Microbial Infections and Diagnostic Methods

. Key Terms and Definitions

Acute Rheumatic Fever: A sequela to streptococcal pharyngitis or tonsillitis, characterized by fever, joint pain, and heart inflammation. This condition typically arises 1-3 weeks after the onset of a streptococcal throat infection. The inflammation can affect the heart (carditis), joints (polyarthritis), brain, and skin.
Anti-DNase B: An antibody produced in response to streptococcal infections, particularly useful in diagnosing glomerulonephritis following skin infections. Unlike ASO antibodies, Anti-DNase B antibodies are stimulated by skin infections caused by Streptococcus pyogenes.
ASO Titer: A test to detect antibodies against streptolysin O, indicating a recent streptococcal infection. Elevated ASO titers suggest a recent or ongoing infection with Group A Streptococcus.
Commensalistic: A symbiotic relationship where one organism benefits and the other is neither harmed nor benefits. An example includes certain bacteria on the skin that feed on dead cells without affecting the host.
Endotoxin: A toxic component (lipid A) of the lipopolysaccharide (LPS) layer in the cell walls of gram-negative bacteria, released upon cell death. Endotoxins can trigger a potent immune response, leading to fever, inflammation, and potentially septic shock.
Exotoxin: A toxic protein secreted by bacteria, often with specific and targeted activity on host cells. Examples include toxins produced by Clostridium tetani (tetanus toxin) and Corynebacterium diphtheriae (diphtheria toxin).
Group A Streptococcus (GAS): A bacterium, also known as Streptococcus pyogenes, responsible for various infections, including pharyngitis and impetigo. GAS infections can lead to serious complications such as rheumatic fever and glomerulonephritis if left untreated.
Helicobacter pylori: A bacterium that colonizes the gastric epithelium and is a major cause of gastric and duodenal ulcers. H. pylori survives in the harsh acidic environment of the stomach by producing urease, which neutralizes stomach acid.
Impetigo: A superficial skin infection, often caused by Streptococcus pyogenes, characterized by vesicular lesions. Impetigo is highly contagious and commonly seen in children.
Indigenous Microbiota: The collection of symbiotic bacteria that reside on and colonize various surfaces of the body, also previously known as "normal flora." The composition of the indigenous microbiota varies depending on the body site and individual factors.
Infectivity: The ability of an organism to establish an infection. High infectivity means that a small number of organisms is sufficient to cause infection in a susceptible host.
Lancefield Groups: A classification system that divides β-hemolytic streptococci into groups based on their group-specific carbohydrate composition. This classification helps in identifying and differentiating various streptococcal species.
Lateral Flow Immunochromatographic Assay (LFA): A rapid and simple assay used to detect the presence of an analyte (e.g., bacterial antigen) in a sample. LFA is commonly used in point-of-care testing for quick and convenient diagnosis.
MALDI-TOF: Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; used for identifying microorganisms based on their protein profiles. MALDI-TOF MS provides rapid and accurate identification of bacteria, fungi, and other microorganisms.
Microbiome: The collection of microorganisms (bacteria, viruses, fungi, etc.) that exists on or within the human body. The microbiome plays a crucial role in human health and disease.
Mutualistic: A symbiotic relationship where both organisms benefit. An example includes the bacteria in the gut that help digest food and produce vitamins, while receiving nutrients and a stable environment from the host.
Mycoplasma pneumoniae: A bacterium that lacks a cell wall and is a common cause of respiratory infections, including atypical pneumonia. Because it lacks a cell wall, M. pneumoniae is resistant to many common antibiotics.
Parasitic: A symbiotic relationship where one organism benefits at the expense of the other. Examples include tapeworms and malaria parasites.
Pathogenicity: The inherent capacity of an organism to cause disease. Pathogenicity is determined by various virulence factors that enable the organism to colonize, invade, and damage the host.
Plasmids: Self-replicating extrachromosomal DNA molecules in bacteria that often carry genes encoding virulence factors. Plasmids can be transferred between bacteria, leading to the spread of antibiotic resistance and virulence traits.
Poststreptococcal Glomerulonephritis: A kidney condition characterized by damage to the glomeruli, often following a streptococcal infection. It typically occurs 1-4 weeks after a Streptococcus pyogenes infection of the throat or skin.
Proteome: The entire set of proteins expressed by an organism or cell.
Proteomics: The analysis and study of the proteins expressed by an organism's genome. Proteomics provides insights into the function and regulation of proteins in health and disease.
Rickettsiae: A group of obligate intracellular, gram-negative bacteria that cause various infections, such as Rocky Mountain spotted fever. Rickettsiae are transmitted to humans through the bite of infected arthropods, such as ticks, fleas, and lice.
Rocky Mountain Spotted Fever (RMSF): A tick-borne disease caused by Rickettsia rickettsii, characterized by a spotted rash. RMSF can be life-threatening if not treated promptly with antibiotics.
Scarlet Fever: A disease resulting from infection with Streptococcus pyogenes that elaborates streptococcal pyrogenic exotoxins, characterized by a distinct rash. Scarlet fever is characterized by a red, sandpaper-like rash, fever, and sore throat.
Streptococcus pyogenes: A bacterium, also known as Group A streptococcus (GAS), responsible for various infections, including pharyngitis and impetigo. S. pyogenes produces several virulence factors that contribute to its pathogenicity.
Streptolysin O: An enzyme produced by Streptococcus pyogenes that is able to lyse RBCs. Streptolysin O is responsible for the beta-hemolysis observed on blood agar plates.
Streptozyme: A test used for the detection of antibodies against multiple streptococcal antigens. The streptozyme test is a rapid screening test for detecting recent streptococcal infections.
Symbiotic: A close and long-term interaction between two different organisms. Symbiotic relationships can be commensalistic, mutualistic, or parasitic.
Typhus: A group of infectious diseases caused by Rickettsial bacteria, often transmitted by arthropods such as lice or fleas. Typhus is characterized by fever, headache, and a rash.
Urease: An enzyme produced by some bacteria, such as Helicobacter pylori, that catalyzes the hydrolysis of urea into ammonia and carbon dioxide. Urease production helps H. pylori survive in the acidic environment of the stomach.
Virulence: The extent of damage, or pathology, caused by an organism. High virulence indicates that the organism is capable of causing severe disease.
Virulence Factors: Properties or features of a bacterium that determine its pathogenicity and ability to cause disease. Virulence factors include toxins, adhesins, and enzymes.

