FY OEB 107 MICROBES IN HUMAN HEALTH UNIT 1.docx

**MICROBES IN HUMAN HEALTH** **Unit 1 Role of microorganisms in causing infection** **(1.1)** a. **Definition of Microbiology and Microorganism** Microbiology is the study of all living organisms that are too small to be visible to the naked eye. This includes bacteria, archaea, viruses, fungi,...

**MICROBES IN HUMAN HEALTH** **Unit 1 Role of microorganisms in causing infection** **(1.1)** a. **Definition of Microbiology and Microorganism** Microbiology is the study of all living organisms that are too small to be visible to the naked eye. This includes bacteria, archaea, viruses, fungi, and algae, collectively known as \'microbes\'. A microorganism, or microbe, is a microscopic organism that is too small to be seen with the naked eye. These organisms can be bacteria, viruses, fungi, protozoa, and some types of algae. Microorganisms are found in nearly every environment on Earth, from soil and water to extreme environments like hot springs and deep-sea vents. Examples of microorganisms  Bacteria Viruses  Fungi Algae Microorganisms play a crucial role in various natural processes and have numerous benefits in different fields. Here are some of the key benefits of microorganisms: Environmental Benefit: Microorganisms break down dead organic matter, recycling nutrients like carbon, nitrogen, and phosphorus back into the ecosystem, which is essential for soil fertility and plant growth. Certain microorganisms can degrade or detoxify pollutants in the environment, such as oil spills, heavy metals, and pesticides, helping to clean up contaminated sites. Health and Medicine**:** Beneficial bacteria, like *Lactobacillu*s found in the human gut and certain foods, help maintain a healthy digestive system and boost the immune system. Many antibiotics, such as penicillin, are derived from microorganisms. These antibiotics are used to treat bacterial infections. Microorganisms are used in the production of vaccines, which are essential for preventing infectious diseases. Food and Beverage Industry: Microorganisms are essential in the fermentation process used to produce foods and beverages like yogurt, cheese, bread, beer, wine, and sauerkraut. For example, yeast (a fungus) is used in bread and alcohol production. While microorganisms offer numerous benefits, they can also have significant disadvantages, especially when they cause diseases or disrupt ecosystems. Here are some of the key disadvantages of microorganisms: Pathogenic Infections: Certain microorganisms, such as bacteria, viruses, fungi, and parasites, can cause a wide range of diseases in humans and animals. Food Spoilage and Contamination: Pathogenic microorganisms can contaminate food and cause foodborne illnesses. For example, *E. coli* can cause severe food poisoning. Microorganisms can spoil food by growing on it, leading to the production of off-flavors, odors, and harmful toxins, which render the food unsafe for consumption. Plant disease: Some microorganisms cause diseases in crops, leading to significant agricultural losses. Environmental impact: Some types of algae can grow excessively in water bodies, leading to harmful algal blooms. These blooms can produce toxins that harm aquatic life, disrupt ecosystems, and pose health risks to humans. b. **Importance of Microbiology in human health.** Microbiology in medicine is significant for several reasons. Microbiologists can recognize, isolate, diagnose, and prevent harmful bacteria due to their expertise in medical microbiology. The term "microbiome" was coined to describe the collective community of bacteria, viruses, fungi, and other microbes that colonize a particular space or ecosystem for example, a microbiome of the soil, or a microbiome of a body of water. In humans, the term is often used to describe the microorganisms that live in or on a particular part of the body, such as the skin or gastrointestinal tract. The microbiome is the collection of all microbes, such as bacteria, fungi, viruses, and their genes, that naturally live on our bodies and inside us. Although microbes are so small that they require a microscope to see them, they contribute in big ways to human health and wellness. The composition of microbiota varies from site to site. Gut microbiota is considered the most significant part of maintaining our health. The gut bacteria serve several functions, such as fermentation of food, protection against pathogens, stimulating immune response, and vitamin production. Altogether, the human microbiome thrives in optimal growth conditions, depending on the natural environment of the body. When the natural environment of the body is altered, this results in microbial composition and diversity shifting to adapt to the changing environment, potentially resulting in disease' The microbiome is important for