EERA Unit "Ecology and Evolution of Antibiotic Resistance", Institut Pasteur - Assistance Publique/Hôpitaux de Paris - University Paris-Saclay, Paris, France.
UMR CNRS 3525, 75015, Paris, France.
BMC Microbiol. 2020 Oct 27;20(1):325. doi: 10.1186/s12866-020-02007-y.
Klebsiella pneumoniae is a leading cause of intractable hospital-acquired multidrug-resistant infections and carbapenemase-producing K. pneumoniae (CPKp) are particularly feared. Most of the clinical isolates produce capsule as a major virulence factor. Recombination events at the capsule locus are frequent and responsible for capsule diversity within Klebsiella spp. Capsule diversity may also occur within clonal bacterial populations generating differences in colony aspect. However, little is known about this phenomenon of phenotypic variation in CPKp and its consequences.
Here, we explored the genetic causes of in vitro switching from capsulated, mucoid to non-mucoid, non-capsulated phenotype in eight clinical CPKp isolates. We compared capsulated, mucoid colony variants with one of their non-capsulated, non-mucoid isogenic variant. The two colony variants were distinguished by their appearance on solid medium. Whole genome comparison was used to infer mutations causing phenotypic differences. The frequency of phenotypic switch was strain-dependent and increased along with colony development on plate. We observed, for 72 non-capsulated variants that the loss of the mucoid phenotype correlates with capsule deficiency and diverse genetic events, including transposition of insertion sequences or point mutations, affecting genes belonging to the capsule operon. Reduced or loss of capsular production was associated with various in vitro phenotypic changes, affecting susceptibility to carbapenem but not to colistin, in vitro biofilm formation and autoaggregation.
The different impact of the phenotypic variation among the eight isolates in terms of capsule content, biofilm production and carbapenem susceptibility suggested heterogeneous selective advantage for capsular loss according to the strain and the mutation. Based on our results, we believe that attention should be paid in the phenotypic characterization of CPKp clinical isolates, particularly of traits related to virulence and carbapenem resistance.
肺炎克雷伯菌是导致难以治疗的医院获得性多重耐药感染的主要原因，产碳青霉烯酶肺炎克雷伯菌（CPKp）尤其令人担忧。大多数临床分离株产生荚膜作为主要毒力因子。荚膜基因座的重组事件频繁发生，导致肺炎克雷伯菌属内荚膜多样性。荚膜多样性也可能在克隆细菌群体中发生，从而导致菌落外观的差异。然而，对于 CPKp 中这种表型变异及其后果的现象知之甚少。
在这里，我们探索了 8 株临床 CPKp 分离株体外从荚膜、粘液型向非粘液型、无荚膜表型转变的遗传原因。我们比较了荚膜粘液型菌落变体与其无荚膜、非粘液型同系变体之一。这两种菌落变体在固体培养基上的外观不同。全基因组比较用于推断引起表型差异的突变。表型转换的频率取决于菌株，并且随着平板上菌落的发育而增加。我们观察到，在 72 个非荚膜变体中，粘液表型的丧失与荚膜缺失和多种遗传事件相关，包括插入序列或点突变的转位，影响属于荚膜操纵子的基因。荚膜产生减少或缺失与各种体外表型变化相关，影响对碳青霉烯的敏感性，但对粘菌素、体外生物膜形成和自动聚集没有影响。
8 个分离株中表型变异的不同影响，包括荚膜含量、生物膜形成和碳青霉烯敏感性，表明根据菌株和突变，荚膜缺失具有不同的选择优势。基于我们的结果，我们认为在 CPKp 临床分离株的表型特征，特别是与毒力和碳青霉烯耐药性相关的特征方面，应引起重视。
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