BACTERIAL PROTIENS

BACTERIAL PROTIENS 

INTRODUCTION: 

A bacterial protein is a protein which is either part of the structure of a bacterium, or produced by a bacterium as part of its life cycle. Proteins are an important part of all living organisms, and bacteria are no exception. Thanks to the fact that many bacteria are easy to culture in the laboratory, a great deal of research on bacterial proteins has been performed with the goal of learning more about specific proteins and their functions. Understanding bacterial proteins is important both because bacteria play a very active role in human health, and because the information can be extrapolated to gather more data about the proteins associated with larger organisms, including humans. 

  

Proteins are lengthy chains of amino acids which are folded back upon them. The nature of a protein is determined both by the amino acid chain and by the way in which the protein is folded. Proteins are encoded in the genes, with certain proteins being expressed while an organism develop, and with others are produced by an organism with the goal of accomplishing specific tasks. The genetic code of an organism holds the blueprints for numerous proteins. 

In addition to being a unique structure, a bacterial protein also has the ability to bind with other proteins. Protein binding involves the formation of very strong links between two different proteins. Once proteins bind, they can trigger a reaction which may vary from an immune system response to an infection to the onset of a disease. Over time, many bacteria have evolved to produce proteins which target particular locations on human and animal cells. 

Bacterial proteins are of interest to humans for a number of reasons. Understanding which proteins are involved in the structure of particular bacteria can help researchers develop medications which identify and target a particular bacterial protein, allowing the researchers to create antibacterial drugs which target specific organisms. Understanding individual proteins can also allow researchers to monitor mutations and to keep track of the ways in which these mutations occurred, and how they can be addressed. 

Some bacteria produce proteins which have a deleterious effect on the human body. A bacterial protein can be toxic, causing illness or death in an organism which has been infected by the bacteria, and bacterial proteins can also bind with specific proteins in the body to cause a variety of symptoms. Researchers can spend years identifying all of the proteins associated with a single type of bacterium, and this process can be complicated by rapid mutations, as seen in the case of the wily Staphylococcus bacterium. 

Bacterial Proteins: 

• AraC Transcription Factor 
• Azurin 
• Bacterial Outer Membrane Proteins 
• Adhesins, Bacterial + 
• Bacterial Transferrin Receptor Complex + 
• Fimbriae Proteins 
• Bacterial Proton-Translocating ATPases 

• Bacterial Transferrin Receptor Complex 
• Bacteriocins 
• Cloacin 
• Megacins 
• Nisin 
• Pyocins 
• Botulinum Toxins 
• Botulinum Toxins, Type A 
• Cell Wall Skeleton 
• Coagulase 
• Colicins 
• DNA Gyrase 
• DNA Topoisomerase IV 
• Escherichia coli Proteins 
• Adhesins, Escherichia coli 
• Colicins 
• Shiga Toxin 1 
• Shiga Toxin 2 
• Exfoliatins 
• Factor For Inversion Stimulation Protein 

• Ferredoxins 
• Molybdoferredoxin 
• Rubredoxins 
• Flagellin 
• Flavodoxin 
• Integration Host Factors 
• Host Factor 1 Protein 
• Lac Repressors 
• Luciferases, Bacterial 
• MutS DNA Mismatch-Binding Protein 
• NADP Transhydrogenase, B-Specific 
• Penicillin-Binding Proteins 
• Periplasmic Proteins 
• Periplasmic Binding Proteins + 
• RNA Polymerase Sigma 54 
• Staphylococcal Protein A 
• Streptavidin 
• Tetanus Toxin 

BACTERIAL PROTEINS & TYPES 

Proteins are the Bacterial most powerful human poisons known and belong to two broad categories: lipopolysaccharides (Gram-negative bacteria) and proteins, which are released from bacterial cells. Endotoxins, which are structural components of bacteria, are cell-associated substances that a located in the cell envelope and can be released from growing bacteria or lysed cells as a consequence of effective host defense mechanisms or antibiotics. The extracellular diffusible toxins are referred to as exotoxins and are usually secreted by bacteria during exponential growth. Exotoxins are usually polypeptides that act at tissue sites remote from the original point of bacterial invasion or growth. The location for activity of a particular toxin, like Botulinium, is determined by the site of damage. Enterotoxin, neurotoxin, leukocidin and hemolsyin are terms that describe the target site of well-defined protein toxins. Although the tissues affected and the target site may be known, the exact mechanism by which toxins cause death is not clear and is subject to debate. 

