Emergence and spread in Taiwan of multidrug-resistant serotypes of nontyphoidal Salmonella.

PURPOSE: Antimicrobial resistance of nontyphoidal Salmonella (NTS) is a threat to public health worldwide. METHODS: A study on longitudinally collected NTS isolates from a medical center in Taiwan from 2011 to 2019 was undertaken. The multidrug resistance (MDR) and extensively drug resistance (XDR) phenotypes were determined according to internationally used definitions. Molecular serotyping was performed on the resistant NTS. RESULTS: Notably 16.1% (870/5412) of the isolates were MDR, while XDR accounted for 2.1% (111/5412). Both MDR and XDR NTS have increased significantly from 2011 to 2019, especially from 2015 to 2017 (MDR from 9.6% in 2015 to 23.1% 2017; XDR from 1.4% in 2016 to 4.7% in 2017). S. Anatum was the commonest NTS serotype expressing MDR and XDR, in 256/559 (45.8%) and 81/111 (73.0%) of the isolates, respectively, followed by S. Typhimurium and S. Goldcoast. Children < 18 years old contributed to 69.0% of all MDR cases and 64.0% of all XDR cases; majority of them aged less than 5 years. CONCLUSIONS: Increasing MDR and XDR NTS is a threat to public health. MDR and XDR NTS usually caused gastroenteritis in children < 5 years old. Multiple NTS serotypes expressing MDR and XDR indicate multiple food vehicles involved in the transmission. Proper food hygiene practice should never be over-reinforced.

Microbiological and genomic investigations of invasive Salmonella enterica serovar Panama from a large outbreak in Taiwan.

BACKGROUND/PURPOSE: Salmonella Panama was considered an invasive non-typhoidal Salmonella (iNTS) serovar. Comprehensive clinical, microbiological, and genomic studies on S. Panama are scarce. We aimed to characterize the clinical and microbiological characteristics of S. Panama infection. Virulence mechanism of S. Panama and other iNTS serovars were also examined. METHODS: Based on data from the longitudinal surveillance system for Salmonella deployed in Taiwan since 2004, a case-control study was undertaken to evaluate clinical characteristics of S. Panama infection during an outbreak in 2015-2016. Cellular experiments were conducted to compare pathogenicity of S. Panama and other iNTS with S. Typhimurium. RESULTS: Most patients (41/44, 93.2%) infected by S. Panama were <5 years old (median, 1.3 years). The case-control study showed that 28 out of the 41 (68.3%) manifested as bacteremia, compared to S. Typhimurium (11.1%). Patients infected by S. Panama had longer durations of fever (P = 0.005) and hospitalization (P < 0.001). Genomic analyses split the isolates into three clades: two clones caused the outbreak, whereas another one accounted for the sporadic infections before 2015. Cellular experiments revealed that S. Panama and other iNTS serovars showed higher monolayer penetration and intracellular survival within macrophages, compared to S. Typhimurium. CONCLUSION: This study confirmed that S. Panama is a clinically invasive serovar. Different iNTS serovars express common virulence phenotypes, but they may acquire invasiveness through distinct expression or combinations of virulence genes.

Characterization and Source Investigation of Multidrug-Resistant Salmonella Anatum from a Sustained Outbreak, Taiwan.

An ongoing outbreak of multidrug-resistant Salmonella enterica serovar Anatum began in Taiwan in 2015. Pork and poultry were identified as vehicles for transmission. Contaminated meat contributed to the high rate of infections among children. Nearly identical Salmonella Anatum strains have been identified in the United Kingdom, the United States, and the Philippines.

Genomic Insight into the Spread of Meropenem-Resistant Streptococcus pneumoniae Spain23F-ST81, Taiwan.

Incidence of invasive pneumococcal disease caused by antimicrobial-resistant Streptococcus pneumoniae types not included in pneumococcal conjugate vaccines has increased, including a penicillin- and meropenem-resistant serotype 15A-ST63 clone in Japan. During 2013-2017, we collected 206 invasive pneumococcal isolates in Taiwan for penicillin and meropenem susceptibility testing. We found serotypes 15B/C-ST83 and 15A-ST63 were the most prevalent penicillin- and meropenem-resistant clones. A transformation study confirmed that penicillin-binding protein (PBP) 2b was the primary meropenem resistance determinant, and PBP1a was essential for high-level resistance. The rate of serotype 15B/C-ST83 increased during the study. All 15B/C-ST83 isolates showed an ermB macrolide resistance genotype. Prediction analysis of recombination sites revealed 12 recombination regions in 15B/C-ST83 compared with the S. pneumoniae Spain23F-ST81 genome. Pneumococcal clones rapidly recombine to acquire survival advantages and undergo local expansion under the selective pressure exerted by vaccines and antimicrobial drugs. The spread of 15B/C-ST83 is alarming for countries with high antimicrobial pressure.

