Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Streptococci and streptococcal infections

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In the 1990s, outbreaks of a virulent strain of group A

Streptococcuswere reported to cause a toxic-shock-like illness

and a severe invasive infection called necrotizing fasciitis,

which destroys skin and muscle tissue. Although these dis-

eases are caused by group AStreptococcus, they rarely begin

with strep throat. Usually the Streptococcus bacteria enter the

body through a skin wound. These complications are rare.

However, since the death rate in necrotizing fasciitis is

30–50%, prompt medical attention for any streptococcal infec-

tion is prudent.

The Streptococcusbacteria are susceptible to antibiotics

such as penicillin. However, in some 10% of infections, peni-

cillin is ineffective. Then, other antibiotics are used, including

amoxicillin, clindamycin, or a cephalosporin.

See alsoBacteria and bacterial infection; Streptococci and

streptococcal infections

SStreptococcal antibody testsTREPTOCOCCAL ANTIBODY TESTS

Species of Gram positive bacteriafrom the genus Strepto-

coccusare capable of causing infections in humans. There are

several disease-causing strains of streptococci. These strains

have been categorized into groups (A, B, C, D, and G), accord-

ing to their behavior, chemistry, and appearance.

Each group causes specific types of infections and

symptoms. For example, group A streptococci are the most

virulent species for humans and are the cause of “strep throat,”

tonsillitis, wound and skin infections, blood infections (sep-

ticemia), scarlet fever, pneumonia, rheumatic fever,

Sydenham’s chorea (formerly called St. Vitus’ dance), and

glomerulonephritis.

While the symptoms affected individuals experience

may be suggestive of a streptococcal infection, a diagnosis

must be confirmed by testing. The most accurate common pro-

cedure is to take a sample from the infected area for culture, a

means whereby the bacteria of interest can be grown and iso-

lated using various synthetic laboratory growth media. This

process can take weeks. A more rapid indication of the pres-

ence of streptococci can be obtained through the detection of

antibodies that have been produced in response to the infecting

bacteria. The antibody-based tests can alert the physician to the

potential presence of living infectious streptococci.

The presence of streptococci can be detected using anti-

body-based assays. Three streptococcal antibodytests that are

used most often are known as the antistreptolysin O titer

(ASO), the antideoxyribonuclease-B titer (anti-Dnase-B, or

ADB), and the streptozyme test.

The antistreptolysin O titer determines whether an

infection with the group A Streptococcus has precluded the

development of post-infection complications. The term titer

refers to the amount of antibody. Thus, this test is quantitative.

That is, the amount of specific antibody in the sample can be

deduced. In an infection the amount of antibody will rise, as

the immune systemresponds to the invading bacteria. These

complications include scarlet fever, rheumatic fever, or a kid-

ney disease termed glomerulonephritis.

The ASO titer is used to demonstrate the body’s reaction

to an infection caused by group A beta-hemolytic streptococci.

The beta-hemolytic designation refers to a reaction produced

by the bacteria when grown in the presence of red blood cells.

Bacteria of this group are particularly important in suspected

cases of acute rheumatic fever (ARF) or acute glomeru-

lonephritis. Group A streptococci produce the enzyme strep-

tolysin O, which can destroy (lyse) red blood cells. Because

streptolysin O is antigenic (contains a protein foreign to the

body), the body reacts by producing antistreptolysin O (ASO),

which is a neutralizing antibody. ASO appears in the blood

serum one week to one month after the onset of a strep infec-

tion. A high titer (high levels of ASO) is not specific for any

type of poststreptococcal disease, but it does indicate if a

streptococcal infection is or has been present.

Tests conducted after therapy starts can reveal if an

active infection was in progress. This will be evident by a

decreasing antibody titer over time, as more and more of the

streptococci are killed.

The anti-DNase-B test likewise detects groups A beta-

hemolytic Streptococcus. This test is often done at the same

time as the ASO titer. This done as the Dnase-based test can

produce results that are more variable than those produced by

the ASO test. This blanket coverage typically detects some

95% of previous strep infections are detected. If both tests are

repeatedly negative, the likelihood is that the patient’s symp-

toms are not caused by a poststreptococcal disease.

The final antibody-based test is a screening test. That is,

the test is somewhat broader in scope than the other tests. The

streptozyme test is often used as a screening test for antibod-

ies to the streptococcal antigens NADase, DNase, streptoki-

nase, streptolysin O, and hyaluronidase. This test is most

useful in evaluating suspected poststreptococcal disease fol-

lowing infection with Streptococcus pyogenes, such as rheu-

matic fever.

The streptozyme assay has certain advantages over the

other two tests. It can detect several antibodies in a single

assay, is quick and easy to perform, and is unaffected by fac-

tors that can produce false-positives in the ASO test. However,

the assay does have some disadvantages. While it detects dif-

ferent antibodies, it does not determine which one has been

detected, and it is not as sensitive in children as in adults.

See alsoAntibody and antigen; Antibody formation and kinet-

ics; Bacteria and bacterial infection

STREPTOCOCCI AND STREPTOCOCCAL

INFECTIONSStreptococci and streptococcal infections

Streptococci are spherical, Gram positive bacteria. Commonly

they are referred to as strep bacteria. Streptococci are normal

residents on the skin and mucous surfaces on or inside humans.

However, when strep bacteria normally found on the skin or in

the intestines, mouth, nose, reproductive tract, or urinary tract

invade other parts of the body—via a cut or abrasion—and

contaminate blood or tissue, infection can be the result.

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