maintaining human health, and when things go wrong it can contribute to disease. To understand how microbes influence human disease, we first need to understand the microbial makeup of a healthy person.E. coli, yeast, Staphylococcus aureus, and Clostridium difficile are all examples of potential pathogens that can be found in healthy individuals, but cause disease only when the microbiome is disturbed or the microbes gain access to a part of the body where they do not normally live. Newborn babies get their first microbiome from their mother during birth. During that journey, a newborn baby gets completely covered with bacteria, giving it a brand-new microbiome. Regarding the immune system, the human microbiota not only protects the host from external pathogens by producing antimicrobial substances but also serves as a significant component in the development of intestinal mucosa and the immune system. The microbiome refers to the totality of microbes (the microbiota, which includes archaea, fungi, and viruses as well as bacteria and is, therefore, to be preferred to the term flora), their genetic elements (genomes), and environmental interactions in a defined environment, in this instance, the human gut. The human microbiome has extensive functions such as the development of immunity, defense against pathogens, and host nutrition including the production of short-chain fatty acids important in host energy metabolism, synthesis of vitamins and fat storage as well as an influence on human behavior, The microbiome is made up of all the bacteria and other tiny organisms that live in and on our bodies. These microbes are not harmful invaders but helpful partners that are essential for our development, immune system, and nutrition. When the microbiome is out of balance, it can lead to autoimmune diseases like diabetes, rheumatoid arthritis, muscular dystrophy, multiple sclerosis, and fibromyalgia. These diseases occur when harmful microbes build up over time, changing how our genes work and how our bodies process food. This can cause the immune system to mistakenly attack the body's tissues. Understanding the wide variety of microbes in the human microbiome could lead to new treatments. For example, instead of just killing \"bad\" bacteria that cause infections, doctors might help people grow more \"good\" bacteria to fight off the bad ones. The Human Microbiome Project (HMP) is a guide for discovering how the microbiome affects health, nutrition, the immune system, and disease. c. **Role of the microbiome in human health** The gastrointestinal microbiota plays a role in host physiology, metabolism, and nutrition. An alteration in the gut microbial community is linked to several intestinal conditions, including cancer, obesity, and a variety of bowel disorders. A biome is a distinct ecosystem characterized by its environment and its inhabitants. Your gut inside your intestines is a miniature biome, populated by trillions of microscopic organisms. These microorganisms include over a thousand species of bacteria, as well as viruses, fungi, and parasites. Your microbiome includes both "good" and "bad" bacteria. They create a healthy balance in the body. Some things, such as certain diets or antibiotics, can disrupt it. But your lifestyle can help it. Trillions of bacteria and other microbes live in your gut, forming your unique gut microbiome. These microbes play a big role in your health, including helping your immune system and digesting your food. Your diet is very important for keeping your gut microbiome healthy. Knowing which microbes are in your gut can help you choose the best foods to nourish them and support your health. 1. Your gut microbiome is the collection of all the genetic material from the microbes in your gut. 2. It contains around 3 million genes. 3. Gut microbes digest fiber from your food, shape your immune system, and help protect you from pathogens. 4. Your diet is a key factor in determining which microbes are in your gut. 5. Fiber-rich, minimally processed foods support a healthy microbiome. 6. Ultra-processed foods promote the growth of harmful microbes. There are trillions of microbes that live in your gut. Bacteria are the most common type. But there are also viruses, fungi, and [protozoa](https://microbiologysociety.org/why-microbiology-matters/what-is-microbiology/protozoa.html), which are small organisms made up of single cells. Together these make the gut microbiota, the collection of all the organisms that live in the gut. Everyone's microbiome is different. Diet, drugs, genetics, the way you were born, and your age all play a role in determining which microbes live in your gut. The gut microbiome has several roles, including extracting nutrients from food, digesting fiber and proteins from food, making vitamins B and K, shaping the immune system, and