Fitness factor genes conserved within the multi-species core genome of Gram-negative Enterobacterales species contribute to bacteremia pathogenesis.

There is a critical gap in knowledge about how Gram-negative bacterial pathogens, using survival strategies developed for other niches, cause lethal bacteremia. Facultative anaerobic species of the Enterobacterales order are the most common cause of Gram-negative bacteremia, including Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Citrobacter freundii, and Enterobacter hormaechei. Bacteremia often leads to sepsis, a life-threatening organ dysfunction resulting from unregulated immune responses to infection. Despite a lack of specialization for this host environment, Gram-negative pathogens cause nearly half of bacteremia cases annually. Based on our existing Tn-Seq fitness factor data from a murine model of bacteremia combined with comparative genomics of the five Enterobacterales species above, we prioritized 18 conserved fitness genes or operons for further characterization. Mutants were constructed for all genes in all five species. Each mutant was used to cochallenge C57BL/6 mice via tail vein injection along with each respective wild-type strain to determine competitive indices for each fitness gene. Five fitness factor genes, when mutated, attenuated mutants in four or five species in the spleen and liver (tatC, ruvA, gmhB, wzxE, arcA). Five additional fitness factor genes or operons were validated as outcompeted by wild-type in three, four, or five bacterial species in the spleen (xerC, prc, apaGH, atpG, aroC). Overall, 17 of 18 fitness factor mutants were attenuated in at least one species in the spleen or liver. Together, these findings allow for the development of a model of bacteremia pathogenesis that may include future targets of therapy against bloodstream infections.

The UDP-GalNAcA biosynthesis genes gna-gne2 are required to maintain cell envelope integrity and in vivo fitness in multi-drug resistant Acinetobacter baumannii.

Acinetobacter baumannii infects a wide range of anatomic sites including the respiratory tract and bloodstream. Despite its clinical importance, little is known about the molecular basis of A. baumannii pathogenesis. We previously identified the UDP-N-acetyl-d-galactosaminuronic acid (UDP-GalNAcA) biosynthesis genes, gna-gne2, as being critical for survival in vivo. Herein, we demonstrate that Gna-Gne2 are part of a complex network connecting in vivo fitness, cell envelope homeostasis and resistance to antibiotics. The ∆gna-gne2 mutant exhibits a severe fitness defect during bloodstream infection. Capsule production is abolished in the mutant strain, which is concomitant with its inability to survive in human serum. In addition, the ∆gna-gne2 mutant was more susceptible to vancomycin and unable to grow on MacConkey plates, indicating an alteration in cell envelope integrity. Analysis of lipid A by mass spectrometry showed that the hexa- and hepta-acylated species were affected in the gna-gne2 mutant. Finally, the ∆gna-gne2 mutant was more susceptible to several classes of antibiotics. Together, this study demonstrates the importance of UDP-GalNAcA in the pathobiology of A. baumannii. By interrupting its biosynthesis, we showed that this molecule plays a critical role in capsule biosynthesis and maintaining the cell envelope homeostasis.

The Staphylococcus aureus Cystine Transporters TcyABC and TcyP Facilitate Nutrient Sulfur Acquisition during Infection.

Staphylococcus aureus is a significant human pathogen due to its capacity to cause a multitude of diseases. As such, S. aureus efficiently pillages vital nutrients from the host; however, the molecular mechanisms that support sulfur acquisition during infection have not been established. One of the most abundant extracellular sulfur-containing metabolites within the host is cysteine, which acts as the major redox buffer in the blood by transitioning between reduced and oxidized (cystine) forms. We therefore hypothesized that S. aureus acquires host-derived cysteine and cystine as sources of nutrient sulfur during systemic infection. To test this hypothesis, we used the toxic cystine analogue selenocystine to initially characterize S. aureus homologues of the Bacillus subtilis cystine transporters TcyABC and TcyP. We found that genetic inactivation of both TcyA and TcyP induced selenocystine resistance. The double mutant also failed to proliferate in medium supplemented with cystine, cysteine, or N-acetyl cysteine as the sole sulfur source. However, only TcyABC was necessary for proliferation in defined medium containing homocystine as the sulfur source. Using a murine model of systemic infection, we observed tcyP-dependent competitive defects in the liver and heart, indicating that this sulfur acquisition strategy supports proliferation of S. aureus in these organs. Phylogenetic analyses identified TcyP homologues in many pathogenic species, implying that this sulfur procurement strategy is conserved. In total, this study is the first to experimentally validate sulfur acquisition systems in S. aureus and establish their importance during pathogenesis.

