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.

中文翻译：

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产碳青霉烯酶肺炎克雷伯菌黏液向非黏液转变的多样性

肺炎克雷伯菌是顽固性医院获得性耐多药感染的主要原因，尤其令人担忧的是产碳青霉烯酶的肺炎克雷伯菌 (CPKp)。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是频繁发生的，并且是克雷伯氏菌属内荚膜多样性的原因。胶囊多样性也可能发生在克隆细菌种群中，从而产生菌落方面的差异。然而，对这种 CPKp 表型变异现象及其后果知之甚少。在这里，我们探讨了在 8 种临床 CPKp 分离株中从有囊状、粘液状到非粘液状、无囊状表型的体外转换的遗传原因。我们比较了有荚膜的粘液集落变异与其中一种非有荚膜的非粘液同基因变异。两种菌落变体的区别在于它们在固体培养基上的外观。全基因组比较用于推断导致表型差异的突变。表型转换的频率是应变依赖性的，并且随着板上菌落的发育而增加。我们观察到，对于 72 个非荚膜变体，粘液样表型的丧失与荚膜缺陷和多种遗传事件相关，包括插入序列的转座或点突变，影响属于荚膜操纵子的基因。荚膜产生的减少或丧失与各种体外表型变化有关，影响对碳青霉烯的敏感性，但不影响对粘菌素的敏感性，体外生物膜形成和自动聚集。8个分离株间表型变异对荚膜含量的不同影响，生物膜的产生和碳青霉烯的敏感性表明，根据菌株和突变，荚膜丢失的异质选择性优势。根据我们的研究结果，我们认为应该关注 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。