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Colonization of the gastrointestinal (GI) tract by Klebsiella pneumoniae is generally considered asymptomatic. However, gut colonization allows K. pneumoniae to either translocate to sterile site within the same host or transmit through the fecal-oral route to another host. K. pneumoniae gut colonization is poorly understood, but knowledge of this first step toward infection and spread is critical for combatting its disease manifestations. K. pneumoniae must overcome colonization resistance (CR) provided by the host microbiota to establish itself within the gut. One such mechanism of CR is through nutrient competition. Pathogens that metabolize a broad range of substrates have the ability to bypass nutrient competition and overcome CR. Herein, we demonstrate that in response to mucin-derived fucose, the conserved fucose metabolism operon (fuc) of K. pneumoniae is upregulated in the murine gut, and we subsequently show that fucose metabolism promotes robust gut colonization. Growth studies using cecal filtrate as a proxy for the gut lumen illustrate the growth advantage that the fuc operon provides K. pneumoniae. We further show that fucose metabolism allows K. pneumoniae to be competitive with a commensal Escherichia coli isolate (Nissle). However, Nissle is eventually able to outcompete K. pneumoniae, suggesting that it can be utilized to enhance CR. Finally, we observed that fucose metabolism positively modulates hypermucoviscosity, autoaggregation, and biofilm formation but not capsule biogenesis. Together, these insights enhance our understanding of the role of alternative carbon sources in K. pneumoniae gut colonization and the complex relationship between metabolism and virulence in this species.

Colonization of the gastrointestinal (GI) tract by Klebsiella pneumoniae (K. pneumoniae) is generally considered asymptomatic. However, gut colonization allows K. pneumoniae to either translocate to sterile site within the same host or transmit through the fecal-oral route to another host. K. pneumoniae gut colonization is poorly understood, but knowledge of this first step toward infection and spread is critical for combatting its disease manifestations. K. pneumoniae must overcome colonization resistance (CR) provided by the host microbiota to establish itself within the gut. One such mechanism of CR is through nutrient competition. Pathogens that metabolizes a broad range of substrates have the ability to bypass nutrient competition and overcome CR. Herein, we demonstrate that in response to mucin derived fucose, the conserved fucose metabolism operon (fuc) of K. pneumoniae is upregulated in the murine gut and subsequently show that fucose metabolism promotes robust gut colonization. Growth studies using cecal filtrate as a proxy for the gut lumen illustrates the growth advantage that the fuc operon provides K. pneumoniae. We further show that fucose metabolism allows K. pneumoniae to be competitive with a commensal E. coli isolate (Nissle). However, Nissle is eventually able to out-compete K. pneumoniae, suggesting that it can be utilized to enhance CR. Lastly, we observed that fucose metabolism positively modulates hypermucoviscosity, auto-aggregation, and biofilm formation, but not capsule biogenesis. Together, these insights enhance our understanding of the role of alternative carbon sources on K. pneumoniae gut colonization and the complex relationship between metabolism and virulence in this species.ImportanceKlebsiella pneumoniae (K. pneumoniae) is a leading cause of hospital-acquired infection. Treatment of infection by K. pneumoniae isolates is becoming difficult as this pathogen becomes increasingly antibiotic resistant. While there has been extensive investigation into the disease states associated with K. pneumoniae, its colonization of the gastro-intestinal (GI) tract is poorly understood. Epidemiological data suggest that in many cases the strain that colonizes the GI tract causes disease manifestations in the same host. Herein, we used our newly developed murine model of K. pneumoniae gut colonization, where colonization is achieved without disrupting the resident gut microbiota. We demonstrate that K. pneumoniae uses fucose as an alternative carbon source present in the gut lumen to overcome the intense nutritional competition. We further illustrate that K. pneumoniae, through fucose metabolism, is initially competitive with the probiotic E coli isolate Nissle 1917 (EcN). Lastly, we show that fucose metabolism modulates several virulence determinants of K. pneumoniae. Thus, our results provide new insight into the role fucose metabolism plays in gut colonization and virulence of K. pneumoniae, and furthermore identify EcN as having the ability to out-compete K. pneumoniae and be used as a probiotic.

Due to its high transmissibility, Klebsiella pneumoniae (Kpn) is one of the leading causes of nosocomial infections. Here, we studied the biological cost of colistin resistance, an antibiotic of last resort, of this opportunistic pathogen using a murine model of gut colonization and transmission. Colistin resistance in Kpn is commonly the result of inactivation of the small regulatory protein MgrB. Without a functional MgrB, the two-component system PhoPQ is constitutively active, leading to increased lipid A modifications and subsequent colistin resistance. Using an engineered MgrB mutant, we observed that MgrB-dependent colistin resistance is not associated with a fitness defect during in vitro growth conditions. However, colistin-resistant Kpn colonizes the murine gut poorly, which may be due to the decreased production of capsular polysaccharide by the mutant. The colistin-resistant mutant of Kpn had increased survival outside the host when compared to the parental colistin-sensitive strain. We attribute this enhanced survivability to dysregulation of the PhoPQ two-component system and accumulation of the master stress regulator RpoS. The enhanced survival of the colistin resistant strain may be a key factor in the observed rapid host-to-host transmission in our model. Together, our data demonstrate that colistin-resistant Kpn experiences a biological cost in gastrointestinal colonization. However, this cost is mitigated by enhanced survival outside the host, increasing the risk of transmission. Additionally, it underscores the importance of considering the entire life cycle of a pathogen to truly determine the biological cost associated with antibiotic resistance.ImportanceThe biological cost associated with colistin resistance in Klebsiella pneumoniae (Kpn) was examined using a murine model of Kpn gut colonization and fecal-oral transmission. A common mutation resulting in colistin resistance in Kpn is a loss-of-function mutation of the small regulatory protein MgrB that regulates the two-component system PhoPQ. Even though colistin resistance in Kpn comes with a fitness defect in gut colonization, it increases bacterial survival outside the host enabling it to more effectively transmit to a new host. The enhanced survival is dependent upon the accumulation of RpoS and dysregulation of the PhoPQ. Hence, our study expands our understanding of the underlying molecular mechanism contributing to the transmission of colistin-resistant Kpn.