In the 2008 and 2010 studies (described in detail here and here), Barthold's group gave doxycycline, ceftriaxone, or tigecycline to mice with disseminated Borrelia burgdorferi infection. As expected, all tissues were culture negative up to three months following antibiotic therapy. Tissues from untreated mice were culture positive. However, B. burgdorferi DNA and mRNA were detected by PCR in up to half the treated mice, and microscopy revealed a few intact spirochetes in collagen-rich tissues from these mice. Ticks allowed to feed on the treated mice even transmitted the spirochetes to other mice (albeit immune deficient ones), where B. burgdorferi DNA was detected by PCR. Clearly, the spirochetes that survived antibiotic treatment were alive despite being unculturable.
Although live spirochetes remained following antibiotic therapy, there was no evidence that they were capable of causing disease. Lyme disease is driven by inflammation, but no inflammatory response in the form of infiltrating immune cells were seen in tissues harboring the spirochetes. A critic of the work also pointed out that the number of spirochetes declined during the 3 months following treatment, implying that any lingering spirochetes would eventually disappear. It seemed unlikely that a similar phenomenon was responsible for persisting symptoms following treatment of Lyme disease in human patients, who may suffer with disabling symptoms for years.
In 2014 Barthold's group came out with another paper, which I'm discussing here for the first time. Again, mice with disseminated B. burgdorferi infections were treated with antibiotics, ceftriaxone in this case. But this time, the mice were left for up to a year before their tissues were examined for the presence of B. burgdorferi. Control mice were mock treated with saline and examined along with the treated mice.
There weren't any surprises when tissues were tested by culture. Most of the control mice were culture positive at all time points (2, 4, 8, and 12 months) with both tissues tested, the urinary bladder and the skin where B. burgdorferi was inoculated to initiate infection. None of the treated mice were culture positive at either site at any time point.
PCR testing for B. burgdorferi DNA was done with tissue obtained from six sites in the mice. Ticks allowed to feed on the mice were also tested for the presence B. burgdorferi DNA by PCR in a method called xenodiagnosis. All saline-treated mice were PCR positive in most tissues tested, and most tested positive by xenodiagnosis.
The results with the mice treated with ceftriaxone are shown in the table below. Each row represents a single mouse. Note that each tissue homogenate was tested three times.
They saw something remarkable with the mice left for 12 months. Although few tissues were positive at earlier time points, most tissues extracted from mice a year after treatment tested positive. 6 of the 8 mice also tested positive by xenodiagnosis. So, instead of eventually disappearing, the spirochetes proliferated starting at some point after 8 months elapsed following treatment. This resurgence occurred even though the spirochetes remained unculturable.
Barthold's group also looked for evidence of inflammation. Despite the resurgence of spirochetes, they did not see much evidence of inflammation by microscopy of the tissues 12 months following antibiotic treatment. However, the researchers pointed out that no conclusions can be drawn about the ability of the persisting spirochetes to cause disease since inflammation was minimal even in saline-treated mice, which harbored culturable spirochetes.
The researchers next looked for molecular evidence of inflammation. They measured transcript levels of 18 cytokines in the base of the heart, heart muscle, quadriceps muscle, and leg joint 12 months after treatment with ceftriaxone or saline. The levels of cytokine transcripts in the two groups were compared to those in age-matched uninfected mice. Not surprisingly, saline-treated mice had what the authors deemed a "proinflammatory" cytokine profile, most likely due to their ongoing infection. Antibiotic-treated mice also had a proinflammatory cytokine profile, although it differed from that of the saline-treated mice. This observation is the first to suggest that the mice were responding to persisting spirochetes that survived antibiotic treatment.
In conclusion, the evidence is convincing that B. burgdorferi persists in mice for a long time after antibiotic treatment. They don't eventually disappear and may even proliferate. Whether these unculturable spirochetes are capable of generating an inflammatory condition necessary for disease is less clear, though mice do appear to generate a unique cytokine profile in response to the persisting spirochetes.
Barthold's group caution readers from applying the findings too broadly:
Because of the controversial nature of these findings, they should not be over-interpreted and certainly not translated directly into clinical management of human Lyme borreliosis.
So is there any relevance of these findings to post-treatment symptoms in humans? I will touch upon this issue in a future post.
Hodzic E, Imai D, Feng S, & Barthold SW (2014). Resurgence of persisting non-cultivable Borrelia burgdorferi following antibiotic treatment in mice. PLOS One, 9 (1) PMID: 24466286
- Chronic Lyme disease in mice?
- Tigecycline fails to eradicate persisting Borrelia burgdorferi
- Long-term antibiotics for those with chronic symptoms that may or may not be related to Lyme disease