Page 127 - Review of Medical Microbiology and Immunology ( PDFDrive )

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drug combinations (see Chapter 10). mebooksfree.com
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PART II Clinical Bacteriology
116
STREPTOCOCCUS
Some strains of staphylococci exhibit tolerance (i.e.,
they can be inhibited by antibiotics but are not killed).
Streptococci of medical importance are listed in Table
(That is, the ratio of minimum bactericidal concentration
15–3. All but one of these streptococci are discussed in this
[MBC] to minimum inhibitory concentration [MIC] is
section; S. pneumoniae is discussed separately at the end of
very high.) Tolerance may result from failure of the drugs
to inactivate inhibitors of autolytic enzymes that degrade
the organism. Tolerant organisms should be treated with
Drainage (spontaneous or surgical) is the cornerstone of this chapter because it is so important.
Streptococci cause a wide variety of infections. Streptococcus
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abscess treatment. Incision and drainage (I&D) is often
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pyogenes (group A streptococcus) is the leading bacterial cause
sufficient treatment for a skin abscess (e.g., furuncle [boil]);
of pharyngitis (Figure 15–8) and cellulitis (Figure 15–9). It is
antibiotics are not necessary in most cases. Previous infec-
tion provides only partial immunity to reinfection.
fasciitis, and streptococcal toxic shock syndrome. It is also the
Staphylococcus epidermidis is highly antibiotic resistant.
inciting factor of two important immunologic diseases,
Most strains produce β-lactamase but are sensitive to
namely, rheumatic fever and acute glomerulonephritis. Strep-
β-lactamase–resistant drugs such as nafcillin. These are
tococcus agalactiae (group B streptococcus) is the leading
called methicillin-sensitive strains (MSSE). Some strains
cause of neonatal sepsis and meningitis. Enterococcus faecalis
are methicillin/nafcillin resistant (MRSE) due to altered
is an important cause of hospital-acquired urinary tract infec-
penicillin-binding proteins. The drug of choice is vanco-
mycin, to which either rifampin or an aminoglycoside can
most common cause of endocarditis (Figure 15–10). Strepto-
be added. Removal of the catheter or other device is often
coccus bovis (also known as Streptococcus gallolyticus) is an
necessary. Staphylococcus saprophyticus urinary tract infec- tions and endocarditis. Viridans group streptococci are the
uncommon cause of endocarditis.
tions can be treated with trimethoprim-sulfamethoxazole
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or a quinolone, such as ciprofloxacin.
Important Properties
Prevention
chains or pairs (Figure 15–11). All streptococci are catalase-
There is no vaccine against staphylococci. Cleanliness, fre-
negative, whereas staphylococci are catalase-positive (see
quent handwashing, and aseptic management of lesions
Table 15–3).
help to control spread of S. aureus. Persistent colonization
One of the most important characteristics for identifica-
of the nose by S. aureus can be reduced by intranasal mupi-
tion of streptococci is the type of hemolysis (Figure 15–12).
rocin or by oral antibiotics, such as ciprofloxacin or
(1) `-Hemolytic streptococci form a green zone around
trimethoprim-sulfamethoxazole, but is difficult to elimi-
nate completely. Shedders may have to be removed from
cells in the agar. The green color is formed when hydrogen
high-risk areas (e.g., operating rooms and newborn nurser-
peroxide produced by the bacteria oxidizes hemoglobin
ies). Cefazolin is often used perioperatively to prevent their colonies as a result of incomplete lysis of red blood
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staphylococcal surgical-wound infections.
(red color) to biliverdin (green color).
TABLE 15–3 Streptococci of Medical Importance
1
Diagnostic Features
Species
S. pyogenes
Bacitracin-sensitive
β
A
B
S. agalactiae
β
Bacitracin-resistant; hippurate hydrolyzed
E. faecalis
3
α or none
D
S. bovis
No growth in 6.5% NaCl
α
Bile-soluble; inhibited by optochin
S. pneumoniae 5 D 4 α or β or none Growth in 6.5% NaCl 2
NA
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NA
Viridans group
α
Not bile-soluble; not inhibited by optochin
1
All streptococci are catalase-negative.
2
Both E. faecalis and S. bovis grow on bile-esculin agar, whereas other streptococci do not. They hydrolyze the esculin, and this results in a characteristic black discoloration of
the agar.
3
Streptococcus bovis is a nonenterococcal group D organism.
4
NA, not applicable.
5
Viridans group streptococci include several species, such as S. sanguinis, S. mutans, S. mitis, S. gordonii, S. salivarius, S. anginosus, S. milleri, and S. intermedius.
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