Antimicrobial Susceptibility Testing and Therapy for Streptococcus,Enterococcus, and Similar Organisms
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p260-262
2025-05-25
577
For S. pyogenes and the other beta-hemolytic streptococci, penicillin is the drug of choice (Table 1). Because penicillin resistance has not been encountered among these organisms, susceptibility testing of clinical isolates for reasons other than resistance surveillance is not necessary. 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 β-lactam therapy and enhance bacterial killing.
Table1. Antimicrobial Therapy and Susceptibility Testing
Table1. Antimicrobial Therapy and Susceptibility Testing—cont’d
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. When testing is performed, methods that produce minimal inhibitory concentration (MIC) data for β-lactams are preferred. The level of resistance (i.e., MIC in µg/mL) can provide important 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 the clinical efficacy of penicillins and cephalosporins. Vancomycin resistance has not been described in either S. pneumoniae or viridans streptococci.
S. pneumoniae or other beta-hemolytic Streptococcus spp. that demonstrate resistance to erythromycin and susceptible or intermediate to clindamycin should be examined for inducible clindamycin resistance as previously described for Staphylococcus spp. in Chapter 14. Disk diffusion using Mueller Hinton or Tryptic soy agar supplemented with 5% sheep blood may be used. Place a 15-µg erythromycin disk and a 2-µg disk 12 mm apart. If inducible resistance is present, the clindamycin zone adjacent to the erythromycin disk will demonstrate the classic flattening or D-zone appearance. Alternately, a broth microdilution using Mueller Hinton containing lysed horse blood (2.5% to 5%) may be used by adding 1 µg/mL erythromycin and 0.5 µg/mL clindamycin within a single well. Any visible growth within the well would indicate inducible clindamycin resistance.
Enterococci 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 gentamicin or streptomycin.
Unfortunately, many E. faecalis and E. faecium isolates have acquired resistance to one or more of these com ponents 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.
All gram-positive bacteria demonstrate intrinsic anti biotic resistance to polymyxin B/colistin, nalidixic acid, and axtreonam. In addition, several species of enterococci are intrinsically resistant to additional antibiotics, including the following: E. faecalis (cephalosporins, aminoglycosides, clindamycin, quinpristin-dalfopristin, trimethoprim, trimethoprim/sulfamethoxazole, and fusidic acid), E. facieum (all of these included for E. faecalis except quinupristin-dalfopristin), E. gallinarum, and E. casseliflavus (all of those included for E. faecalis and, in addition, vancolycin). Careful consideration should be taken when reporting susceptibilities, cephalosporins, aminoglycosides (except for high-level resistance screening), clindamycin, and trimethoprim-sulfamethoxazole may appear to be effective in the laboratory using in vitro methods, but they are not clinically effective and should not be reported as susceptible.
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