A prospective review of all enterococcal isolates for 13 months showed that 9.0% were resistant to ampicillin (MIC, .16 ,ug/ml; zone diameter, <15 mm), as determined by the Vitek system, disk diffusion, microdilution MIC testing, and macrodilution MIC testing. All were I8-lactamase negative. A total of 19 and 3 resistant isolates were from urine and intravascular sites, respectively. Ampicillin-resistant enterococci appear to be a growing clinical problem.MICs of ampicillin against enterococci usually range from 1 to 8 p.g/ml, although MICs for Enterococcus faecium may be as high as 32 ,ug/ml (2, 6,13,14). From the 1960s to the early 1980s, there was no change in the susceptibility of enterococci to either penicillin or ampicillin (6). Resistance of isolates to ampicillin by P-lactamase production (8, 10) or unknown mechanisms (1) Uncertain as to whether this result was due to methodology or a change in susceptibility, we studied all the enterococcal isolates to determine the incidence of ampicillin resistance by using four different methods. Ampicillin-resistant isolates also were tested for P-lactamase production, high-level gentamicin resistance, and susceptibility to other antibiotics possibly effective for the treatment of enterococcal infections.

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A prospective study identified 9 (32%) of 28 ampicillin-resistant (MIC 2 16 ,ug/ml) enterococcus isolates as Enterococcus raffinosus. A case-control study found no significant differences with respect to underlying diseases, catheterization, or surgery between patients with ampicillin-resistant E. raffinosus and those with ampicillin-susceptible Enterococcus spp. Prior treatment with antibiotics and prolonged hospitalization were more frequent among patients with ampicillin-resistant E. raffinosus. Patients with the same strain (determined by plasmid analysis) were frequently hospitalized concurrently. From the 1960s to the early 1980s, susceptibility of enterococci to ampicillin and penicillin did not change (6). Resistance of a few Enterococcusfaecalis isolates to ampicillin by ,B-lactamase production (10, 13) was reported in the 1980s. Enterococcus faecium, Enterococcus raffinosus, and Enterococcus gallinarum were reported subsequently (2, 16), with ampicillin resistance probably due to decreased penicillin-binding affinity of penicillin-binding proteins (3, 17). In recent reports, E. raffinosus has constituted a relatively large

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The risk of significant hyperbilirubinemia increased progressively with increasing percentile. Newborns >75th percentile groups are at high risk for phototherapy and should be closely monitored.