Infections, antibiotic treatment and the Microbiome in relation to schizophrenia

SCHRES-08580; No of Pages 7 Schizophrenia Research xxx (xxxx) xxx

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Infections, antibiotic treatment and the mircrobiome in relation to schizophrenia Amalie W. Klein-Petersen a,⁎, Ole Köhler-Forsberg a,b,c, Michael E. Benros a,⁎ a b c

Mental Health Centre Copenhagen, Copenhagen University Hospital, Denmark Psychosis Research Unit, Aarhus University Hospital - Psychiatry, Denmark Department of Clinical Medicine, Aarhus University, Denmark

a r t i c l e

i n f o

Article history: Received 23 September 2019 Received in revised form 19 November 2019 Accepted 22 November 2019 Available online xxxx Keywords: Infection Antibiotic Intestinal microbiome Microbiota Schizophrenia

a b s t r a c t Schizophrenia is a heterogeneous disorder with several potential pathophysiological mechanisms, including immune activation. Infections have been identified as a significant contributing risk factor for schizophrenia; this association is reviewed together with the potential impact of antibiotic treatment and alterations of the intestinal microbiota. Both infections and the treatment with antibiotics may alter the composition of the gut microbiota, causing dysbiosis, which in animal studies has been associated with alterations of behavior. Of the few studies that have been conducted on humans, some have suggested alterations in the microbial composition of individuals with schizophrenia compared to healthy controls, albeit with conflicting results. Recently, increased attention has emerged regarding potential adverse effects from antibiotics, as a number of these have been associated with an increased risk of psychotic episodes. Particularly, the fluoroquinolones have been associated with neurotoxic adverse events. The association between schizophrenia and infections, antibiotic treatment and dysbiosis, may be an epiphenomenon, which could be explained by other confounding factors. However, these associations could be causal and could therefore be important risk factors in a subgroup of patients. Large-scale well-matched longitudinal studies are needed with measurements of immune markers from multiple biological samples, ranging from material close to the brain, as cerebrospinal fluid and brain-scans targeting neuroinflammation, to analysis of blood and intestinal microbiota. This would help to obtain more definite results on the association between infections, immune components and microbiota alterations in relation to schizophrenia. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Schizophrenia is a severe mental disorder with a high heritability but the etiological origin is still vastly unknown (Sullivan et al., 2003). In addition to the strong genetic component, the environmental impact and gene-environmental interactions are believed to be important risk factors (van Os et al., 2008). Amongst environmental contributing factors to the development schizophrenia, particularly infections (Arias et al., 2012; Benros et al., 2012; Khandaker et al., 2012; KohlerForsberg et al., 2018; Kohler et al., 2017; Lydholm et al., 2019; Nielsen et al., 2014, 2016; Torrey et al., 2012) and immune dysregulation have been highlighted in recent years. Anti-infective agents (Bhattacharyya et al., 2016; Grill and Maganti, 2011; Kohler et al., 2017; Slykerman et al., 2017; Tandan et al., 2018; Yolken and Torrey, 2008;

⁎ Corresponding authors at: Mental Health Centre Copenhagen, Copenhagen University Hospital, Gentofte Hospital, Kildegaardsvej 28, Entrance 15, 4th floor, 2900 Hellerup, Denmark. E-mail addresses: [email protected] (A.W. Klein-Petersen), [email protected] (M.E. Benros).

Zareifopoulos and Panayiotakopoulos, 2017) and microbiome alterations (Castro-Nallar et al., 2015; Nguyen et al., 2018; Schwarz et al., 2018) have also been suggested as potential contributing factors. Support for the immunological-dysregulation in schizophrenia comes from large-scale genome-wide association studies. Some of these studies report increased polymorphisms in immune-regulatory genes, including the major histocompatibility complex regions on chromosome 6, as well as a specific variant of the gene C4 that encodes the complement protein C4, amongst patients with schizophrenia (Pardiñas et al., 2018; Schizophrenia Working Group of the Psychiatric Genomics Consortium, 2014; Sekar et al., 2016; Shi et al., 2009). Infections and an activated immune response may compromise the blood brain barrier (BBB), permitting inflammatory markers to enter the brain. These may potentially contribute to an increased risk of mental disorders amongst a subgroup of patients with schizophrenia (Khandaker et al., 2012, 2015; Wang and Miller, 2018). Serum cytokines have been found to be at significantly elevated levels in medicationnaïve patients with first episode psychosis compared to healthy controls (Upthegrove et al., 2014). However, the findings may be partly explained through a more sedentary and unhealthier lifestyle amongst

