Type VI Secretion Systems: Environmental and Intra-host Competition of Vibrio cholerae

Functional role of rpoN in regulating the virulence of non-O1/O139 Vibrio cholerae

Non-O1/O139 Vibrio cholerae is widely distributed in brackish and estuarine ecosystems, which can infect many aquatic animals. RpoN, an alternative sigma factor, plays a critical role in regulating cell functions such as motility, quorum sensing, and virulence. However, the function of rpoN in non-O1/O139 V. cholerae has rarely been reported. In the present study, we constructed the deletion mutant ΔrpoN of non-O1/O139 V. cholerae GXFL1-4 using recombination technology and investigated the function of rpoN through transcriptomic and phenotypic analyses. RNA-seq results showed that many major virulence-related genes were down-regulated in the ΔrpoN mutant, including the type VI secretion system (tssJ, tssA, tagO, tssG), type IV pilus assembly proteins (pilM, pilB), biofilm formation genes (vpsC, cheC), and hemolysin-related genes (hlyD, hlyD-PA). Additionally, phenotypic assays showed that the growth and motility of the ΔrpoN had no apparent change. The deletion of rpoN in non-O1/O139 V. cholerae led to decreased biofilm formation and reduced hemolytic activity. Furthermore, artificial infection tests showed that the virulence of the ΔrpoN mutant toward Macrobrachium rosenbergii was decreased. Our study provides essential insights into the regulatory function of rpoN, revealing that rpoN is a key determinant of virulence regulation in non-O1/O139 V. cholerae.

Assessing toxicity and competitive fitness of Vibrio isolates from coastal waters in Israel

Several species of aquatic bacteria belonging to the genus Vibrio are emerging pathogens of humans and marine animals. Vibrio-associated infections have been shown to correlate with the increase in the oceans' surface water temperatures. A slow yet steady increase in Vibrio isolates from clinical settings in Israel over the past decade led us to investigate their pathogenic potential in Israel's coastal waters. We sequenced the genomes of 23 Vibrio isolates from the Mediterranean and Red Seas. Analysis of these genomes revealed the presence of diverse toxin secretion systems and toxins, as well as mobile genetic elements known to facilitate the dissemination of fitness-enhancing determinants. Moreover, we showed that at least 10 of these isolates induce cell death in bone marrow-derived macrophages and that at least 12 isolates intoxicate a rival Vibrio strain in interbacterial competition. Lastly, we determined the susceptibility profiles of these isolates to common antibiotics used to treat Vibrio infections and found widespread resistance to azithromycin. Taken together, our results reveal pathogenic potential within the Vibrio population of Israel's coastal waters and underline the need for continued environmental monitoring of emerging pathogens.

IMPORTANCE

The ocean's surface water temperatures have increased in the past decades due to climate change. This increase correlates with the spread of Vibrio, a genus of aquatic bacteria, many of which are pathogens of humans and marine animals. Since Vibrio-associated illnesses are rising in Israel, we set out to investigate the Vibrio population in Israel's coastal environments and monitor their pathogenic potential. We found diverse repertoires of predicted toxins, the ability to kill mammalian immune cells, and traits that enhance bacterial fitness, such as antibacterial toxicity and resistance to antibiotics commonly used to treat Vibrio infections. These findings indicate that pathogenic traits are circulating within the environmental Vibrio population in Israel's coastal waters and suggest that continued monitoring is essential to identify emerging pathogenic strains.

Case report: Characterization and bioinformatics analysis of non-O1/O139 Vibrio cholerae strain isolated from a choledochoduodenal fistula patient with septicemia

The gram-negative bacterium Vibrio cholerae (VC) is divided into multiple serogroups, with groups O1 and O139 responsible for cholera. Conversely, Vibrio cholerae belonging to the non-O1/non-O139 group (NOVC) does not produce cholera-causing toxins. Insufficient understanding of the frequency of NOVC causes fear during the early detection phase. Acute gastroenteritis is often caused by NOVC, while extra gastrointestinal infections are less common. In the case described here, the patient had a postoperative choledochoduodenal fistula due to prior choledochotomy. In August 2023, he was hospitalized with fever and diarrhea. The gram-negative bacilli Vibrio cholerae was isolated from a blood specimen using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The strain was identified as non-O1/O139 by serum agglutination tests. Subsequent whole-genome sequencing and database analysis revealed that the strain possessed resistance genes such as CRP, varG, almG, and QnrVC4, as well as various virulence factors such as RTX, hlyA, VAS, and T3SS. The phylogenetic tree analysis indicated that CQ23-0008VC had close relationship with cholerae strains isolated from aquatic environments. The patient was treated promptly and discharged after being admitted with severe symptoms. However, Bioinformatics analysis indicated that the virulence factors that were identified in the bacteria were significant; thus, these virulence factors can indicate to medical professionals that a patient could have a septicemia caused by NOVC.

The role of the type VI secretion system in the stress resistance of plant-associated bacteria

The type VI secretion system (T6SS) is a powerful bacterial molecular weapon that can inject effector proteins into prokaryotic or eukaryotic cells, thereby participating in the competition between bacteria and improving bacterial environmental adaptability. Although most current studies of the T6SS have focused on animal bacteria, this system is also significant for the adaptation of plant-associated bacteria. This paper briefly introduces the structure and biological functions of the T6SS. We summarize the role of plant-associated bacterial T6SS in adaptability to host plants and the external environment, including resistance to biotic stresses such as host defenses and competition from other bacteria. We review the role of the T6SS in response to abiotic factors such as acid stress, oxidation stress, and osmotic stress. This review provides an important reference for exploring the functions of the T6SS in plant-associated bacteria. In addition, characterizing these anti-stress functions of the T6SS may provide new pathways toward eliminating plant pathogens and controlling agricultural losses.