Human Microbiome and Its Significance

The human microbiome is the collection of microorganisms, including bacteria, viruses, and single-celled prokaryotic organisms (e.g., yeast and fungi), that exists on the human body. These microorganisms play a crucial role in maintaining human health by:

Protecting against disease-causing bacteria by competing for nutrients and attachment sites.
Aiding in the digestion of food, particularly complex carbohydrates that the human body cannot break down on its own.
Producing certain vitamins, such as vitamin K and B vitamins.
Stimulating both the innate and adaptive immune systems, helping to regulate immune responses and prevent autoimmune diseases.

The establishment of an organism that leads to host injury is referred to as an “infection.” When a microbe causes damage to host cells or altered physiology that results in clinical signs and symptoms of disease, the phrase “infectious disease” applies. Infectious diseases can range from mild to life-threatening.

The microbial populations outnumber human cells by 10 to 1 and may account for 2 to 6 pounds of an individual’s body weight. Greater than 90% of the bacteria that comprise the human microbiome cannot be cultured in vitro, making it challenging to study their roles and functions.

Human-Microbe Relationships

When an individual is born, a relationship begins between the human host and the bacteria in the environment. Bacteria establish themselves on the surfaces of an individual, including the gastrointestinal tract, creating a symbiotic relationship. Symbiotic bacteria that reside on and colonize these surfaces are referred to as the indigenous microbiota (previously known as “normal flora”). The composition of the microbiota can be influenced by factors such as diet, environment, and antibiotic use.

Three types of symbiotic relationships can exist between humans and bacteria:

Commensalistic Relationships: There is no apparent benefit or harm to either organism. Staphylococcus epidermidis, which colonizes human skin, is an example. These bacteria may benefit from the nutrients and environment provided by the skin, while the human host is neither harmed nor benefited.
Mutualistic Relationships: Both humans and the bacteria benefit. Lactobacillus species in the vaginal canal produce lactic acid, preventing colonization of harmful bacteria and yeast. In exchange, the human host provides appropriate temperature, atmosphere, and nutrients for the bacteria to grow and multiply. Another example is the gut microbiota that aids in the digestion of complex carbohydrates and produces essential vitamins.
Parasitic Relationships: The encounter with specific organisms or viruses occasionally results in harm to the host. An example is infection with Chlamydia trachomatis, a sexually transmitted organism where only about 10% of infected males and 5% to 30% of infected females will show symptoms. Parasitic relationships can lead to infectious diseases.

