Page 167 - Concise Pathology for Exam Preparation ( PDFDrive )

152 SECTION I General Pathology

Morphology
S. aureus causes pyogenic inflammation with a tendency for local destruction. Histopa-
thology usually shows deep-seated suppuration that tends to expand laterally to form
multiple sinuses in the adjacent skin.

Streptococcal Infections
• Streptococci are Gram-positive pathogens that divide in a single plane and thus occur
in pairs or chains of varying lengths (Fig. 7.1).
• Streptococcal species are classified based on their haemolytic properties. b-haemolytic
streptococci can be further subclassified based on the antigenic differences in group-
specific polysaccharides (Lancefield antigens) located in the bacterial cell wall (desig-
nated by letters A, B and C).
• Streptococcus pyogenes is a Group A (beta-haemolytic) streptococcus that colonizes throat
or skin. It is covered by a hyaluronic acid capsule and a layer of carbohydrate and is an
important cause of many invasive and noninvasive infections.
• Amongst the a-haemolytic streptococci, Streptococcus pneumoniae is the most important.
It is a well-known cause of community-acquired pneumonia and meningitis in adults.
Streptococcus viridians, another a-haemolytic streptococcus, are not only part of normal
oral flora but are also a common cause of endocarditis. Streptococcus mutans is major
cause of dental caries.
• Streptococcus pyogenes and Streptococcus pneumoniae have capsules that are resistant to
phagocytosis. S. pyogenes also has M protein which contributes to its resistance to
phagocytosis and a pyogenic exotoxin that is responsible for fever and rash in scarlet
fever.
Clinical Manifestations
• Otherwise part of normal flora, in immunosuppressed individuals, S. pyogenes can cause
a variety of suppurative infections such as puerperal sepsis, pharyngitis (formation of
microabscesses in the tonsillar crypts), erysipelas (infection of the dermal lymphatics
characterized by rapidly spreading erythematous cutaneous swelling), impetigo or
cellulitis.
• Scarlet fever (streptococcal pharyngitis with an erythematous rash) is caused by a strain
of S. pyogenes which produces the ‘erythrogenic toxin’. The toxin induces a rash having a
sandpaper consistency that originates on the trunk and limbs, and resolves with desqua-
mation. Accompanying the rash are some changes in tongue (initially ‘white-strawberry’
followed by a ‘red-strawberry’ appearance).
• Acute streptococcal infection may sometimes result in immune-mediated sequelae,
such as acute rheumatic fever and acute glomerulonephritis (antistreptococcal M
protein antibodies and T cells cross react with cardiac and renal proteins).
• Invasive infection with toxin-producing strains may result in necrotizing fasciitis,
muscle necrosis and streptococcal toxic shock syndrome (TSS).
• S. pneumoniae is an important cause of lobar pneumonia (produces toxin ‘pneumolysin’
which causes membrane lysis and tissue destruction).
Morphology
Although streptococcus causes lesions similar to those caused by staphylococcus (furun-
cles, carbuncles and impetigo), it shows lesser fewer tendency for destruction. Classic
histopathological finding is diffuse interstitial neutrophilic infiltrate.

Diphtheria
Diphtheria is caused by Corynebacterium diphtheria, a Gram-positive, aerobic, non-
motile, rod-shaped bacteria with clubbed ends classified as Actinobacteria, which
undergo snapping movements just after cell division resulting in characteristic
Chinese letter like forms. It is transmitted from person-to-person through aerosols/
skin shedding.

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