Chapter 130. Streptococcal and Enterococcal Infections (Part 12)

Enterococci may be resistant to penicillins via two distinct mechanisms. The first is β-lactamase production (mediating resistance to penicillin and ampicillin), which has been reported for E. faecalis isolates from several locations in the United States and other countries. Because the amount of β-lactamase produced may be insufficient for detection by routine antibiotic susceptibility testing, isolates from serious infections should be screened specifically for βlactamase production with a chromogenic cephalosporin or another method. For the treatment of β-lactamase–producing strains, vancomycin,ampicillin/sulbactam, amoxicillin/clavulanate, imipenem, or meropenem may be used in combination with gentamicin. ...
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Chapter 130. Streptococcal and Enterococcal Infections (Part 12) Chapter 130. Streptococcal and Enterococcal Infections (Part 12) Enterococci may be resistant to penicillins via two distinct mechanisms.The first is β-lactamase production (mediating resistance to penicillin andampicillin), which has been reported for E. faecalis isolates from several locationsin the United States and other countries. Because the amount of β-lactamaseproduced may be insufficient for detection by routine antibiotic susceptibilitytesting, isolates from serious infections should be screened specifically for β-lactamase production with a chromogenic cephalosporin or another method. Forthe treatment of β-lactamase–producing strains, vancomycin,ampicillin/sulbactam, amoxicillin/clavulanate, imipenem, or meropenem may beused in combination with gentamicin. The second mechanism of penicillin resistance is not mediated by β-lactamase and may be due to altered penicillin-binding proteins. This intrinsicpenicillin resistance is common among E. faecium isolates, which routinely aremore resistant to β-lactam antibiotics than are isolates of E. faecalis. Moderatelyresistant enterococci (MICs of penicillin and ampicillin, 16–64 µg/mL) may besusceptible to high-dose penicillin or ampicillin plus gentamicin, but strains withMICs of ≥200 µg/mL must be considered resistant to clinically achievable levelsof β-lactam antibiotics, including imipenem and meropenem. Vancomycin plusgentamicin is the recommended regimen for infections due to enterococci withhigh-level intrinsic resistance to β-lactams. Vancomycin-resistant enterococci (VRE), first reported from clinicalsources in the late 1980s, have become common in many hospitals. Three majorvancomycin resistance phenotypes have been described: VanA, VanB, and VanC.The VanA phenotype is associated with high-level resistance to vancomycin andto teicoplanin, a related glycopeptide antibiotic not currently available in theUnited States. VanB and VanC strains are resistant to vancomycin but susceptibleto teicoplanin, although teicoplanin resistance may develop during treatment inVanB strains. For enterococci resistant to both vancomycin and β-lactams, noestablished therapies provide uniformly bactericidal activity. Two newer agentsactive against VRE are quinupristin/dalfopristin and linezolid, which wereapproved for use in the United States in 1999 and 2000, respectively.Quinupristin/dalfopristin is a streptogramin combination with in vitrobacteriostatic activity against E. faecium, including VRE, but not against E.faecalis or other enterococcal species. Disadvantages of quinupristin/dalfopristinare its limited spectrum of activity against enterococcal species and its relativelyfrequent side effects of phlebitis and myalgia. Linezolid is an oxazolidinoneantibiotic with good bacteriostatic activity against nearly all enterococci, includingVRE. Limited clinical experience suggests that linezolid is at least as efficaciousas quinupristin/dalfopristin, and linezolid is usually preferred because of itsbroader activity against all enterococci and the availability of both parenteral andoral formulations. Bone marrow toxicity (especially thrombocytopenia) andperipheral neuropathy are potential side effects. Two other antibiotics are active invitro against VRE (both E. faecalis and E. faecium), although neither had beenapproved for treatment of these infections as of May 2006: daptomycin, a cycliclipopeptide, and tigecycline, a glycylcycline related to tetracycline. Other Group D Streptococci The main nonenterococcal group D streptococcal species that causes humaninfections is S. bovis. S. bovis endocarditis is often associated with neoplasms ofthe gastrointestinal tract—most frequently, a colon carcinoma or polyp—but isalso reported in association with other bowel lesions. When occult gastrointestinallesions are carefully sought, abnormalities are found in ≥60% of patients with S.bovis endocarditis. In contrast to the enterococci, nonenterococcal group Dstreptococci like S. bovis are reliably killed by penicillin as a single agent, andpenicillin is the agent of choice for S. bovis infections. Viridans and Other Streptococci Viridans Streptococci Consisting of multiple species of α-hemolytic streptococci, the viridansstreptococci are a heterogeneous group of organisms that are important agents ofbacterial endocarditis (Chap. 118). Several species of viridans streptococci,including S. salivarius, S. mitis, S. sanguis, and S. mutans, are part of the normalflora of the mouth, where they live in close association with the teeth and gingiva.Some species contribute to the development of dental caries. Previously known as S. morbillorum, Gemella morbillorum has been placedin a separate genus, along with G. haemolysans, on the basis of genetic-relatednessstudies. These species resemble viridans streptococci with respect to habitat in thehuman host and associated infections. The transient viridans streptococcal bacteremia induced by eating, tooth-brushing, flossing, and other sources of minor trauma, together with adherence tobiologic surfaces, is thought to account for the predilection of these organisms tocause endocarditis (see Fig. 118-1). Viridans streptococci are also isolated, oftenas part of a mixed flora, from sites of sinusitis, brain abscess, and liver abscess. Viridans streptococcal bacteremia occurs relatively frequently inneutropenic patients, particularly after bone marrow transplantation or high-dosechemotherapy for cancer. Some of these patients develop a sepsis syndrome withhigh fever and shock. Risk fac ...