Essential Amino Acid Metabolites as Chemical Mediators of Host-Microbe Interaction in the Gut.

Amino acids are indispensable substrates for protein synthesis in all organisms and incorporated into diverse aspects of metabolic physiology and signaling. However, animals lack the ability to synthesize several of them and must acquire these essential amino acids from their diet or perhaps their associated microbial communities. The essential amino acids therefore occupy a unique position in the health of animals and their relationships with microbes. Here we review recent work connecting microbial production and metabolism of essential amino acids to host biology, and the reciprocal impacts of host metabolism of essential amino acids on their associated microbes. We focus on the roles of the branched-chain amino acids (valine, leucine, and isoleucine) and tryptophan on host-microbe communication in the intestine of humans and other vertebrates. We then conclude by highlighting research questions surrounding the less-understood aspects of microbial essential amino acid synthesis in animal hosts.

Diversity of plant DNA in stool is linked to dietary quality, age, and household income.

Eating a varied diet is a central tenet of good nutrition. Here, we develop a molecular tool to quantify human dietary plant diversity by applying DNA metabarcoding with the chloroplast trnL-P6 marker to 1,029 fecal samples from 324 participants across two interventional feeding studies and three observational cohorts. The number of plant taxa per sample (plant metabarcoding richness or pMR) correlated with recorded intakes in interventional diets and with indices calculated from a food frequency questionnaire in typical diets (ρ = 0.40 to 0.63). In adolescents unable to collect validated dietary survey data, trnL metabarcoding detected 111 plant taxa, with 86 consumed by more than one individual and four (wheat, chocolate, corn, and potato family) consumed by >70% of individuals. Adolescent pMR was associated with age and household income, replicating prior epidemiologic findings. Overall, trnL metabarcoding promises an objective and accurate measure of the number and types of plants consumed that is applicable to diverse human populations.

The intestine is a major contributor to circulating succinate in mice.

The tricarboxylic acid (TCA) cycle is the epicenter of cellular aerobic metabolism. TCA cycle intermediates facilitate energy production and provide anabolic precursors, but also function as intra- and extracellular metabolic signals regulating pleiotropic biological processes. Despite the importance of circulating TCA cycle metabolites as signaling molecules, the source of circulating TCA cycle intermediates remains uncertain. We observe that in mice, the concentration of TCA cycle intermediates in the portal blood exceeds that in tail blood indicating that the gut is a major contributor to circulating TCA cycle metabolites. With a focus on succinate as a representative of a TCA cycle intermediate with signaling activities and using a combination of gut microbiota depletion mouse models and isotopomer tracing, we demonstrate that intestinal microbiota is not a major contributor to circulating succinate. Moreover, we demonstrate that endogenous succinate production is markedly higher than intestinal succinate absorption in normal physiological conditions. Altogether, these results indicate that endogenous succinate production within the intestinal tissue is a major physiological source of circulating succinate. These results provide a foundation for an investigation into the role of the intestine in regulating circulating TCA cycle metabolites and their potential signaling effects on health and disease.

An orphaned Mce-associated membrane protein of Mycobacterium tuberculosis is a virulence factor that stabilizes Mce transporters.

Mycobacterium tuberculosis proteins that are exported out of the bacterial cytoplasm are ideally positioned to be virulence factors; however, the functions of individual exported proteins remain largely unknown. Previous studies identified Rv0199 as an exported membrane protein of unknown function. Here, we characterized the role of Rv0199 in M. tuberculosis virulence using an aerosol model of murine infection. Rv0199 appears to be a member of a Mce-associated membrane (Mam) protein family leading us to rename it OmamA, for orphaned Mam protein A. Consistent with a role in Mce transport, we showed OmamA is required for cholesterol import, which is a Mce4-dependent process. We further demonstrated a function for OmamA in stabilizing protein components of the Mce1 transporter complex. These results indicate a function of OmamA in multiple Mce transporters and one that may be analogous to the role of VirB8 in stabilizing Type IV secretion systems, as structural similarities between Mam proteins and VirB8 proteins are predicted by the Phyre 2 program. In this study, we provide functional information about OmamA and shed light on the function of Mam family proteins in Mce transporters.

Early-Life Intranasal Colonization with Nontypeable Haemophilus influenzae Exacerbates Juvenile Airway Disease in Mice.

