Corynebacterium pseudodiphtheriticum: More Likely a Pathogen than a Probiotic?

Relevance. Corynebacterium pseudodiphtheriticum has recently been increasingly considered as an etiologic agent of inflammatory diseases of various localizations, despite the fact that it is part of the microbiota of various biotopes of the human body. According to the literature, non- diphtheria corynebacteria and, in particular, C. pseudodiphtheriticum, are characterized by a dual nature, which is manifested by the presence of not only pathogenic, but also beneficial properties for the human body. In this regard, an important task is to differentiate colonization and infection caused by C. pseudodiphtheriticum, which may become possible when studying the genetic structure and virulence phenotype that determine the pathogenic effect on the human body. 
Aims: analysis of data from a study of the genome structure and phenotype characterizing the pathogenic and beneficial potential of phylogenetically closely related clinical isolates of C. pseudodiphtheriticum and C. propinquum from the oropharyngeal mucosa of healthy people. 
Materials and methods. Strains of C. pseudodiphtheriticum SX2, SX3, SX6 and C.propinquum SX4 were identified by mass spectrometry. Their whole genome sequencing and search for genes of pathogenicity, resistance to antimicrobial drugs (AMP), synthesis of amino acids, vitamins and terpenes were carried out. Virulence was determined on the model of wax moth larvae Galleria mellonella, sensitivity to AMP – by disk diffusion method, amino acid production – by tandem mass spectrometry. 
Results and discussion. It was found that all the studied isolates contain a wide range of polyfunctional genes regulating metabolism, pathogenicity (adhesion, survival inside macrophages, biofilm formation, etc.), resistance to AMP, as well as the rpf2 gene encoding the transition of corynebacteria from commensalism to parasitism. All the studied strains are low-virulent, produce glycine and valine, isolates of C. pseudodiphtheriticum SX2, SX6 and C.propinquum SX4 – alanine. The studied strains did not have a complete match in the pheno- and genotype of resistance to AMP. C. pseudodiphtheriticum SX6 belongs to the MDR category, showing phenotypic resistance to benzylpenicillin, ciprofloxacin and rifampicin. 
Conclusions. Thus, the duality of the nature of closely related strains of C. pseudodiphtheriticum and C.propinquum is characterized by the presence of a close relationship between pathogenic and beneficial properties. The implementation of the pathogenic potential of these microorganisms in the absence of true pathogenicity genes can occur with the activation of genes presumably associated with pathogenicity. This suggests that caution is needed when evaluating C. pseudodiphtheriticum (C.propinquum) strains as potential probiotics, even if they have a wide range of beneficial properties, given their ability to switch from commensalism to parasitism.

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