Infectivity, Pathogenicity, and Virulence

Several terms are used to describe the interaction between infecting organisms and the host:

Infectivity: Refers to an organism’s ability to establish an infection and describes the proportion of individuals exposed to a pathogen through horizontal transmission who will become infected. For example, the measles virus is extremely contagious and has a high degree of infectivity. Individuals exposed to the measles virus are highly likely to become infected if they are not immune.
Pathogenicity: Refers to the inherent capacity of an organism to cause disease. This is a qualitative trait of the organism determined by its genetic makeup. Some organisms, such as the HIV virus, are considered to be primary or true pathogens. These pathogens can cause disease in healthy individuals.
Virulence: A quantitative trait that refers to the extent of damage, or pathology, caused by the organism. For example, Yersinia pestis, the causative agent of bubonic and pneumonic plague, is considered to be extremely virulent. Highly virulent organisms can cause severe and life-threatening diseases.

The degree of damage is mediated by specific virulence factors. Virulence factors may increase an organism’s pathogenicity by contributing to the organism’s ability to establish itself on or in the host, invade or damage host tissue, or evade the host immune response. These factors determine the severity and outcome of the infection.

Bacterial Virulence Factors

Bacterial properties or features that determine whether an organism is pathogenic and able to cause disease are referred to as “virulence factors.” These factors can be structural, enzymatic, or toxic in nature.

Genetic determinants located on the bacterial chromosome are responsible for the production of structural or surface molecules, which help the organism to attach to and colonize the host. For example, genes encoding for adhesins are found on the bacterial chromosome.
The genetic information needed to produce the extracellular substances that are virulence factors is most frequently located on independent genetic elements called plasmids. Plasmids can be transferred between bacteria, leading to the spread of virulence traits.

Structural Virulence Features

Bacterial cells are classified as prokaryotic cells. Bacterial chromosome is not enclosed inside of a membrane-bound nucleus but instead resides inside the bacterial cytoplasm. Bacteria have a cell wall as their outermost feature. The bacterial cell wall prevents osmotic lysis and confers shape and rigidity to the bacteria. Peptidoglycan is the primary component providing the shape and rigidity. The majority of antibiotics used to treat bacterial infections target the production of peptidoglycan in the cell wall, disrupting its synthesis and leading to bacterial cell death.
The bacterial cell wall structure has two different variations:
- gram-positive: characterized by a thick layer of peptidoglycan, which retains the crystal violet stain during Gram staining, giving the bacteria a purple color.
- gram-negative: characterized by a thin layer of peptidoglycan surrounded by an outer membrane containing lipopolysaccharide (LPS). Gram-negative bacteria appear pink or red after Gram staining.

Gram-negative cell walls have a lipopolysaccharide (LPS) layer. Lipid A, also referred to as endotoxin, is a powerful stimulator of cytokine production that leads to a variety of systemic manifestations and potentially fatal endotoxic (gram-negative) shock. Endotoxin is released when the bacterial cell dies or undergoes lysis.

Features are generally involved in the adherence of bacteria or the evasion of the immune system.
- Whip-like structures called flagella can facilitate adherence as well as movement of the bacteria toward a host cell Flagella enable bacteria to swim towards nutrients or away from harmful substances.
- Bacteria have surface structures or molecules that can bind specifically to complementary receptors on specific cells of host tissues. The most common surface structures involved in attachment are fimbriae, which are composed of protein. Fimbriae allow bacteria to adhere to specific host cells, initiating the infection process.
- Attachment may also occur via adhesions or surface molecules on the bacterial cell. Attachment caused by the presence of surface molecules is referred to as afimbrial (non–pili dependent) attachment. Afimbrial adhesions mediate tight binding to host cells.
- A major structural feature that plays an important role in increasing an organism’s virulence is the presence of a capsule, which is usually polysaccharide in nature. Capsules contribute to the organism’s ability to resist innate and adaptive immune responses through a variety of means. Capsules protect bacteria from phagocytosis and complement-mediated killing.