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Please cite this article as: A.W. Klein-Petersen, O. Köhler-Forsberg and M.E. Benros, Infections, antibiotic treatment and the mircrobiome in relation to schizophrenia, Schizophrenia Research, https://doi.org/10.1016/j.schres.2019.11.033

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patients with schizophrenia. Cytokine alterations may vary according to the patient's clinical status, as interleukin-1 β, IL-6 and Transforming growth factor-β have been observed at significantly increased levels during acute exacerbations of schizophrenia, whilst normalized during antipsychotic treatment (Miller et al., 2011). Further, a meta-analysis reported elevated levels of IL-6 and IL-8 in the CSF (Orlovska-Waast et al., 2018) in patients with schizophrenia compared to healthy controls. Moreover, CSF meta-analysis have indicated increased albumin ratios, indicating a dysfunctional BBB in this patient group (OrlovskaWaast et al., 2018). The gut-brain axis has drawn increased focus over the past years. The gut contains approximately 70% of the immune system cells within the gut-associated lymphoid tissue (Vighi et al., 2008), whilst the gastrointestinal (GI) tract has been estimated to have at least 100 trillion (1 × 1014) microorganisms, containing up to a thousand distinct bacterial species, along with viruses, fungi, and archaea (Gill et al., 2006). Altogether, the complex community of the microbiota contains 150 times as many genes as the human genome (Gill et al., 2006) and plays a crucial role in maintaining homeostasis (Clemente et al., 2012; Gill et al., 2006; Hooper et al., 2012). Some studies suggest a significantly altered microbial diversity in patients with schizophrenia compared to nonpsychiatric controls (Castro-Nallar et al., 2015; Cuomo et al., 2018; Nguyen et al., 2018, 2019; Schwarz et al., 2018; Yolken et al., 2015; Yuan et al., 2018) and a dysregulation of the microbiota, dysbiosis, has been associated with behavioral alterations in rodent studies (Bercik et al., 2011; Leclercq et al., 2017; Tochitani et al., 2016). This review aims to provide an overview of the association between infections, antibiotic treatment and microbiome alterations in connection with schizophrenia. 2. Association between infections and the risk of developing schizophrenia Severe infections requiring hospitalization have been associated with an increased risk of schizophrenia spectrum disorders by 60% (Benros et al., 2011). The risk increases proportionally to a doseresponse relationship with the number of infections, where three or more hospital contacts increased the incidence rate ratio of schizophrenia by 2.56-times (Benros et al., 2011). Further, an epidemiological study found that 45% of patients with schizophrenia had hospital contact with an infection prior to their schizophrenia diagnosis (Nielsen et al., 2014). In relation to specific infectious agents, meta-analyses have found significant associations between patients with schizophrenia and the increased prevalence of Herpes simplex virus (HSV)-1 and -2 (Arias et al., 2012; Khandaker et al., 2013; Tucker and Bertke, 2019), Borna Disease Virus and Human Endogenous Retrovirus-W (retroviruses characterized by integration in the genome) (Arias et al., 2012; Azami et al., 2018). Amongst the pathogenic agents most frequently associated with schizophrenia is the parasite Toxoplasma gondii (Arias et al., 2012; Gutiérrez-Fernández et al., 2015; Monroe et al., 2015; Sølvsten Burgdorf et al., 2019; Torrey et al., 2012; Yolken and Torrey, 2008). Meta-analyses, including studies from areas with low and high prevalence of Toxoplasmosis gondii, found an odds ratio of 2.73 for developing schizophrenia when exposed to the parasite (Torrey et al., 2007, 2012). However, after adjusting for publication bias, the odds ratio was reduced to 1.43 in a more recent meta-analysis (Sutterland et al., 2015). A recent large-scale Danish serological case-control study examined the temporality of Toxoplasma gondii exposure in blood donors and their corresponding psychiatrics outcomes. The study found that toxoplasma infection was associated with schizophrenia by an odds ratio of 1.47 (Sølvsten Burgdorf et al., 2019). The same study found higher levels of Cytomegalovirus (CMV) associated with any psychiatric disorder, although not significantly associated with schizophrenia specifically (Sølvsten Burgdorf et al., 2019). In patients diagnosed with schizophrenia, the presence of CMV has been found to impair cognitive function (Shirts et al., 2008), possibly intensifying psychiatric