Accumulating evidence suggests a connection between asthma development and colonization with nontypeable Haemophilus influenzae (NTHi). Specifically, nasopharyngeal colonization of human infants with NTHi within 4 weeks of birth is associated with an increased risk of asthma development later in childhood. Monocytes derived from these infants have aberrant inflammatory responses to common upper respiratory bacterial antigens compared to those of cells derived from infants who were not colonized and do not go on to develop asthma symptoms in childhood. In this study, we hypothesized that early-life colonization with NTHi promotes immune system reprogramming and the development of atypical inflammatory responses. To address this hypothesis in a highly controlled model, we tested whether colonization of mice with NTHi on day of life 3 induced or exacerbated juvenile airway disease using an ovalbumin (OVA) allergy model of asthma. We found that animals that were colonized on day of life 3 and subjected to induction of allergy had exacerbated airway disease as juveniles, in which exacerbated airway disease was defined as increased cellular infiltration into the lung, increased amounts of inflammatory cytokines interleukin-5 (IL-5) and IL-13 in lung lavage fluid, decreased regulatory T cell-associated FOXP3 gene expression, and increased mucus production. We also found that colonization with NTHi amplified airway resistance in response to increasing doses of a bronchoconstrictor following OVA immunization and challenge. Together, the murine model provides evidence for early-life immune programming that precedes the development of juvenile airway disease and corroborates observations that have been made in human children.

The Haemophilus cryptic genospecies Cha adhesin has at least two variants that differ in host cell binding, bacterial aggregation, and biofilm formation properties.

The Haemophilus cryptic genospecies (HCG) causes genital tract infections in pregnant and postpartum women and respiratory infections in neonates. The major surface adhesin in HCG is called Cha, which mediates bacterial adherence to cultured human epithelial cells. In this study, we report that there are two antigenically distinct variants of Cha, dubbed Cha1 and Cha2. These variants are encoded by the same genetic locus in diverse strains and have nearly identical N-terminal export and C-terminal surface anchoring domains but significantly different internal adhesive domains. Based on the comparison of derivatives of a laboratory strain of Haemophilus influenzae expressing either surface-associated Cha1 or surface-associated Cha2, Cha1 mediates a higher level of adherence to cultured human epithelial cells and Cha2 mediates a higher level of adherence to abiotic surfaces. We hypothesize that variation in the Cha1 and Cha2 internal region results in changes in binding specificity or binding affinity and may be associated with adaptation to different host environments during colonization and disease.

Spontaneous episodic inflammation in the intestines of mice lacking HNF4A is driven by microbiota and associated with early life microbiota alterations.

The inflammatory bowel diseases (IBD) occur in genetically susceptible individuals who mount inappropriate immune responses to their microbiota leading to chronic intestinal inflammation. Whereas IBD clinical presentation is well described, how interactions between microbiota and host genotype impact early subclinical stages of the disease remains unclear. The transcription factor hepatocyte nuclear factor 4 alpha (HNF4A) has been associated with human IBD, and deletion of Hnf4a in intestinal epithelial cells (IECs) in mice (Hnf4aΔIEC) leads to spontaneous colonic inflammation by 6-12 mo of age. Here, we tested if pathology in Hnf4aΔIEC mice begins earlier in life and if microbiota contribute to that process. Longitudinal analysis revealed that Hnf4aΔIEC mice reared in specific pathogen-free (SPF) conditions develop episodic elevated fecal lipocalin 2 (Lcn2) and loose stools beginning by 4-5 wk of age. Lifetime cumulative Lcn2 levels correlated with histopathological features of colitis at 12 mo. Antibiotic and gnotobiotic tests showed that these phenotypes in Hnf4aΔIEC mice were dependent on microbiota. Fecal 16S rRNA gene sequencing in SPF Hnf4aΔIEC and control mice disclosed that genotype significantly contributed to differences in microbiota composition by 12 mo, and longitudinal analysis of the Hnf4aΔIEC mice with the highest lifetime cumulative Lcn2 revealed that microbial community differences emerged early in life when elevated fecal Lcn2 was first detected. These microbiota differences included enrichment of a novel phylogroup of Akkermansia muciniphila in Hnf4aΔIEC mice. We conclude that HNF4A functions in IEC to shape composition of the gut microbiota and protect against episodic inflammation induced by microbiota throughout the lifespan. IMPORTANCE The inflammatory bowel diseases (IBD), characterized by chronic inflammation of the intestine, affect millions of people around the world. Although significant advances have been made in the clinical management of IBD, the early subclinical stages of IBD are not well defined and are difficult to study in humans. This work explores the subclinical stages of disease in mice lacking the IBD-associated transcription factor HNF4A in the intestinal epithelium. Whereas these mice do not develop overt disease until late in adulthood, we find that they display episodic intestinal inflammation, loose stools, and microbiota changes beginning in very early life stages. Using germ-free and antibiotic-treatment experiments, we reveal that intestinal inflammation in these mice was dependent on the presence of microbiota. These results suggest that interactions between host genotype and microbiota can drive early subclinical pathologies that precede the overt onset of IBD and describe a mouse model to explore those important processes.

