Review: Microbial endocrinology: intersection of microbiology and neurobiology matters to swine health from infection to behavior.

From birth to slaughter, pigs are in constant interaction with microorganisms. Exposure of the skin, gastrointestinal and respiratory tracts, and other systems allows microorganisms to affect the developmental trajectory and function of porcine physiology as well as impact behavior. These routes of communication are bi-directional, allowing the swine host to likewise influence microbial survival, function and community composition. Microbial endocrinology is the study of the bi-directional dialogue between host and microbe. Indeed, the landmark discovery of host neuroendocrine systems as hubs of host-microbe communication revealed neurochemicals act as an inter-kingdom evolutionary-based language between microorganism and host. Several such neurochemicals are stress catecholamines, which have been shown to drastically increase host susceptibility to infection and augment virulence of important swine pathogens, including Clostridium perfringens. Catecholamines, the production of which increase in response to stress, reach the epithelium of multiple tissues, including the gastrointestinal tract and lung, where they initiate diverse responses by members of the microbiome as well as transient microorganisms, including pathogens and opportunistic pathogens. Multiple laboratories have confirmed the evolutionary role of microbial endocrinology in infectious disease pathogenesis extending from animals to even plants. More recent investigations have now shown that microbial endocrinology also plays a role in animal behavior through the microbiota-gut-brain axis. As stress and disease are ever-present, intersecting concerns during each stage of swine production, novel strategies utilizing a microbial endocrinology-based approach will likely prove invaluable to the swine industry.

Altered Schaedler flora mice: A defined microbiota animal model to study the microbiota-gut-brain axis.

Despite considerable attention, the mechanisms by which the microbiota affect brain function and host behaviour via the gut-brain axis remain undefined. Identifying microbe-specific pathways that influence neuronal function and bi-directional communication between the gut microbiota and the host central nervous system is challenging due to the extreme microbial diversity in the gut of conventionally-reared mice. Herein, we describe the use of the altered Schaedler flora (ASF) mouse model as an alternative to conventionally-reared and germ-free animals. Colonized with only 8 bacterial species, use of ASF mice greatly simplifies the examination of microbiota-host interactions. We assessed the extent to which behaviour differed between mice with a limited consortium of bacteria compared with a complex, conventional microbiota. The elevated plus maze and open-field assays were utilized to assess murine behaviour. Histological analysis of ileum and colon was performed to evaluate intestinal morphology, and 16 s rRNA gene taxonomic profiling was performed to determine host-stress induced changes in fecal microbial communities. Behavioural and serum corticosterone differences were observed between ASF and conventionally-reared mice, while no differences were found between the intestinal morphology of these two groups. The stress of the behavioural tests induced significant changes in the ASF fecal microbial community but not in that of the conventionally-reared mice. In contrast to the conventionally-reared mice, the results indicated that the ASF mice displayed a marked anxiogenic-like behaviour. These data indicate that ASF mice represent a unique model to elucidate mechanisms governing microbiota-gut-brain communication affecting behaviour.

Social stress-enhanced severity of Citrobacter rodentium-induced colitis is CCL2-dependent and attenuated by probiotic Lactobacillus reuteri.

Psychological stressors are known to affect colonic diseases but the mechanisms by which this occurs, and whether probiotics can prevent stressor effects, are not understood. Because inflammatory monocytes that traffic into the colon can exacerbate colitis, we tested whether CCL2, a chemokine involved in monocyte recruitment, was necessary for stressor-induced exacerbation of infectious colitis. Mice were exposed to a social disruption stressor that entails repeated social defeat. During stressor exposure, mice were orally challenged with Citrobacter rodentium to induce a colonic inflammatory response. Exposure to the stressor during challenge resulted in significantly higher colonic pathogen levels, translocation to the spleen, increases in colonic macrophages, and increases in inflammatory cytokines and chemokines. The stressor-enhanced severity of C. rodentium-induced colitis was not evident in CCL2(-/-) mice, indicating the effects of the stressor are CCL2-dependent. In addition, we tested whether probiotic intervention could attenuate stressor-enhanced infectious colitis by reducing monocyte/macrophage accumulation. Treating mice with probiotic Lactobacillus reuteri reduced CCL2 mRNA levels in the colon and attenuated stressor-enhanced infectious colitis. These data demonstrate that probiotic L. reuteri can prevent the exacerbating effects of stressor exposure on pathogen-induced colitis, and suggest that one mechanism by which this occurs is through downregulation of the chemokine CCL2.