TYPES OF PROTEINS: 

Botulinium neurotoxins: 

Botulinum neurotoxins (BoNTs), a family of bacterial proteins produced by the anaerobic bacteria Clostridium Botulinum, and the causative agent of botulism, is acknowledged to be the most poisonous protein known. Botulism poisoning is a serious and life-threatening illness in humans and animals. BoNT proteases disable synpathic vesicle exocytosis by cleaving their cytosolic SNARE substrates. There are seven distinct BoNT isoforms (A-G), which show strong amino acid sequence similarity. Human botulism is caused by the BoNT serotypes A, B, E and F. Interestingly, type A is used for various cosmetic and medical procedures, more commonly known as Botox.  

BoNTs exert their neurotoxic effect by a multistep mechanism: binding, internalization, membrane translocation, intracellular traffic and proteolytic degradation. The activated mature toxin consists of 3 parts: the N-terminal light chain (~50 kDa), the heavy chain (100 kDa) that encompases the light chain (HN) and the receptor-binding doman (HC). HC determines the cellular specificity with a protein receptor (SV2 or Syt depending on the isoform) and a ganglioside. HN is a helical bundle that chaperones the light chains across endosomes where it is driven by a transmembrane proton gradient. Then, BoNTs enter the cells via receptor-mediated endocytosis, induces a conformational change and the light chains (LCs) cleave the unique components of the synaptic vesicle docking-fusion complex known as SNARE. As a result, cleavage of SNARE nullifies vesicle fusion and synaptic transmission, which causes the severe paralysis characteristic of botulism. 

Tetanus toxin: 

Tetanus toxin is a very powerful neurotoxin produced by the vegetative cell of Clostridium tetani in conditions that lack oxygen (anaerobic). As the bacterium matures, it developed its characteristic terminal spores which also give them advantage by increasing the bacteria's resistance to heat and most antiseptics. The toxin causes tetanus, a fatal disease that involves unfavorable muscle spasms that can cause respiratory failure and even death. The LD50 of this toxin has been measured to be approximately 1 ng/kg, making it the second most deadliest toxin in the world after the Botulinium neurotoxins. 

The mechanism of the toxin is it first travels through the vascular and lymphatic systems of the body, disrupting the neuromuscular junctions and the central nervous system. Tetanus toxin blocks the release of inhibitory gamma-aminobutyric acid (GABA) and glycine by degrading the protein synaptobrevin. This causes the failure of regulating motor reflexes by sensory stimulation, which leads to the muscle depolarizing even with the smallest of action potentials. This continued depolarization causes the antagonist and agonist muscles to contract simultaneously and this generalized contraction causes the symptom known as tetanic spasm. 

Diphtheria Toxin: 

Diphtheria Toxin is a bacterial exotoxin caused by Corynebacterium Diphtheriae. This toxin exists as a single polypeptide chain, about 60,000 daltons in molecular weight. Outside of the cell, the toxin is produced in its inactive form, later to be activated by trypsin, a proteolytic enzyme, in the presence of thiol. Thiol acts as a reducing agent during this activation process. The toxin consists of two parts: Fragment A and Fragment B. Fragment A, which is responsible for the catalytic activity of the toxin, is masked until it reaches the target cell. The hydrophobic portion of the toxin, named Fragment B, is responsible for interacting with cell membrane receptors on the target cell surface. 

Diphtheria toxin may enter a target cell via direct entry or receptor mediated endocytosis. In direct entry, the toxin binds to a target cell surface receptor. This binding induces the formation of a pore on the cell membrane, allowing the catalytic chain of the toxin to enter the target cell’s cytoplasm. During receptor-mediated endocytosis, the toxin is placed in a vesicle, where the pH drops, allowing both Fragment A and Fragment B to unfold. The hydrophobic regions of both chains then enter the vesicle membrane. Next, reduction and proteolytic cleavage of the A chain is released into the cytoplasm, where it regains enzymatic conformation. 

Diphtheria toxin utilizes NAD as a substrate, and catalyzes ADP ribosylation, where the ADP-ribose portion of NAD combines with elongation factor-2 (EF-2). This process inactivates protein synthesis (in animal cells), resulting in cell death. 

