Bacterial Growth Rate and Host Factors as Determinants of Intracellular Bacterial Distributions in SystemicSalmonella entericaInfections

Abstract

Bacteria of the species Salmonella enterica cause a range of life-threatening diseases in humans and animals worldwide. The within-host quantitative, spatial, and temporal dynamics of S. enterica interactions are key to understanding how immunity acts on these infections and how bacteria evade immune surveillance. In this study, we test hypotheses generated from mathematical models of in vivo dynamics of Salmonella infections with experimental observation of bacteria at the single-cell level in infected mouse organs to improve our understanding of the dynamic interactions between host and bacterial mechanisms that determine net growth rates of S. enterica within the host. We show that both bacterial and host factors determine the numerical distributions of bacteria within host cells and thus the level of dispersiveness of the infection.Bacteria of the species Salmonella enterica are a threat to public health, causing enteric systemic diseases (typhoid and paratyphoid fever), gastroenteritis and nontyphoidal septicemia in humans and similar diseases in many other animal species worldwide. The emergence of multidrug-resistant Salmonella strains and the insufficient efficacy of some of the currently available Salmonella vaccines highlight the urgent need for improved prevention strategies to combat salmonellosis in humans and animals.S. enterica typhoidal infections have a truly dispersive nature. In mouse infections, the clonal and independent expansion of bacteria derived from the initial inoculum is paralleled by an increase in the number of infected cells and of the multicellular pathological lesions that form at foci of infection (11,15,16). Control of the growth and spread of the bacteria requires recruitment and activation of phagocytes at the foci of infection with an influx of bone marrow-derived cells into the tissues and the concerted action of key cytokine networks: tumor necrosis factor alpha, gamma interferon (IFN-␥), and interleukin-12 (IL-12), . In addition, reactive oxygen intermediates and reactive nitrogen intermediates act in phagocytes to control the growth of ingested bacteria (10,17).A key feature of the spread of S. enterica in the tissues is the heterogeneous numerical distribution of intracellular bacteria with low bacterial numbers seen at any one time in the majority of infected phagocytes, while a minority of phagocytes can be heavily infected (16). The combination of intracellular bacterial distributional data with "branching process" mathematical models allowed the discrimination of a number of mechanistic hypotheses on the cause of this strikingly skewed distribution of bacteria among host cells (1). In particular, the analysis gave scant support to the hypothesis that highly infected cells were intrinsically more susceptible to infection. Rather, the model supported the idea that all phagocytes were equivalently susceptible and that the skewed distribution was generated by the epidemic spread of the bacteria through the host cell population. The model gave further discrimination at ...

Enhanced Virulence of Salmonella enterica Serovar Typhimurium after Passage through Mice

“…Intracellular distributions of in vivo-and in vitro-grown Salmonella within phagocytes in infected livers. Our previous work on the spread and distribution of S. Typhimurium in the tissues of infected mice indicated that bacterial growth is paralleled by increases in the number of infected cells resulting in dispersive infections (6,7,17). This is indicated by the fact that intracellular bacterial loads per cell remain generally skewed toward low numbers, despite the observed increases in total bacterial numbers in a given organ.…”

“…infection, a decrease in total viable bacterial numbers is observed in infected livers and spleens, as the result of a high rate of reactive oxygen radical-mediated killing that exceeds the bacterial division rate (6). After this initial phase, killing becomes negligible and the bacteria divide intracellularly at variable rates, depending on both the virulence of the infecting isolate and the level of resistance of the host (6,7,18). Growth of virulent S. Typhimurium in vivo is also associated with escape from infected macrophages and dissemination to other uninfected cells (1,18).…”

Resistance and survival strategies of Salmonella enterica to environmental stresses

“…As a result of their life cycle, salmonellae often suffer periods of nutrient starvation as they voyage through different natural, commercial, and host microenvironments they encounter (Abshire & Neidhardt, 1993;Dodd et al, 2007;Fang et al, 1992;Grant et al, 2009;Humphreys, Stevenson, Bacon, Weinhardt, & Roberts, 1999;Koch, 1971;Roszak et al, 1984;Rychlik & Barrow, 2005;Spector, 1998;Testerman et al, 2002;Turpin et al, 1993;Winfield & Groisman, 2003). Unlike endosporeforming bacteria, Salmonella and other enterobacteria depend upon different types of "programmed" physiologic responses for survival during periods of nutrient starvation, that are functionally analogous to sporulation but do not technically result in a structurally distinct "differentiated" cell form (i.e., an endospore).…”

“…This is particularly vital for foodborne microbial pathogens that can encounter potentially life-threatening conditions in virtually every environment they may find themselves including: natural (e.g., soil, water systems), commercial (e.g., slaughter houses, food processing plants) and host (e.g., animals, humans) settings (Winfield & Groisman, 2003). Responses to these conditions not only impact growth and survival but can also influence virulence and resistance to multiple antimicrobics (Altier, 2005;Clements, Ericksson, Tezcan-Merdol, Hinton, & Rhen, 2001;Dodd, Richards, & Aldsworth, 2007;Grant et al, 2009;Kenyon & Spector, 2011;McMahon, Xu, Moore, Blair, & McDowell, 2007;McMeechan et al, 2007;Rowley, Spector, Kormanec, & Roberts, 2006). Few microorganisms are as capable of coping with the range of stresses present in natural, commercial and host microenvironments as Salmonella enterica serovars (Cabello, Hormaeche, Mastroeni, & Bonina, 1993;D' Aoust, Maurer, & Bailey, 2001;Kenyon & Spector, 2011;Rychlik & Barrow, 2005;Stocker & Makela, 1986).…”

Taming the Elephant: Salmonella Biology, Pathogenesis, and Prevention

“…Evidence for bacterial growth and NADPH phagocyte oxidase (Nox2)-mediated bacterial killing was observed within the first 6 h of infection (64). At later time points, host factors, including natural resistance-associated macrophage protein 1 (Nramp1, Slc11a1), gamma interferon (IFN-␥), tumor necrosis factor alpha (TNF-␣), and inducible nitric oxide synthase (iNOS), exhibited predominantly bacteriostatic effects on S. Typhimurium.…”