Immunomodulatory and protective roles of quorum-sensing signaling molecules N-acyl homoserine lactones during infection of mice with Aeromonas hydrophila.

Aeromonas hydrophila leads to both intestinal and extraintestinal infections in animals and humans, and the underlying mechanisms leading to mortality are largely unknown. By using a septicemic mouse model of infection, we showed that animals challenged with A. hydrophila die because of kidney and liver damage, hypoglycemia, and thrombocytopenia. Pretreatment of animals with quorum-sensing-associated signaling molecules N-acyl homoserine lactones (AHLs), such as butanoyl and hexanoyl homoserine lactones (C(4)- and C(6)-HSLs), as well as N-3-oxododecanoyl (3-oxo-C(12))-HSL, prevented clinical sequelae, resulting in increased survivability of mice. Since little is known as to how different AHLs modulate the immune response during infection, we treated mice with the above AHLs prior to lethal A. hydrophila infection. When we compared results in such animals to those in controls, the treated animals exhibited a significantly reduced bacterial load in the blood and other mouse organs, as well as various levels of cytokines/chemokines. Importantly, neutrophil numbers were significantly elevated in the blood of C(6)-HSL-treated mice compared to those in animals given phosphate-buffered saline and then infected with the bacteria. These findings coincided with the fact that neutropenic animals were more susceptible to A. hydrophila infection than normal mice. Our data suggested that neutrophils quickly cleared bacteria by either phagocytosis or possibly another mechanism(s) during infection. In a parallel study, we indeed showed that other predominant immune cells inflicted during A. hydrophila infections, such as murine macrophages, when they were pretreated with AHLs, rapidly phagocytosed bacteria, whereas untreated cells phagocytosed fewer bacteria. This study is the first to report that AHL pretreatment modulates the innate immune response in mice and enhances their survivability during A. hydrophila infection.

Cethromycin-mediated protection against the plague pathogen Yersinia pestis in a rat model of infection and comparison with levofloxacin.

The Gram-negative plague bacterium, Yersinia pestis, has historically been regarded as one of the deadliest pathogens known to mankind, having caused three major pandemics. After being transmitted by the bite of an infected flea arthropod vector, Y. pestis can cause three forms of human plague: bubonic, septicemic, and pneumonic, with the latter two having very high mortality rates. With increased threats of bioterrorism, it is likely that a multidrug-resistant Y. pestis strain would be employed, and, as such, conventional antibiotics typically used to treat Y. pestis (e.g., streptomycin, tetracycline, and gentamicin) would be ineffective. In this study, cethromycin (a ketolide antibiotic which inhibits bacterial protein synthesis and is currently in clinical trials for respiratory tract infections) was evaluated for antiplague activity in a rat model of pneumonic infection and compared with levofloxacin, which operates via inhibition of bacterial topoisomerase and DNA gyrase. Following a respiratory challenge of 24 to 30 times the 50% lethal dose of the highly virulent Y. pestis CO92 strain, 70 mg of cethromycin per kg of body weight (orally administered twice daily 24 h postinfection for a period of 7 days) provided complete protection to animals against mortality without any toxic effects. Further, no detectable plague bacilli were cultured from infected animals' blood and spleens following cethromycin treatment. The antibiotic was most effective when administered to rats 24 h postinfection, as the animals succumbed to infection if treatment was further delayed. All cethromycin-treated survivors tolerated 2 subsequent exposures to even higher lethal Y. pestis doses without further antibiotic treatment, which was related, in part, to the development of specific antibodies to the capsular and low-calcium-response V antigens of Y. pestis. These data demonstrate that cethromycin is a potent antiplague drug that can be used to treat pneumonic plague.

Progress on plague vaccine development.

Yersinia pestis (YP), the gram-negative plague bacterium, has shaped human history unlike any other pathogen known to mankind. YP (transmitted by the bite of an infected flea) diverged only recently from the related enteric pathogen Yersinia pseudotuberculosis but causes radically different diseases. Three forms of plague exist in humans: bubonic (swollen lymph nodes or bubos), septicemic (spread of YP through the lymphatics or bloodstream from the bubos to other organs), and contagious, pneumonic plague which can be communicated via YP-charged respiratory droplets resulting in person-person transmission and rapid death if left untreated (50-90% mortality). Despite the potential threat of weaponized YP being employed in bioterrorism and YP infections remaining prevalent in endemic regions of the world where rodent populations are high (including the four corner regions of the USA), an efficacious vaccine that confers immunoprotection has yet to be developed. This review article will describe the current vaccine candidates being evaluated in various model systems and provide an overall summary on the progress of this important endeavor.