# Reactivation of hidden-latent Brucella infection after doxycycline and streptomycin treatment in mice

## TL;DR

This study shows that even high antibiotic doses in mice can't prevent relapse of Brucella infection, despite no antibiotic resistance detected.

## Contribution

The study identifies a high relapse rate in mice after antibiotic treatment, which is much higher than in humans and lacks resistance mutations.

## Key findings

- High doxycycline and streptomycin doses achieved therapeutic levels in mice tissues without immediate relapse.
- 100% of mice relapsed after 182 days despite no antibiotic resistance detected in relapsed bacteria.
- Relapse rates in mice (100%) are much higher than in humans (15%).

## Abstract

Brucellosis has therapeutic challenges due to 3%–15% relapses/therapeutic failures (R/TF) after antibiotic treatment. Therefore, determining the antibiotic concentration in tissues, the physiopathological parameters, and the R/TF after treatment is relevant. After exploring different antibiotic quantities, we found that a combined dose of 100 µg/g of doxycycline (for 45 days) and 7.5 µg/g of streptomycin (for 14 days), respectively, achieved therapeutic levels of more than fourfold minimum inhibitory concentrations (MICs) against Brucella abortus in the spleen, liver, bone marrow, and plasma of mice, causing minimal pathophysiological effects. After 30 days of infection, mice received antibiotics, and hematological, histopathological, biochemical, and immunological analyses were performed. After antibiotic therapy, the pathological, hematological, immunological, and physiological profiles paralleled those described in human brucellosis. Treatment lowered antibody titers, reduced proinflammatory cytokines, and reduced inflammation in the target organs for a protracted period. No bacteria were detected in tissues 8 weeks after treatment, suggesting complete recovery. However, despite high doxycycline and streptomycin concentrations in tissues, relapses appeared in 100% of the animals after 182 days post-infection, estimated by the bacterial counts and PCR from organs. This proportion contrasts with the 15% R/TF observed in humans after antibiotic treatments. None of the B. abortus isolated from relapses showed augmented MICs or mutations coding for antibiotic resistance in chromosomal-relevant regions. We discuss whether our findings constitute a general phenomenon or differences in the exhaustive screening method for bacteria detection related to the murine model. Along these lines, we envision likely mechanisms of bacterial persistence in tissues after antibiotic treatment.