Extracellular Virulence Factors

Extracellular substances produced by bacteria also contribute to an organism’s virulence by breaking down primary or secondary defenses of the body, damaging the host tissues and cells, or facilitating the growth and spread of the organism. Substances that perform the latter function are called invasins. Invasins allow bacteria to penetrate host tissues and spread to other sites in the body.

Bacteria may also produce two types of toxins—endotoxin and exotoxins.

Endotoxin:
- Found in the LPS layer of the cell walls of all gram-negative bacteria.
- Released when the cell dies, leading to its dissemination into the host.
- Induces a variety of host responses, including inflammation, increased heart rate, increased body temperature (fever), and a decrease in blood pressure. These responses are mediated by the release of cytokines.
- Activates the complement cascade, resulting in vasodilation and increased vascular permeability, leading to inflammation and edema.
- Activates the alternative coagulation cascade, producing coagulation, thrombosis, and acute disseminated intravascular coagulation (DIC), leading to hemorrhage and shock. DIC is a life-threatening condition characterized by widespread clotting and bleeding.
- A potential consequence of endotoxin release is septic shock, a severe and often fatal condition characterized by hypotension, organ failure, and death.
- Unlike endotoxin, which has multiple effects on the body, exotoxins have a very specific and targeted activity.
Exotoxins:
- Protein molecules that are released from living bacteria (mostly gram- positive bacteria).
- Classified as neurotoxins, cytotoxins, or enterotoxins, according to their effect on cells. Neurotoxins affect the nervous system, cytotoxins damage or kill cells, and enterotoxins affect the gastrointestinal tract.
- Bind to specific receptors on host cells, leading to their targeted effects.
- Examples include diphtheria toxin (cytotoxin), tetanus toxin and botulism toxin (neurotoxins), and toxins A and B produced by Clostridium difficile (enterotoxins). Diphtheria toxin inhibits protein synthesis, tetanus toxin blocks inhibitory neurotransmitters, botulism toxin blocks acetylcholine release, and toxins A and B cause intestinal inflammation and diarrhea.
- Superantigens can induce activation of up to 20% of T cells, resulting in a massive release of cytokines, leading to toxic shock syndrome. Superantigens bypass normal antigen processing and directly stimulate T cells, leading to an excessive immune response.

Immune Defenses Against Bacterial Infections and Mechanisms of Evasion

Immune Defense Mechanisms

Both innate and adaptive responses may occur after an encounter with foreign antigens. The immune system works to eliminate bacterial infections through various mechanisms.

The first line of defense against potential pathogens involves intact skin and mucosal surfaces that serve as structural barriers. In addition, the epithelial surface may have enzymes (e.g., lysozyme), nonspecific antimicrobial defense peptides (ADPs), or defensins, and proteins that have antimicrobial activity. These components provide a physical and chemical barrier against bacterial invasion.

Bacteria, fungi, and viruses possess pathogen-associated molecular patterns (PAMPs), which are structural patterns consisting of carbohydrates, nucleic acids, or bacterial peptides. PAMPs are recognized by pattern recognition receptors (PRRs) expressed on the cells of the innate immune system. PRRs include Toll-like receptors (TLRs) and NOD-like receptors (NLRs).

Adaptive immune responses include the production of antibodies directed against bacterial antigens or extracellular products produced by bacteria such as exotoxins. Antibody formation is the main defense against extracellular bacteria. Antibodies neutralize toxins, opsonize bacteria for phagocytosis, and activate the complement system.

Cell-mediated immunity (CMI) is helpful in attacking intracellular bacteria, such as Mycobacterium tuberculosis, Legionella pneumophila, Listeria monocytogenes, and Rickettsia species. CMI involves the activation of T cells, which kill infected cells or release cytokines to activate macrophages.

Bacterial Evasion Mechanisms

Bacteria have developed several ways to inhibit the immune system or make it more difficult for immune responses to occur. Three main mechanisms used by bacteria involve avoiding antibody, blocking phagocytosis, and inactivating the complement cascade. These mechanisms allow bacteria to persist and cause chronic infections.