Persistence of immunity in children immunised with 13-valent pneumococcal conjugate vaccine and impact on nasopharyngeal carriage: a cross-sectional study.

In 500 children aged ≤10 years after 13-valent pneumococcal conjugate vaccine (PCV)13 immunisation in different schedules, serotypes 19A-specific and 19F-specific immunoglobulin G (IgG) were predicted to persist above 0.35 µg/mL for ≥10 years in all groups, likely due to PCV13-induced memory with natural boosting from residual diseases and colonisation. Generally, serotype-specific IgG could persist above 0.35 µg/mL longer (≥5 years) in the catch-up group than in the 2+1 and 3+1 immunisation groups. 14.5% of the carriage isolates belonged to PCV13 serotypes; statistical analysis revealed that a high serum IgG level (>10.96 µg/mL) will be required to eliminate the point-prevalence nasopharyngeal carriage of serotype 19A.

Transcription Factor Atf1 Regulates Expression of Cellulase and Xylanase Genes during Solid-State Fermentation of Ascomycetes.

Transcriptional regulation of cellulolytic and xylolytic genes in ascomycete fungi is controlled by specific carbon sources in different external environments. Here, comparative transcriptomic analyses of Penicillium oxalicum grown on wheat bran (WB), WB plus rice straw (WR), or WB plus Avicel (WA) as the sole carbon source under solid-state fermentation (SSF) revealed that most of the differentially expressed genes (DEGs) were involved in metabolism, specifically, carbohydrate metabolism. Of the DEGs, the basic core carbohydrate-active enzyme-encoding genes which responded to the plant biomass resources were identified in P. oxalicum, and their transcriptional levels changed to various extents depending on the different carbon sources. Moreover, this study found that three deletion mutants of genes encoding putative transcription factors showed significant alterations in filter paper cellulase production compared with that of a parental P. oxalicum strain with a deletion of Ku70 (ΔPoxKu70 strain) when grown on WR under SSF. Importantly, the ΔPoxAtf1 mutant (with a deletion of P. oxalicumAtf1, also called POX03016) displayed 46.1 to 183.2% more cellulase and xylanase production than a ΔPoxKu70 mutant after 2 days of growth on WR. RNA sequencing and quantitative reverse transcription-PCR revealed that PoxAtf1 dynamically regulated the expression of major cellulase and xylanase genes under SSF. PoxAtf1 bound to the promoter regions of the key cellulase and xylanase genes in vitro This study provides novel insights into the regulatory mechanism of fungal cellulase and xylanase gene expression under SSF.IMPORTANCE The transition to a more environmentally friendly economy encourages studies involving the high-value-added utilization of lignocellulosic biomass. Solid-state fermentation (SSF), that simulates the natural habitat of soil microorganisms, is used for a variety of applications such as biomass biorefinery. Prior to the current study, our understanding of genome-wide gene expression and of the regulation of gene expression of lignocellulose-degrading enzymes in ascomycete fungi during SSF was limited. Here, we employed RNA sequencing and genetic analyses to investigate transcriptomes of Penicillium oxalicum strain EU2101 cultured on medium containing different carbon sources and to identify and characterize transcription factors for regulating the expression of cellulase and xylanase genes during SSF. The results generated will provide novel insights into genetic engineering of filamentous fungi to further increase enzyme production.

Development of carbapenem resistance in Pseudomonas aeruginosa is associated with OprD polymorphisms, particularly the amino acid substitution at codon 170.