symptoms (Mohagheghi et al., 2018). Moreover, increased prevalence of both Chlamydophila psittaci and Chlamydophila pneumoniae has been found in the blood and brains of patients compared with healthy controls (Arias et al., 2012; Fellerhoff et al., 2007; Gutiérrez-Fernández et al., 2015). Childhood infections in general (Blomstrom et al., 2014; Kohler-Forsberg et al., 2018), childhood CNS infections (Abrahao et al., 2005; Blomstrom et al., 2014; Dalman et al., 2008; Khandaker et al., 2012; Rantakallio et al., 1997), and Epstein-barr virus (Khandaker et al., 2014) have been associated with an increased risk of schizophrenia and psychotic experiences, as well as mental disorders throughout adolescence and adulthood. Furthermore, urinary tract infections during childhood (Bayliss et al., 2017; Carson et al., 2017) and adulthood (Graham et al., 2014) have been associated with an increased vulnerability to develop schizophrenia or psychosis. Studies have suggested that maternal infections during pregnancy may increase the risk of developing schizophrenia in the offspring. During fetal life, certain maternal viral, bacterial and parasitic infections (Khandaker et al., 2013), such as toxoplasmosis (Arias et al., 2012; Brown et al., 2009; Khandaker et al., 2013; Mortensen et al., 2007), rubella (Brown et al., 2001), HSV-1, HSV-2, CMV (Buka et al., 2001; Khandaker et al., 2013; Mortensen et al., 2010) and influenza (Brown et al., 2009; Cai et al., 2015) have been associated with an increased risk of psychotic disorders in the offspring. However, the results have been conflicting and other meta-analyses have not found a similar association between schizophrenia and influenza (Selten and Termorshuizen, 2017). Instead, some studies have found associations between maternal bacterial infection in the first trimester of pregnancy and offspring risk of schizophrenia (Sørensen et al., 2009), as well as between maternal fever and genitourinary infections at any point in pregnancy with increased risk of psychosis-like experiences (about 10–15%) amongst offspring (Dreier et al., 2018). Large epidemiological studies have found an increased risk of schizophrenia after maternal exposure to infections during pregnancy (Lydholm et al., 2019; Nielsen et al., 2013). However, the risk was similarly increased for maternal infections occurring outside the pregnancy period (Lydholm et al., 2019; Nielsen et al., 2013). Other epidemiological studies on maternal infection during pregnancy did not find an increased risk of psychotic disorders in offspring (AlHaddad et al., 2019; Blomström et al., 2016); instead, they observed associations in the offspring with a maternal history of mental disorder (Blomström et al., 2016). 3. Association between antibiotic treatment with psychosis and schizophrenia Serious adverse drug reactions affecting the CNS have been reported in less than 1% of all antibiotic classes (Bhattacharyya et al., 2016). Given the widespread use of antibiotics and the few cases of antibioticinduced psychosis, the phenomenon is considered rare (Grill and Maganti, 2011). Nonetheless, neurotoxic effects of antibiotics have reportedly been underestimated, as the symptoms from patients receiving antibiotics may arise from multiple potential causes, with an obvious cause being infection induced delirium (Grill and Maganti, 2011). Recent studies have suggested that antibiotic-induced neurotoxic effects can occur from therapeutical doses (Bhattacharyya et al., 2016; Grill and Maganti, 2011; Zareifopoulos and Panayiotakopoulos, 2017). The adverse effects of antibiotics include confusion and encephalopathy (Grill and Maganti, 2011; Tomé and Filipe, 2011a, 2011b), the latter of which may induce psychotic reactions, seizures, myoclonus, magnetic resonance imaging and electroencephalography changes (Bhattacharyya et al., 2016). A review of 391 cases with antibioticassociated encephalopathy (Bhattacharyya et al., 2016) found psychosis present in 47% of the cases, most of which were associated with sulfonamides, fluoroquinolones, macrolides and intramuscular penicillin treatment. Symptoms evolved during the first days after the initiation of therapy, with resolution typically ranging from 2 to 10 days after

