Causative Agent

Diphtheria is caused by Corynebacterium diphtheriae, a variably shaped, non-motile, non-spore-forming, Gram-positive rod that often stains irregularly. Slide preparations of C. diphtheriae commonly show the organisms arranged in "Chinese letter" patterns (figure 23.5) or side by side in "palisades." Different colony types can be identified, a property that is useful in tracing epi-

10 mm

Figure 23.5 Corynebacterium diphtheriae Note the "Chinese letter'" pattern.

10 mm

Figure 23.5 Corynebacterium diphtheriae Note the "Chinese letter'" pattern.

Nester-Anderson-Roberts: IV. Infectious Diseases 23. Respiratory System © The McGraw-Hill

Microbiology, A Human Infections Companies, 2003

Perspective, Fourth Edition

Table 23.3 Strep Throat (Streptococcal Pharyngitis)

23.3 Bacterial Infections of the Upper Respiratory System 569

© Streptococcus pyogenes enters by inhalation (nose), or by ingestion (mouth).

(2 Pharyngitis, fever, enlarged lymph nodes; sometimes tonsillitis, abcess; scarlet fever with strains that produce erythrogenic toxin

@ Symptoms go away.

@ S. pyogenes exits by nose and mouth.

Late complications appear:

<5 glomerulonephritis

© rheumatic fever

@ chorea

Complications subside.

® Damaged heart valves leak, heart failure develops.

Symptoms Red throat, often with pus and tiny hemorrhages, enlargement and tenderness of lymph nodes in the neck; less frequently, abscess formation involving tonsils; occasionally, rheumatic fever and glomerulonephritis as sequels

Incubation period 2 to 5 days

Causative agent Streptococcus pyogenes, Lancefield group A b-hemolytic streptococci

Pathogenesis Virulence associated with hyaluronic acid capsule and M protein, both of which inhibit phagocytosis; protein G binds Fc segment of IgG; protein F for mucosal attachment; multiple enzymes

Epidemiology Direct contact and droplet infection;

ingestion of contaminated food

Prevention and Avoidance of crowding; adequate treatment ventilation; daily penicillin to prevent recurrent infection in those with a history of rheumatic heart disease. Treatment: 10 days of penicillin or erythromycin demic spread of the organisms. These bacteria require special media and microbiological techniques to aid their recovery from the mixture of bacteria present in throat material. It is unlikely that they would be recovered by the usual methods for culturing S. pyogenes from sore throats.

Most but not all strains of C. diphtheriae release diphtheria toxin, a powerful exotoxin responsible for the seriousness of diphtheria. Production of this toxin requires that the bacterium be lysogenized by one of a certain group of bacteriophages, an example of lysogenic conversion. Strains that are non-toxigenic become toxin producers when lysogenized with the bacterio-phage. Toxin production can be demonstrated in vitro, or in vivo by its effect on guinea pigs. The toxin is produced only when the medium on which C. diphtheriae is growing has too little iron for optimal growth. The gene for toxin production is under the control of a repressor that is only active when bound to iron. In a medium containing adequate iron, the repressor shuts down toxin production; when the concentration of iron is very low, the iron molecule leaves the repressor and the toxin gene becomes active. ■ lysogenic conversion, p. 331 ■ repressors, p. 183

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