The lytic transglycosylase MltB connects membrane homeostasis and in vivo fitness of Acinetobacter baumannii.

Acinetobacter baumannii has emerged as a leading nosocomial pathogen, infecting a wide range of anatomic sites including the respiratory tract and the bloodstream. In addition to being multi-drug resistant, little is known about the molecular basis of A. baumannii pathogenesis. To better understand A. baumannii virulence, a combination of a transposon-sequencing (TraDIS) screen and the neutropenic mouse model of bacteremia was used to identify the full set of fitness genes required during bloodstream infection. The lytic transglycosylase MltB was identified as a critical fitness factor. MltB cleaves the MurNAc-GlcNAc bond of peptidoglycan, which leads to cell wall remodeling. Here we show that MltB is part of a complex network connecting resistance to stresses, membrane homeostasis, biogenesis of pili and in vivo fitness. Indeed, inactivation of mltB not only impaired resistance to serum complement, cationic antimicrobial peptides and oxygen species, but also altered the cell envelope integrity, activated the envelope stress response, drastically reduced the number of pili at the cell surface and finally, significantly decreased colonization of both the bloodstream and the respiratory tract.

Carbapenem-Resistant Enterobacter hormaechei Uses Mucus Metabolism to Facilitate Gastrointestinal Colonization.

The emergence and global spread of carbapenem-resistant Enterobacter cloacae complex species presents a pressing public health challenge. Carbapenem-resistant Enterobacter species cause a wide variety of infections, including septic shock fatalities in newborns and immunocompromised adults. The intestine may be a major reservoir for these resistant strains, either by facilitating contamination of fomites and transfer to susceptible individuals, or through translocation from the gut to the bloodstream. For this reason, we sought to establish a neonatal mouse model to investigate the mechanisms underpinning gut colonization by carbapenem-resistant Enterobacter hormaechei. We describe a new mouse model to study gut colonization by Enterobacter species, leading to vital insights into the adaptation of carbapenem-resistant E. hormaechei to the gut environment during the early stages of intestinal colonization. We observed successful colonization and proliferation of E. hormaechei in the five-day old infant mouse gut, with primary localization to the colon following oral inoculation. We also uncovered evidence that E. hormaechei uses mucus as a carbon source during colonization of the colon. Our findings underscore the importance of oxygen-dependent metabolic pathways, including the pyruvate dehydrogenase complex, and N-acetyl-D-glucosamine metabolism, in gut colonization and proliferation, which aligns with previous human studies. These insights are essential for developing novel therapeutic strategies that can serve as decolonization therapies in at-risk populations. Importance: Bloodstream infections caused by Enterobacter species pose a significant clinical threat. The intestine acts as the primary site for colonization and serves as a reservoir for infection. To combat this pathogen, it is crucial to understand how carbapenem-resistant Enterobacter species colonize the gut, as such knowledge can pave the way for alternative therapeutic targets. In this study, we developed a novel neonatal mouse model for gastrointestinal colonization by Enterobacter species and discovered that mucus plays a key role as a carbon source during colonization. Additionally, we identified two mucus catabolism pathways that contribute to intestinal colonization by carbapenem-resistant E. hormaechei. This new mouse model offers valuable insights into host-pathogen interactions and helps identify critical gastrointestinal fitness factors of Enterobacter, potentially guiding the development of vaccines and alternative therapeutic strategies to minimize intestinal carriage in patient populations at risk for infection with Enterobacter species.