Please cite this article as: A.W. Klein-Petersen, O. Köhler-Forsberg and M.E. Benros, Infections, antibiotic treatment and the mircrobiome in relation to schizophrenia, Schizophrenia Research, https://doi.org/10.1016/j.schres.2019.11.033

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discontinuation depending on the class of antibiotic (Bhattacharyya et al., 2016; Warstler and Bean, 2016). A meta-analysis of Randomized controlled trials (RCTs) found significantly higher risk of CNS-related adverse effects in fluoroquinolones compared to other antimicrobial agents, specifically macrolides, cefuroxime axetil and co-amoxiclav. Fluoroquinolones are able to cross the BBB and inhibit gamma-aminobutyric acid (GABA) receptors, a major inhibitory neurotransmitter in the CNS, causing a glutamate up-regulation and stimulation of the CNS (Tomé and Filipe, 2011a, 2011b). In line with this, fluoroquinolones have also been linked to severe neurological side effects, including delirium and psychosis in several case reports (Bhattacharyya et al., 2016; Grimm et al., 2007; James and Demian, 1998; Owens and Ambrose, 2005; Ranjan and Praharaj, 2014; Reeves, 2007; Tomé and Filipe, 2011a, 2011b). A systematic review found the treatment of urinary tract infections with fluoroquinolones, penicillin or trimethoprim-sulfamethoxazole to be associated with development of psychosis (Mostafa and Miller, 2014); however, the significance of the results is limited, as they were based on 15 cases of psychosis where 53% had a psychiatric history (including previous antibiotic-associated psychosis (n = 2)). Onset and resolution of psychosis occurred within one week of treatment initiation and discontinuation (Mostafa and Miller, 2014). Particularly betalactams, fluoroquinolones and the macrolide clarithromycin act as CNS stimulants via GABA-A agonism, potentially triggering mania or seizures in rare cases (Zareifopoulos and Panayiotakopoulos, 2017). Several studies have investigated the use of anti-infective agents as a proxy for infection in relation to the risk of psychosis and schizophrenia (Kohler-Forsberg et al., 2018; Kohler et al., 2017; Lurie et al., 2015; Wong et al., 2016). Thus, it cannot be determined whether the neurotoxic adverse effect was due to the infection or the treatment with anti-infective agents. A Danish population-based study found an association between antibiotic exposure, particularly broad-spectrum antibiotic (Kohler et al., 2017). The authors reported an increased risk of mental disorders amongst children and adolescents (Kohler et al., 2017), also when compared to antivirals, antimycotics and antiparasitic, which had no significant effect on the risk estimates (Kohler-Forsberg et al., 2018). The risk of mental disorders was augmented in a doseresponse relationship with the quantity of infections and treatment with different anti-infective groups (Kohler-Forsberg et al., 2018). A nested case-control study from the United Kingdom (n = 2690 with psychosis) found an increased risk of anxiety and depression, though not psychosis, when exposed to antibiotics (Lurie et al., 2015). The study's primary area of interest was any applied antibiotic treatment that had occurred more than one year prior to index date of the psychiatric diagnosis. Furthermore, a population-based study from Hong Kong on treatment of Helicobacter pylori found a short-term increased risk of neuropsychiatric events associated with the use of clarithromycin, as well as an increased incidence rate ratio of psychotic events of 5.42 (95% CI, 2.77–10.60) during treatment with clarithromycin (Wong et al., 2016). However, it is worth noting that the register-based studies were built on registered redemptions of prescriptions and do not necessarily indicate the actual consumption, as compliance amongst patients for completing the antibiotic regimes as prescribed is estimated at a mean of 62.2% according to meta-analysis of (Kardas et al., 2005). 4. Schizophrenia and the intestinal microbiome Recent years studies have explored the bidirectional communication between the gut microbiota and the CNS (Cryan and Dinan, 2012; Dinan and Cryan, 2017). Alterations of the microbial composition have been associated with several diseases, ranging from local GI-disorders to respiratory, cardiovascular and neurological diseases (Lynch and Pedersen, 2016). The microbiota is susceptible to endogenous and exogenous factors including genetics (Goodrich et al., 2017), delivery mode (Rutayisire et al., 2016), diet (David et al., 2014), along with infections and anti-infective treatment (Severance et al., 2016; Sullivan et al.,

