Chapter eleven notes microbiology | Study notes Microbiology

Chapter 11

11.1 Characteristics of Infectious Disease

Signs and Symptoms of Disease

An infection occurs when a microorganism successfully colonizes a host, potentially leading to

disease. Disease results in signs (objective, measurable indicators like fever, abnormal vital signs,

or antibodies in the blood) and symptoms (subjective experiences like pain, nausea, or loss of

appetite). Signs can be clinically observed, while symptoms rely on patient reporting.

A syndrome is a specific set of signs and symptoms associated with a disease. Diagnosing

diseases can be challenging since different pathogens can cause similar signs and symptoms.

Some diseases may also be asymptomatic, meaning they present no noticeable signs or

symptoms, as seen in many herpes simplex virus infections.

Cyto: cell; hepat- of the liver; -pathy disease; -emia of the blood; -itis inflammation; -lysis

destruction; -oma tumor; -osis diseased or abnormal condition; -derma of the skin.

Classifications of Disease

The World Health Organization (WHO) uses the International Classification of Diseases

(ICD) to classify diseases and track morbidity (cases) and mortality (deaths). Diseases can be

categorized as infectious or non-infectious. Infectious diseases result from pathogens, which can

be cellular (bacteria, fungi, parasites) or acellular (viruses, viroid’s, prions). Some are

communicable, meaning they spread from person to person, while contagious diseases are

highly transmissible, like measles. Others, like Gonorrhea is not as contagious as measles, it

requires close intimate contact (usually sexual). Certain diseases are acquired in healthcare

settings. Iatrogenic diseases result from medical procedures involving wound treatments,

catheterization, or surgery becomes contaminated while nosocomial diseases (hospital-acquired

infections) occur due to contaminated equipment, improper hygiene, or antibiotic-resistant

bacteria. First, sick patients bring numerous pathogens into hospitals, and some of these

Pathogens can be transmitted easily via improperly sterilized medical equipment, bedsheets, call

buttons, door handles, or by clinicians, nurses, or therapists who do not wash their hands before

touching a patient. Second, many hospital patients have weakened immune systems, making them

more susceptible to infections. Some infectious diseases originate from animals, known as

zoonotic diseases (e.g., rabies, Rocky Mountain spotted fever). A zoonosis is a disease that

occurs when a pathogen is transferred from a vertebrate animal to a human; however, sometimes

the term is defined more broadly to include diseases transmitted by all animals (including

invertebrates). Noncommunicable infectious diseases, like tetanus caused by Clostridium tetani,

(a bacterium that produces endospores) and Legionnaires’ disease, caused by Legionella

pneumophila, (a bacterium that lives within amoebae in moist locations like water-cooling

towers) do not spread from person to person but come from environmental sources. Non

infectious diseases are not caused by pathogens but result from genetic, environmental, or

immune system factors. For example, sickle cell anemia is an inherited genetic disorder.

Periods of Disease:

The five periods of disease include incubation, prodromal, illness, decline, and convalescence.

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Chapter 11 11.1 Characteristics of Infectious Disease Signs and Symptoms of Disease An infection occurs when a microorganism successfully colonizes a host, potentially leading to disease. Disease results in signs (objective, measurable indicators like fever, abnormal vital signs, or antibodies in the blood) and symptoms (subjective experiences like pain, nausea, or loss of appetite). Signs can be clinically observed, while symptoms rely on patient reporting. A syndrome is a specific set of signs and symptoms associated with a disease. Diagnosing diseases can be challenging since different pathogens can cause similar signs and symptoms. Some diseases may also be asymptomatic, meaning they present no noticeable signs or symptoms, as seen in many herpes simplex virus infections. Cyto: cell; hepat- of the liver; -pathy disease; -emia of the blood; -itis inflammation; -lysis destruction; -oma tumor; -osis diseased or abnormal condition; -derma of the skin. Classifications of Disease The World Health Organization (WHO) uses the International Classification of Diseases (ICD) to classify diseases and track morbidity (cases) and mortality (deaths). Diseases can be categorized as infectious or non-infectious. Infectious diseases result from pathogens, which can be cellular (bacteria, fungi, parasites) or acellular (viruses, viroid’s, prions). Some are communicable , meaning they spread from person to person, while contagious diseases are highly transmissible, like measles. Others, like Gonorrhea is not as contagious as measles, it requires close intimate contact (usually sexual). Certain diseases are acquired in healthcare settings. Iatrogenic diseases result from medical procedures involving wound treatments, catheterization, or surgery becomes contaminated while nosocomial diseases (hospital-acquired infections) occur due to contaminated equipment, improper hygiene, or antibiotic-resistant bacteria. First, sick patients bring numerous pathogens into hospitals, and some of these Pathogens can be transmitted easily via improperly sterilized medical equipment, bedsheets, call buttons, door handles, or by clinicians, nurses, or therapists who do not wash their hands before touching a patient. Second, many hospital patients have weakened immune systems, making them more susceptible to infections. Some infectious diseases originate from animals, known as zoonotic diseases (e.g., rabies, Rocky Mountain spotted fever). A zoonosis is a disease that occurs when a pathogen is transferred from a vertebrate animal to a human; however, sometimes the term is defined more broadly to include diseases transmitted by all animals (including invertebrates). Noncommunicable infectious diseases , like tetanus caused by Clostridium tetani, (a bacterium that produces endospores) and Legionnaires’ disease, caused by Legionella pneumophila , (a bacterium that lives within amoebae in moist locations like water-cooling towers) do not spread from person to person but come from environmental sources. Non infectious diseases are not caused by pathogens but result from genetic, environmental, or immune system factors. For example, sickle cell anemia is an inherited genetic disorder. Periods of Disease: The five periods of disease include incubation, prodromal, illness, decline, and convalescence.

