Study: Lactobacillus rhamnosus is resistant to antibiotics

Metronidazole is used to treat bacterial vaginosis, caused primarily by the Gardnerella vaginalis biofilm. A recent study looked into the effects of the most common antibiotic used for BV, metronidazole, on the levels of three types of bacteria, including two species of good bacteria and Gardnerella vaginalis.

Gardnerella vaginalis, Lactobacillus rhamnosus, and Lactobacillus plantarum.

(We’ve written a great book on how to solve your BV once and for all  – Killing BV – the definitive way to clean up the mess left by bacterial vaginosis.)

The research was in a lab, so the bacteria go into little agar Petri dishes, get loaded up with antibiotics, then watch and wait to see what happens. This study is interesting because the researchers looked at not just the amount of antibiotic given, but how much was actually reaching the vagina where it was needed, and how long it took, which nobody really talks about. There is a difference in the amounts ingested and the amount that reaches the vagina.

The results of the study include:

Growth of all strains was slowed significantly at high antibiotic concentrations, but remarkably, at certain concentrations, L. rhamnosus, a probiotic bacteria, growth was stimulated during the process. After a certain level, however, the stimulatory effect declined, as you would expect.

Keep in mind that a standard dose of antibiotic for BV is 500mg twice daily.

L. rhamnosus

64mg/ml   Stimulated (possibly using the antibiotic as a fuel source) 264mg/ml   Stimulated *4,000mg/ml   Inhibited by over 71 per cent (which means there was still 29 per cent still alive, probably not by much though)

G. vaginalis

*3,000mg/ml   Inhibited by 70 per cent (most dead, 30 per cent still alive)

L. plantarum

64mg/ml   Inhibited

What this means is that if most G. vaginalis die at 3,000mg/ml, but most L. rhamnosus die at 4,000mg/ml, there is a one thousand mg/ml gap whereby some lactobacilli species can stay alive, while G. vaginalis does not. This offers evidence for a combined treatment of antibiotics and probiotics at the same time, despite the fact that the G. vaginalis biofilm is generally resistant to antibiotics, and the study didn’t separate the distinct difference in G. vaginalis concerning planktonic and cohesive forms of the bacteria.

The paper goes on to say that the recommended dose of metronidazole is 500mg by mouth, or 7.5mg vaginally twice a day for 7 days, which would be expected to kill both the bad and the good germs, but 2,000mg of antibiotic has been found to only offer a mere 26mg/ml after six hours vaginally. This means it isn’t really doing anything, to any bacteria, including lactobacilli. Is it possible that antibiotics aren’t really doing anything at all?

The specific species studied were Gardnerella vaginalis ATCC 14018, Lactobacillus rhamnosus GR-1, and Lactobacillus plantarum KCA.  



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Jessica Lloyd - Vulvovaginal Specialist Naturopathic Practitioner, BHSc(N)

Jessica is a degree-qualified naturopath (BHSc) specialising in vulvovaginal health and disease, based in Melbourne, Australia.

Jessica is the owner and lead naturopath of My Vagina, and is a member of the:

  • International Society for the Study of Vulvovaginal Disease (ISSVD)
  • International Society for the Study of Women's Sexual Health (ISSWSH)
  • National Vulvodynia Association (NVA) Australia
  • New Zealand Vulvovaginal Society (ANZVS)
  • Australian Traditional Medicine Society (ATMS)
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