From the FDA Drug Label
Because many strains of the following groups of gram-positive microorganisms have been shown to be resistant to tetracycline, culture and susceptibility testing are recommended Up to 44 percent of strains of Streptococcus pyogenes and 74 percent of Streptococcus faecalis have been found to be resistant to tetracycline drugs. Enterococcus group (Streptococcus faecalis and Streptococcus faecium)
Doxycycline may not be effective against Enterococcus faecalis as up to 74 percent of strains have been found to be resistant to tetracycline drugs. Therefore, culture and susceptibility testing are recommended before using doxycycline to treat infections caused by this organism 1.
From the Research
Doxycycline does not reliably cover Enterococcus faecalis infections. Enterococcus faecalis is generally resistant to doxycycline and other tetracycline antibiotics, making it a poor choice for treating infections caused by this organism. According to a study published in 2004 2, only 28% of E. faecalis strains were sensitive to tetracycline, indicating a high level of resistance. For Enterococcus faecalis infections, preferred antibiotics include ampicillin (2 grams IV every 4-6 hours), penicillin G (3-4 million units IV every 4 hours), or vancomycin (15-20 mg/kg IV every 8-12 hours) for penicillin-allergic patients. More serious infections often require combination therapy with an aminoglycoside like gentamicin. The resistance of Enterococcus faecalis to tetracyclines occurs because the bacterium possesses efflux pumps that actively remove the drug from bacterial cells and ribosomal protection mechanisms that prevent doxycycline from binding to bacterial ribosomes.
When treating suspected or confirmed Enterococcus faecalis infections, it's essential to select antibiotics with proven activity against this organism rather than doxycycline. A recent review published in 2023 3 highlights the importance of appropriate antibiotic therapy for enterococcal infections, including UTIs. The review emphasizes the need for guidelines-based treatment recommendations, which do not support the use of doxycycline for Enterococcus faecalis infections. Another study published in 2019 4 found that amoxicillin was associated with a better outcome in patients with enterococcal bacteraemia, further supporting the use of alternative antibiotics.
Key points to consider when treating Enterococcus faecalis infections include:
- The high prevalence of resistance to tetracyclines, including doxycycline
- The importance of selecting antibiotics with proven activity against E. faecalis, such as ampicillin, penicillin G, or vancomycin
- The potential need for combination therapy with an aminoglycoside for more serious infections
- The need to follow guidelines-based treatment recommendations, which do not support the use of doxycycline for E. faecalis infections.