Pathogenesis--Thoughts from the Front Line

Capsular polysaccharide enables Klebsiella pneumoniae to evade phagocytosis by blocking host-bacteria interactions

Capsule polysaccharide (CPS) is among the most important virulence factors of Klebsiella pneumoniae. Previous studies demonstrated that CPS plays multiple functional roles, but the mechanism by which this virulence factor enhances the survival fitness of K. pneumoniae remains unclear. In this work, we demonstrate that CPS is the main cellular component that not only elicits the host immune response to K. pneumoniae but also enables this pathogen to survive for a prolonged period under adverse environmental conditions. Consistently, our in vitro experiments suggest that CPS prevents K. pneumoniae from phagocytosis, rendering the encapsulated strain more difficult to be eradicated by the host. We also found that phagocytosis of K. pneumoniae is partially mediated by LOX-1, a scavenger receptor of the host, and that CPS may impede interaction between LOX-1 and this pathogenic bacteria, therefore reducing the phagocytosis process. These findings provide insights into the pathogenic mechanisms of this important clinical pathogen and should facilitate the design of new strategies to combat K. pneumoniae infections.

IMPORTANCE

Klebsiella pneumoniae has become one of the most important clinical bacterial pathogens due to its evolution into hyperresistant and hypervirulent phenotypes. The mechanism of virulence of this pathogen is not well understood, particularly because it differs from other Enterobacteriaceae pathogens such as Escherichia coli and Salmonella. The capsule polysaccharide (CPS) of this pathogen is well recognized for contributing to the virulence of K. pneumoniae, but the exact mechanisms underlying its contribution are unclear. In this study, we demonstrated that CPS does not directly contribute to the host response; rather, it forms an external coat that blocks host recognition and prevents immune cells from binding to receptor proteins on K. pneumoniae, thus inhibiting phagocytosis, which makes it more challenging for the body to fight off infections. Understanding these mechanisms is vital for developing new treatments against K. pneumoniae infections, ultimately improving patient outcomes and public health.

Occurrence of carbapenem-resistant hypervirulent Klebsiella pneumoniae in oysters in Egypt: a significant public health issue

BACKGROUND

The global dissemination of critical-priority carbapenem-resistant hypervirulent Klebsiella pneumoniae (CR-hvKp) via food sources represents a significant public health concern. Epidemiological data on CR-hvKp in oysters in Egypt is limited. This study aimed to investigate the potential role of oysters sold in Egypt as a source for carbapenem-resistant K. pneumoniae (CRKP), hypervirulent K. pneumoniae (hvKp), and CR-hvKp and assess associated zoonotic risks.

METHODS

A sample of 330 fresh oysters was randomly purchased from various retail fish markets in Egypt and divided into 33 pools. Bacteriological examination and the identification of Klebsiella pneumoniae were performed. Carbapenem resistance in K. pneumoniae isolates was determined by phenotypic and molecular methods. Additionally, the presence of hypervirulent K. pneumoniae was identified based on virulence gene markers (peg-344, rmpA, rmpA2, iucA, and iroB), followed by a string test. The clustering of CR-hvKp strains was carried out using R with the pheatmap package.

RESULTS

The overall prevalence of K. pneumoniae was 48.5% (16 out of 33), with 13 isolates displaying carbapenem resistance, one intermediate resistance, and two sensitive. Both carbapenem-resistant K. pneumoniae and carbapenem-intermediate-resistant K. pneumoniae strains exhibited carbapenemase production, predominantly linked to the blaVIM gene (68.8%). HvKp strains were identified at a rate of 62.5% (10/16); notably, peg-344 was the most prevalent gene. Significantly, 10 of the 13 CRKP isolates possessed hypervirulence genes, contributing to the emergence of CR-hvKp. Moreover, cluster analysis revealed the clustering of two CR-hvKp isolates from the same retail fish market.

CONCLUSION

This study provides the first insight into the emergence of CR-hvKp among oysters in Egypt. It underscores the potential role of oysters as a source for disseminating CR-hvKp within aquatic ecosystems, presenting a possible threat to public health.

