Buchanan John T, Colvin Kelly M, Vicknair Mike R, Patel Silpa K, Timmer Anjuli M, Nizet Victor
Department of Pediatrics, University of California, San Diego, La Jolla, CA 92093, USA.
Vet Microbiol. 2008 Sep 18;131(1-2):145-53. doi: 10.1016/j.vetmic.2008.02.027. Epub 2008 Mar 7.
Streptococcus iniae is a major fish pathogen producing invasive infections that result in economic losses in aquaculture. Development of in vitro models of S. iniae virulence may provide insight to the pathogenesis of infection in vivo. Three S. iniae strains (K288, 94-426, and 29178) were tested for virulence in a hybrid-striped bass (HSB) model using intraperitoneal injection. S. iniae strains K288 and 94-426 caused high levels of mortality in HSB (lethal dose 2x10(5)CFU) while strain 29178 was avirulent even upon IP challenge with 1000-fold higher inocula. In vitro assays were developed to test for the presence of characteristics previously associated with virulence in other species of pathogenic Streptococcus in animals and humans. In vitro differences relevant to virulence were not detected for beta-hemolysin activity, sensitivity to antimicrobial peptides, or adherence and invasion of epithelial cell layers. However, in whole-blood killing assays, the pathogenic strains were resistant to blood clearance, while 29178 was cleared (P<0.001) and more sensitive to complement (P<0.001). The avirulent strain 29178 was most efficiently phagocytosed and was most susceptible to intracellular killing (P<0.01) by the carp leukocyte cell line (CLC). When exposed to reactive oxygen species, strain 29178 was most susceptible. When the oxidative burst of CLC cells was inhibited, intracellular survival of 29178 was rescued fivefold, while no significant enhancement in survival of K288 or 94-426 was detected. Our results indicate that resistance to phagocytosis, oxidative killing, and associated phagocytic clearance is a significant factor in S. iniae virulence.
海豚链球菌是一种主要的鱼类病原体，会引发侵袭性感染，给水产养殖业造成经济损失。建立海豚链球菌毒力的体外模型可能有助于深入了解体内感染的发病机制。使用腹腔注射法，在杂交条纹鲈（HSB）模型中测试了三株海豚链球菌（K288、94 - 426和29178）的毒力。海豚链球菌K288和94 - 426菌株在HSB中导致了高水平的死亡率（致死剂量为2×10⁵CFU），而29178菌株即使在腹腔注射1000倍高接种量时也无致病性。开展了体外试验，以检测先前在动物和人类的其他致病性链球菌物种中与毒力相关的特征。在β - 溶血素活性、对抗菌肽的敏感性或上皮细胞层的黏附与侵袭方面，未检测到与毒力相关的体外差异。然而在全血杀伤试验中，致病菌株对血液清除具有抗性，而29178菌株被清除（P<0.001）且对补体更敏感（P<0.001）。无毒力的29178菌株最易被吞噬，并且最易被鲤鱼白细胞细胞系（CLC）进行细胞内杀伤（P<0.01）。当暴露于活性氧时，29178菌株最敏感。当CLC细胞的氧化爆发被抑制时，29178菌株的细胞内存活率提高了五倍，而未检测到K288或94 - 426菌株的存活率有显著提高。我们的结果表明，对吞噬作用、氧化杀伤及相关吞噬清除的抗性是海豚链球菌毒力的一个重要因素。
