The benefit of having a bioluminescent strain is that infections of small laboratory rodents can be monitored without sacrificing the animals. Instead, the animals are placed in a special whole-body imager that detects light emitted from the bodies. The location of the infection in the animals can be determined from the images, and the light intensity measured by the imager gives an idea of the bacterial load in infected tissues. After imaging, the animal can be returned to its cage, and additional images of the same animal can be taken later as the infection progresses.
Another advantage of using bioluminescent bacteria is that the luciferase reaction requires ATP, meaning that the bacteria must be metabolically active to light up. Dead bacteria containing luciferase will not generate light, unlike green fluorescent protein (GFP), another reporter used to image bacteria (see this post that describes an experiment with Borrelia burgdorferi expressing GFP).
To generate bioluminescent L. interrogans, the researchers hooked the firefly luciferase gene up to a strong L. interrogans promoter and inserted the construct into a transposon carried on a suicide plasmid. The plasmid was then introduced into L. interrogans by conjugation (see this blog post for details of the process). Cultures of the engineered spirochetes lit up when luciferin, the luciferase substrate, was added. The amount of light emitted depended on the culture density: more light was detected at higher culture densities.
Mice are ideal models to study persistent infection of the kidney since many rodents are chronic carriers of Leptospira out in nature. These rodents don't get sick from the infection, but they contaminate the environment with the spirochete every time they urinate.
To see how chronic infection is established, the investigators injected a sublethal dose of 107 bioluminescent L. interrogans cells into the abdominal cavity of C57BL6/J mice and took sequential images of the animals over the following months. Albino mice were used because dark fur blocks the signal emitted by the bioluminescent bacteria. (They later showed that standard C57BL6/J mice with black fur could be used as long as they shaved the fur off before placing them in the imager.) The mice were injected with luciferin 10 minutes prior to imaging.
There turned out to be two phases of infection (see the "MFlum1" plot in the graph below). In the acute phase, the bioluminescent signal rose to a peak by day 4 and quickly declined to background levels by day 7. The signal then started increasing again slowly and plateaued after a month.
|Figure 2A from Ratet et al., 2014. Images of a single mouse taken sequentially are shown below the graph. Click for larger image. Source.|
Next, the investigators tested the effectiveness of antibiotics in treating mice infected with the bioluminescent L. interrogans. Several antibiotics are used to treat acute leptospirosis in humans, including penicillin and azithromycin. It is generally believed that antibiotics are more effective if provided early in acute illness. Therefore, the investigators tested whether the timing of antibiotic treatment was important for effectiveness.
As expected, penicillin treatment was most effective when treatment was started at the beginning of the acute phase. In mice treated with daily injections of penicillin for 5 days starting a day after infection, no bioluminescence was detected in the kidneys, and urine was free of L. interrogans as measured by qPCR. However, if treatment was delayed until three days after inoculation, during the peak of acute infection, a low level of L. interrogans was detected in urine by qPCR even though no bioluminescence was detected in the kidneys. It is likely L. interrogans was present in the kidneys but at levels too low to be detected by the imager. The bioluminescence approach clearly does not have the sensitivity of qPCR. Additional experiments revealed that the limit of detection was 100 bioluminescent L. interrogans cells in 100 μl of buffer.
Penicillin was even less effective when administered after the spirochetes settled in the kidneys. When penicillin treatment was initiated at peak bacterial load in the kidneys, day 25 of infection, the signal diminished by over 90% but then bounced back to the level observed before treatment began (see figure below). Ciprofloxacin also failed to eradicate the bacteria.
|Figure 5A from Ratet et al., 2014. Antibiotics were administered for 5 days started on day 25 of infection. Cipr, ciprofloxacin; Pen, penicillin. Source.|
On the other hand, azithromycin managed to knock the signal in the kidneys down to background levels (see graph below). However, the signal came back within a week, although not to the high levels seen in untreated mice. A second course of antibiotics starting on day 112 knocked the signal back down to near background levels, but again, spirochete numbers rebounded, although not to the levels seen before retreatment.
|Figure 5B from Ratet et al., 2014. Azithromycin was administered for 5 days starting on day 25 and day 112 of infection. Source.|
I should caution readers from concluding that tolerance accounts for the poor effectiveness of antibiotics in treating human cases of acute leptospirosis. As the authors point out, leptospirosis patients die because the infection severely injure vital organs. By the time lethal damage occurs, it does not matter whether antibiotics kill all of the spirochetes.
So does the mouse model have any relevance to human leptospirosis? The authors argue that asymptomatic carriage of Leptospira has been overlooked. A 2013 study from the Netherlands revealed that 21% of patients who contracted leptospirosis continued to suffer from headaches, muscle aches, and extreme fatigue two years later. This may reflect unrepaired tissue damage inflicted during acute infection, but no one checked for the presence of Leptospira in these patients. Another study from Peru (see this post) describes asymptomatic individuals who may have persistent Leptospira infection. Kidney function was not checked in the Peruvians, but there is reason to believe that chronic infection affects the kidneys despite the lack of symptoms. Mice chronically infected with L. interrogans are not visibly sick, but they end up with scarred kidneys (fibrosis), as explained in this study.
If persistent asymptomatic infections really do occur in humans, it may be sensible to treat with antibiotics. The chronically-infected mouse will serve as a nice model for testing antibacterial regimens that target Leptospira living in the kidneys.
Ratet G, Veyrier FJ, Fanton d'Andon M, Kammerscheit X, Nicola MA, Picardeau M, Boneca IG, & Werts C (2014). Live imaging of bioluminescent Leptospira interrogans in mice reveals renal colonization as a stealth escape from the blood defenses and antibiotics. PLoS Neglected Tropical Diseases, 8 (12) PMID: 25474719
Goris MG, Kikken V, Straetemans M, Alba S, Goeijenbier M, van Gorp EC, Boer KR, Wagenaar JF, & Hartskeerl RA (2013). Towards the burden of human leptospirosis: duration of acute illness and occurrence of post-leptospirosis symptoms of patients in the Netherlands. PloS One, 8 (10) PMID: 24098528
Fanton d'Andon M, Quellard N, Fernandez B, Ratet G, Lacroix-Lamandé S, Vandewalle A, Boneca IG, Goujon JM, & Werts C (2014). Leptospira Interrogans induces fibrosis in the mouse kidney through Inos-dependent, TLR- and NLR-independent signaling pathways. PLoS Neglected Tropical Diseases, 8 (1) PMID: 24498450
- Life after leptospirosis, a pilot study
- Healthy human carriers of the spirochete Leptospira in the Peruvian Amazon
- The magic of antibiotic tolerance