Major mechanisms:

Avoiding Antibody: Bacteria can evade antibodies by altering their bacterial antigens (antigenic variation), coating themselves with host proteins, or hiding their surface molecules through antigenic disguise. Antigenic variation involves changing the surface antigens to avoid recognition by existing antibodies. Coating with host proteins allows bacteria to blend in with the host and evade immune detection. Antigenic disguise involves masking bacterial antigens with host molecules.
Blocking Phagocytosis: Bacteria can inhibit chemotaxis, block adherence of phagocytes, block digestion, or inhibit complement C3b binding. Inhibition of chemotaxis prevents phagocytes from migrating to the site of infection. Blocking adherence prevents phagocytes from attaching to bacteria. Blocking digestion prevents the killing of bacteria within phagocytes. Inhibition of complement C3b binding prevents opsonization and phagocytosis.
Inactivating the Complement Cascade: Some organisms express molecules that disrupt one or more of the complement pathways. These molecules interfere with the activation and function of the complement system, reducing its ability to kill bacteria and promote inflammation.

Toll-Like Receptors (TLRs)

TLRs are expressed on key cells of the innate immune system, such as macrophages and dendritic cells. They recognize molecules that are commonly found in microbial pathogens but not on host cells. Once TLRs have bound to their ligands, cell- signaling pathways are triggered that result in the production of cytokines that enhance the inflammatory response, resulting in more efficient pathogen destruction. TLR activation leads to the recruitment of immune cells, increased phagocytosis, and the production of antimicrobial substances.

Laboratory Detection and Diagnosis of Bacterial Infections

Five general ways can be used to detect the causative agent of a bacterial infection:

Culture or growth of the causative agent: allows for the isolation and identification of bacteria from clinical samples.
Microscopy: enables direct visualization of bacteria in clinical samples using various staining techniques.
Detection of bacterial antigens in the clinical sample: provides rapid and specific identification of bacteria.
Molecular detection of bacterial DNA or RNA: allows for highly sensitive and specific detection of bacteria.
Serology to detect antibodies produced in response to the infection: helps in diagnosing infections that are difficult to culture or detect directly.

Bacterial Culture

Traditional means of determining the cause of a bacterial infection rely largely on growing the organism in culture. Various broth and solid media may be used to recover the organism. Culture allows for the isolation, identification, and antimicrobial susceptibility testing of bacteria.

Microscopic Visualization

Visualization of the causative agents using microscopic techniques is most often done using differential or fluorescent stains. One example is the Gram stain for differentiating gram-positive bacteria from gram-negative bacteria. Microscopic examination can provide rapid preliminary information about the presence and type of bacteria in a sample.

Antigen Detection

Antigen detection assays are available for a wide variety of bacteria, viruses, parasites, and fungi in clinical samples. Testing methodologies include latex agglutination (LA), enzyme-linked immunosorbent assay (ELISA), and lateral flow immunochromatographic assays (LFA). Antigen detection assays are rapid, specific, and easy to perform, making them suitable for point-of-care testing.

Molecular Detection

Rapid developments in the field of molecular diagnostics have allowed for the increased availability and use of nucleic acid–based assays in the clinical laboratory to detect pathogenic microorganisms. The most widely known molecular technology is the polymerase chain reaction (PCR), in which specific genetic sequences are amplified and detected. Molecular detection assays are highly sensitive and specific, allowing for the detection of low levels of bacteria in clinical samples.

Serological Diagnosis

Serology has historically been used to detect and confirm infections from organisms that are difficult to grow or for which other laboratory methods of diagnosing the infection are not available. Serological tests detect antibodies produced by the host in response to bacterial antigens.

Antibody Response Curve

A serological pattern of IgM+, IgG- generally indicates an early-stage, acute infection, whereas an IgM–, IgG+ pattern usually signifies a past exposure. The best indication of a current infection is a four-fold rise in antibody titer. Monitoring antibody titers over time can help determine the stage and course of an infection.

Proteomics

The proteome is the total complement, or set, of proteins expressed by an organism or cell. Proteomics is the analysis and study of the proteins expressed by an organism’s genome. Mass spectrometry (MS) platforms such as MALDI-TOF MS and surface- enhanced laser desorption ionization time-of-flight MS (SELDI-TOF MS) are used to generate a characteristic “protein fingerprint” called a profile. Proteomics allows for the rapid and accurate identification of microorganisms based on their protein profiles.

