Rakovitsky Nadya, Lurie-Weinberger Mor N, Hameir Amichay, Wulffhart Liat, Keren Paz Alona, Schwartz David, Carmeli Yehuda
National Institute for Antibiotic Resistance and Infection Control, Israel Ministry of Health, Tel Aviv, Israel.
Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel.
Microbiol Spectr. 2023 Jan 31;11(2):e0300222. doi: 10.1128/spectrum.03002-22.
A positive "string test" indicates the ability of bacterial colonies grown on agar plates to form viscous strings of >5 mm when stretched. This phenotype is strongly associated with hypervirulence in Klebsiella pneumoniae but has never been described in carbapenem-resistant Acinetobacter baumannii (CRAB), an emerging human pathogen of high clinical significance. In this work, we screened 1,000 CRAB isolates, among which we identified and characterized 9 string-positive CRAB (stCRAB) isolates. Phenotypic and genotypic analyses revealed that the isolates were not phylogenetically related and possessed different antibiotic resistance and virulence profiles. Transmission electron microscopy (TEM) showed the presence of capsule in string-positive isolates. String-positive isolates were more motile but did not form more biofilm than non-string-positive isolates. They were less virulent in a murine thigh fitness model and a Galleria mellonella survival assay. In conclusion, here, we describe string-positive A. baumannii isolates and their phenotypic and molecular characteristics. We found that unlike K. pneumoniae, stCRAB isolates were not associated with increased virulence. Acinetobacter baumannii has been considered a major health care threat in recent years. Despite many efforts, the pathogenesis and molecular mechanism of A. baumannii virulence remain poorly understood. Moreover, the plasticity of its genome frequently gives rise to new and more virulent isolates. Our current study is of significant importance as it concerns a previously undescribed A. baumannii phenotype. The string-positive phenotype is strongly associated with increased fitness and virulence in other Gram-negative bacteria such as K. pneumoniae. Although no clear correlation with virulence or fitness was found in our 9 stCRAB isolates, this could have been due to the limited statistical power of our research. We suggest that this phenotype should be taken into consideration as due to its genome plasticity, the next change can give rise to string-positive and hypervirulent strains, as is known for K. pneumoniae. Additional future research is needed regarding its possible consequences.
阳性“拉丝试验”表明在琼脂平板上生长的细菌菌落拉伸时能够形成长度大于5毫米的粘性丝状物。这种表型与肺炎克雷伯菌的高毒力密切相关，但在耐碳青霉烯鲍曼不动杆菌（CRAB）中从未被描述过，CRAB是一种具有高度临床意义的新出现的人类病原体。在这项研究中，我们筛选了1000株CRAB分离株，从中鉴定并表征了9株拉丝阳性CRAB（stCRAB）分离株。表型和基因型分析表明，这些分离株在系统发育上没有相关性，并且具有不同的抗生素耐药性和毒力谱。透射电子显微镜（TEM）显示拉丝阳性分离株存在荚膜。拉丝阳性分离株的运动性更强，但与非拉丝阳性分离株相比，形成的生物膜并不更多。在小鼠大腿适应性模型和大蜡螟存活试验中，它们的毒力较低。总之，我们在此描述了拉丝阳性鲍曼不动杆菌分离株及其表型和分子特征。我们发现，与肺炎克雷伯菌不同，stCRAB分离株与毒力增加无关。近年来，鲍曼不动杆菌一直被视为主要的医疗威胁。尽管付出了很多努力，但鲍曼不动杆菌毒力的发病机制和分子机制仍知之甚少。此外，其基因组的可塑性经常产生新的、毒力更强的分离株。我们目前的研究具有重要意义，因为它涉及一种以前未被描述过的鲍曼不动杆菌表型。在其他革兰氏阴性菌如肺炎克雷伯菌中，拉丝阳性表型与适应性和毒力增加密切相关。尽管在我们的9株stCRAB分离株中未发现与毒力或适应性有明确的相关性，但这可能是由于我们研究的统计能力有限。我们建议应考虑这种表型，因为由于其基因组可塑性，下一次变化可能会产生如肺炎克雷伯菌那样的拉丝阳性和高毒力菌株。关于其可能的后果，还需要进一步的未来研究。