Actin assembly-inducing protein (ActA): 

The Actin assembly-inducing protein (ActA) is a protein encoded and used by Listeria monocytogenes to propel itself through a mammalian host cell. ActA is a bacterial surface protein comprising a membrane-spanning region.In a mammalian cell the bacterial ActA interacts with the Arp2/3 complex and actin monomers to induce actin polymerization on the bacterial surface generating an actin comet tail. The gene encoding ActA is named actA or prtB.  

Arp2/3 complex is a seven-subunit protein that plays a major role in the regulation of the actin cytoskeleton. It is a major component of the actin cytoskeleton and is found in most actin cytoskeleton-containing eukaryotic cells. 

BACTERIAL PROTEIN EXTRACTION (MINI-SCALE) USING B-PER 

A useful procedure for initial screening of expression conditions instead of sonication of bacteria; not recommended for large scale preparation of protein.  Not always the protein activity is conserved after B-Per extraction. 

• Spin bacterial cells 10min 5000rpm in a microcentrifuge 4°C (cells can either be used fresh or frozen at -70°C). 
• Resuspend cells in 300µl of B-Per reagent (Pierce) by either vigorous vortexing or by pipetting up and down until the cell suspension is homogenous. Vortex for 1 more min. (if suspension is too viscous add Dnase 100U/ml or 25-50µg/ml (SIGMA DN-25). Incubate 10min 4°C in the presence of 10mMMgCl2 
• Spin 13000rpm 5min 4°C, to separate soluble proteins from the insoluble proteins in the pellet. Collect supernatant. Keep 40µl sample of supernatant for PAGE-SDS or western blot: soluble proteins. 

• Resuspend pellet in 300µl lysis buffer (25mM TrisHCl pH8.0; 0.1M NaCl; 10% glycerol; 0.1% Triton X-100 and 1mM PMSF). Spin 13000rpm 5min 4°C, to separate detergent-soluble proteins from inclusion bodies in the pellet. Collect supernatant. Keep 40ul sample of supernatant for PAGE-SDS or western blot: detergent-soluble proteins. 
• For inclusion bodies purification, add (lysozime 6µl of 10mg/ml) to the resuspended pellet to a f.c. of 200µg/ml, vortex 1min. Add 1ml of B-Per 1:10 to the suspension and vortex for another 1min. Collect inclusion bodies by centrifugation 13000rpm 10min 4°C. Resuspend pellet in 1ml of B-Per 1:10. Spin and repeat extraction again. Pellet can be resuspended in sample buffer for PAGE-SDS: inclusion bodies fraction or resuspended in denaturant buffer of Urea or Guanidine-HCl (see next point). 
• For inclusion bodies resuspension prepare lysis buffer (without detergent) containing 8M urea or 6M Guanidine-HCl. Resuspend cells in 300µl buffer + Urea or Guanidine-HCl by pipetting up and down until the cell suspension is homogenous. Spin 13000rpm 5min 4°C. Collect supernatant. Keep 40µl sample of supernatant for PAGE-SDS or western blot: inclusion bodies proteins. Precipitate Guanidine-HCl from samples with 9 volumes of ethanol: resuspended Inclusion Bodies. 
  
  
  
• You can try first 8M of Urea, and if protein is soluble titer down in the next experiments the urea concentration till minimal urea is required for protein solubilization.  

SMART BACTERIAL PROTEIN EXTRACTION: 

The SMART Bacterial Protein Extraction Solution is designed for fast and easy extraction of total proteins from bacteria without the need for sonication or precipitation. This solution utilizes a proprietary non-ionic detergent in 20mM Tris/HCl(pH7.5). In addition, the SMART Bacterial Protein Extraction Solution offers several folding increase in the yield of soluble protein when compared with other commercial lysisreagent. Depending on the particular application, additional components, such as lysozyme, protease inhibitors, salts, reducing agents and chelating agents may be added to the solution. The solution may be used for both soluble protein extraction and inclusion body purification from bacterial cell lysates. In fact, yields obtained with ( ) 1. Harvest cells from 1.5 ml bacterial culture (OD600 1.5 ~ 3.0) at 10,000 rpm for 10 minutes in a microcentrifuge. NOTE: For larger volumes, e.g. 40~250 ml of bacterial culture, pellet cells by centrifugation at 13,000 rpm for 15 minutes. 2. Remove all media by aspiration. The cells can either be used fresh or frozen at – 80ºC. 3. Resuspend cells in 350 μlofSMARTTMTotal Protein Extraction Solution by vigorouslyvortexing sample for 1 minute. NOTE: If the pellet was harvested from 40~250 ml of culture, resuspend in 8 ~ y p y, y SMART Bacterial Protein Extraction Solution greatly exceeds those obtained using standard sonication methods! Bacteria often over-express recombinant proteins and form inclusion bodies, which are insoluble aggregates of misfolded protein. Centrifugation separates inclusion bodies from soluble proteins; however, Lysozyme is required for purification of inclusion bodies. Lysozyme significantly improves inclusion body purity by digesting the cell debris. 