A mini case report: Klebsiella pneumoniae-induced metastatic neck abscess following laparoscopic appendectomy

Common complications following laparoscopic appendectomy include wound infection, bleeding, intra-abdominal abscess, small bowel obstruction, stump leakage, and stump appendicitis. Here, we presented a case reporting detailing a rare complication following laparoscopic appendectomy: the development of a metastatic neck abscess induced by Klebsiella pneumoniae(K. pneumoniae). A 49-year-old male underwent emergency laparoscopic surgery with prophylactic antibiotic administration for acute appendicitis. Subsequently, he experienced persistent neck pain and fever postoperatively, prompting further investigation. Pus and blood cultures revealed K. pneumoniae, with magnetic resonance imaging confirming the presence of a neck abscess. Antibiotic therapy was adjusted, and surgical drainage of the abscess was performed after multidisciplinary consultation. The patient was discharged without complications. While rare, metastatic abscesses following appendectomy warrant consideration, particularly in K. pneumoniae infections. Comprehensive clinical assessment, imaging, and laboratory evaluation are crucial for timely diagnosis and management of such complications.

The molecular characteristics, clinical manifestations, 44 and risk factors of hypervirulent Klebsiella pneumoniae infections in a large teaching hospital in southwest China

Hypervirulent Klebsiella pneumoniae (hvKp) has been globally disseminated recently, especially in Asia. The purpose of this study was to identify the molecular characteristics, clinical manifestations, and clinical risk factors of hvKp infections among patients in a large teaching hospital. A retrospective study was conducted in 123 patients infected with K. pneumoniae at the Affiliated Hospital of Southwest Medical University (Luzhou, China) from October 2016 to November 2018. An isolate that positive for both PCR amplification of aerobactin gene and Galleria mellonella infection model was defined as hvKp. Overall, 43.1% (53/123) of K. pneumoniae isolates were hvKp. String tests were performed on all isolates, and MLSTs of all hvKp were conducted. The K1 ST23 isolates were the dominant clone of hvKp (35.8%). Univariate analysis revealed the following risk factors for hvKp: hepatic abscess (OR = 41.818 [95% CI, 5.379-335.086]), bacteremia (OR = 19.94 [95% CI, 5.565-71.446]), metastatic spread (OR = 19.938 [95% CI, 6.344-62.654]), CRP (OR = 1.008 [95% CI, 1.001-1.015]), nitroimidazole treatment (OR = 7.907 [95% CI, 1.652-37.843]), diabetes (OR = 3.067 [95% CI, 1.38-6.817]), and admission to positive culture interval (OR = 3.636 [95% CI, 1.524-8.678]). Moreover, Multivariate analysis implicated hepatic abscess (OR = 74.332 [95% CI, 3.121-1769.588]), bacteremia (OR = 28.388 [95% CI, 3.039-264.200]), and metastatic spread (OR = 19.391 [95% CI, 3.633-103.498]) as independent risk factors for hvKp infections. Thirteen of twenty-one tested antibiotics were founded resistance to non-hvKp, which is significantly greater than hvKp. Importantly, the ESBL-hvKp and MDR-hvKp were responsible for 7.5% and 15.1% in the hvKp group, respectively.

Brucella abortus Depends on l-Serine Biosynthesis for Intracellular Proliferation

l-Serine is a nonessential amino acid and a key intermediate in several relevant metabolic pathways. In bacteria, the major source of l-serine is the phosphorylated pathway, which comprises three enzymes: d-3-phosphoglycerate dehydrogenase (PGDH; SerA), phosphoserine amino transferase (PSAT; SerC), and l-phosphoserine phosphatase (PSP; SerB). The Brucella abortus genome encodes two PGDHs (SerA-1 and SerA-2), involved in the first step in l-serine biosynthesis, and one PSAT and one PSP, responsible for the second and third steps, respectively. In this study, we demonstrate that the serA1 serA2 double mutant and the serC and serB single mutants are auxotrophic for l-serine. These auxotrophic mutants can be internalized but are unable to replicate in HeLa cells and in J774A.1 macrophage-like cells. Replication defects of auxotrophic mutants can be reverted by cell medium supplementation with l-serine at early times postinfection. In addition, the serB mutant is attenuated in the murine intraperitoneal infection model and has an altered lipid composition, since the lack of l-serine abrogates phosphatidylethanolamine synthesis in this strain. Taken together, these results reveal that limited availability of l-serine within the host cell impairs proliferation of the auxotrophic strains, highlighting the relevance of this biosynthetic pathway in Brucella pathogenicity.

