individuals with disease caused by S. pyogenes produce antibodies against various antigens. The most common are antistreptolysin 0 (ASO), anti-DNase B, antistrepto-kmase, and antihyaluronidase. Pharyngitis seems to be followed by rises in antibody titers against all antigens, whereas patients with pyoderma, an infection of the Pa- only show a significant response to anti-DNase B. Use of sérodiagnostic tests is most useful to demonstrate prior streptococcal infection in patients from whom group A Streptococcus has not been cultured but who present with sequelae suggestive of rheumatic fever or acute glomerulonephritis. Serum obtained as long as 2 months after infection usually demonstrates increased antibodies. As with other serologic tests, an increasing titer over time is most useful for diagnosing previous streptococcal infection.
Commercial products are available for detection of antistreptococcal antibodies. Streptozyme (Wampole Laboratories, Princeton, NJ), which detects a mixture of antibodies, is a commonly used test. Unfortunately, no commercial system has been shown to accurately detect all streptococcal antibodies.
antimicrobial susceptibility testing amd therapy
For S. pyogenes and the other beta-hemolytic streptococci, penicillin is the drug of choice (Table 17-7). Because penicillin resistance has not been encountered among these organisms, susceptibility testing of clinical isolates for reasons other than resistance surveillance is not necessary.21 However, if a macrolide such as erythromycin, is being considered for use, as is the case with patients who are allergic to penicillin, testing is needed to detect resistance that has emerged among these organisms. For serious infections caused by S. agalactiae, an aminoglycoside may be added to supplement beta-lactam therapy and enhance bacterial killing, In contrast to beta-hemolytic streptococci, the emergence of resistance to a variety of different antimicrobial classes in S. pneumoniae and viridans streptococci dictates that clinically relevant isolates be subjected to in vitro susceptibility testing.27 When testing is performed, methods that produce minimal inhibitory concentration (MIC) data for beta-Iactams are preferred. The level of resistance (i.e., MIC in ng/mL) can provide importent information regarding therapeutic management of the patient, particularly in cases of pneumococcal meningitis in which relatively slight increases in MIC can have substantial impact on clinical efficacy of penicillins and cephalosporins. Vancomycin resistance has not been described in either S. pneumoniae or in viridans streptococci.
Enterococd are intrinsically resistant to a wide array of antimicrobial agents, and they generally are resistant to killing by any of the single agents (e.g., ampicillin or vancomycin) that are bactericidal for most other gram-positive cocci. Therefore, effective bactericidal activity can only be achieved with the combination of a cell wall-active agent, such as ampicillin or vancomycin, and an aminoglycoside, such as gentamidn or streptomycin.
Unfortunately, many B. faecalis and E. faedum isolates have acquired resistance to one or more of these components of combination therapy. This resistance generally eliminates any contribution that the target antimicrobial agent could make to the synergistic killing of the organism. Therefore, performance of in vitro susceptibility testing with clinical isolates from systemic infections is critical for determining which combination of agents may still be effective therapeutic choices.
For uncomplicated urinary tract infections, bactericidal activity is usually not required for clinical efficacy, so that single agents such as ampicillin, nitrofurantoin, or a quinolone are often sufficient.
A single-dose, 23-valent vaccine (Pneumovax, Merck fr Co. Inc., West Point, Pa) to prevent infection by the most common serotypes of S. pneumoniae is available in the United States. Vaccination is recommended for individuals older than 65 years of age and for patients with (1) chronic pulmonary, cardiac liver, or renal disease; (2) no spleen (asplenic); (3) sickle cell disease; (4) diabetes; (5) HTV infection; or (6) any other diseases that compromise the immune system. The vaccine is not effective in children younger than 2 years of age. A heptavalent (seven serotypes) vaccine (Prevnar, distributed by Wyeth-Ayerst Pharmaceuticals, Philadelphia, Pa) is available for children younger than 2 years of age. The seven serotypes in this vaccine account for the majority of cases of bacteremia, meningitis, and otitis media in children younger than 6 years of age. Moreover, 80% of penicillin-resistant strains are one of these seven serotypes.
lifetime chemoprophylaxis with penicillin, given either monthly (intramuscular administration) or daily (oral administration), is recommended for patients with rheumatic heart disease to prevent development of bacteria] endocarditis on a damaged heart valve. Likewise, penicillin may be indicated to control outbreaks of S. pyogenes in individuals in close physical contact, such as in households, military populations, or newborn nurseries.
A 76-year-old man with atherosclerosis had been, previously admitted for abdominal aneurysm and resection of the perirenal aorta. He had several follow-up admissions over the next year for postoperative wound infections, with accompanying bacteremia, alternating between Pseudomonas aeruginosa, vancomycin-resistant Enterococcus faedum, and Candida glabrata. On his final admission, blood cultures were positive, with numerous gram-positive coca in pairs and chains in the smear, but subculture of the bottle showed no growth aerobically with increased C02 on blood agar or chocolate agar or anaerobically on Brucella agar.
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