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2001). Broad spectrum antibiotics significantly decrease the taxonomic richness and diversity of the intestinal microbiota (Dethlefsen et al., 2008) even for short term exposure (Doan et al., 2017; Sullivan et al., 2001), which may cause long-term alterations ranging from months (Palleja et al., 2018) to several years post-treatment (Jakobsson et al., 2010; Jernberg et al., 2007). Interestingly, a recent study extended the classic gene set enrichment analysis approach using published genome-wide association study (GWAS) data of 33,426 cases with schizophrenia and 32,541 healthy controls. This study found an association between specific genus levels with schizophrenia such as the genus Desulfovibrio and Bacteroides, suggesting these may contribute in the development of schizophrenia (Cheng et al., 2019). In general, studies on the intestinal microbiome's impact on schizophrenia spectrum disorders consist of small sample sizes (Nguyen et al., 2018), but collectively report alterations in the microbial diversity of individuals with schizophrenia compared to healthy controls, although with conflicting results (Castro-Nallar et al., 2015; He et al., 2018; Nguyen et al., 2019; Schwarz et al., 2018; Shen et al., 2018; Yolken et al., 2015; Yuan et al., 2018). A set of 12 significant microbiota biomarkers were significantly different in a study of patients with the schizophrenia diagnosis less than 10 years (n = 64) compared to healthy controls (n = 53) and the authors point to a potential future for microbiota-based diagnosis for schizophrenia (Shen et al., 2018). However, the study was a cross-sectional study with a small sample size that could not eliminate the effect of antipsychotic treatment on the intestinal microbiota. One study observed increased levels of Lactobacillales in patients with ultra-high risk of schizophrenia (cases = 19, healthy controls = 69) (He et al., 2018), whilst another reported decreased levels of Lactobacillus in patients with first-episode psychosis (cases = 41, healthy controls = 41) (Yuan et al., 2018). In fact, the microbial composition, particularly the family of Lactobacillus, has been significantly correlated with the severity of psychotic symptoms in a study on cases with firstepisode psychosis (n = 28) (Schwarz et al., 2018). Further, a lower rate of disease remission was found in patients with the greatest difference in microbiota composition compared to healthy controls (n = 16) at the one-year follow-up (Schwarz et al., 2018). Additionally, decreased levels of Bifidobacterium and Escherichia coli have been observed in patients compared to healthy controls, which increased post antipsychotic treatment (24 weeks risperidone), whilst Lactobacillus levels decreased (Yuan et al., 2018). With regards to the psychopathology, a study on patients (n = 25) with more than ten years of illness (range: 12–56 years) found fewer negative symptoms associated with decreased levels of Ruminococcaceae (clostridia family) (Nguyen et al., 2019). The study further reported increased depressive symptoms correlated with a greater presence of genus Bacteroides (n = 25) (Nguyen et al., 2019). Whilst a healthy microbiome is characterized by a high microbial diversity (Yatsunenko et al., 2012), a decreased biodiversity of the oropharyngeal microbiome has been observed in patients with schizophrenia (n = 16) compared to controls (n = 16) (Castro-Nallar et al., 2015). Further, a study of the oropharyngeal bacteriophages (viruses) indicated elevated levels of the phage, Lactobacillus phiadh in patients with schizophrenia (n = 41) compared with non-psychiatric controls (n = 33), which was positively associated with increased risk of co-morbid immunological diseases (Yolken et al., 2015). RNA-sequencing has demonstrated the translocation of intestinal microbial products to the systemic circulation in patients with schizophrenia (n = 48) compared to non-psychiatric controls (n = 49) (Olde Loohuis et al., 2018). Further, this study pointed to an increased microbial diversity in the blood of the patients (Olde Loohuis et al., 2018). Further, increased levels of a phyla closely related to the Chlamydia phylum has been observed in individuals with schizophrenia compared to controls (Olde Loohuis et al., 2018). Other clinical studies have investigated the permeability of the GI-tract-based blood

