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Listeria monocytogenes is a Gram-positive, intracellular pathogen responsible for the highly fatal foodborne illness listeriosis. Establishing intracellular infections requires the coordinated expressions of a variety of virulence factors, such as the pore-forming toxin listeriolysin O (LLO), in response to various intra- and extracellular signals. For example, we previously reported that L. monocytogenes differentially modulated LLO production in response to exogenous propionate, a short chain fatty acid either used in salt form as a human food ingredient or produced endogenously by gut microbial fermentation. Therefore, propionate is likely a continuously present signal throughout the L. monocytogenes transmission and infection process. However, little is known about the role of propionate in modulating L. monocytogenes-host interactions. Here we investigated the impact of propionate treatment on L. monocytogenes intracellular infections using cell culture infection models. Propionate treatment was performed separately on L. monocytogenes or host cells before or during infections to better distinguish pathogen-versus-host responses to propionate. Intracellular CFU in RAW264.7 macrophages and plaque diameters in L-fibroblasts were measured as proxy for intracellular infection outcomes. Nitrite levels and cellular morphology were also measured to assess host responses to propionate. We found that propionate pretreatment of anaerobic, but not aerobic, L. monocytogenes significantly enhanced subsequent intracellular infections in both cell types and nitrite production by infected macrophages. Propionate treatment of uninfected macrophages significantly altered cell morphology, seen by longer cells and greater migration, and reduced nitrite concentration in activated macrophages. Treatment of macrophages with propionate prior to or during infections significantly inhibited intracellular growth of L. monocytogenes, including those pre-treated with propionate. These results showcased an opposing effect of propionate on L. monocytogenes intracellular infections and strongly support propionate as an important signaling molecule for both the pathogen and the host cell that can potentially alter the outcome of L. monocytogenes-host interactions.

Insect gut microbiomes consist of bacteria, fungi, and viruses that can act as mutualists to influence the health and fitness of their hosts. While much has been done to increase understanding of the effects of environmental factors that drive insect ecology, there is less understanding of the effects of environmental factors on these gut microbial communities. For example, the effect of environmental nutrients on most insect gut microbiomes is poorly defined. To address this knowledge gap, we investigated the relationship between environmental nutrients and the gut microbial communities in a small study of katydids (n = 13) of the orthopteran species Orchelimum vulgare collected from a costal prairie system. We sampled O. vulgare from unfertilized plots, as well as from plots fertilized with added nitrogen and phosphorus or sodium separately and in combination. We found significantly higher Shannon diversity for the gut bacterial communities in O. vulgare from plots fertilized with added sodium as compared to those collected from plots without added sodium. In contrast, diversity was significantly lower in the gut fungal communities of grasshoppers collected from plots with added nitrogen and phosphorus, as well as those with added sodium, in comparison to those with no added nutrients. There was also a strong positive correlation between the gut bacterial and gut fungal community diversity within each sample. Indicator group analysis for added sodium plots included several taxa with known salt-tolerant bacterial and fungal representatives. Therefore, despite the small sample number, these results highlight the potential for the gut bacterial and fungal constituents to respond differently to changes in environmental nutrient levels. Future studies with a larger sample size will help identify mechanistic determinants driving these changes. Based on our findings and the potential contribution of gut microbes to insect fitness and function, consideration of abiotic factors like soil nutrients along with characteristic gut microbial groups is necessary for better understanding and conservation of this important insect herbivore.

Listeria monocytogenes is a foodborne pathogen capable of secreting listeriolysin O (LLO), a pore-forming toxin encoded by the hly gene. While the functions of LLO have been studied extensively, how the production of LLO is modulated by the intestinal environment, devoid of oxygen and enriched in short chain fatty acids (SCFAs), is not completely understood. Using L. monocytogenes strain 10403s, we found that hly transcription was moderately decreased by aerobic SCFA exposures but significantly increased by anaerobic SCFA exposures. Moreover, aerobic, but not anaerobic, exposure to low levels of SCFAs resulted in a significantly higher LLO activity. These results demonstrated that transcriptional and post-transcriptional regulations of LLO production were separately modulated by SCFAs and were responsive to oxygen levels. Examining isogenic mutants revealed that PrfA and SigB play a role in regulating LLO production in response to SCFAs. Effects of SCFAs were also present in the cardiotropic strain 07PF0776 but distinctly different from those in strain 10403s. For both strains, prior exposures to SCFAs altered intracellular infections in Caco-2 and RAW264.7 cells and the plaque sizes in L fibroblasts, a result confirming the ability of L. monocytogenes to adapt to SCFAs in ways that impact its subsequent infection outcomes.

