organism, the immune status of the host and age. The four-year mortality rate of individuals
successfully treated for non-IVDA IE was 33% (216) (Table 15). There was no difference in
survival between patients with NVIE and PVE or between those who underwent surgery in the
hospital and those who did not. Mortality was associated with increased age and comorbid
diagnoses.
Relapse of IE most frequently occurs within the first two months of cessation of treatment
of (215–217). No grave relapse is chiefly dependent on the infecting organism. Well-treated
NVIE, due toS. viridans, rarely relapses. Four percent ofS. aureusIE and 30% of enterococcal IE
do relapse. Gram-negative organisms, especiallyP. aeruginosa,have higher rates of relapse
(218). Untreated IE for greater three months’ duration has a significant relapse rate. The
greatest risk factor for recurrent IE is a previous valvular infection, especially IVDA IE (219).
Forty percent of these cases represent recurrence.
ORGANISM DIRECTED ANTIBIOTIC THERAPY
The gram-positive organisms have clearly become the major challenge antibiotic therapy of IE.
Classically,S. viridanshas been extremely sensitive to theb-lactam antibiotics and vancomycin
[minimum inhibitory concentration (MIC) for penicillin less than 0.12mg/mL). IE due to the
viridans streptococci may be cured by a two-week course of theb-lactam antibiotic combined
with gentamicin (220–222). The shortened regimen is appropriate to the following conditions:
(i) a sensitive asS. viridans(MIC<0.1mg/mL); (ii) NVIE of less than three months’ duration;
(iii) vegetation size less than 10 mm in diameter; (iv) no cardiac or extracardiac complications;
(v) a low risk for developing aminoglycoside nephrotoxicity; and (vi) a good clinical response
during the first week of therapy.
Increasing amounts ofS. viridansare becoming resistant to penicillin (MIC>0.1mg/mL).
Highly resistant isolates are categorized as having a MIC> 1 mg/mL. Some 13.4% ofS. viridans,
retrieved from BSIs, are highly resistant. Seventeen percent of these are also highly resistant to
ceftriaxone (MIC> 2 mg/mL) (223).
AllAbiotrophiaspp. are resistant to penicillin, many highly so. Even the penicillin
sensitive strains may be tolerant to theb-lactam compounds (224). Tolerance is a phenomenon
in which the MBC of an antibiotic exceeds its MIC by a factor 10 (225). Groups B, C, and G
streptococci are less sensitive to penicillin thanS. viridansor group A. streptococci (222).
Penicillin alone can cure most cases ofS. viridansIE. Because of its pharmacokinetics,
ceftriaxone has become antibiotic choice because of its twice-a-day dosing regimen. The
combined use of ab-lactam or a glycopeptide with gentamicin is required to eradicate resistant
streptococci. Such a combination is beneficial in the treatment of tolerant streptococci as well.
Table 16 summarizes the recommendations for the treatment of non-enterococcal streptococci.
Since the beginning of the antibiotic era, enterococci have posed a significant therapeutic
challenge because of their ability to raise multiple resistance mechanisms. These organisms are
resistant to all cephalosporins and to the penicillinase-resistant penicillins. When used alone,
penicillin and ampicillin are ineffective against serious enterococcal infection. Likewise,
aminoglycosides fail to treat these infections when used alone because of their inability to
penetrate the bacterial cell wall. The combination of ab-lactam agents (with the exception of
the cephalosporins) is able to effectively treat severe enterococcal infections. The cell wall
active component plus penetration of the aminoglycoside into the interior of the enterococcus
in so reach its target, the ribosome. A serum concentration of 3mg/mL is necessary is necessary
Table 15 Mortality Rates of Left-sided Native Valve IE Due to Various Organisms
Organism Mortality Rates
Streptococcus viridansandStreptococcus bovis 4%–16%
Enterococci 15%–25%
Staphylococcus aureus 25%-–47%
Groups B, C, G streptococci 13%–50%
Coxiella burnetti 5%–37%
Pseudomonas aeruginosa,Enterobacteriaceae, fungi >50%
Source: From Ref. 222.
Infective Endocarditis and Its Mimics in Critical Care 241