The incubation period begins after the pathogen enters the host and starts multiplying, but there are not enough pathogen particles to cause symptoms. This period varies in length depending on factors such as pathogen strength, immune response, and infection site. The prodromal period follows, during which the pathogen continues to multiply, and the host experiences general symptoms like fever, pain, or inflammation, but these are too vague to diagnose a specific disease.The illness period is when symptoms are most severe, followed by the decline period , during which pathogen levels decrease, and symptoms lessen. However, patients are vulnerable to secondary infections due to a weakened immune system. The convalescence period is the recovery stage, though some diseases may cause lasting damage. Infectious diseases can be contagious during any of these periods, depending on the pathogen and transmission method. For example, bacterial meningitis can be transmitted during the incubation period, while viral meningitis becomes contagious in the prodromal stage. Respiratory infections, such as colds and flu, are typically contagious from the prodromal period onward. Some diseases can still spread even after symptoms disappear. Acute and Chronic Diseases: Diseases can be classified based on their duration and progression as acute, chronic, or latent. Acute diseases develop quickly and last for a short time, typically days to weeks. For example, influenza has a rapid onset, an incubation period of 1–2 days, and a contagious period of about 5 days. Symptoms peak within a week before declining. Chronic diseases progress slowly and can last for months, years, or a lifetime. For example, chronic gastritis caused by Helicobacter pylori can persist indefinitely unless treated with antibiotics, as the bacteria survive in the stomach’s acidic environment. Latent diseases involve a pathogen that remains dormant for extended periods before reactivating. For example, the varicella-zoster virus (VZV) causes chickenpox in childhood but can stay inactive in the nervous system for years. It may later reactivate as shingles in adulthood, especially during stress or immunosuppression. 11.2 How Pathogens Cause Disease Pathogenicity and Virulence Pathogenicity refers to a microorganism’s ability to cause disease, while virulence describes its degree of pathogenicity. Virulence is a continuum. Virulence exists on a spectrum, ranging from avirulent (harmless) organisms to highly virulent pathogens that almost always cause severe disease, sometimes leading to organ failure. Less virulent pathogens may only cause mild symptoms like fever or headache, and some individuals may remain asymptomatic. Virulence is measured using median infectious dose (ID50) and median lethal dose (LD50) in controlled animal studies. ID50 is the number of pathogen cells or virions needed to infect 50% of a test group, while LD50 is the amount required to kill 50% of infected animals. These values help determine a pathogen’s potency and potential severity. Primary Pathogens versus Opportunistic Pathogens Pathogens are classified as primary or opportunistic. Primary pathogens can cause disease in healthy individuals, regardless of their immune system or microbiota. For example, E. coli (EHEC) produces Shiga toxin, leading to severe diarrhea and kidney failure.

However, some intracellular pathogens can survive and multiply inside phagocytes, evading immune defenses.