Insect microbiomes play an important role in the health and fitness of insect hosts by contributing to nutrient absorption, immune health, and overall ecological fitness. As such, research interests in insect microbiomes have focused on agriculturally and industrially important organisms such as honey bees and termites. Orthopterans, on the other hand, have not been well explored for their resident microbial communities. Grasshoppers are an integral part of grassland ecosystems and provide important ecosystem services. Conversely, grasshoppers can be an agricultural pest requiring management with broad spectrum pesticides. However, little is known about the microbiomes of grasshoppers and their potential contribution to grasshopper biology. Here we examine the gut microbiome of six species of grasshoppers (n = 60) from a coastal tallgrass prairie ecosystem to gain a better understanding of the microbial communities present across the orthopteran order in this ecosystem. We found that there are bacterial phyla common to all six grasshopper species: Actinobacteria, Proteobacteria, Firmicutes, and to a lesser degree, Tenericutes. Although the grasshopper species shared a high relative abundance of these groups, there were notable shifts in dominant phyla depending on the grasshopper species. Moreover, measures of alpha diversity revealed a more diverse microbiome in males than females. Our observations support the hypothesis that there is a "core" group of bacterial families in these grasshopper species and factors such as trophic behaviors and the evolution of the host may contribute to the shifts in prevalence among these core microbial groups.

Listeria monocytogenes is a human pathogen and a facultative anaerobe. To better understand how anaerobic growth affects L. monocytogenes pathogenesis, we first showed that anaerobic growth led to decreased growth and changes in surface morphology. Moreover, compared to aerobically grown bacteria, anaerobically grown L. monocytogenes established higher level of invasion but decreased intracellular growth and actin polymerization in cultured cells. The production of listeriolysin O (LLO) was significantly lower in anaerobic cultures-a phenotype observed in wild type and isogenic mutants lacking transcriptional regulators SigB or CodY or harboring a constitutively active PrfA. To explore potential regulatory mechanisms, we established that the addition of central carbon metabolism intermediates, such as acetate, citrate, fumarate, pyruvate, lactate, and succinate, led to an increase in LLO activity in the anaerobic culture supernatant. These results highlight the regulatory role of central carbon metabolism in L. monocytogenes pathogenesis under anaerobic conditions.

Fatty acids (FAs) are the major structural component of cellular membranes, which provide a physical and chemical barrier that insulates intracellular reactions from environmental fluctuations. The native composition of membrane FAs establishes the topological and chemical parameters for membrane-associated functions and is therefore modulated diligently by microorganisms especially in response to environmental stresses. However, the consequences of altered FA composition during host-pathogen interactions are poorly understood. The food-borne pathogen Listeria monocytogenes contains mostly saturated branched-chain FAs (BCFAs), which support growth at low pH and low temperature. In this study, we show that anteiso-BCFAs enhance bacterial resistance against phagosomal killing in macrophages. Specifically, BCFAs protect against antimicrobial peptides and peptidoglycan hydrolases, two classes of phagosome antimicrobial defense mechanisms. In addition, the production of the critical virulence factor, listeriolysin O, was compromised by FA modulation, suggesting that FAs play a key role in virulence regulation. In summary, our results emphasize the significance of FA metabolism, not only in bacterial virulence regulation but also in membrane barrier function by providing resistance against host antimicrobial stress.

