From the Guidelines
Enterococcus faecium with an MIC of 2 is considered resistant to penicillin, and alternative antimicrobial agents should be considered for treating infections caused by this organism. For E. faecium, the susceptibility breakpoint for penicillin is typically ≤0.5-1 μg/mL, depending on the specific guidelines followed, as stated in the guidelines for the management of infective endocarditis 1. With an MIC of 2, this isolate demonstrates reduced susceptibility and would not be expected to respond adequately to penicillin therapy.
Key Points to Consider
- Enterococcus faecium is generally more resistant to beta-lactams than Enterococcus faecalis, often requiring combination therapy with a cell-wall active agent plus an aminoglycoside for synergy, or treatment with agents like vancomycin, linezolid, or daptomycin, depending on the susceptibility profile, as recommended by the American Heart Association 1.
- The resistance in E. faecium is due to altered penicillin-binding proteins that have lower affinity for beta-lactam antibiotics, making them less effective at disrupting bacterial cell wall synthesis.
- It is essential to test enterococci routinely in vitro for susceptibility to penicillin and vancomycin (MIC determination) and for high-level resistance to gentamicin to predict synergistic interactions, as recommended by the American Heart Association 1.
Treatment Options
- Vancomycin-based regimens can be used as an alternative to penicillin for treating E. faecium infections, as suggested in the guidelines for the management of infective endocarditis 1.
- Other treatment options, such as daptomycin, linezolid, or quinupristin-dalfopristin, may be considered for multidrug-resistant E. faecium strains, as recommended by the American Heart Association 1.
From the Research
Susceptibility of Enterococcus faecium to Penicillin
- The susceptibility of Enterococcus faecium to penicillin is a complex issue, with various studies indicating that many strains of E. faecium are resistant to penicillin 2, 3, 4.
- A study from 1989 found that one third of E. faecium isolates were highly resistant to penicillin G, with a minimum inhibitory concentration (MIC) of greater than or equal to 200 micrograms/mL 3.
- Another study from 2014 noted that most modern isolates of E. faecium are highly resistant to ampicillin, a type of penicillin 4.
- A study from 1985 found that the presence of low affinity penicillin-binding proteins, such as PBP5, may be responsible for the range of susceptibility of E. faecium to benzylpenicillin 5.
MIC Value of 2
- There is no direct information in the provided studies about the susceptibility of E. faecium to penicillin with a MIC value of 2.
- However, a study from 1985 found that E. faecium strains with MIC values of 0.031-0.5 microgram ml-1 were considered susceptible to benzylpenicillin, while those with MIC values of 16-64 micrograms ml-1 were considered resistant 5.
- It is unclear how a MIC value of 2 would be classified, but it is likely that E. faecium strains with this MIC value would be considered susceptible to penicillin, based on the findings of the 1985 study 5.
Clinical Implications
- The resistance of E. faecium to penicillin has important clinical implications, as it can limit the treatment options for infections caused by this bacterium 2, 3, 4.
- Clinicians may need to consider alternative antibiotics, such as glycopeptides or newer agents like tigecycline or daptomycin, to treat E. faecium infections 2, 4.
- Further research is needed to understand the mechanisms of resistance and to develop effective treatment strategies for E. faecium infections 2, 3, 4.