Gentamicin Treatment in a Murine Infection Model Suggests Extracellular Orientia tsutsugamushi Contributes to Systemic Dissemination

Technical Summary: Gentamicin Treatment in a Murine Infection Model Suggests Extracellular Orientia tsutsugamushi Contributes to Systemic Dissemination

Problem Statement
Orientia tsutsugamushi (Ot), the obligate intracellular bacterium causing scrub typhus, is known to replicate within host cells and disseminate systemically via infected leukocytes. While the bacterium is known to bud off infected cells as a metabolically inactive, membrane-encased extracellular form (EB), the presence and functional role of this extracellular population in systemic dissemination during in vivo infection remain unclear. Specifically, it is unknown whether EB bacteria exist in detectable quantities in the cell-free plasma fraction of infected hosts or if they contribute significantly to the spread of infection to distant organs.

Methodology
The study employed a combination of clinical sample analysis, in vitro cell culture models, and a murine infection model to investigate the extracellular form of Ot.

1. Clinical and Murine Sample Analysis: Blood samples were collected from infected mice (9 days post-infection) and human scrub typhus patients. Samples were separated into buffy coat (white blood cells) and cell-free plasma fractions. Bacterial load was quantified using qPCR targeting the conserved tsa47 (47 kDa) gene. To confirm that DNA in plasma was associated with intact bacteria rather than free DNA or residual cells, human plasma samples underwent differential centrifugation to separate host cells from bacterial pellets.
2. In Vitro Gentamicin Sensitivity Assays: The study utilized gentamicin, a membrane-impermeable aminoglycoside antibiotic known to have poor penetration into eukaryotic cells. Intracellular (IB) and extracellular (EB) Ot populations were isolated and exposed to varying concentrations of gentamicin. Following treatment, bacteria were used to infect naïve host cells, and subsequent growth was measured via qPCR at 1 and 5 days.
3. Fluorescent Uptake Visualization: To confirm antibiotic penetration into the extracellular form, EB bacteria were incubated with Gentamicin-Texas Red (GTTR) and visualized via fluorescence microscopy to assess uptake.
4. In Vivo Murine Model: C57Bl/6NJcl mice were intradermally inoculated with a sublethal dose of Ot (Karp strain). To isolate the effect of the extracellular form, gentamicin was administered intravenously (low dose: 6 mg/kg; high dose: 24 mg/kg) starting at day 6 post-infection, allowing initial dissemination before treatment. Mice were monitored for weight loss, clinical scores, and cytokine profiles. At day 9, bacterial loads were quantified in blood fractions and tissues (brain, kidney, liver, lung, spleen) via qPCR.

Key Results

• Presence of Extracellular Bacteria: Ot DNA was detected in the cell-free plasma fraction of both infected mice (7/8 samples) and human patients (29/64 samples). Differential centrifugation confirmed that this DNA was associated with bacterial pellets rather than free DNA or mammalian cells.
• Gentamicin Specificity In Vitro: Gentamicin treatment significantly inhibited the subsequent growth of EB Ot in a dose-dependent manner but had no effect on IB Ot. Fluorescence microscopy confirmed that Gentamicin-Texas Red penetrated approximately 54.8% of the EB population.
• Reduction in Tissue Burden In Vivo: In the murine model, gentamicin treatment resulted in a statistically significant reduction of bacterial copy numbers in the brain, kidney, lung, and spleen compared to untreated controls. However, bacterial levels in the liver showed only a small, non-significant reduction.
• Blood Levels and Disease Outcomes: Gentamicin treatment did not significantly reduce bacterial loads in the buffy coat or plasma fractions. Furthermore, despite the reduction in tissue bacterial burden, there were no significant differences in weight change, clinical disease scores (lethargy, ruffled fur), or plasma cytokine profiles (IFN-γ, IL-6, MCP-1) between treated and untreated groups.

Significance and Claims
The authors claim that their findings demonstrate the presence of O. tsutsugamushi in the cell-free plasma of infected hosts and provide evidence that this extracellular form contributes to systemic dissemination. The study suggests that the extracellular bacteria are accessible to membrane-impermeable antibiotics like gentamicin, which can reduce bacterial loads in specific tissues (brain, kidney, lung, spleen) but not in the liver, potentially due to differences in interstitial fluid volume or tissue architecture.

The paper posits that these findings advance the understanding of bacterial dissemination mechanisms, distinguishing between spread via infected leukocytes and the role of cell-free extracellular bacteria. The authors suggest that the accessibility of the extracellular form to antibodies and complement has implications for vaccine development. Additionally, they propose that combining gentamicin with conventional antibiotics targeting the intracellular form (e.g., doxycycline) could potentially augment therapeutic efficacy, although the current study did not demonstrate improved disease outcomes with gentamicin monotherapy in their sublethal model. The authors remain modest regarding the clinical impact, noting that the lack of disease improvement in their model may be due to the sublethal nature of the infection and the short duration of treatment.