Infection : Infection occurs when a pathogen successfully multiplies within the host after invasion. Infections can be classified based on their spread and severity. -A local infection remains confined to a specific area near the portal of entry, such as a Staphylococcus aureus infection in a hair follicle that results in a boil. Other examples include urinary tract infections limited to the bladder and pneumonia affecting only the lungs. -A focal infection begins as a localized infection but spreads to a secondary site, often through the bloodstream. For instance, bacteria from an oral infection caused by Streptococcus may enter the bloodstream and travel to other parts of the body, leading to a secondary infection. -In systemic infections , the pathogen disseminates throughout the body, affecting multiple organs or systems. An example is varicella-zoster virus, which initially infects the respiratory system before spreading through the bloodstream, resulting in the characteristic skin lesions of chickenpox. -Primary infections , caused by an initial pathogen, can sometimes lead to secondary infections. For example, HIV weakens the immune system, making the patient more susceptible to opportunistic infections such as oral thrush. Example: UTI leading to yeast infection. Cipro to Tx UTI but it killed the normal microbiota and lead to Yeast infection. -Secondary infections can also arise due to treatment for a primary infection. The use of antibiotics may disrupt the normal microbiota, allowing opportunistic pathogens to flourish, as seen in antibiotic-associated yeast infections. Transmission of Disease For a pathogen to continue spreading, it must exit the infected host through a portal of exit and reach a new host. The most common portals of exit include the skin, respiratory tract, urogenital tract, and gastrointestinal tract. Pathogens can be expelled through coughing and sneezing , with a single sneeze releasing thousands of virus particles into the air. Other bodily secretions and excretions such as feces, urine, semen, vaginal secretions, tears, sweat, and shed skin cells also serve as exit routes. Some pathogens use insect vectors for transmission, exiting through blood extracted by biting insects , while others leave the body through blood drawn by needles. 11.3 Virulence Factors of Bacterial and Viral Pathogens Bacterial Exoenzymes and Toxins as Virulence Factors After exposure and adhesion , the next stage of pathogenesis is invasion , which often involves the use of exoenzymes and toxins. Many pathogens spread by entering the bloodstream , which allows them to reach nearly every cell in the body. However, this also exposes them to the immune system. Different terms describe the presence of pathogens in the blood: bacteremia refers to bacteria in the blood , viremia refers to viruses in the blood , and toxemia occurs when

toxins are present in the blood. If bacteria are both present and multiplying , it is called septicemia , and patients with this condition are described as septic. Exoenzymes are extracellular enzymes that help pathogens invade host cells and spread deeper into tissues. These enzymes target different tissue structures to promote invasion, growth, and immune evasion. For example, hyaluronidase S , produced by Staphylococcus aureus , breaks down hyaluronic acid , a substance that holds connective tissue cells together. This allows the pathogen to penetrate deeper into the body. Other exoenzymes can degrade phospholipid membranes, collagen, or DNA released by dying cells , further aiding in the infection process. Toxins: Certain pathogens produce toxins , biological poisons that help them invade and damage host tissues. The ability to produce toxins is called toxigenicity. Toxins are classified into endotoxins and exotoxins. 1) Endotoxins are found in the lipopolysaccharide (LPS) outer membrane of gram-negative bacteria. When these bacteria die or undergo binary fission, they release lipid A, the toxic component of LPS. Lipid A triggers an immune inflammatory response. In low concentrations, this can help fight infection, but high levels may lead to severe inflammation, multi-organ failure, and death. Endotoxins are heat stable and require extreme temperatures (121°C for 45 minutes) for inactivation. 2) Exotoxins , primarily produced by gram-positive bacteria, are protein-based and cause specific cellular damage. Unlike endotoxins, exotoxins target cells and work through precise molecular mechanisms. They are generally heat-labile, meaning they lose function at high temperatures above 41°C. Exotoxins are far more potent than endotoxins; for instance, botulinum toxin has an LD50 (lethal dose) of 0.000001 mg/kg, making it 240,000 times more lethal than endotoxin. Endotoxin: Source: Gram negative bacteria ; Composition: Lipid A component of LPS ; Effect on host: General systemic symptoms of inflammation and fever ; Heat stability: heat stable ; LD50: High Exotoxin: Source: Gram-positive (primarily) and gram-negative bacteria ; Composition: Protein ; Effect on host: Specific damage to cells dependent upon receptor-mediated targeting of cells and specific mechanisms of action; Heat stability: Most are heat labile, but some are heat stable ; LD50: Low Exotoxins fall into three categories:

Intracellular Targeting (A-B Exotoxins) – These toxins have two subunits : o A (active) subunit , which enters the cell and disrupts functions. o B (binding) subunit , which helps the toxin attach to specific receptors. Once the A-B toxin binds to the host cell, it is brought into the cell by endocytosis and entrapped in a vacuole. The A and B subunits separate as the vacuole acidifies. The A subunit then enters the cell cytoplasm and interferes with the specific internal cellular function that it targets. o Examples include diphtheria, cholera, botulinum, and tetanus toxins.
Membrane-Disrupting Toxins – These damage host cell membranes by: o Forming pores that cause leakage.

o Influenza viruses rapidly evolve , requiring new vaccines annually. These immune evasion strategies allow pathogens to persist, spread, and cause disease.

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