Stress and microbial endocrinology: prospects for ruminant nutrition.

The feed efficiency of ruminant meat and dairy livestock can be significantly influenced by factors within their living environments. In particular, events perceived by the animals as stressful (such as parturition, transport or handling) have been found to affect susceptibility to infection. It has been well documented that even minor stress such as weighing can result in an increase in colonisation and faecal shedding of enteric pathogens such as Salmonella enterica and Escherichia coli O157:H7. Such infections affect both ruminant overall health and therefore performance, and are a particular problem for the meat production industries. Prior explanations for stress enhancing the likelihood of infection is that activation of the sympathetic nervous system under stress leads to the release of neuroendocrine mediators such as the catecholamine stress hormones noradrenaline and adrenaline, which may impair innate and adaptive immunity. More recently, however, another equally compelling explanation, viewed through the lens of the newly recognised microbiological discipline of microbial endocrinology is that the myriad of bacteria within the ruminant digestive tract are as responsive to the hormonal output of stress as the cells of their host. Work from our laboratories has shown that enteric pathogens have evolved systems for directly sensing stress hormones. We have demonstrated that even brief exposure of enteric pathogens to physiological concentrations of stress hormones can result in massive increases in growth and marked changes in expression of virulence factors such as adhesins and toxins. Happy, less stressed ruminants may therefore be better-nourished animals and safer sources of meat. This article reviews evidence that stress, as well as affecting nutrition, in ruminants is correlated with increased risk of enteric bacterial infections, and examines the molecular mechanisms that may be at work in both processes.

Assessment of a new selective chromogenic Bacillus cereus group plating medium and use of enterobacterial autoinducer of growth for cultural identification of Bacillus species.

A new chromogenic Bacillus cereus group plating medium permits differentiation of pathogenic Bacillus species by colony morphology and color. Probiotic B. cereus mutants were distinguished from wild-type strains by their susceptibilities to penicillin G or cefazolin. The enterobacterial autoinducer increased the sensitivity and the speed of enrichment of B. cereus and B. anthracis spores in serum-supplemented minimal salts medium (based on the standard American Petroleum Institute medium) and buffered peptone water.

Resuscitation of Salmonella enterica serovar typhimurium and enterohemorrhagic Escherichia coli from the viable but nonculturable state by heat-stable enterobacterial autoinducer.

Salmonella enterica serovar Typhimurium and enterohemorrhagic Escherichia coli were stressed by prolonged incubation in water microcosms until it was no longer possible to observe colony formation when samples were plated on nonselective medium. Overnight incubation of samples in nutrient-rich broth medium supplemented with growth factors, however, allowed resuscitation of stressed and viable but nonculturable cells so that subsequent plating yielded observable colonies for significantly extended periods of time. The growth factors were (i) the trihydroxamate siderophore ferrioxamine E (for Salmonella only), (ii) the commercially available antioxidant Oxyrase, and (iii) the heat-stable autoinducer of growth secreted by enterobacterial species in response to norepinephrine. Analysis of water microcosms with the Bioscreen C apparatus confirmed that these supplements enhanced recovery of cells in stressed populations; enterobacterial autoinducer was the most effective, promoting resuscitation in populations that were so heavily stressed that ferrioxamine E or Oxyrase had no effect. Similar results were observed in Bioscreen analysis of bacterial populations stressed by heating. Patterns of resuscitation of S. enterica serovar Typhimurium rpoS mutants from water microcosms and heat stress were qualitatively similar, suggesting that the general stress response controlled by the sigma(s) subunit of RNA polymerase plays no role in autoinducer-dependent resuscitation. Enterobacterial autoinducer also resuscitated stressed populations of Citrobacter freundii and Enterobacter agglomerans.