Objectives: Pan-susceptible Pseudomonas aeruginosa (PSPA) clinical isolates carrying an OprD with loop 7 shortening (the group-1A allele) were found to rapidly develop carbapenem resistance under continuous selection pressure. We further studied whether OprD polymorphisms are associated with the potential to develop carbapenem resistance. Methods: OprD amino acid sequences of 126 PSPA clinical isolates were analysed to determine their STs using P. aeruginosa strain PAO1 as the control strain. Site-directed mutagenesis was performed in PAO1 to generate polymorphisms of interest. A disc diffusion method was used to select carbapenem-resistant variants from the mutant strains. Expression levels of oprD were determined by quantitative RT-PCR. MICs of carbapenems were determined by Etest. Results: Forty-eight (38.1%) of the tested isolates carried the group-1A allele. Another two major STs, C1 and C2, both of which harboured an F170L polymorphism, were found in 21 (16.7%) and 39 (31.0%) isolates, respectively. The PAO1 type was also found in 14 (11.1%) isolates. Under continuous selective pressure, isolates of most STs developed carbapenem resistance at different numbers of passaging events; only those belonging to the PAO1 type remained susceptible. However, PAO1 mutants carrying either the oprD group-1A allele or the OprD-F170L polymorphism were able to develop carbapenem resistance. Reduced oprD expression triggered by continuous imipenem challenge was found in PAO1 mutants, but not in the PAO1 WT strain. Conclusions: OprD polymorphisms, particularly the F170L substitution and the specific shortening in loop 7, appear to determine the potential for P. aeruginosa to develop carbapenem resistance.

Cellulase with high ß-glucosidase activity by Penicillium oxalicum under solid state fermentation and its use in hydrolysis of cassava residue.

In this study, we investigated cellulase production by Penicillium oxalicum EU2106 under solid-state fermentation (SSF) and its hydrolysis efficiency toward NaOH-H2O2-pretreated cassava residue (NHCR) produced after bioethanol fermentation. Optimization of SSF cultivation conditions for P. oxalicum EU2106 using a Box-behnken design-based response-surface methodology resulted in maximal cellulase activity of 34.0 ± 2.8 filter-paper units/g dry substrate, exhibiting a ~ twofold increase relative to activities obtained under non-optimized conditions. Furthermore, SSF-derived cellulase converted 94.3 ± 1.5% of NHCR cellulose into glucose within 96 h. Interestingly, P. oxalicum EU2106 produced higher ß-glucosidase activity under SSF conditions than that under submerged-state fermentation conditions, resulting in the elimination of cellobiose inhibition during the early stages of NHCR cellulose hydrolysis. Overall, this work provided an alternative for a potential cellulase source and a preferred option for cassava residue biotechnological application.

Aspirin enhances opsonophagocytosis and is associated to a lower risk for Klebsiella pneumoniae invasive syndrome.

BACKGROUND: Klebsiella pneumoniae (KP) expressing hypermucoviscosity phenotype (HV-KP) has abundant capsular polysaccharide (CPS) and is capable of causing invasive syndrome. Sodium salicylate (SAL) reduces the production of CPS. The study was aimed to investigate the relationship between aspirin usage and KP-mediated invasive syndrome and the effect of SAL on HV-KP. METHODS: Patients with community-acquired KP bacteraemia were prospectively enrolled. KP-M1, a serotype-K1 HV-KP clinical isolate, was used in the following experiments: CPS production, HV-KP phenotype, and the effect of SAL on neutrophils phagocytosis. The effect of oral aspirin intake on the leukocyte bactericidal activity was evaluated. RESULTS: Patients infected by HV-KP and diabetic patients with poor glycemic control were at an increased risk for invasive syndrome (p < 0.01); those who had recent use of aspirin (p = 0.02) were at a lower risk. CPS production was significantly reduced in the presence of SAL. The HV-KP phenotype and resistance to neutrophil phagocytosis were both significantly reduced in the KP-M1 after incubation with SAL (p < 0.01). Aspirin treatment significantly enhanced the killing of KP-M1 by leukocytes (p < 0.01). CONCLUSION: Treatment with SAL significantly reduces CPS production in HV-KP, thereby contributing to leukocyte phagocytosis and bactericidal activity against this pathogen.

Antimicrobial susceptibility patterns among Escherichia coli urinary isolates from community-onset health care-associated urinary tract infection.

Urinary tract infection (UTI) is traditionally classified as community-acquired (CA) and hospital-acquired (HA). Community-onset health care-associated (HCA) infection is a new category that has gained increasing attention. The study aimed to compare the disk susceptibility of nonrepetitive Escherichia coli urinary isolates from HCA-UTI (n = 100) with that of E. coli isolates from CA-UTI (n = 85) and HA-UTI (n = 106). We found that the susceptibility pattern of HCA-UTI E. coli isolates was similar to that of HA-UTI E. coli isolates, but significantly different from that of CA-UTI E. coli isolates. In particular, the proportion of extended-spectrum ß-lactamase-producing isolates was significantly higher in HCA-UTI than that in CA-UTI (30.0% vs. 3.5%, p < 0.001). We recommend that when treating HCA-UTI, it is necessary to take urine cultures for susceptibility testing to guide definite antibiotic therapy.