The Mycobacterium tuberculosis SecA2 system subverts phagosome maturation to promote growth in macrophages.

The ability of Mycobacterium tuberculosis to grow in macrophages is critical to the virulence of this important pathogen. One way M. tuberculosis is thought to maintain a hospitable niche in macrophages is by arresting the normal process of phagosomes maturing into acidified phagolysosomes. The process of phagosome maturation arrest by M. tuberculosis is not fully understood, and there has remained a need to firmly establish a requirement for phagosome maturation arrest for M. tuberculosis growth in macrophages. Other intracellular pathogens that control the phagosomal environment use specialized protein export systems to deliver effectors of phagosome trafficking to the host cell. In M. tuberculosis, the accessory SecA2 system is a specialized protein export system that is required for intracellular growth in macrophages. In studying the importance of the SecA2 system in macrophages, we discovered that SecA2 is required for phagosome maturation arrest. Shortly after infection, phagosomes containing a ΔsecA2 mutant of M. tuberculosis were more acidified and showed greater association with markers of late endosomes than phagosomes containing wild-type M. tuberculosis. We further showed that inhibitors of phagosome acidification rescued the intracellular growth defect of the ΔsecA2 mutant, which demonstrated that the phagosome maturation arrest defect of the ΔsecA2 mutant is responsible for the intracellular growth defect. This study demonstrates the importance of phagosome maturation arrest for M. tuberculosis growth in macrophages, and it suggests there are effectors of phagosome maturation that are exported into the host environment by the accessory SecA2 system.

Advanced Obesity Treatment Selection among Adolescents in a Pediatric Weight Management Program.

Background: Treatment options for adolescents with obesity are limited. Yet, therapies previously reserved for adults, such as medications and bariatric surgery, are increasingly available to adolescents in tertiary obesity treatment settings. We aimed to identify the factors associated with selecting an advanced obesity treatment (diets, medications, and surgery) beyond lifestyle therapy among adolescents presenting to a tertiary, pediatric weight management program. Methods: We conducted a secondary analysis of adolescents (N = 220) who participated in a longitudinal, observational case-control study within a pediatric weight management program. The exposures were potential individual and clinical factors, including sociodemographic characteristics and comorbidities. The outcome was treatment selection, dichotomized into lifestyle vs. advanced treatment. We modeled associations between these factors and treatment selection using logistic regression, controlling for confounding variables (age, race/ethnicity, sex, and insurance). Results: The study population included a majority of non-Hispanic Black (50.5%) and Hispanic/Latino (19.5%) adolescents, of whom 25.5% selected advanced treatment. Adolescents were more likely to choose an advanced treatment option if they had a greater BMI [odds ratio (OR) 1.09, 95% confidence interval (95% CI) 1.04-1.15], lived further from the clinic (OR 1.03, 95% CI 1.00-1.05), and had an elevated glycohemoglobin level (OR 2.46, 95% CI 1.24-4.92). Conclusions: A significant fraction of adolescents seeking obesity treatment in a specialized care setting chose new and emerging obesity treatments, particularly those at high risk of developing diabetes. These findings can inform patient-clinician obesity treatment discussions in specialty care settings. Clinical Trial Registration number: NCT03139877.

Genome-wide identification of Mycobacterium tuberculosis exported proteins with roles in intracellular growth.

The exported proteins of Mycobacterium tuberculosis that are localized at the bacterial cell surface or secreted into the environment are ideally situated to interact with host factors and to function in virulence. In this study, we constructed a novel ß-lactamase reporter transposon and used it directly in M. tuberculosis for genome-wide identification of exported proteins. From 177 ß-lactam-resistant transposon mutants, we identified 111 different exported proteins. The majority of these proteins have no known function, and for nearly half of the proteins, our demonstration that they are exported when fused to a ß-lactamase reporter is the first experimental proof of their extracytoplasmic localization. The transposon mutants in our banked library were of further value as a collection of mutants lacking individual exported proteins. By individually testing each of 111 mutants for growth in macrophages, six attenuated mutants with insertions in mce1A, mce1B, mce2F, rv0199, ctaC, and lppX were identified. Given that much of the M. tuberculosis genome encodes proteins of unknown function, our library of mapped transposon mutants is a valuable resource for efforts in functional genomics. This work also demonstrates the power of a ß-lactamase reporter transposon that could be applied similarly to other bacterial pathogens.

