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Group B Streptococcus (group B strep, GBS) emerged in the 1970s as the most common cause of sepsis in newborns. Invasive adult disease (invasion of the bacteria into a site where there usually are no bacteria, like the blood) was recognized more recently and the incidence has been increasing. Early-onset disease (occurs in babies younger than 1 week old) has declined by 80% since increased use of intrapartum prophylaxis has occurred.

Active surveillance for invasive group B strep disease is ongoing in a multistate population of approximately 33 million, including approximately 450,000 live births annually. The disease is not reportable in most states.

To be considered a case for this active surveillance system, the following case definition must be met:

  • GBS must be isolated from a normally sterile site, such as blood, cerebrospinal fluid (CSF), pleural fluid, peritoneal fluid, pericardial fluid, bone, joint/synovial fluid, or internal body site (e.g., lymph node, brain)
  • Case patient must be a resident of one of the defined surveillance areas

The following is an exception to this sterile site case definition and would be counted as cases for this surveillance system:

  • GBS isolated from the placenta and/or amniotic fluid with fetal death

Resistance in GBS

The widespread use of intrapartum antibiotic prophylaxis to prevent early-onset GBS disease has raised concern about the development of antibiotic resistance among GBS isolates. GBS continues to be susceptible to penicillin, ampicillin, and first-generation cephalosporins (19,105–109). However, an increasing number of isolates with reduced susceptibility to penicillin have been found in Japan (1) and isolates with increasing minimum inhibitory concentrations (MICs) to penicillin or ampicillin have been found in the US including  11 (0.2%) of 5,631 invasive isolates recovered from 1999–2005 in (111). Alterations in a penicillin-binding protein (PBP 2X) were found in all of the isolates from Japan and four of those from the United States. The measured MICs from the 11 invasive isolates from the United States are just at the threshold of susceptibility (≤0.12 μg/ml for penicillin and ≤0.25 μg/ml for ampicillin) (112), but the clinical significance of these MIC values is as yet unclear.

Relatively elevated MICs to cefazolin (1 μg/ml) also were reported among three (0.05%) of 5,631 invasive GBS isolates collected through CDC’s active surveillance during 1999–2005; two of the three isolates also had elevated MICs to penicillin (0.12 μg/ml) (111). Although Clinical and Laboratory Standards Institute guidelines do not specify susceptibility breakpoints for cefazolin, they recommend that all isolates susceptible to penicillin be considered susceptible to cefazolin (112). As with the elevated MICs to penicillin and ampicillin, the clinical significance of higher MICs to cefazolin among GBS isolates remains unclear.

The proportions of GBS isolates with in vitro resistance to clindamycin or erythromycin have increased over the past 20 years. The prevalence of resistance among invasive GBS isolates in the United States ranged from 25% to 32% for erythromycin and from 13% to 20% for clindamycin in reports published during 2006–2009 (19,106,108). Resistance to erythromycin is associated frequently but not always with resistance to clindamycin. One longitudinal study of GBS early-onset sepsis found that although the overall rate of GBS early-onset disease declined over time, erythromycin-resistant GBS caused an increasing proportion of disease during this interval; however, the incidence of antibiotic-resistant GBS early-onset sepsis remained stable (105).

Antimicrobial Susceptibility Testing

Antimicrobial susceptibility testing of GBS isolates is crucial for appropriate antibiotic prophylaxis selection for penicillin-allergic women who are at high risk for anaphylaxis because resistance to clindamycin, the most common agent used in this population, is increasing among GBS isolates. In addition, appropriate methodologies for susceptibility testing are important because inducible clindamycin resistance can occur in some strains that appear susceptible in broth susceptibility tests (193,194). D-zone testing using the double-disk diffusion method has been used to identify isolates that are erythromycin-resistant and clindamycin-susceptible, yet have inducible resistance to clindamycin (195). Isolates that are D-zone positive are considered to have inducible clindamycin resistance and are presumed to be resistant although the clinical significance of this resistance is not clear (196).



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