Factors associated with female pattern hair loss and its prevalence in Taiwanese women: a community-based survey.

BACKGROUND: Although female pattern hair loss (FPHL) has been considered simply the female counterpart of male pattern hair loss in men, the risk factors may differ. OBJECTIVE: We sought to evaluate factors associated with FPHL and to estimate its prevalence in women. METHOD: In total, 26,226 subjects aged 30 years and older participated in a cross-sectional survey. Ludwig and Norwood classifications were used to assess the degree of hair loss. Information on possible risk factors for FPHL was collected using a questionnaire interview. RESULTS: The prevalence of FPHL (Ludwig grade >I) for all ages was 11.8% (95% CI 11.5%-12.2%), increasing with advancing age. After controlling for age and family history, statistically significant associations were noted between FPHL and high fasting glucose (odds ratio [OR] 1.15, 95% confidence interval [CI] 1.04-1.28), fewer childbirths (OR 1.24, 95% CI 1.12-1.38), breast-feeding (OR 0.88, 95% CI 0.78-0.98), oral contraceptive use (OR 1.21, 95% CI 1.01-1.45), and ultraviolet exposure more than 16 hours per week (OR 1.12, 95% CI 1.02-1.22). LIMITATIONS: The validity and reliability of FPHL classification may be not perfect in this survey and may need to be verified. Information on family history may be still subject to recall bias. CONCLUSIONS: Risk factors for FPHL and male androgenic alopecia may differ.

Microbiological and clinical characteristics of Streptococcus pneumoniae serotype 3 infection and risk factors for severe outcome: A multicenter observational study.

BACKGROUND/PURPOSE: Serotype 3 has persisted to be an important cause of invasive pneumococcal disease in adults in the post-vaccine era. We aimed to investigate clinical and microbiological characteristics of Streptococcus pneumoniae serotype 3 infection in Taiwan and identify the risk factors associated with severe clinical outcome. METHODS: A multicenter observational study was conducted to analyze serotype 3 isolates collected between 2012 and 2021. Demographics, comorbidities, and risk categories were statistically compared with clinical outcome. Antimicrobial susceptibility testing and multilocus sequence typing were performed. RESULTS: A total of 146 isolates were collected, including 12 isolates regarded as colonizers. Among 134 infected cases, 54 (40.3%) were aged 65 and older. Mortality was significantly associated with diabetes mellitus, immunosuppression, immunodeficiency, high-risk status, and older age. Susceptibility rates were high to levofloxacin (98.9%), moxifloxacin (100%), vancomycin (100%), and ceftriaxone (97.3%). 25.3% (37/146) of the isolates showed intermediate susceptibility and 0.7% (1/146) showed resistance to penicillin. ST180 was the dominant sequence type. ST13 and ST9625 isolates were less susceptible to penicillin and ceftriaxone. CONCLUSIONS: Serotype 3 infection showed a high mortality rate, especially in patients with older ages and comorbidities. Although the incidence rates decreased during the COVID-19 pandemic, serotype 3 remained as an important cause of infection after the implementation of PCV13. Developing a more effective vaccine against serotype 3 and monitoring the antimicrobial-resistant sequence types are necessary.

A cluster of methicillin-sensitive Staphylococcus aureus colonization in a nursery.

BACKGROUND: Staphylococcus aureus is among the most important pathogens of bacteremia in hospitals. Colonizing strains may spread to other patients. METHODS: Ninety-two mothers visiting delivery rooms were included in this study. From the mothers, specimens were obtained from the nares and vagina for the detection of S. aureus. From the babies, specimens were obtained from the nares and umbilicus within 24 h in the nursery. RESULTS: The carriage rates of S. aureus were 25% in the parturient mothers and 30.9% in their babies. The majority (55 isolates, 94.8%) of the isolates were methicillin-sensitive S. aureus (MSSA). Of the 55 MSSA isolates, 11 genotypes were identified for isolates from the mothers and five geno-types for isolates from the infants. A major clone was identified and accounted for 82% of 34 isolates from the babies. Nine pairs of mothers and babies were colonized with MSSA; by molecular methods, the paired isolates were indistinguishable in two pairs. CONCLUSION: Newborn babies acquire S. aureus colonization soon after birth, partly from their mothers. Once S. aureus is introduced into a nursery, spread of the strain may occur if health-care workers do not execute infection control measures strictly.