Genotypic and Phenotypic Diversity among Human Isolates of Akkermansia muciniphila.

The mucophilic anaerobic bacterium Akkermansia muciniphila is a prominent member of the gastrointestinal (GI) microbiota and the only known species of the Verrucomicrobia phylum in the mammalian gut. A high prevalence of A. muciniphila in adult humans is associated with leanness and a lower risk for the development of obesity and diabetes. Four distinct A. muciniphila phylogenetic groups have been described, but little is known about their relative abundance in humans or how they impact human metabolic health. In this study, we isolated and characterized 71 new A. muciniphila strains from a cohort of children and adolescents undergoing treatment for obesity. Based on genomic and phenotypic analysis of these strains, we found several phylogroup-specific phenotypes that may impact the colonization of the GI tract or modulate host functions, such as oxygen tolerance, adherence to epithelial cells, iron and sulfur metabolism, and bacterial aggregation. In antibiotic-treated mice, phylogroups AmIV and AmII outcompeted AmI strains. In children and adolescents, AmI strains were most prominent, but we observed high variance in A. muciniphila abundance and single phylogroup dominance, with phylogroup switching occurring in a small subset of patients. Overall, these results highlight that the ecological principles determining which A. muciniphila phylogroup predominates in humans are complex and that A. muciniphila strain genetic and phenotypic diversity may represent an important variable that should be taken into account when making inferences as to this microbe's impact on its host's health.IMPORTANCE The abundance of Akkermansia muciniphila in the gastrointestinal (GI) tract is linked to multiple positive health outcomes. There are four known A. muciniphila phylogroups, yet the prevalence of these phylogroups and how they vary in their ability to influence human health is largely unknown. In this study, we performed a genomic and phenotypic analysis of 71 A. muciniphila strains and identified phylogroup-specific traits such as oxygen tolerance, adherence, and sulfur acquisition that likely influence colonization of the GI tract and differentially impact metabolic and immunological health. In humans, we observed that single Akkermansia phylogroups predominate at a given time but that the phylotype can switch in an individual. This collection of strains provides the foundation for the functional characterization of A. muciniphila phylogroup-specific effects on the multitude of host outcomes associated with Akkermansia colonization, including protection from obesity, diabetes, colitis, and neurological diseases, as well as enhanced responses to cancer immunotherapies.

The Pediatric Obesity Microbiome and Metabolism Study (POMMS): Methods, Baseline Data, and Early Insights.

OBJECTIVE: The purpose of this study was to establish a biorepository of clinical, metabolomic, and microbiome samples from adolescents with obesity as they undergo lifestyle modification. METHODS: A total of 223 adolescents aged 10 to 18 years with BMI ≥95th percentile were enrolled, along with 71 healthy weight participants. Clinical data, fasting serum, and fecal samples were collected at repeated intervals over 6 months. Herein, the study design, data collection methods, and interim analysis-including targeted serum metabolite measurements and fecal 16S ribosomal RNA gene amplicon sequencing among adolescents with obesity (n = 27) and healthy weight controls (n = 27)-are presented. RESULTS: Adolescents with obesity have higher serum alanine aminotransferase, C-reactive protein, and glycated hemoglobin, and they have lower high-density lipoprotein cholesterol when compared with healthy weight controls. Metabolomics revealed differences in branched-chain amino acid-related metabolites. Also observed was a differential abundance of specific microbial taxa and lower species diversity among adolescents with obesity when compared with the healthy weight group. CONCLUSIONS: The Pediatric Metabolism and Microbiome Study (POMMS) biorepository is available as a shared resource. Early findings suggest evidence of a metabolic signature of obesity unique to adolescents, along with confirmation of previously reported findings that describe metabolic and microbiome markers of obesity.

Conserved anti-inflammatory effects and sensing of butyrate in zebrafish.