Cytokines and the pathogenesis of nosocomial pneumonia.

BACKGROUND: Nosocomial pneumonia (NP) in injured patients is a significant clinical problem. We hypothesize that the pathogenesis of NP in injured patients involves an imbalanced cytokine response within the alveolar airspace that may inhibit effector cell function. METHODS: Proinflammatory (IL-8) and anti-inflammatory (IL-10) levels were measured in bronchoalveolar lavage (BAL) fluid from multitrauma patients on admission, 24, 48, and 72 hours post-injury and following lipopolysaccharide (LPS) induction of alveolar cells. Patients were compared based on IL-8 levels and the development of NP. RESULTS: A high level of IL-8 on admission was associated with the development of NP. In addition, levels of IL-8 were significantly greater in NP-positive patients at all time points. The IL-10 levels decreased from admission values in NP-negative patients but increased in NP-positive patients. Furthermore, a high level of IL-10 ( > 120 pg/mL) at 72 hours post-injury was associated with the development of NP. Alveolar cells from NP-positive patients produced significantly more IL-10 in response to LPS than cells from NP-negative patients. CONCLUSIONS: The pathogenesis of NP in injured patients involves an early and severe IL-8 process within the lung followed by an exaggerated IL-10 response that may inhibit effector cell function.

Catecholamine inotropes as growth factors for Staphylococcus epidermidis and other coagulase-negative staphylococci.

Drugs commonly used in intensive care settings were assayed for their ability to affect the growth of Staphylococcus epidermidis in a minimal salts medium containing 30% serum. Of 28 compounds tested, the inotropic catecholamines adrenaline, dobutamine, dopamine, isoprenaline and noradrenaline significantly stimulated bacterial growth. These drugs, but not structurally similar compounds lacking a dihydroxybenzoyl moiety (such as tyramine, phenylephrine and salbutamol), were able to remove iron from iron-saturated transferrin and to supply transferrin-bound 55Fe to S. epidermidis cells. Similar results were observed with a range of coagulase-negative staphylococci associated with line infections, but not with Staphylococcus aureus (including MRSA).

The mammalian neuroendocrine hormone norepinephrine supplies iron for bacterial growth in the presence of transferrin or lactoferrin.

Norepinephrine stimulates the growth of a range of bacterial species in nutritionally poor SAPI minimal salts medium containing 30% serum. Addition of size-fractionated serum components to SAPI medium indicated that transferrin was required for norepinephrine stimulation of growth of Escherichia coli. Since bacteriostasis by serum is primarily due to the iron-withholding capacity of transferrin, we considered the possibility that norepinephrine can overcome this effect by supplying transferrin-bound iron for growth. Incubation with concentrations of norepinephrine that stimulated bacterial growth in serum-SAPI medium resulted in loss of bound iron from iron-saturated transferrin, as indicated by the appearance of monoferric and apo- isoforms upon electrophoresis in denaturing gels. Norepinephrine also caused the loss of iron from lactoferrin. The pharmacologically inactive metabolite norepinephrine 3-O-sulfate, by contrast, did not result in iron loss from transferrin or lactoferrin and did not stimulate bacterial growth in serum-SAPI medium. Norepinephrine formed stable complexes with transferrin, lactoferrin, and serum albumin. Norepinephrine-transferrin and norepinephrine-lactoferrin complexes, but not norepinephrine-apotransferrin or norepinephrine-albumin complexes, stimulated bacterial growth in serum-SAPI medium in the absence of additional norepinephrine. Norepinephrine-stimulated growth in medium containing (55)Fe complexed with transferrin or lactoferrin resulted in uptake of radioactivity by bacterial cells. Moreover, norepinephrine-stimulated growth in medium containing [(3)H]norepinephrine indicated concomitant uptake of norepinephrine. In each case, addition of excess iron did not affect growth but significantly reduced levels of radioactivity ((55)Fe or (3)H) associated with bacterial cells. A role for catecholamine-mediated iron supply in the pathophysiology of infectious diseases is proposed.

