RNA sequencing maps ToxR-dependent genes in seafood pathogen

RNA sequencing has identified 603 genes whose expression in the seafood-borne pathogen Vibrio parahaemolyticus depends on the transcriptional regulator ToxR when the bacterium is growing at low cell density. Researchers compared the wild-type RIMD2210633 strain with an isogenic strain lacking toxR at an optical density at 600 nanometers of 0.2, the growth stage at which toxR transcription reaches its maximum. They found that 348 genes were down-regulated and 255 were up-regulated in the mutant, indicating that ToxR acts predominantly as an activator under the tested conditions. The affected genes form a broad network associated with virulence, stress adaptation, biofilm development, motility, metabolism and the bacterial second messenger cyclic di-GMP, but the experiment measured transcriptional changes rather than direct regulator binding or disease-related outcomes.

The study used three independent biological replicates for each strain, cultivated aerobically at 37 degrees Celsius in heart infusion broth. All six libraries passed stringent sequencing-quality criteria and were aligned to the 4,832-gene reference genome of strain RIMD2210633. After filtering, total mapped-read proportions ranged from 98.93% to 99.38%, while unique mapping ranged from 95.23% to 95.45%. Genes were classified as differentially expressed when the absolute fold change exceeded two and the false-discovery-rate-adjusted q value was no greater than 0.05. Functional annotation using orthologous groups, Gene Ontology terms and Kyoto Encyclopedia of Genes and Genomes pathways placed the largest group of affected genes in metabolism, with additional representation from cellular processes and environmental information processing. Among the enriched biological-process categories, pathogenesis and ion transport contained high numbers of differentially expressed genes.

Thirty-two genes encoding transcriptional regulators were differentially expressed in the mutant, including 18 down-regulated and 14 up-regulated. The strongest reductions included calR, which fell 29.4-fold, and scvE, which fell 31.5-fold. CalR is known to repress genes for the first type III secretion system while activating other loci connected with the second secretion system, the thermostable direct hemolysin tdh2 and biofilm-associated expression. Its reduction in the toxR mutant is therefore consistent with a hierarchy in which ToxR-dependent expression of calR influences downstream regulatory programs. The quorum-sensing regulator opaR also declined 2.5-fold in the mutant. Other affected regulators included a TetR/AcrR-family gene, VP1229, that increased fourfold and an AraC-family gene, VP2424, that decreased 12.1-fold. Because the downstream targets of several regulators remain unknown, their altered expression cannot by itself be assigned to a specific antibiotic-resistance, metabolic or virulence function.

ToxR also connected the pathogen’s transcriptional state to cyclic di-GMP signaling. Thirteen genes involved in the synthesis or degradation of this second messenger changed expression, with nine down-regulated and four up-regulated in the absence of ToxR. The pattern was consistent with prior evidence that ToxR promotes the expression of phosphodiesterases, including VP1423, VP1427, VP1768 and VP1979, while repressing diguanylate cyclases such as VP2888, VPA0184, VPA0360 and VPA0556. If the direction of these changes translates into cellular activity, it would favor lower intracellular cyclic di-GMP in wild-type cells. The researchers did not measure cyclic di-GMP concentration, however, and therefore did not directly test that biochemical consequence or its expected links with biofilm formation and motility.

The results revealed a marked reciprocal pattern in the expression of the bacterium’s two type III secretion systems. All 34 genes in the T3SS1 cluster were elevated in the ΔtoxR strain, indicating that ToxR normally represses this apparatus. In contrast, all eight detected T3SS2 genes and the tdh2 hemolysin gene were down-regulated, indicating that ToxR normally promotes this second set of virulence-associated genes. The pattern accords with earlier findings that ToxR directly activates calR and opaR, which repress T3SS1, while its direct and indirect regulatory effects support T3SS2 and thermostable direct hemolysin expression. The separation between the two secretion systems makes ToxR a potential switch in the pathogen’s transcriptional priorities, but the present experiment did not measure cytotoxicity, enterotoxicity, secretion activity or intestinal colonization. It also cannot establish how much of each change resulted from direct ToxR binding rather than regulation through intermediaries such as CalR.