Hypervirulent Klebsiella pneumoniae

Hypervirulent K. pneumoniae (hvKp) is an evolving pathotype that is more virulent than classical K. pneumoniae (cKp). hvKp usually infects individuals from the community, who are often healthy. Infections are more common in the Asian Pacific Rim but are occurring globally. hvKp infection frequently presents at multiple sites or subsequently metastatically spreads, often requiring source control. hvKp has an increased ability to cause central nervous system infection and endophthalmitis, which require rapid recognition and site-specific treatment. The genetic factors that confer hvKp's hypervirulent phenotype are present on a large virulence plasmid and perhaps integrative conjugal elements. Increased capsule production and aerobactin production are established hvKp-specific virulence factors. Similar to cKp, hvKp strains are becoming increasingly resistant to antimicrobials via acquisition of mobile elements carrying resistance determinants, and new hvKp strains emerge when extensively drug-resistant cKp strains acquire hvKp-specific virulence determinants, resulting in nosocomial infection. Presently, clinical laboratories are unable to differentiate cKp from hvKp, but recently, several biomarkers and quantitative siderophore production have been shown to accurately predict hvKp strains, which could lead to the development of a diagnostic test for use by clinical laboratories for optimal patient care and for use in epidemiologic surveillance and research studies.

Erythritol Availability in Bovine, Murine and Human Models Highlights a Potential Role for the Host Aldose Reductase during Brucella Infection

Erythritol is the preferential carbon source for most brucellae, a group of facultative intracellular bacteria that cause a worldwide zoonosis. Since this polyol is abundant in genital organs of ruminants and swine, it is widely accepted that erythritol accounts at least in part for the characteristic genital tropism of brucellae. Nevertheless, proof of erythritol availability and essentiality during Brucella intracellular multiplication has remained elusive. To investigate this relationship, we compared ΔeryH (erythritol-sensitive and thus predicted to be attenuated if erythritol is present), ΔeryA (erythritol-tolerant but showing reduced growth if erythritol is a crucial nutrient) and wild type B. abortus in various infection models. This reporting system indicated that erythritol was available but not required for B. abortus multiplication in bovine trophoblasts. However, mice and humans have been considered to lack erythritol, and we found that it was available but not required for B. abortus multiplication in human and murine trophoblastic and macrophage-like cells, and in mouse spleen and conceptus (fetus, placenta and envelopes). Using this animal model, we found that B. abortus infected cells and tissues contained aldose reductase, an enzyme that can account for the production of erythritol from pentose cycle precursors.

Brucella central carbon metabolism: an update

The brucellae are facultative intracellular pathogens causing brucellosis, an important zoonosis. Here, we review the nutritional, genetic, proteomic and transcriptomic studies on Brucella carbon uptake and central metabolism, information that is needed for a better understanding of Brucella virulence. There is no uniform picture across species but the studies suggest primary and/or secondary transporters for unknown carbohydrates, lactate, glycerol phosphate, erythritol, xylose, ribose, glucose and glucose/galactose, and routes for their incorporation to central metabolism, including an erythritol pathway feeding the pentose phosphate cycle. Significantly, all brucellae lack phosphoenolpyruvate synthase and phosphofructokinase genes, which confirms previous evidence on glycolysis absence, but carry all Entner-Doudoroff (ED) pathway and Krebs cycle (and glyoxylate pathway) genes. However, glucose catabolism proceeds through the pentose phosphate cycle in the classical species, and the ED pathway operates in some rodent-associated brucellae, suggesting an ancestral character for this pathway in this group. Gluconeogenesis is functional but does not rely exclusively on classical fructose bisphosphatases. Evidence obtained using infection models is fragmentary but suggests the combined or sequential use of hexoses/pentoses, amino acids and gluconeogenic substrates. We also discuss the role of the phosphotransferase system, stringent reponse, quorum sensing, BvrR/S and sRNAs in metabolism control, an essential aspect of the life style of facultative intracellular parasites.