Group A Streptococci (Streptococcus pyogenes)

Classification and Structure

Gram-positive cocci arranged in pairs or chains. Streptococcus pyogenes appears as spherical cells arranged in chains when viewed under a microscope.
Classified as β-hemolytic, α-hemolytic, or γ-hemolytic based on their effect on sheep red blood cells (RBCs). Beta-hemolytic streptococci cause complete lysis of RBCs, alpha-hemolytic streptococci cause partial lysis, and gamma-hemolytic streptococci cause no lysis.
β-hemolytic streptococci are further classified into Lancefield groups (A through H and K through V). Lancefield grouping is based on the carbohydrate composition of the bacterial cell wall.
S. pyogenes is a member of the β-hemolytic streptococci and is also referred to as Group A streptococcus (GAS). GAS is a common cause of bacterial infections in humans.
Additional cell wall components, the M and T proteins, allow for further classification and differentiation. M protein is a major virulence factor of S. pyogenes.
Serotyping and molecular techniques can be used to identify a particular strain of GAS. These methods allow for strain-level discrimination, which is useful for epidemiological studies.

Virulence Factors

M protein is the major virulence factor. M protein inhibits phagocytosis and complement activation, allowing S. pyogenes to evade the immune system.
Pyrogenic exotoxins A, B, and C are responsible for the rash seen in scarlet fever. These toxins act as superantigens, stimulating a massive immune response.
Additional extracellular substances include the enzymes streptolysin O, deoxyribonuclease B (DNase B), hyaluronidase, nicotinamide adenine dinucleotide (NAD), and streptokinase. These enzymes contribute to the spread and invasiveness of S. pyogenes.

Clinical Manifestations of Group A Streptococcal Infection

S. pyogenes can be responsible for infections ranging from pharyngitis (a throat infection) to life- threatening illnesses such as necrotizing fasciitis and streptococcal toxic shock syndrome. GAS infections can cause a wide spectrum of diseases, ranging from mild to life-threatening.

The two major sites of infections in humans are the upper respiratory tract and the skin, with pharyngitis (“strep throat”) and streptococcal pyoderma (a skin infection) being the most common clinical manifestations. Pharyngitis is characterized by sore throat, fever, and swollen tonsils. Pyoderma is characterized by skin lesions with pus and crusting.
A small percentage of individuals develop scarlet fever. Scarlet fever is characterized by a red, sandpaper-like rash, fever, and sore throat.
Toxic shock syndrome is a life-threatening multisystem disease that often initiates as a skin or soft tissue infection and may proceed to shock and renal failure because of overproduction of cytokines. Streptococcal toxic shock syndrome is similar to staphylococcal toxic shock syndrome.
Necrotizing fasciitis may occur when a GAS skin infection invades the muscles in the extremities or the trunk. The onset is quite acute and is a medical emergency. Necrotizing fasciitis is a severe and rapidly progressive infection that can lead to tissue destruction and death.

Group A Streptococcal Sequelae

The reason GAS receives so much attention is the potential for the development of two serious sequelae, acute rheumatic fever and poststreptococcal glomerulonephritis. These sequelae are immune-mediated complications that can occur after GAS infections.

Acute rheumatic fever develops as a sequela to pharyngitis or tonsillitis. The latency period is typically 1 to 3 weeks after onset of the sore throat. Characteristic features of acute rheumatic fever include fever, pain caused by inflammation in the joints, and inflammation of the heart. Rheumatic fever can lead to chronic heart valve damage.
The second main complication following a streptococcal infection is acute glomerulonephritis, a condition characterized by damage to the glomeruli in the kidneys. This condition may follow infection of either the skin or the pharynx, whereas rheumatic fever follows only upper respiratory tract infections. Glomerulonephritis can lead to kidney failure.

