The proteobacteria Bdellovibrio bacteriovorus and Micavibrio aeruginosavorus are obligate predators of Gram-negative bacteria, and have been proposed to be used to treat multidrug-resistant bacterial infections. The ability of predatory bacteria to reduce bacterial burden in vivo within the lungs of rats has been demonstrated, but it was unknown if predatory bacteria can attenuate systemic bacterial burden administered intravenously. In this study, we first assessed the safety of intravenous inoculation of predatory bacteria in rats. No rat morbidity or adverse histopathology of various organs due to predatory bacteria administration was observed. An increase in proinflammatory cytokines (TNFα and KC/GRO) was observed at two hours post-inoculation; however, cytokines returned to baseline levels by 18 hours. Furthermore, bacterial dissemination analysis demonstrated that predatory bacteria were efficiently cleared from the host by 20 days post-injection. To determine whether predatory bacteria could reduce bacterial burden in vivo , Klebsiella pneumoniae was injected into the tail veins of rats and followed with multiple doses of predatory bacteria over 16 or 24 hours. Predatory bacteria were unable to significantly reduce K. pneumoniae burden in the blood or prevent dissemination to other organs. The results suggest that predatory bacteria may not be effective for treatment of acute blood infections.

Bdellovibrio bacteriovorus and Micavibrio aeruginosavorus are Gram-negative proteobacteria that are obligate predators of other Gram-negative bacteria and are considered potential alternatives to antibiotics. Most studies focusing on predatory bacteria have been performed in vitro, thus the effect of predatory bacteria on a live host, including the impact on the ecology of the native microbiota, has yet to be fully examined. In this study, intrarectal inoculations of Sprague-Dawley rats with predatory bacteria were performed. Additionally, feces were collected for seven days postinoculation to determine the effect on gut bacterial diversity. Rat colonic tissue exhibited no abnormal histopathological effects due to predatory bacteria. A modest increase in pro-inflammatory cytokines was measured in the colons of rats inoculated with predatory bacteria by 24 and 48 hours, with all but IL-13 returning to baseline by seven days. V4 16S rRNA gene sequencing of fecal DNA demonstrated minimal shifts in taxonomic representation over the week due to predatory bacteria. Changes in bacterial populations due to exposure to B. bacteriovorus are predicted to contribute to health, however, an overgrowth of Prevotella was observed due to exposure to M. aeruginosavorus. This study further addresses safety concerns associated with the potential use of predatory bacteria to treat infections.

ABSTRACTBdellovibrio bacteriovorusandMicavibrio aeruginosavorusare predatory bacteria that naturally—and obligately—prey on other Gram-negative bacteria, and their use has been proposed as a potential new approach to control microbial infection. The ability of predatory bacteria to prey on Gram-negative human pathogensin vitrois well documented; however, thein vivosafety and efficacy of predatory bacteria have yet to be fully assessed. In this study, we examined whether predatory bacteria can reduce bacterial burden in the lungs in anin vivomammalian system. Initial safety studies were performed by intranasal inoculation of rats with predatory bacteria. No adverse effects or lung pathology were observed in rats exposed to high concentrations of predatory bacteria at up to 10 days postinoculation. Enzyme-linked immunosorbent assay (ELISA) of the immune response revealed a slight increase in inflammatory cytokine levels at 1 h postinoculation that was not sustained by 48 h. Additionally, dissemination experiments showed that predators were efficiently cleared from the host by 10 days postinoculation. To measure the ability of predatory bacteria to reduce microbial burdenin vivo, we introduced sublethal concentrations ofKlebsiella pneumoniaeinto the lungs of rats via intranasal inoculation and followed with multiple doses of predatory bacteria over 24 h. Predatory bacteria were able to reduceK. pneumoniaebacterial burden, on average, by more than 3.0 log10in the lungs of most rats as measured by CFU plating. The work presented here provides further support for the idea of developing predatory bacteria as a novel biocontrol agent.IMPORTANCEA widely held notion is that antibiotics are the greatest medical advance of the last 50 years. However, the rise of multidrug-resistant (MDR) bacterial infections has become a global health crisis over the last decade. As we enter the postantibiotic era, it is crucial that we begin to develop new strategies to combat bacterial infection. Here, we report one such new approach: the use of predatory bacteria (Bdellovibrio bacteriovorusandMicavibrio aeruginosavorus) that naturally—and obligately—prey on other Gram-negative bacteria. To our knowledge, this is the first study that demonstrated the ability of predatory bacteria to attenuate the bacterial burden of a key human pathogen in anin vivomammalian system. As the prevalence of MDR infections continues to rise each year, our results may represent a shift in how we approach treating microbial infections in the future.