Molecular determinants of peaceful coexistence versus invasiveness of non-Typhoidal Salmonella: Implications in long-term side-effects

Abstract

The genus Salmonella represents a wide range of strains including Typhoidal and Non-Typhoidal Salmonella (NTS) isolates that exhibit illnesses of varied pathophysiologies. The more frequent NTS ensues a self-limiting enterocolitis with rare occasions of bacteremia or systemic infections. These self-limiting Salmonella strains are capable of subverting and dampening the host immune system to achieve a more prolonged survival inside the host system thus leading to chronic manifestations. Notably, emergence of new invasive NTS isolates known as invasive Non-Typhoidal Salmonella (iNTS) have worsened the disease burden significantly in some parts of the world. NTS strains adapt to attain persister phenotype intracellularly and cause relapsing infections. These chronic infections, in susceptible hosts, are also capable of causing diseases like IBS, IBD, reactive arthritis, gallbladder cancer and colorectal cancer. The present understanding of molecular mechanism of how these chronic infections are manifested is quite limited. The current work is an effort to review the prevailing knowledge emanating from a large volume of research focusing on various forms of NTS infections including those that cause localized, systemic and persistent disease. The review will further dwell into the understanding of how this pathogen contributes to the associated long term sequelae.

Introduction

Salmonella, Gram-negative, rod shaped, facultative anaerobe and enteric pathogen which belongs to the family bacteriaceae, was first isolated and discovered in the porcine intestine in the year 1885 by Theobald Smith and was named after his colleague who worked with him, Dr. Daniel E. Salmon (Smith, 1894.; Su and Chiu, 2007). Salmonella infection is either host specific or includes multiple hosts depending upon the species and serotypes. Within the genus Salmonella, more than 2500 serovars exist and are known to cause infections in multiple hosts including humans, poultry and cattle (Tanner and Kingsley, 2018). In a broader sense, Salmonella is classified into human restricted typhoid causing serovars (eg. S. Typhi and S. Paratyphi) and wide host range non typhoidal serovars (NTS) such as S. Typhimurium and S. Entritidis. The main source of Salmonella infection in humans is contaminated food or water, however indirect contact through droppings and water from aquariums where infected reptiles, amphibians live also pose a risk for salmonellosis. In humans, ailments caused by Salmonella range from gastroenteritis, a localised intestinal infection to a systemic enteric fever called typhoid or enteric fever. Infection with S. Typhi and S. Paratyphi results in typhoid, while that with NTS serovars result in a localized gut inflammation referred as gastroenteritis. Together, the typhoidal strains and NTS possess a huge global burden in terms of mortality and morbidity. Among these, the NTS strains have higher incidence of infectivity, with about 95 million cases of enterocolitis reported in the year 2017 (Roth et al., 2018). NTS mediated gastroenteritis is a self-limiting infection lasting for no more than 5–7 days, nonetheless in rare cases, such as immunocompromised and malnourished patients, these infections can lead to chronic infections and systemic spread. Around 0.15% and 3.9% of overall infections in adults and in children respectively can contribute to chronic state. Chronic carriage lead to recurrent intestinal infections and thus shedding of bacteria in faecal matter, which can thereof enter into the environment, hence becoming source of outbreaks (Gunn et al., 2014). Chronic states can also lead to bacteremia and sepsis in susceptible genetic background resulting in increased morbidity and mortality. The prevalence of Salmonella as the most frequently isolated pathogen from hospitalized patients suffering from bacteremia in Africa and Asia was recently established by a systematic review and meta-analysis study (reviewed in Marchello et al., 2019). The emergence of invasive NTS strains (iNTS) over the last 30–40 years, have further aggravated the situation. Among 5,35,000 cases of iNTS reported in the year 2017, ~78000 contributed to death indicating association of huge mortality rate with iNTS (Stanaway et al., 2019). A clear understanding of the mechanisms of systemic spread as well as chronic states of the typhoidal strains of Salmonella exist, however that of NTS is less well explored.

Salmonella has evolved and adapted to become host restricted via multiple horizontal gene transfer (HGT) events over longer time scales of several years resulting in typhoid causing strains of Salmonella. The invasive strains of NTS have emerged over short time scales resulting from point mutations, SNPs and genome degradation events (Fig. 1). Further, these invasive NTS strains have acquired multiple drug resistance cassettes in its virulence plasmids which make associated infections difficult to contain. They are adept at causing a more febrile disease by circumventing the host gut immune system and thus residing for longer duration in the host in specific niches such as reticulo-endothelial system and gallbladder (Ilyas et al., 2017; Tanner and Kingsley, 2018).

Chronic Salmonella infection is either mediated by concerted efforts of bacterial effector proteins encoded by its genome or host genetic and geographical surrounding which contribute to bacterial persistence. The effector proteins are critically important for bacterial entry, its intracellular survival and modulation of host cellular processes (Srikanth et al., 2011). In addition, these proteins also help the bacterium to circumvent the host innate and adaptive immune system and reside predominantly within macrophages (reviewed in Bernal-Bayard and Ramos-Morales, 2018; Monack et al., 2004). Salmonella reprograms the macrophages from microbicidal M1 subtype to a more tolerable niche of M2 subtype specifically by means of its Salmonella pathogenicity island II encoded effectors (Stapels et al., 2018). This allows the bacterium to not only reside for long duration but also to disseminate further to other permissible niches such as gallbladder and liver via the reticulo-endothelial system. Salmonella can persist for long durations, up to two years or more, in gallbladder as a consequence of biofilm formation and eventually trigger severe outcomes such as chronic inflammation and even cancerous transformation (Gunn et al., 2014; Scanu et al., 2015). In addition, emergence of antimicrobial resistant strains in response to extensive use of antimicrobial therapy have contributed to bacterial persistence (reviewed in Bakkeren et al., 2020).

Following the resolution of disease, in case of chronic state, patients fail to fully clear the infection and they progress to a state of long term carriage. These chronic infections not only contribute to above mentioned recurrent infections and bacteremia but can also lead to other deleterious and debilitating diseases in susceptible hosts. Chronic Salmonella infections have been associated with several unusual outcomes like reactive arthritis (ReA) (Thomson et al., 1995), inflammatory bowel disease (IBD) (Schultz et al., 2017), gallbladder cancer (GBC) (Iyer et al., 2016) and colorectal cancer (CRC) (Lu et al., 2017). Even though association studies between Salmonella infection and these diseases are predominant, studies emphasizing on the molecular mechanistic details of how these diseases are acquired upon persistent infection are very limited. Salmonella mediated molecular mechanisms and disease pathophysiology has been extensively studied, yet there is limited mechanistic information about the new invasive NTS (iNTS) strains and how persistent infections which is responsible for its transmission is achieved.

This review will provide a holistic overview about the NTS and iNTS serotypes, their origin, evolution and associated clinical manifestations. The review will dwell on the available literature elucidating how NTS mediated gastroenteritis is established and can lead to chronic infections. We will also discuss how iNTS mediated disease pathogenesis and chronicity differs from that of NTS according to the available limited studies. Further, the review will entail in understanding the different mechanisms employed by Salmonella to survive inside the host for longer duration and how these could contribute to associated chronic sequelae.