Bacterial enteric pathogens are responsible for a tremendous amount of foodborne illnesses every year through the consumption of contaminated food products. During their transit from contaminated food sources to the host gastrointestinal tract, these pathogens are exposed and must adapt to fluctuating oxygen levels to successfully colonize the host and cause diseases. However, the majority of enteric infection research has been conducted under aerobic conditions. To raise awareness of the importance in understanding the impact of oxygen, or lack of oxygen, on enteric pathogenesis, we describe in this review the metabolic and physiological responses of nine bacterial enteric pathogens exposed to environments with different oxygen levels. We further discuss the effects of oxygen levels on virulence regulation to establish potential connections between metabolic adaptations and bacterial pathogenesis. While not providing an exhaustive list of all bacterial pathogens, we highlight key differences and similarities among nine facultative anaerobic and microaerobic pathogens in this review to argue for a more in-depth understanding of the diverse impact oxygen levels have on enteric pathogenesis.

Anteiso-branched-chain fatty acids (BCFA) represent the dominant group of membrane fatty acids and have been established as crucial determinants in resistance against environmental stresses in Listeria monocytogenes , a facultative intracellular pathogen. Here, we investigate the role of anteiso-BCFA in L. monocytogenes virulence by using mutants deficient in branched-chain alpha-keto acid dehydrogenase (BKD), an enzyme complex involved in the synthesis of BCFA. In tissue culture models of infection, anteiso-BCFA contributed to intracellular growth and survival in macrophages and significantly enhanced plaque formation upon prolonged infection in L2 fibroblasts. The intracellular defects observed could be attributed partially to insufficient listeriolysin O (LLO) production, indicating a requirement for anteiso-BCFA in regulating virulence factor production. In a murine model of infection, the BKD-deficient mutant was highly attenuated, further emphasizing the importance of BKD-mediated metabolism in L. monocytogenes virulence. This study demonstrates an underappreciated role for BCFA in bacterial pathogenesis, which may provide insight into the development and application of antimicrobial agents.

The human gut microbiota is inextricably linked to health and disease. One important function of the commensal organisms living in the intestine is to provide colonization resistance against invading enteric pathogens. Because of the complex nature of the interaction between the microbiota and its host, multiple mechanisms likely contribute to resistance. In this review, we dissect the biological role of short-chain fatty acids (SCFA), which are fermentation end products of the intestinal microbiota, in host–pathogen interactions. SCFA exert an extensive influence on host physiology through nutritional, regulatory, and immunomodulatory functions and can also affect bacterial fitness as a form of acid stress. Moreover, SCFA act as a signal for virulence gene regulation in common enteric pathogens. Taken together, these studies highlight the importance of the chemical environment where the biology of the host, the microbiota, and the pathogen intersects, which provides a basis for designing effective infection prevention and control.

The response behavior of three dissimilatory perchlorate-reducing bacteria to different electron acceptors (nitrate, chlorate, and perchlorate) was investigated with two different assays. The observed response was species-specific, dependent on the prior growth conditions, and was inhibited by oxygen. We observed attraction toward nitrate when Dechloromonas aromatica strain RCB and Azospira suillum strain PS were grown with nitrate. When D. aromatica and Dechloromonas agitata strain CKB were grown with perchlorate, both responded to nitrate, chlorate, and perchlorate. When A. suillum was grown with perchlorate, the organism responded to chlorate and perchlorate but not nitrate. A gene replacement mutant in the perchlorate reductase subunit (pcrA) of D. aromatica resulted in a loss of the attraction response toward perchlorate but had no impact on the nitrate response. Washed-cell suspension studies revealed that the perchlorate grown cells of D. aromatica reduced both perchlorate and nitrate, while A. suillum cells reduced perchlorate only. Based on these observations, energy taxis was proposed as the underlying mechanism for the responses to (per)chlorate by D. aromatica. To the best of our knowledge, this study represents the first investigation of the response behavior of perchlorate-reducing bacteria to environmental stimuli. It clearly demonstrates attraction toward chlorine oxyanions and the unique ability of these organisms to distinguish structurally analogous compounds, nitrate, chlorate, and perchlorate and respond accordingly. Electronic supplementary material The online version of this article (doi:10.1007/s00253-009-2051-3) contains supplementary material, which is available to authorized users.