In vivo adaptation of attenuated Salmonella typhimurium results in increased growth upon exposure to norepinephrine.

Two studies were conducted to determine whether attenuated strains of Salmonella typhimurium, currently being investigated as possible vectors for mucosal vaccines, are able to respond to norepinephrine (NE). Bacteria were tested for NE responsiveness before and for 1 week after passage through juvenile rhesus monkeys. NE significantly increased the growth of the attenuated bacteria after being shed from the animal, but not before animal infection. Follow-up in vitro tests were performed by passaging the bacteria in Lauria-Bertani (LB) broth with or without selective antibiotic for the attenuation insert and supplementing with NE. NE increased the growth of bacteria passaged in LB broth with no selective antibiotic, but not in bacteria passaged in LB broth with selective antibiotic. These results show that the attenuated bacteria assumed to be safe for use as a vaccine are able to respond to environmental stimuli, such as NE, and change their characteristics. The results suggest that there may be problems with the stability of attenuated bacteria used as vectors for mucosal vaccines.

Effects of social conflict on immune responses and E. coli growth within closed chambers in mice.

Social conflict has been shown to affect the neuroendocrine stress response in rodents. The current study was designed to characterize the effects of social conflict on leukocyte subset distribution and function as well as in vivo bacterial growth. Male DBA/2 mice implanted or not implanted with a closed chamber containing Escherichia coli were repeatedly challenged by temporary placement in the territory of a dominant CF-1 mouse five times a day for 2 consecutive days. Nonstressed animals were similarly handled, but were not exposed to social conflict. Effects on immune responses and E. coli growth were analyzed 13 h after the last social conflict session. Social conflict alone was associated with an increase in plasma corticosterone concentration and decreases in thymocyte numbers and splenocyte ability to proliferate in vitro in the presence of lipopolysaccharide (p < 0.05). After social conflict, immature CD4+CD8+ thymocytes decreased, whereas mature T cells increased (p < 0.05). In the presence of E. coli, social conflict induced a significant increase in plasma concentration of interleukin-1beta, and a decrease in the number of thymocytes and the percentage of CD4+CD8+ T cells in the thymus (p < 0.05). In addition to the lymphocyte subpopulation changes observed with social conflict alone, the proportion of CD3+ and major histocompatibility complex (MHC) class II IAd+ cells were significantly higher in stressed mice implanted with a closed chamber containing E. coli (p < 0.05). Social conflict tended to favor E. coli growth in the closed chamber, indicating possible direct bacterial-neuroendocrine hormone interactions. Taken together, these results suggest that stress may modulate the host immune response by altering both bacterial growth and resistance to infection.

Stimulation of bacterial growth by heat-stable, norepinephrine-induced autoinducers.

The ability of norepinephrine to increase the growth of Escherichia coli in a serum-based medium has previously been shown to be due to the production of an autoinducer of growth during early log phase. Seventeen Gram-negative and 6 Gram-positive clinical isolates were examined for a similar ability to respond to norepinephrine, and to synthesise autoinducer. The majority of Gram-negative strains both produced and responded to heat-stable norepinephrine-induced autoinducers of growth. Most of these autoinducers showed a high degree of cross-species activity, suggesting the existence of a novel family of Gram-negative bacterial signalling molecules. In contrast, although a number of Gram-positive strains were able to respond to norepinephrine, the majority failed to produce autoinducers in the presence of norepinephrine.

Anxiogenic effect of subclinical bacterial infection in mice in the absence of overt immune activation.