RESEARCH ARTICLE 

“Bacterial protein found in yogurt may alleviate inflammatory bowel disorders” 

• A protein isolated from beneficial bacteria found in yogurt and dairy products could offer a new, oral therapeutic option for inflammatory bowel disorders, suggests a study led by Vanderbilt University Medical Center researcher Fang Yan. 
• The study, published May 23 in the Journal of Clinical Investigation, shows that the protein, called p40, was effective as an intervention in animal models of colitis (colon inflammation). The investigators demonstrated that the protein supports intestinal epithelial cell growth and function, and reduces inflammatory responses that can cause intestinal cells to die. Importantly, the investigators showed that oral consumption of p40 by mice in a protective delivery system prevents and treats colitis in multiple models of the disease. 
• Many of the hundreds of bacterial species that live in our gut (known as the “human microbiome”) are helpful to us. They help us digest certain substances, produce vitamins and fight off more dangerous bacteria. But miscommunication between these bacteria and our gut lining can lead to conditions like ulcerative colitis and Crohn’s disease. According to the Centers for Disease Control and Prevention, as many as 1.4 million persons in the United States alone may suffer from these diseases. 
• One type of helpful bacteria often used in yogurt production and in nutritional supplements, Lactobacillus rhamnosus GG (LGG), has been used in attempts to prevent intestinal disorders such as IBD and diarrhea, as well as other conditions such as dermatitis (skin inflammation). However, results generated using whole bacteria have been mixed. 
• This research was sparked when a colleague in pediatric infectious diseases asked him, “Is there anything to this probiotic stuff?” said Polk, co-author on the study and currently director of the Saban Research Institute of Children’s Hospital Los Angeles. 
• “Probiotic bacterial function is not very clear right now,” said Yan, a research associate professor of pediatrics at Vanderbilt. 
• Polk and Yan showed that LGG prevented epithelial cells from inflammation-induced apoptosis – a kind of cell suicide. They then isolated and characterized two specific proteins secreted by LGG (which they called p75 and p40) responsible for the bacterium’s beneficial effects.                                                                                                                  In the current study, Yan investigated the mechanisms by which one of these proteins, p40, prevents and treats colitis.                                                                                                                                        In cell experiments, Yan and colleagues showed that p40 activates the epidermal growth factor receptor (EGFR), a protein critical for cell survival and growth.                                                     Activation of EGFR protected epithelial cells in two ways: by preventing both apoptosis and inflammation-induced disruption of the “tight junctions” between epithelial cells, which form a barrier to keep toxic substances and pathogens out of the bloodstream.                                                      To test the isolated protein’s effectiveness in animal models of disease, the investigators developed a gel bead system to deliver the protein specifically to the colon while protecting the protein from being degraded by stomach acid and digestive enzymes.                                                                                                                                      In three different mouse models of intestinal inflammation, they showed that p40 prevented and treated intestinal injury and acute colitis.                                                                                                 This study is one of the few to identify and use individual molecules from beneficial microbes as potential therapeutics. In clinical applications, Yan says that the isolated protein could provide at least two advantages to using whole bacteria.                                                                                           “One is bioavailability,” she said. “Even if you eat live bacteria (as in yogurt), that does not mean 100 percent of bacteria will still be alive (and active) in your body.”                                          Another advantage is safety. Although LGG is generally safe for most people, “in patients with immune deficiency, it could be a problem because it may induce an abnormal immune response. 

PROTEIN PURIFICATION STRATEGIES AND PREPARATION OF CELL-FREE EXTRACTS 

An important part of biotechnology research is to use protein engineering techniques to design or modify proteins with optimized properties for specific industrial applications. In order to do this, scientists must be able to isolate and purify proteins of interest so their conformations, substrate specificities, reactions with other ligands, and activities can be studied. 