Short-chain fatty acids (SCFAs) are produced by microbial fermentation of dietary fiber in the gut. Butyrate is a particularly important SCFA with anti-inflammatory properties and is generally present at lower levels in inflammatory diseases associated with gut microbiota dysbiosis in mammals. We aimed to determine if SCFAs are produced by the zebrafish microbiome and if SCFAs exert conserved effects on zebrafish immunity as an example of the non-mammalian vertebrate immune system. We demonstrate that bacterial communities from adult zebrafish intestines synthesize all three main SCFA in vitro, although SCFA were below our detectable limits in zebrafish intestines in vivo. Immersion in butyrate, but not acetate or propionate, reduced the recruitment of neutrophils and M1-type pro-inflammatory macrophages to wounds. We found conservation of butyrate sensing by neutrophils via orthologs of the hydroxycarboxylic acid receptor 1 (hcar1) gene. Neutrophils from Hcar1-depleted embryos were no longer responsive to the anti-inflammatory effects of butyrate, while macrophage sensitivity to butyrate was independent of Hcar1. Our data demonstrate conservation of anti-inflammatory butyrate effects and identify the presence of a conserved molecular receptor in fish.

Fetal exposure to the maternal microbiota in humans and mice.

Previous studies have demonstrated the presence of microbial DNA in the fetal environment. However, it remains unclear whether this DNA represents viable bacteria and how it relates to the maternal microbiota across body sites. We studied the microbiota of human and mouse dyads to understand these relationships, localize bacteria in the fetus, and demonstrate bacterial viability. In human preterm and full-term mother-infant dyads at the time of cesarean delivery, the oral cavity and meconium of newborn infants born as early as 24 weeks of gestation contained a microbiota that was predicted to originate from in utero sources, including the placenta. Using operative deliveries of pregnant mice under highly controlled, sterile conditions in the laboratory, composition, visualization, and viability of bacteria in the in utero compartment and fetal intestine were demonstrated by 16S rRNA gene sequencing, fluorescence in situ hybridization, and bacterial culture. The composition and predicted source of the fetal gut microbiota shifted between mid- and late gestation. Cultivatable bacteria in the fetal intestine were found during mid-gestation but not late gestation. Our results demonstrate a dynamic, viable mammalian fetal microbiota during in utero development.

Identification of functional Tat signal sequences in Mycobacterium tuberculosis proteins.

The twin-arginine translocation (Tat) pathway is a system used by some bacteria to export proteins out from the cytosol to the cell surface or extracellular environment. A functional Tat pathway exists in the important human pathogen Mycobacterium tuberculosis. Identification of the substrates exported by the Tat pathway can help define the role that this pathway plays in the physiology and pathogenesis of M. tuberculosis. Here we used a reporter of Tat export, a truncated beta-lactamase, 'BlaC, to experimentally identify M. tuberculosis proteins with functional Tat signal sequences. Of the 13 proteins identified, one lacks the hallmark of a Tat-exported substrate, the twin-arginine dipeptide, and another is not predicted by in silico analysis of the annotated M. tuberculosis genome. Full-length versions of a subset of these proteins were tested to determine if the native proteins are Tat exported. For three proteins, expression in a Deltatat mutant of Mycobacterium smegmatis revealed a defect in precursor processing compared to expression in the wild type, indicating Tat export of the full-length proteins. Conversely, two proteins showed no obvious Tat export in M. smegmatis. One of this latter group of proteins was the M. tuberculosis virulence factor phospholipase C (PlcB). Importantly, when tested in M. tuberculosis a different result was obtained and PlcB was exported in a twin-arginine-dependent manner. This suggests the existence of an M. tuberculosis-specific factor(s) for Tat export of a proven virulence protein. It also emphasizes the importance of domains beyond the Tat signal sequence and bacterium-specific factors in determining if a given protein is Tat exported.

ATPase activity of Mycobacterium tuberculosis SecA1 and SecA2 proteins and its importance for SecA2 function in macrophages.

The Sec-dependent translocation pathway that involves the essential SecA protein and the membrane-bound SecYEG translocon is used to export many proteins across the cytoplasmic membrane. Recently, several pathogenic bacteria, including Mycobacterium tuberculosis, were shown to possess two SecA homologs, SecA1 and SecA2. SecA1 is essential for general protein export. SecA2 is specific for a subset of exported proteins and is important for M. tuberculosis virulence. The enzymatic activities of two SecA proteins from the same microorganism have not been defined for any bacteria. Here, M. tuberculosis SecA1 and SecA2 are shown to bind ATP with high affinity, though the affinity of SecA1 for ATP is weaker than that of SecA2 or Escherichia coli SecA. Amino acid substitution of arginine or alanine for the conserved lysine in the Walker A motif of SecA2 eliminated ATP binding. We used the SecA2(K115R) variant to show that ATP binding was necessary for the SecA2 function of promoting intracellular growth of M. tuberculosis in macrophages. These results are the first to show the importance of ATPase activity in the function of accessory SecA2 proteins.