Please cite this article as: A.W. Klein-Petersen, O. Köhler-Forsberg and M.E. Benros, Infections, antibiotic treatment and the mircrobiome in relation to schizophrenia, Schizophrenia Research, https://doi.org/10.1016/j.schres.2019.11.033

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biomarkers of microbial translocation (Nguyen et al., 2018; Severance et al., 2013). A serological surrogate marker of bacterial translocation, soluble cluster of differentiation (sCD) 14, is found more prevalently (3.1-fold increased odds) amongst patients with schizophrenia compared to healthy controls (Severance et al., 2013). Both sCD14 and lipopolysaccharide-binding protein were significantly correlated with C-reactive protein in individuals with schizophrenia, suggesting a common pathway of associated inflammation (Severance et al., 2013). Thus, a compromised gut barrier may enable microbes and surrogate markers to enter the systemic circulation and elicit a state of low-grade inflammation (Severance et al., 2013). 5. Probiotic treatment for schizophrenia? A recent meta-analysis of RCTs investigating probiotic treatment compared to placebo in patients with schizophrenia found no significant difference in psychotic (positive and negative) symptoms postintervention, based on the three included RCTs with small sample sizes (n = 172) (Ng et al., 2019). Interestingly, a systematic-review of seven RCTs on healthy adults (n = 315) concluded that the existing literature lacks sufficient evidence of probiotic's impact on the fecal microbial composition (Kristensen et al., 2016). However, a small openlabel, single-arm study on patients with schizophrenia (n = 29) found a potentially beneficial effects regarding negative symptoms after receiving Bifidobacterium for four weeks compared to baseline (Okubo et al., 2019). Given the limited scale of this study, the efficacy of probiotics in individuals with schizophrenia remains to be validated by future, larger clinical studies. 6. Potential etiological factors Relevant potential confounding factors, such as smoking, body mass index, diet, socioeconomic status, medical comorbidities and lifestyle, throughout both the prodromal phase and manifest schizophrenia, must be acknowledged, as these factors may influence the abovementioned associations on immune-related markers to a certain degree (Goldsmith et al., 2016; Miller et al., 2011; Orlovska-Waast et al., 2018; Wang and Miller, 2018). Also, substance abuse is a frequent comorbidity amongst patients with schizophrenia and this patient group is prone to acquiring severe infections, which influences immune-markers and the microbiome (Hunt et al., 2018). This, in turn, compromises the immune system, which may lead to an increased vulnerability to infections. Additionally, antipsychotic medication has been shown to alter several interleukins, as well as inhibit infective agents (Gutiérrez-Fernández et al., 2015; Miller et al., 2011; Orlovska-Waast et al., 2018). Clozapine, for instance, may increase IL-6 levels in patients (Miller et al., 2011). Haloperidol, risperidone and fluphenazine demonstrate anti-toxoplasmosis activity by inhibiting the replication in infected individuals (GutiérrezFernández et al., 2015). Although several novel insights on the human microbiome are emerging, interpretation of the literature generally remains inconsistent and inconclusive, as the inter- and intra-individual variability of the microbiome, study designs, drug treatment and lifestyle represent confounding elements influencing the outcome of each individual study. Nonetheless, growing evidence indicates dysregulations of the immune response within schizophrenia (Frydecka et al., 2018). Indeed, a singularly causal microbe in relation to the development of schizophrenia has not been identified. Instead, an epidemiological study reported an association between an increased risk of schizophrenia and infections, irrespective of the site of infection (Benros et al., 2011). This implies that the increased risk could be due to a general concurrent activation of the immune response for at least a subgroup of patients with schizophrenia. Furthermore, the study found clear temporal associations, with the risk of schizophrenia and psychosis increasing the most with the temporal proximity to the infection diagnosis (Benros et al., 2011). This could, in part, be due to increased health-seeking behavior in the prodromal phase.

