If daptomycin therapy is selected, then doses of 10 may be considered (Class IIb; Level of Evidence C). 3. Combination therapy with daptomycin and ampi- cillin or ceftaroline may be considered, especially in patients with persistent bacteremia or enterococcal strains with high MICs (ie, 3 g/mL) to daptomy- cin within the susceptible range (Class IIb; Level of Evidence C). HACEK Microorganisms IE caused by fastidious Gram-negative bacilli of the HACEK group (HACEK indicates Haemophilus species, Aggregatibacter species, Cardiobacterium hominis, Eikenella corrodens, and Kingella species) accounts for 5% to 10% of community-acquired NVE in patients who are not IDUs.218 These microorganisms grow slowly in standard blood cul- ture media, and recovery may require prolonged incubation. Typically, only a small fraction of blood culture bottles in patients with HACEK IE demonstrate growth. Bacteremia caused by HACEK microorganisms in the absence of an obvi- ous focus of infection is highly suggestive of IE even without typical physical findings of IE. Previously, the HACEK group of microorganisms was uniformly susceptible to ampicillin. However, -lactamase producing strains of HACEK are appearing with increased frequency; rarely, resistance to ampicillin can occur in Moreover, difficulty in per- forming antimicrobial susceptibility testing as a result of failure of growth in in vitro susceptibility testing is common- place. In 1 survey, 60% of isolates did not grow adequately in control wells, and no valid in vitro susceptibility results were Therefore, unless growth is adequate for in vitro screening, then HACEK microorganisms should be consid- ered ampicillin resistant, and penicillin and ampicillin should not be used to treat patients with IE in these cases. Almost all strains of the HACEK group are susceptible to ceftriaxone (or other third- or fourth-generation cephalosporins). Ceftriaxone has commonly been used to treat HACEK IE220 and is reason- able for treatment (Table 16). 