The degree of protein purity required depends on the intended end use of the protein. For some applications, a crude extract is sufficient. However, for other uses, such as in foods and pharmaceuticals, a high level of purity is required. In order to achieve this, several protein purification methods are typically used, in a series of purification steps. 

Each protein purification step usually results in some degree of product loss. Therefore, an ideal protein purification strategy is one in which the highest level of purification is reached in the fewest steps. The selection of which steps to use is dependent on the size, charge, solubility and other properties of the target protein. The following techniques are most appropriate for purifying a single cytosolic protein. Purification of cytosolic protein complexes is more complicated and usually requires that different methods be applied. 

1. First step: Preparation of a crude extract 

• The first step in purifying intracellular (inside the cell) proteins is preparation of a crude extract. The extract will contain a complex mixture of all the proteins from the cell cytoplasm, and some additional macromolecules, cofactors and nutrients. 
•  Crude extract may be used for some applications in biotechnology, however, if purity is an issue, subsequent purification steps must be followed. 
•  Crude protein extracts are prepared by removal of cellular debris generated by cell lysis, which is achieved using chemicals and enzymes, sonication or a French Press.  
• Debris are removed by centrifugation and the supernatant is recovered.  
• Crude preparations of extracellular proteins may be obtained by simply removing the cells by centrifugation. 
• For certain biotechnology applications, there is a demand for thermostable enzymes. Organisms that produce them are sometimes called extremophiles.  
• An easy approach to purifying a heat-resistant protein is to denature the other proteins in the mixture by heating, then cooling the solution (thus allowing the thermostable enzyme to reform or redissolve, if necessary. The denatured proteins can then be removed by centrifugation. 

2. Intermediate Purification Steps: purifying a protein from a crude extract by precipitation 

• In the past, a common second step to purifying a protein from a crude extract was by precipitation in a solution with high osmotic strength (i.e. salt solutions). 
•  Nucleic acids in the crude extract can be removed by precipitating aggrigates formed with streptomycin sulfate or protamine sulfate. Protein precipitation is usually done using ammonium sulfate as the salt. 
• Different proteins will precipitate in different concentrations of ammonium sulfate.  
• In general, proteins of higher molecular weight precipitate in lower concentrations of ammonium sulfate. Salt precipitation does not usually lead to a highly purified protein, but can assist in eliminating some unwanted proteins in a mixture and concentrating the sample. Salts in the solution are then removed by dialysis through porous cellulose tubing, filtration, or gel exclusion chromatography. 

Modern biotech protocols often take advantage of the many commercially-available kits that provide ready-made solutions for standard procedures. Protein purification is often performed using filters and prepared gel filtration columns. All you have to do is follow the instructions and add the right volume of the right solution and wait the specified length of time while collecting the eluant (what comes out the other end of the column) in a fresh test tube. 

Chromatographic methods can be applied using bench-top columns or automated HPLC equipment. Separation by HPLC can be done by reverse-phase, ion-exchange or size-exclusion methods, and samples detected by diode array or laser technology. 

• Reverse-phase chromatography (RPC) separates proteins based on their relativehydrophobicities. This technique is highly selective but requires the use of organic solvents. Some proteins are permanently denatured by solvents and will lose functionality during RPC, therefore this method is not recommended for all applications, particularly if it is necessary for the target protein to retain activity. 

• Ion-exchange chromatography refers to separation of proteins based on charge. Columns can either be prepared for anion exchange or cation exchange. Anion exchange columns contain a stationary phase with a positive charge that attracts negatively charged proteins. Cation exchange columns are the reverse, negatively charged beads which attract positively charged proteins. Elution of the target protein(s) is done by changing the pH in the column, which results in a change or neutralization of the charged functional groups of each protein. 

Size-exclusion chromatography (gel filtration) separates larger proteins from small ones, since the larger molecules travel faster through the cross-linked polymer in the chromatography column. The large proteins do not fit into the pores of the polymer whereas smaller proteins do, and take longer to travel through the chromatography column, via their less direct route. Eluate is collected in a series of tubes separating proteins based on elution time. Gel filtration is a useful tool for concentrating a protein sample, since the target protein is collects in a smaller elution volume than was initially added to the column. Similar filtration techniques might be used during large scale protein production because of their cost-effectiveness. 

