Salmonella Extracellular Polymeric Substances Modulate Innate Phagocyte Activity and Enhance Tolerance of Biofilm-Associated Bacteria to Oxidative Stress

Abstract

Salmonella enterica serovar Typhi causes 14.3 million acute cases of typhoid fever that are responsible for 136,000 deaths each year. Chronic infections occur in 3%-5% of those infected and S. Typhi persists primarily in the gallbladder by forming biofilms on cholesterol gallstones, but how these bacterial communities evade host immunity is not known. Salmonella biofilms produce several extracellular polymeric substances (EPSs) during chronic infection, which are hypothesized to prevent pathogen clearance either by protecting biofilm-associated bacteria from direct humoral attack or by modulating innate phagocyte interaction with biofilms. Using wild-type and EPS-deficient planktonic and biofilm Salmonella, the direct attack hypothesis was tested by challenging biofilms with human serum and antimicrobial peptides. Biofilms were found to be tolerant to these molecules, but these phenotypes were independent of the tested EPSs. By examining macrophage and neutrophil responses, new roles for biofilm-associated capsular polysaccharides and slime polysaccharides were identified. The S. Typhi Vi antigen was found to modulate innate immunity by reducing macrophage nitric oxide production and neutrophil reactive oxygen species (ROS) production. The slime polysaccharides colanic acid and cellulose were found to be immune-stimulating and represent a key difference between non-typhoidal serovars and typhoidal serovars, which do not express colanic acid. Furthermore, biofilm tolerance to the exogenously-supplied ROS intermediates hydrogen peroxide (H 2 O 2 ) and hypochlorite (ClO − ) indicated an additional role of the capsular polysaccharides for both serovars in recalcitrance to H 2 O 2 but not ClO − , providing new understanding of the stalemate that arises during chronic infections and offering new directions for mechanistic and clinical studies.

Sulfate import inSalmonellaTyphimurium impacts bacterial aggregation and the neutrophil respiratory burst

“…Furthermore, the lack of an altered neutrophil respiratory burst in response to the colanic acid operon deletion mutant (∆wza) suggests that colanic acid deletion is not the likely cause for the altered neutrophil respiratory burst in response to the ∆STM2112 mutant in our work. Consistent with our findings, a mutant in colanic acid biosynthesis (∆wcaM) elicited no change in neutrophil respiratory burst when in planktonic growth [16]. Since mutations in enzymes downstream of the initial glycosyl transferase in the colanic acid biosynthesis pathway (WcaJ) lead to accumulation of toxic intermediates [29], we hypothesize that the ∆STM2112 mutant has reduced viability in the presence of neutrophils, leading to the observed reduction in neutrophil respiratory burst.…”

“…Disruption of sulfate reduction, cysteine biosynthesis, and cysteine catabolism is associated with biofilm development in E. coli in the absence of canonical environmental cues [48][49][50]. Biofilm-associated Salmonella Typhimurium elicit a blunted neutrophil respiratory burst as compared with planktonic cells and the blunted respiratory burst is linked to curli fimbriae and O-antigen capsule [16]. Curli fimbriae are a major component of the Salmonella biofilm extracellular matrix and are recognized by innate immune receptors tolllike receptor-2 (TLR-2) and TLR9 and the nod-like receptor protein-3 inflammasome [51][52][53][54].…”

“…In addition, both the Salmonella Typhimurium T3SS-1 and flagellar motility are important agonists of the neutrophil respiratory burst in vitro [14]. However, biofilm-associated NTS, encased in an extracellular matrix composed of proteins, carbohydrates, and extracellular DNA (reviewed in [15]), elicit a diminished neutrophil respiratory burst when compared to planktonic cells [16]. Taken together, these findings suggest that NTS have numerous mechanisms to alter the magnitude of the neutrophil respiratory burst, which likely help to facilitate its survival in different host compartments.…”

Salmonella Biofilm Formation, Chronic Infection, and Immunity Within the Intestine and Hepatobiliary Tract

“…Importantly, the chronic mouse model of infection involves treatment of the mice for 8 weeks prior to infection with a lithogenic diet to induce gallstone formation, thus providing an optimal environment for Salmonella biofilm formation. In this context, biofilms provide Salmonella with increased recalcitrance to the harsh gallbladder environment (which includes detergentlike bile salts) and other potential threats such as the host immune system or antibiotic therapy, allowing for long-term survival within the host and prolonged transmissibility via intermittent fecal shedding (Jolivet-Gougeon and Bonnaure-Mallet, 2014; Gonzaĺez et al, 2018;Hahn and Gunn, 2020;Tsai et al, 2020). Chronic gallstone biofilm carriers can shed S. Typhi for at least a year or longer (Gonzalez-Escobedo et al, 2011).…”

Special Issue “Salmonella: Pathogenesis and Host Restriction”

“…Biofilms are defined as a highly organized multicellular community of bacteria embedded in a self-induced extracellular matrix. Here, Hahn and Gunn [ 12 ] describe the role of extracellular polymeric substances in biofilms and in host innate immunity. Furthermore, Sokaribo et al [ 13 ] describe the expression of CsgD, the major biofilm regulator, under several conditions.…”