutrophils, and many bacteria in the alveoli. Within a few
Table 12–6 The Pneumonia Syndromes and Implicated Pathogens days, the stage of red hepatization ensues, in which the
lung lobe has a liver-like consistency; the alveolar spaces are
Community-Acquired Acute Pneumonia packed with neutrophils, red cells, and fibrin (Fig. 12–32, A).
Streptococcus pneumoniae In the next stage, gray hepatization, the lung is dry, gray,
Haemophilus influenzae and firm, because the red cells are lysed, while the fibrinosup-
Moraxella catarrhalis purative exudate persists within the alveoli (Fig. 12–33; see
Staphylococcus aureus also Fig. 12–32, B). Resolution follows in uncomplicated
Legionella pneumophila cases, as exudates within the alveoli are enzymatically digested
Enterobacteriaceae (Klebsiella pneumoniae) and Pseudomonas spp. to produce granular, semifluid debris that is resorbed, ingested
Community-Acquired Atypical Pneumonia by macrophages, coughed up, or organized by fibroblasts
Mycoplasma pneumoniae growing into it (Fig. 12–32, C). The pleural reaction (fibrinous
Chlamydia spp.—Chlamydia pneumoniae, Chlamydia psittaci, Chlamydia or fibrinopurulent pleuritis) may similarly resolve or under
go organization, leaving fibrous thickening or permanent
trachomatis adhesions.
Coxiella burnetii (Q fever)
Viruses: respiratory syncytial virus, human metapneumovirus, In the bronchopneumonic pattern, foci of inflammatory
consolidation are distributed in patches throughout one or
parainfluenza virus (children); influenza A and B (adults); adenovirus several lobes, most frequently bilateral and basal. Well-
(military recruits) developed lesions up to 3 or 4 cm in diameter are slightly
Nosocomial Pneumonia elevated and are gray-red to yellow; confluence of these foci
Gram-negative rods belonging to Enterobacteriaceae (Klebsiella spp., may occur in severe cases, producing the appearance of a
Serratia marcescens, Escherichia coli) and Pseudomonas spp. lobar consolidation. The lung substance immediately sur-
S. aureus (usually methicillin-resistant) rounding areas of consolidation is usually hyperemic and
Aspiration Pneumonia edematous, but the large intervening areas are generally
Anaerobic oral flora (Bacteroides, Prevotella, Fusobacterium, normal. Pleural involvement is less common than in lobar
Peptostreptococcus), admixed with aerobic bacteria (S. pneumoniae, pneumonia. Histologically, the reaction consists of focal sup-
S. aureus, H. influenzae, and Pseudomonas aeruginosa) purative exudate that fills the bronchi, bronchioles, and adja-
Chronic Pneumonia cent alveolar spaces.
Nocardia
Actinomyces With appropriate therapy, complete restitution of the lung
Granulomatous: Mycobacterium tuberculosis and atypical mycobacteria, is the rule for both forms of pneumococcal pneumonia, but
Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis in occasional cases complications may occur: (1) tissue
Necrotizing Pneumonia and Lung Abscess destruction and necrosis may lead to abscess formation; (2)
Anaerobic bacteria (extremely common), with or without mixed suppurative material may accumulate in the pleural cavity,
aerobic infection producing an empyema; (3) organization of the intra-
S. aureus, K. pneumoniae, Streptococcus pyogenes, and type 3 alveolar exudate may convert areas of the lung into solid
pneumococcus (uncommon) fibrous tissue; and (4) bacteremic dissemination may lead
Pneumonia in the Immunocompromised Host to meningitis, arthritis, or infective endocarditis.
Cytomegalovirus Complications are much more likely with serotype 3
Pneumocystis jiroveci pneumococci.
Mycobacterium avium complex (MAC)
Invasive aspergillosis
Invasive candidiasis
“Usual” bacterial, viral, and fungal organisms (listed above)
MORPHOLOGY Examination of gram-stained sputum is an important
step in the diagnosis of acute pneumonia. The presence of
With pneumococcal lung infection, either pattern of pneu- numerous neutrophils containing the typical gram-positive,
monia, lobar or bronchopneumonia, may occur; the latter is lancet-shaped diplococci is good evidence of pneumococ-
much more prevalent at the extremes of age. Regardless of cal pneumonia; of note, however, S. pneumoniae is a part of
the distribution of the pneumonia, because pneumococcal the endogenous flora, so false-positive results may be
lung infections usually are acquired by aspiration of pharyn- obtained by this method. Isolation of pneumococci from
geal flora (20% of adults harbor S. pneumoniae in the throat), blood cultures is more specific. During early phases of
the lower lobes or the right middle lobe is most frequently illness, blood cultures may be positive in 20% to 30% of
involved. persons with pneumonia. Whenever possible, antibiotic
sensitivity should be determined. Commercial pneumococ-
In the era before antibiotics, pneumococcal pneumonia cal vaccines containing capsular polysaccharides from the
involved entire or almost entire lobes and evolved through common serotypes of the bacteria are available, and their
four stages: congestion, red hepatization, gray hepati- proven efficacy mandates their use in persons at risk for
zation, and resolution. Early antibiotic therapy alters or pneumococcal infections (see earlier).
halts this typical progression.
Pneumonias Caused by Other Important Pathogens
During the first stage, that of congestion, the affected
lobe(s) is (are) heavy, red, and boggy; histologically, vascular Other organisms commonly implicated in community-
acquired acute pneumonias include the following.