• Affinity chromatography is a very useful technique for "polishing" or completing the protein purification process. Beads in the chromatography column are cross-linked toligands that bind specifically to the target protein. The protein is then removed from the column by rinsing with a solution containing free ligands. This method generally gives the purest results and highest specific activity compared to other techniques. 

PROTEIN VISUALIZATION AND ASSESSMENT OF PURIFICATION 

SDS-PAGE is polyacrylamide gel electrophoresis, performed in the presence of SDS (sodium dodecyl sulfate) which binds to proteins giving them a large net negative charge. Since the charges of all proteins are fairly equal, this method separates them almost entirely based on size. SDS-PAGE is often used to test the purity of a protein after each step in a series. As unwanted proteins are gradually removed from the mixture, the number of bands visualized on the SDS-PAGE gel, is reduced, until there is only one band representing the desired protein. Immunoblotting is a protein visualization technique applied in combination with affinity chromatography. Antibodies for a specific protein are used as ligands on an affinity chromatography column. The target protein is retained on the column, then removed by rinsing the column with a salt solution or other agents. Antibodies linked to radioactive or dye labels aid in detection of the target protein once it is separated from the rest of the mixture. 

  

CONCLUSION: 

Once the protein extract is prepared, in order to purify a single protein from this extract different fractionation methodologies. Protein purification is a series of processes intended to isolate one or a few proteins from a complex mixture, usually cells, tissues or whole organisms. Protein purification is vital for the characterization of the function, structure and interactions of the protein of interest. The purification process may separate the protein and non-protein parts of the mixture, and finally separate the desired protein from all other proteins. Separation of one protein from all others is typically the most laborious aspect of protein purification. Separation steps usually exploit differences in protein size, physico-chemical properties, binding affinity and biological activity. 

AN ATYPICAL GIANT MANTOUX REACTION 

2012.3-20 

 ARTICLE: 

Mantoux tuberculin skin test is used for routine screening of individuals with a high risk of Tuberculosis infection and also for diagnosis of tubercular etiology in various illnesses [1]. A standardized 5 tuberculin units (TU) of purified protein derivative (PPD) is injected intradermally into the volar aspect of the left forearm and the delayed hypersensitivity reaction is noted by measuring the indurations after 48-72 hours. Severe reactions with the formation of blisters and necrosis are very rare, we present a child who developed a very rapid and abnormally large lesion after Mantoux testing. An eight year old child was admitted with history of fever and since the past 2 months. The child had a normal growth for her age. On examination the left posterior cervical lymph nodes were significantly enlarged, discrete, mobile and non tender. Systemic examination was normal. Her blood picture and Erythrocyte Sedimentation Rate (ESR) was normal. A Mantoux test was done with the standard 5 TU Purified Protein Derivative (PPD) given intradermally. An immediate and exaggerated reaction with blisters and indurations measuring 25X30mm was noticed within 6 hours after administrating the test Enzyme linked immunosorbent assay (ELISA) for human immunodeficiency virus (HIV) was nonreactive. Chest radiograph looked normal and sputum tested did not show any Acid Fast Bacilli (AFB). Her lymph node biopsy showed features of reactive lymphadenitis. Child was not started on antitubercular therapy but was treated with a short course of antibiotics and was asymptomatic at follow up child had demonstrated an atypical and uncommon phenomenon since tubercular response is a delayed type of hypersensitivity reaction. Active tuberculosis, high mycobacterial antigen load or lepromatous leprosy may cause an exaggerated Mantoux response.  Patients who have indurations of more than 20mm have a higher chance of developing active tuberculosis than those with 10mm indurations. Tuberculin testing is useful for assessing the prevalence of tubercular infection in the developing countries. It should be administered and interpreted with caution and the decision of starting on antitubercular therapy is finally based on the clinical scenario and the results of the other tests for confirming tuberculosis. 

Who Should Get a PPD Skin Test? 

Tuberculosis is a highly contagious (spreadable) disease. The World Health Organization estimates that 2 billion people worldwide have inactive TB and about 3 million people worldwide die of TB every year. (WHO) However, the disease is relatively rare in the United States and most people in the U.S. who are infected with the bacteria do not show symptoms. 

You need a PPD skin test if:  

The TB antigens used in a tuberculin skin test are called purified protein derivative (PPD). A measured amount of PPD in a shot is put under the top layer of skin on your forearm. This is a good test for finding a TB infection. It is often used when symptoms, screening, or testing, such as a chest X-ray, show that a person may have TB.