However, an increased risk was also observed several years after the infection had occurred (Nørgaard et al., 2016). Peripheral inflammation and dysbiosis of the microbiota can affect the CNS through inflammatory/immune components, as cytokines may enter the CNS (Cuomo et al., 2018; Wang and Miller, 2018). The CNS may also be impacted through peripheral alterations of the metabolism of tryptophan and its metabolite, kynurenic acid, which is a N-methyl-D-aspartate receptor antagonist affecting the brain through the kynurenic pathway (Cuomo et al., 2018; Wang and Miller, 2018). Hypofunction of this receptor leads to a lack of glutamate neurotransmission, which has been proposed as a contributing factor of psychotic symptoms and cognitive dysfunction in schizophrenia (Khandaker et al., 2015; Wang and Miller, 2018). Moreover, individuals with schizophrenia had increased cerebrospinal fluid albumin ratios and total protein, suggestive of BBB dysfunction (OrlovskaWaast et al., 2018). Furthermore, a meta-analysis of postmortem studies of patients with schizophrenia found significantly increased levels of microglia and pro-inflammatory markers in brains compared to healthy controls (Pandey et al., 2018; Trépanier et al., 2016; van Kesteren et al., 2017). The effect has been found to be more pronounced in the temporal cortex, which is known to be involved in higher processing of sensory, emotional and cognitive information (van Kesteren et al., 2017). However, due to the heterogeneity across postmortem study designs, such as stage of disorder, brain region measured, medication, conclusive results on the potential association have not been obtained (Trépanier et al., 2016). Although evidence of the microbiome's impact on the brain and mental health is expanding considerably, the majority of studies performed have been based on a small group of patients, with study designs varying greatly. Nonetheless, evidence points to a reduced microbial diversity and a significant difference in the composition and diversity found in individuals with schizophrenia compared to healthy controls (Nguyen et al., 2018). A higher degree of microbial diversity is correlated with increased resistance towards infections and anti-inflammatory profiles, whereas dysbiosis is characterized by an upregulation of pro-inflammatory pathways (Li et al., 2016; Mu et al., 2016). The microbiome acts as a barrier against colonization of pathogens and overgrowth of organisms of the intestinal tract (Lynch and Pedersen, 2016); hence, a compromised gut barrier may enable microbes and surrogate markers to enter the systemic circulation and elicit a state of low-grade inflammation (Severance et al., 2013). The microbiome is substantially affected by several factors such as lifestyle and diet (Cryan and Dinan, 2012), which are wellknown to impact individuals with schizophrenia. Therefore, the associations with the microbiome may also be explained by these confounding factors. However, experimental mouse studies have revealed behavioral changes induced by antibiotic agents, where administration during pregnancy was associated with behavioral alterations in offspring (Tochitani et al., 2016). For instance, oral penicillin administration in early life of mice is associated with cytokine elevation in the frontal cortex, as well as increased aggressiveness and decreased anxiety (Leclercq et al., 2017). Further, when exposed to oral antibiotics, alterations in the behavior were observed (Bercik et al., 2011). Contrarily, no behavioral change was observed when exposed to intraperitoneal administration or amongst germ-free mice, thereby excluding a direct effect of antibiotics on the CNS and underlining the important role of the microbiota (Bercik et al., 2011). With regards to human microbiota, a transplant of fecal microbiota from schizophrenic patients to antibiotic-treated mice induced behavioral abnormalities in mice (psychomotor hyperactivity, impaired learning and memory) (Zhu et al., 2019) along with degradation of tryptophan pathways and decreased levels of dopamine (in prefrontal cortex and hippocampus) compared to control mice receiving transplants from healthy controls (Zhu et al., 2019). 7. Conclusion and perspectives Infections and antibiotic treatments have been associated with the etiology and pathology of schizophrenia in a subgroup of individuals.