Challenge of animals with infectious microorganisms is well documented to affect a number of behavioral measures through activation of immune-neural mechanisms. In the present study, the ability of an infectious microorganism to directly alter behavioral responses in the absence of an overt immunologic response was examined. Eight-week-old CF-1 male mice were infected orally with the Gram-negative pathogen Campylobacter jejuni in order to establish a subclinical infection that did not result in immune activation. Microbiological examination of cecal contents revealed the presence of C. jejuni in all infected, but not control, animals 1 and 2 days post-oral challenge. Measurement of interleukin-6 (IL-6) levels and peripheral blood leukocyte populations did not reveal the activation of an overt immune response in 1 or 2 day infected animals as compared to controls. Infected mice demonstrated altered levels of anxiety-like behaviors on the elevated plus-maze as compared to controls on Day 2, but not Day 1, as reflected by a significant decrease in exploratory and an increase in nonexploratory behaviors. The anxiogenic effect of a subclinical infection in the absence of an overt immunologic response suggests that the direct activation of neural pathways by microorganisms may play a role in behavior.

Induction of gram-negative bacterial growth by neurochemical containing banana (Musa x paradisiaca) extracts.

Bananas contain large quantities of neurochemicals. Extracts from the peel and pulp of bananas in increasing stages of ripening were prepared and evaluated for their ability to modulate the growth of non-pathogenic and pathogenic bacteria. Extracts from the peel, and to a much lesser degree the pulp, increased the growth of Gram-negative bacterial strains Escherichia coli O157:H7, Shigella flexneri, Enterobacter cloacae and Salmonella typhimurium, as well as two non-pathogenic E. coli strains, in direct relation to the content of norepinephrine and dopamine, but not serotonin. The growth of Gram-positive bacteria was not altered by any of the extracts. Supplementation of vehicle and pulp cultures with norepinephrine or dopamine yielded growth equivalent to peel cultures. Total organic analysis of extracts further demonstrated that the differential effects of peel and pulp on bacterial growth was not nutritionally based, but due to norepinephrine and dopamine. These results suggest that neurochemicals contained within foodstuffs may influence the growth of pathogenic and indigenous bacteria through direct neurochemical-bacterial interactions.

Neuroendocrine-bacterial interactions in a neurotoxin-induced model of trauma.

BACKGROUND: The destruction of noradrenergic nerve cell innervation and resultant release of norepinephrine into the systemic circulation accompany severe tissue trauma. To examine whether destruction of noradrenergic neurons may directly influence the growth of indigenous bacteria in vivo, the selective noradrenergic neurotoxic agent 6-hydroxydopamine (6-OHDA) was employed in a murine model of trauma-induced norepinephrine release. MATERIALS AND METHODS: Following 6-OHDA administration, the cecums of 6- to 8-week-old male CF-1 mice were excised and examined for total bacterial counts and identification of bacterial species present in both the luminal space and intestinal wall. Lipopolysaccharide levels were also measured. RESULTS: An increase of 3-5 logs in the total gram-negative population, most notably Escherichia coli, compared to controls on a per gram equivalent basis was observed at 1 day post-6-OHDA. Neurotoxin-induced alterations in cecal flora were completely inhibited by the prior administration of the catecholamine uptake blocker desipramine hydrochloride, indicating the specificity of the effect being due to the released norepinephrine. Within 14 days following chemical sympathectomy, during which regeneration of noradrenergic neurons occurs, the cecal flora returned to the distribution observed in controls. Levels of lipopolysaccharide were not increased in either the luminal contents or cecal tissue at any of the time points. CONCLUSIONS: These results suggest that the destruction of noradrenergic neurons during trauma and consequent release of norepinephrine into the systemic circulation may influence the in vivo growth of the indigenous bacterial population within the gastrointestinal system.

Norepinephrine-induced expression of the K99 pilus adhesin of enterotoxigenic Escherichia coli.