Laboratory Diagnosis

Culture Diagnosis is made by culture of the organism from the infected site. The specimen is plated on sheep blood agar and incubated. If GAS is present, small translucent colonies surrounded by a clear zone of β hemolysis will be visible. Culture is the gold standard for diagnosing GAS infections.
Detection of Group A Streptococcal Antigens rapid assays have been commercially developed to detect Group A streptococcal antigens directly from throat swabs. Rapid antigen tests are convenient for point-of-care testing but have lower sensitivity than culture.
Detection of Streptococcal Antibodies serological diagnosis must be used to identify acute rheumatic fever and post- streptococcal glomerulonephritis. Serological tests detect antibodies produced by the host in response to GAS antigens.
The most diagnostically important antibodies are the following: anti-streptolysin O (ASO), anti-DNase B

Antistreptolysin O (ASO) Testing

ASO tests detect antibodies to the streptolysin O enzyme produced by GAS. The presence of antibodies to streptolysin O indicates recent streptococcal infection. ASO testing is currently performed by nephelometric methods. Elevated ASO titers are indicative of recent GAS infection.

Anti-DNase B (Anti-Deoxyribonuclease B) Testing

Testing for the presence of anti-DNase B is clinically useful in patients suspected of having glomerulonephritis preceded by streptococcal skin infections because ASO antibodies often are not stimulated by this type of disease. Anti-DNase B antibodies are more sensitive for detecting GAS infections of the skin. Nephelometry provides an automated means of testing that can be used for rapid quantitation of anti-DNase B and has largely replaced the enzyme-neutralization method.

Streptozyme Testing

The streptozyme test is a slide agglutination screening test for the detection of antibodies against streptococcal antigens. The streptozyme test measures antibodies against five extracellular streptococcal antigens: ASO, anti-hyaluronidase (AHase), anti-streptokinase (ASKase), anti- nicotinamide-adenine dinucleotide (anti-NAD), and anti-DNAase B antibodies. The streptozyme test is a rapid screening test for detecting recent streptococcal infections but has lower specificity than individual antibody tests.

Helicobacter pylori

Helicobacter pylori is now recognized as a major cause of both gastric and duodenal ulcers. The organism resides in the mucus layer, the gastric epithelium, and occasionally the duodenal epithelium. This gram-negative, microaerophilic spiral bacterium is observed in 30% of the population in developed countries and more than 90% of the population in developing countries. H. pylori infection is more prevalent in developing countries due to factors such as poor sanitation and hygiene.

Helicobacter pylori Virulence Factors

A major virulence factor of H. pylori is the production of the protein CagA, which is highly immunogenic. CagA is injected into host cells, where it disrupts cellular signaling pathways.
A second virulence factor is vacuolating cytotoxin, or VacA. VacA induces apoptosis and inhibits T cell proliferation.

Pathology and Pathogenesis

Unlike most other bacteria, H. pylori is able to survive and multiply in the gastric environment. This occurs because of several characteristics of the bacteria:

Its spiral shape and flagella help the organism to be highly motile and to penetrate the viscous mucus layer in the stomach. The motility of H. pylori allows it to reach the gastric epithelium, where it can colonize and cause damage.
The organism produces large amounts of urease, providing a buffering zone around the bacteria that protects it from the effects of the stomach acid. Urease hydrolyzes urea into ammonia and carbon dioxide, neutralizing the acidic environment.
In addition, the acid-labile flagella are coated with a flagellar sheath, protecting them from the acidic environment of the stomach. The flagellar sheath prevents the denaturation and inactivation of flagella.

Diagnosis of H. pylori Infection

Detection of H. pylori may be achieved by the invasive techniques of endoscopy or biopsy or through noninvasive techniques, including serological analysis, fecal antigen detection, and demonstration of urease production with urea breath tests. Diagnosis requires a combination of methods to ensure accurate detection of H. pylori.

Endoscopy and Biopsy One method of testing for H. pylori involves the detection of urease from a biopsy taken from the antrum of the stomach. Biopsy specimens can also be used for culture and histology.
Noninvasive Detection Methods Procedures that do not require the use of endoscopy include urea breath testing. Urea breath testing measures the amount of carbon dioxide produced after the patient drinks a solution containing urea. If H. pylori is present, it will hydrolyze the urea, producing carbon dioxide that can be detected in the breath.
Detection of H. pylori Antibodies Serological testing is a primary screening method of determining infection with H. pylori. Infections from this organism result in production of IgG, IgA, and IgM antibodies. Most serological tests in clinical use detect H. pylori–specific antibodies of the IgG class. Serological tests are noninvasive and can be used for large-scale screening.