Please cite this article as: A.W. Klein-Petersen, O. Köhler-Forsberg and M.E. Benros, Infections, antibiotic treatment and the mircrobiome in relation to schizophrenia, Schizophrenia Research, https://doi.org/10.1016/j.schres.2019.11.033

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The association between a wide range of infections and subsequent development of schizophrenia is well established through an extensive literature, whereas the associations observed between antibiotic exposure and neurotoxic adverse effects are only supported by a limited number of case reports, case-control-, and population-based-studies, with insufficient data from large RCTs. Furthermore, several of the included studies used antibiotic treatment a as proxy for infections; thus it cannot be determined whether the neurotoxic adverse effect was due to the infection or treatment. It remains essential to underline that the vast majority of individuals exposed to infections and antibiotics do not develop schizophrenia or other psychotic disorders. Moreover, alterations of the microbiota may play an essential role in mental health (Sullivan et al., 2001). Infections and gut dysbiosis are both preventable and treatable conditions, requiring increased attention when dealing with psychiatric patients, and may comprise a novel contribution in personalized treatment of schizophrenia. Life expectancy of patients with schizophrenia is on average 15–25 years lower than the general population, primarily due to somatic causes of death (Nordentoft et al., 2013). Thus, screening for somatic diseases and infections, preferably before initiation of antipsychotic medication, is of utmost importance for both the differential diagnostic process as well as the ability to provide the optimal treatment for individuals with onset symptoms of schizophrenia. In order to advance the field and to identify individuals with an immunological contribution to psychosis, large-scale well-matched longitudinal studies are needed with longitudinal measurements of immune markers from multiple biological samples, ranging from material close to the brain, as cerebrospinal fluid and brain-scans targeting neuroinflammation, to analysis of blood and intestinal microbiota. This would help to obtain more definite results on the association between infections, immune components and microbiota alterations in relation to schizophrenia. Addressing these issues a cross-disciplinary collaborative work is needed, including immunologic, genetic, microbiological and psychiatric expertise, contributing to a clarification of the etiological disease mechanisms behind potential subgroups of schizophrenia where the immune system may play a role. Moreover, future intervention studies should increasingly be based on immune markers to identify individuals more likely to benefit from anti-inflammatory treatment. Contributors MEB conceived and supervised the work. AWKP conducted the literature search and made the first draft. All authors have read and approved the manuscript.

Role of the funding source The sponsor had no role in the acquisition of the data, interpretation of the results or the decision to publish the findings. Declaration of competing interest We declare no conflicts of interest. Acknowledgements The work was funded by the Independent Research Fund Denmark (grant number 7025-00078B) and by an unrestricted grant from The Lundbeck Foundation (grant number R268-2016-3925). The sponsor had no role in the acquisition of the data, interpretation of the results or the decision to publish the findings.

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Please cite this article as: A.W. Klein-Petersen, O. Köhler-Forsberg and M.E. Benros, Infections, antibiotic treatment and the mircrobiome in relation to schizophrenia, Schizophrenia Research, https://doi.org/10.1016/j.schres.2019.11.033