This study examined whether the provision of norepinephrine, as would be encountered within the highly innervated gastrointestinal system, affected the growth rate of enterotoxigenic Escherichia coli (ETEC) and the expression of the K99 pilus adhesin virulence-related factor. The addition of norepinephrine to serum-containing medium resulted in a 3- to 7-fold increase in the growth rate of the K99+ ETEC strain B44 as compared to growth in vehicle supplemented medium or medium supplemented with normetanephrine, a norepinephrine metabolite that contains one more methyl group than norepinephrine. ELISA analysis revealed that K99 pilus adhesin expression was increased in norepinephrine supplemented culture as compared to normetanephrine and vehicle supplemented controls. This increase occurred from 9 to 15 hours of incubation which represented the exponential growth phase for the norepinephrine supplemented culture. These results indicate that addition of norepinephrine affects both ETEC growth and expression of a specific virulence factor.

Alteration of Escherichia coli O157:H7 growth and molecular fingerprint by the neuroendocrine hormone noradrenaline.

The neuroendocrine hormone noradrenaline has previously been shown to increase the growth of Gram-negative bacteria. To determine whether these results were due to noradrenaline-induced genetic rearrangements, arbitrarily-primed PCR utilizing a large number of primers of varying G + C content was performed on Escherichia coli O157:H7 grown in serum-based media at low initial inocula in order to approximate in vivo conditions. In addition to increased growth, bacteria grown in noradrenaline supplemented media displayed a different molecular fingerprint compared with growth in non-noradrenaline supplemented media with a minority of the primers tested. Differences in fingerprint patterns between noradrenaline supplemented and non-supplemented growth bacteria were greatest in the late logarithmic to early stationary phase of growth compared with the late stationary phase. These results suggest that the neuroendocrine hormonal environment of the host may provide environmental signals which lead to genetic rearrangements in the infectious agent and possibly alterations in subsequent pathogenicity.

Norepinephrine induced growth and expression of virulence associated factors in enterotoxigenic and enterohemorrhagic strains of Escherichia coli.

The small intestine is richly innervated by the sympathetic nervous system. High concentrations of monoamines, most notably norepinephrine, are found throughout the various intestinal layers. In order to determine whether norepinephrine is capable of influencing bacterial pathogenesis, the growth and production of virulence factors in ETEC and EHEC were examined in a physiologically relevant medium utilizing very low initial bacterial inoculums to more closely mimie in vivo conditions. The growth of ETEC strain B44 and the production of the K99 pilus adhesin on a protein equivalent basis was greatly increased in the presence of norepinephrine. Growth of EHEC O157:H7 was also increased in norepinephrine containing medium as well as production of SLT-I and SLT-II. The ability of norepinephrine to increase both bacterial growth and expression of virulence factors was shown to be non-nutritional in nature. Given the abundant adrenergic innervation in the small intestine, these in vitro results suggest that the neurohumoral environment of the host may play a role in bacterial growth and expression of virulence factors.

Production of Shiga-like toxins by Escherichia coli O157:H7 can be influenced by the neuroendocrine hormone norepinephrine.

To examine whether the neuroendocrine hormone norepinephrine may influence the production of the Shiga-like toxins (SLTs), several Escherichia coli O157:H7 clinical isolates were grown in the presence or absence of norepinephrine. An in vitro culture system consisting of low (<1500 colony-forming units/ml) initial concentrations of inocula into a serum-based medium was used to more closely approximate in vivo conditions. The growth of all isolates was increased several logs in the presence of norepinephrine, as compared with the growth in controls, during a 24-hour growth period. Controls included additional dextrose as well as the use of the norepinephrine metabolite normetanephrine, which contains one more methyl group than norepinephrine and hence would serve as a better energy source for growth if the effect were solely nutritionally mediated. During the 24 hours of growth, the production of cell-associated SLT-I on a protein-equivalent basis was shown to be increased over 100-fold in norepinephrine-cultured bacteria as compared with controls. SLT-II elaboration into culture supernatants was also greatly increased in norepinephrine-cultured bacteria as compared with controls. Maximal detection of cell-associated SLT-II occurred at least 12 hours before maximum levels were achieved in culture supernatants. Because norepinephrine represents one of the largest pools of monoamines present throughout the small intestine, these results suggest that the neuroendocrine environment of the small intestine may play a role in the growth of O157:H7 and the production of SLTs.