Two type VI secretion systems showed another split regulatory response. Most of the six affected T6SS1 genes were up-regulated in the mutant, with the exception of VP1418, whereas five T6SS2 genes were down-regulated. This is compatible with earlier evidence that ToxR represses T6SS1 directly while supporting T6SS2 expression through CalR. Because T6SS1 is associated with interbacterial competition and T6SS2 with host-pathogen interactions, the transcriptional pattern is consistent with ToxR favoring host-associated functions over antibacterial competition at low density. That interpretation remains a model rather than a measured outcome: the researchers did not test either secretion system, competing bacteria or host interactions, and they did not resolve which affected genes are direct ToxR targets.

Biofilm-associated and motility-related loci also changed in opposing directions. All five genes in the scv extracellular-polysaccharide locus and the mshF gene involved in MSHA pilus biogenesis were down-regulated when toxR was deleted, indicating that ToxR promotes their expression. By contrast, the complete mfpABC operon was up-regulated in the mutant, suggesting repression by ToxR. That result differs from an earlier report that ToxR does not regulate mfpABC or cpsQ-mfpABC transcription. The authors noted several possible reasons for the discrepancy, including indirect regulation through CalR or another regulator, as well as differences in strain, medium, growth phase or data normalization. All eight affected lateral flagellar genes were also up-regulated in the ΔtoxR strain, suggesting that ToxR represses this swarming-associated program at the sampled density. These findings did not include measurements of extracellular polysaccharide production, attachment, swimming, swarming or biofilm maturation, so the inferred phenotypic consequences remain untested.

Independent quantitative real-time PCR provided partial validation of the sequencing results. Investigators selected 17 genes distributed across functional groups and genomic locations, then compared log2-transformed fold changes from the two methods. The results were positively correlated, with a Pearson coefficient of 0.65 and an R-squared value of 0.42, both significant at P values below 0.01. The rank correlation was also positive, with a Spearman coefficient of 0.72 and P below 0.01. Five genes called as up-regulated by RNA sequencing did not meet a twofold threshold in qPCR, while six genes called as down-regulated by sequencing had qPCR fold changes above 0.5. The researchers therefore described the validation as directional rather than complete, acknowledging threshold-level discrepancies among the tested genes.

Beyond secretion systems and surface-associated traits, ToxR-dependent expression involved several outer-membrane proteins, antioxidant genes and metabolic pathways. ToxR strongly activated chiP and VP2467 while reducing expression of porin genes VP0802, VP1008, VPA0527 and VPA1644. The two catalase genes katG1 and katG2 were also down-regulated in the mutant, suggesting that ToxR activates part of an oxidative-stress response program. Changes involving sulfur, amino acid and fatty acid metabolism pointed to additional effects on cellular physiology. These observations connect ToxR with environmental sensing and adaptation, but bile resistance, acid tolerance, oxidative-stress survival and metabolic performance were not directly assessed. Taken together, the single-time-point transcriptomic comparison supports a model in which ToxR coordinates quorum sensing, cyclic di-GMP signaling, virulence, biofilm-associated traits, motility and stress responses within a multilayered regulatory network. Resolving which targets it binds directly and linking those transcriptional changes to behavior in host and environmental settings will require chromatin-binding assays, regulator-mutant epistasis experiments, phenotypic measurements and time courses across cell densities.

Subject of Research: Genome-wide ToxR regulation in Vibrio parahaemolyticus at low cell density

Article Title: Transcriptomic dissection of the ToxR regulon in Vibrio parahaemolyticus at low cell density

Article References: Wang, Z., Zhang, N., Zhang, Y., & Lu, R. (2026). Transcriptomic dissection of the ToxR regulon in Vibrio parahaemolyticus at low cell density. International Microbiology. https://doi.org/10.1007/s10123-026-00910-4

Image Credits: AI Generated

DOI: 10.1007/s10123-026-00910-4

Keywords: Vibrio parahaemolyticus, ToxR, RNA sequencing, bacterial gene regulation, type III secretion, type VI secretion, cyclic di-GMP, biofilm regulation, virulence genes, low cell density, Transcriptomic, dissection