Mycoplasma pneumoniae

Mycoplasma is a member of a unique group of organisms that belong to the class Mollicutes. These extracellular parasites attach to and exist on the surface of host cells using attachment organelles and adhesion molecules specific for their host cells. Mycoplasma lacks a cell wall, making it resistant to antibiotics that target cell wall synthesis.

Mycoplasma pneumoniae Pathogenesis

The best-known Mycoplasma is M. pneumoniae, which is a leading cause of respiratory infections worldwide. M. pneumoniae infections are found in all age groups, with a majority of the infections involving the upper respiratory tract. M. pneumoniae causes atypical pneumonia, also known as "walking pneumonia."

Unlike most respiratory infections, the incubation period is 1 to 3 weeks. This long incubation period can make it difficult to trace the source of infection.

Immunology of Mycoplasma pneumoniae Infection

In addition to stimulating the production of many proinflammatory and anti-inflammatory cytokines and chemokines, several classes of antibodies are produced in the course of a M. pneumoniae infection. These antibodies play a role in clearing the infection and providing immunity.

Laboratory Diagnosis of Mycoplasma pneumoniae Infection

Laboratory means of detecting Mycoplasma infection may involve culturing of the organism, detection of M. pneumoniae
-specific antibodies in serum, and detection of M. pneumoniae- specific antigens or nucleotide sequences directly in patient specimens. Diagnosis requires a combination of methods due to the fastidious nature of Mycoplasma.

Detection of Mycoplasma pneumoniae by Culture: Culture is difficult and time-consuming due to the slow growth rate of Mycoplasma.
Detection of Antibodies to Mycoplasma pneumoniae: Serological tests, such as complement fixation and ELISA, are commonly used to detect antibodies to M. pneumoniae.
Detection of Cold Agglutinins: Cold agglutinins are antibodies that cause red blood cells to agglutinate at low temperatures. Cold agglutinin testing is nonspecific but can provide a clue to M. pneumoniae infection.
Molecular Diagnosis of Mycoplasma Infections: PCR and other molecular methods provide rapid and sensitive detection of M. pneumoniae DNA in clinical samples.

Rickettsial Infections

Members of the Rickettsiaceae family cause a variety of infections in man and animals. These obligate intracellular, gram-negative bacteria now include the genera Rickettsia (rickettsiosis) and Orientia (Orientia tsutsugamushi causing scrub typhus). Rickettsial infections are transmitted to humans by arthropod vectors.

Agents of Rickettsia-Related Disease

The genus Rickettsia is made up of two distinct groups: the spotted fever group (SFG) and the typhus group (TG). The SFG includes R. rickettsii (Rocky Mountain spotted fever) and R. conorii (Mediterranean spotted fever). The TG includes R. prowazekii (epidemic typhus) and R. typhi (endemic typhus).

Each of the species responsible for the various Rickettsial diseases has a variety of animal reservoirs. The vectors responsible for the transmission are arthropods (ticks, mites, lice, or fleas) that transmit the organism through its bite after feeding on an infected animal. Understanding the animal reservoirs and vectors is important for preventing and controlling Rickettsial infections.

Rocky Mountain Spotted Fever

RMSF is caused by R. rickettsii. The organism is transmitted to the human host by the bite of a tick. The symptoms observed with RMSF occur approximately 2 to 14 days (median 7 days) after a tick bite. RMSF is a serious and potentially fatal disease if not treated promptly.

Pathogenesis

Once introduced into the skin, the organisms spread via the lymphatic and circulatory system, where they attach to and invade their target cells. R. rickettsii primarily infects endothelial cells, leading to vasculitis and damage to blood vessels.

Diagnosis of RMSF

Initial diagnosis is often made clinically after ruling out a large variety of other conditions. Clinical features of RMSF include fever, headache, rash, and history of tick bite. The rash typically starts on the wrists and ankles and spreads to the trunk.

Serological and Molecular Diagnosis

Serology is the usual method for confirming the diagnosis of RMSF, but this is a retrospective diagnosis. The gold standard for the serological diagnosis of RMSF is the indirect immunofluorescence assay (IFA) with R. rickettsii antigen, performed on two paired serum samples to demonstrate a significant (four-fold) rise in antibody titers. For many years, antibodies produced in patients with Rickettsial infections were detected by an agglutination test known as the Weil-Felix test, which was based on cross-reactivity of

Group A Streptococci (Streptococcus pyogenes)