Incidence rates, emerging serotypes and genotypes, and antimicrobial susceptibility of pneumococcal disease in Taiwan: A multi-center clinical microbiological study after PCV13 implementation.

Objectives The multi-center clinical microbiological study in Taiwan aimed to evaluate the impact of childhood PCV13 immunization on pneumococcal disease, and the magnitude of serotype replacement in invasive and non-invasive pneumococcal disease among all age groups. Methods The study of culture-confirmed pneumococcal disease (CCPD) was conducted at four hospitals across Taiwan in 2015-2018. Pneumococcal pneumonia was defined as clinical diagnosis with positive sputum or bronchoalveolar lavage culture. Serotyping, multi-locus sequence typing, and antimicrobial susceptibility testing for penicillin and ceftriaxone were performed. Results A total of 1413 CCPD cases were identified. Invasive pneumococcal disease (IPD) accounted for 13.4% (190/1413) of CCPD. PCV7-type CCPD incidence declined among all age groups between 2015 and 2018. In adults aged 50-64 years, PCV7-type pneumococcal pneumonia incidence in 2018 was 72% lower than that in 2015, and all pneumococcal pneumonia incidence was 35% lower than that in 2015. In children, CCPD incidence was higher in 2018 than in 2015 (IRR 1.75 for age < 5 years, IRR 1.56 for age 5-17 years). Incidence of CCPD caused by non-PCV13-types, mainly 15A and 23A, increased significantly in those younger than 50 years. Serotypes 19A and 19F constituted the largest clonal complex, CC236/320 (n = 280, 19.8%). The rates of penicillin and ceftriaxone non-susceptibility were higher in PCV13-type isolates. Conclusions Childhood PCV13 immunization exerted an indirect protection to vaccine serotype clinically defined non-bacteremic pneumococcal pneumonia among adults, especially those between 50 and 64 years of age. Emerging non-PCV13 serotypes mainly caused non-invasive mucosal disease among children.

Increasing ceftriaxone resistance in Salmonellae, Taiwan.

In Taiwan, despite a substantial decline of Salmonella enterica serotype Choleraesuis infections, strains resistant to ciprofloxacin and ceftriaxone persist. A self-transferable bla(CMY-2)-harboring IncI1 plasmid was identified in S. enterica serotypes Choleraesuis, Typhimurium, Agona, and Enteritidis and contributed to the overall increase of ceftriaxone resistance in salmonellae.

Spread of ISCR1 elements containing blaDHA-1 and multiple antimicrobial resistance genes leading to increase of flomoxef resistance in extended-spectrum-beta-lactamase-producing Klebsiella pneumoniae.

Increasing resistance to quinolones, aminoglycosides, and/or cephamycins in extended-spectrum-ß-lactamase (ESBL)-producing Enterobacteriaceae exacerbates the already limited antibiotic treatment options for infections due to these microbes. In this study, the presence of resistance determinants for these antimicrobial agents was examined by PCR among ESBL-producing Klebsiella pneumoniae (ESBL-KP) isolates that caused bacteremia. Pulsed-field gel electrophoresis was used to differentiate the clonal relationship among the isolates studied. Transferability and the location of the resistance genes were analyzed by conjugation experiments, followed by DNA-DNA hybridization. Among the 94 ESBL-KP isolates studied, 20 isolates of flomoxef-resistant ESBL-KP were identified. They all carried a DHA-1 gene and were genetically diverse. CTX-M genes were found in 18 of the isolates. Among these DHA-1/CTX-M-producing K. pneumoniae isolates, ISCR1 was detected in 13 (72%) isolates, qnr genes (1 qnrA and 17 qnrB genes) were detected in 18 (100%), aac(6')-Ib-cr was detected in 11 (61%), and 16S rRNA methylase (all armA genes) was detected in 14 (78%). Four transconjugants were available for further analysis, and qnrB4, aac(6')-Ib-cr, armA, and bla(DHA-1) were all identified on these self-transferable bla(CTX-M)-carrying plasmids. The genetic environments of ISCR1 associated with armA, bla(DHA-1), and qnrB4 genes in the four transconjugants were identical. Replicon-type analysis revealed a FIIA plasmid among the four self-transferable plasmids, although the other three were nontypeable. The cotransfer of multiple resistance genes with the ISCR1 element-carrying plasmids has a clinical impact and warrants close monitoring and further study.