Granuloma formation and host defense in chronic Mycobacterium tuberculosis infection requires PYCARD/ASC but not NLRP3 or caspase-1.

The NLR gene family mediates host immunity to various acute pathogenic stimuli, but its role in chronic infection is not known. This paper addressed the role of NLRP3 (NALP3), its adaptor protein PYCARD (ASC), and caspase-1 during infection with Mycobacterium tuberculosis (Mtb). Mtb infection of macrophages in culture induced IL-1beta secretion, and this requires the inflammasome components PYCARD, caspase-1, and NLRP3. However, in vivo Mtb aerosol infection of Nlrp3(-/-), Casp-1(-/-), and WT mice showed no differences in pulmonary IL-1beta production, bacterial burden, or long-term survival. In contrast, a significant role was observed for Pycard in host protection during chronic Mtb infection, as shown by an abrupt decrease in survival of Pycard(-/-) mice. Decreased survival of Pycard(-/-) animals was associated with defective granuloma formation. These data demonstrate that PYCARD exerts a novel inflammasome-independent role during chronic Mtb infection by containing the bacteria in granulomas.

The Accessory SecA2 System of Mycobacteria Requires ATP Binding and the Canonical SecA1.

In bacteria, the majority of exported proteins are transported by the general Sec pathway from their site of synthesis in the cytoplasm across the cytoplasmic membrane. The essential SecA ATPase powers this Sec-mediated export. Mycobacteria possess two nonredundant SecA homologs: SecA1 and SecA2. In pathogenic Mycobacterium tuberculosis and the nonpathogenic model mycobacterium Mycobacterium smegmatis, SecA1 is essential for protein export and is the "housekeeping" SecA, whereas SecA2 is an accessory SecA that exports a specific subset of proteins. In M. tuberculosis the accessory SecA2 pathway plays a role in virulence. In this study, we uncovered basic properties of the mycobacterial SecA2 protein and its pathway for exporting select proteins. By constructing secA2 mutant alleles that encode proteins defective in ATP binding, we showed that ATP binding is required for SecA2 function. SecA2 mutant proteins unable to bind ATP were nonfunctional and dominant negative. By evaluating the subcellular distribution of each SecA, SecA1 was shown to be equally divided between cytosolic and cell envelope fractions, whereas SecA2 was predominantly localized to the cytosol. Finally, we showed that the canonical SecA1 has a role in the process of SecA2-dependent export. The accessory SecA2 export system is important to the physiology and virulence of mycobacteria. These studies help establish the mechanism of this new type of specialized protein export pathway.

Beta-lactamase can function as a reporter of bacterial protein export during Mycobacterium tuberculosis infection of host cells.

Mycobacterium tuberculosis is an intracellular pathogen that is able to avoid destruction by host immune defences. Exported proteins of M. tuberculosis, which include proteins localized to the bacterial surface or secreted into the extracellular environment, are ideally situated to interact with host factors. As a result, these proteins are attractive candidates for virulence factors, drug targets and vaccine components. Here we describe a beta-lactamase reporter system capable of identifying exported proteins of M. tuberculosis during growth in host cells. Because beta-lactams target bacterial cell-wall synthesis, beta-lactamases must be exported beyond the cytoplasm to protect against these drugs. When used in protein fusions, beta-lactamase can report on the subcellular location of another protein as measured by protection from beta-lactam antibiotics. Here we demonstrate that a truncated TEM-1 beta-lactamase lacking a signal sequence for export ('BlaTEM-1) can be used in this manner directly in a mutant strain of M. tuberculosis lacking the major beta-lactamase, BlaC. The 'BlaTEM-1 reporter conferred beta-lactam resistance when fused to both Sec and Tat export signal sequences. We further demonstrate that beta-lactamase fusion proteins report on protein export while M. tuberculosis is growing in THP-1 macrophage-like cells. This genetic system should facilitate the study of proteins exclusively exported in the host environment by intracellular M. tuberculosis.