Analysis of pSC138, the multidrug resistance plasmid of Salmonella enterica serotype Choleraesuis SC-B67.

Salmonella enterica serotype Choleraesuis (S. Choleraesuis) usually causes systemic infections in man and needs antimicrobial treatment. Multidrug resistance (MDR) in S. Choleraesuis is thus a great concern in the treatment of systemic non-typhoid salmonellosis. A large plasmid, pSC138, was identified in 2002 from a S. Choleraesuis strain SC-B67 that was resistant to all antimicrobial agents commonly used to treat salmonellosis, including ciprofloxacin and ceftriaxone. Complete DNA sequence of the plasmid had been determined previously (Chiu et al., 2005). In the present study, the sequence of pSC138 was reannotated in detail and compared with several newly sequenced plasmids. Some transposable elements and drug resistance genes were further delineated. Plasmid pSC138 was 138,742 bp in length and consisted of 177 open reading frames (ORFs). While 134 of the ORFs displayed significant identity levels to other plasmid and prokaryotic sequences, the remaining 43 ORFs have not been previously reported. Mobile elements, including two integrons, seven insertion sequences and eight transposons, and a truncated prophage together encompass at least 66,781 bp (48.1%) of the plasmid genome. The sequence of pSC138 consists of three major regions: a large composite transposable region Tn6088 with a Tn21-like backbone inserted by a variety of integrons or transposable elements; a transfer/maintenance region that contains a conserved ISEcp1-mediated transposon-like element Tn6092, carrying an AmpC gene, bla(CMY-2), that confers the ceftriaxone resistance; and a Rep_3 type of replication region. Another seven bacteremic strains of S. Choleraesuis that expressed the same MDR phenotype were identified during 2003-2008. The same Rep_3 type replicase and the bla(CMY-2)-containing, ISEcp1-mediated transposon-like element were found in the MDR isolates, suggesting a successful preservation and dissemination of the MDR plasmid. Comparison of pSC138 with other recently published plasmids revealed a high identity level between partial sequences of pSC138 and plasmids of the same or different incompatibility groups. The large MDR region found in pSC138 may provide a niche for the future evolution of the plasmid by acquisition of relevant resistance genes through the panoply of mobile elements and illegitimate recombination events.

Comparison of virulence variations on MDCK monolayers by Escherichia coli isolated from acute lobar nephronia and acute pyelonephritis.

Escherichia coli is the most common pathogen associated with acute lobar nephronia (ALN), a clinically more severe parenchymal inflammatory disease that requires a longer duration of antibiotic treatment than acute pyelonephritis (APN). This study was conducted to unravel the virulence differences between clinical isolates of E. coli from pediatric ALN and APN patients. A total of 88 urinary isolates of E. coli were investigated. They were identified from radiologically diagnosed ALN and APN patients and had previously been molecularly characterized for important urovirulence genes. Madin-Darby canine kidney (MDCK) epithelial cells were used as an in vitro model. Multivariate logistic regression analyses indicated that ALN isolates were more likely to show adhesion (p<0.05; odds ratio [OR], 3.81; 95% confidence interval [CI], 1.23-11.80) and cytotoxicity (p<0.001; OR, 10.42; 95% CI, 3.03-35.89). However, no difference in the penetration ability was noted. Henceforth, the ability to adhere to and produce cytotoxicity against uroepithelial cells appears a prerequisite factor for E. coli to cause more severe bacterial kidney infection, such as ALN.

RssAB-FlhDC-ShlBA as a major pathogenesis pathway in Serratia marcescens.

Serratia marcescens has long been recognized as an important opportunistic pathogen, but the underlying pathogenesis mechanism is not completely clear. Here, we report a key pathogenesis pathway in S. marcescens comprising the RssAB two-component system and its downstream elements, FlhDC and the dominant virulence factor hemolysin ShlBA. Expression of shlBA is under the positive control of FlhDC, which is repressed by RssAB signaling. At 37°C, functional RssAB inhibits swarming, represses hemolysin production, and promotes S. marcescens biofilm formation. In comparison, when rssBA is deleted, S. marcescens displays aberrant multicellularity favoring motile swarming with unbridled hemolysin production. Cellular and animal infection models further demonstrate that loss of rssBA transforms this opportunistic pathogen into hypervirulent phenotypes, leading to extensive inflammatory responses coupled with destructive and systemic infection. Hemolysin production is essential in this context. Collectively, a major virulence regulatory pathway is identified in S. marcescens.