protecting the body from pathogens. One important job of the gut microbiome is to help digest fiber from the food you eat since your body can't break down fiber on its own. The microbes in your gut turn fiber into molecules called short-chain fatty acids (SCFAs). These molecules are important for gut health, controlling blood sugar and blood fat levels, managing appetite, and supporting the immune system. Additionally, your microbes protect you by preventing harmful germs (pathogens) from growing in your gut. They do this by competing for space, keeping the intestinal barrier healthy, and producing molecules that kill harmful invaders. The precise mix of microbes in your gut directly influences your health. This is because some microbes are beneficial and others harmful. Tipping the balance in favor of harmful species can contribute to many health conditions, including [obesity](https://www.bmj.com/content/361/bmj.k2179) and [autoimmune diseases](https://www.nature.com/articles/d41586-021-01837-8). A typical Western diet which is high in sugar, fats, and ultra-processed foods and low in fiber can be detrimental to the microbial diversity in the gut. Eating [ultra-processed foods](https://joinzoe.com/post/processed-food-gut-health) which are typically high in chemical additives, sugar, fat, and salt and low in fiber was associated with higher levels of potentially harmful, or "[bad](https://joinzoe.com/post/good-bugs-bad-bugs)," microbes but also have more beneficial, or "good," microbes in the guts of volunteers who ate a diet that heavily featured [plant-based and minimally processed foods](https://joinzoe.com/post/best-foods-for-gut-bacteria-microbiome). However, there is no one-size-fits-all approach to nurturing the beneficial microbes in the gut because we all have unique gut microbiomes. Understanding which microbes live in your gut can help you understand what foods can help them thrive. Many bacteria in our bodies are beneficial, such as those in the gut that help digest food, produce vitamins, and protect against harmful microbes. d. **Types of microbes that affect humans** Microbes that affect humans can be classified into several types based on their biological characteristics. Each type includes various species or strains that can cause different effects, ranging from beneficial interactions to harmful diseases. Here are the main types of microbes that affect humans: 1\. Bacteria: Bacteria are single-celled organisms that can be found in various environments, including the human body. While many bacteria are harmless or beneficial, some can cause infections. Pathogenic Bacteria: *Escherichia coli (E. coli), Streptococcus pneumoniae* 2\. Viruses: Viruses are microscopic agents that can only replicate inside the living cells of a host organism. They are responsible for a wide range of diseases. Example - *Influenza Viru, Human Immunodeficiency Virus (HIV)*, Corona virus 3\. Fungi: Fungi are a diverse group of organisms that include yeasts, molds, and mushrooms. Some fungi can cause infections in humans, particularly in those with weakened immune systems. example-*Candida albicans, Aspergillus* 4\. Algae: While most algae are harmless, some produce toxins that can affect humans, especially through contaminated water or seafood. example - Cyanobacteria Some bacteria can cause infections and diseases, like *Staphylococcus* (which can cause skin infections) and *Streptococcus* (which can cause strep throat). Viruses like the flu virus and the common cold virus infect cells and can cause illnesses ranging from mild to severe. Some fungi are helpful, like *Saccharomyces* used in baking and brewing, or those that live on our skin and help protect against infections. Other fungi can cause infections, such as *Candida* (which can cause yeast infections) and *Aspergillus* (which can cause lung infections). **e**. **Ways in which humans are infected with microbes** Transmission via contact includes direct skin-to-skin or mucous membrane-to-mucous membrane contact or fecal-oral transmission of intestinal bacteria. Transfusion of contaminated blood products also transmits several bacterial infections, such as syphilis. 1.Direct contact Direct contact can be categorized as vertical, horizontal, or droplet transmission. Vertical direct contact transmission occurs when pathogens are transmitted from mother to child during pregnancy, birth, or breastfeeding. Other kinds of direct contact transmission are called horizontal direct contact transmission. Often, contact between mucous membranes is required for entry of the pathogen into the new host, although skin-to-skin contact can lead to mucous membrane contact if the new host subsequently touches a mucous membrane. Contact transmission may also be site-specific; for example, some diseases can be transmitted by sexual contact but not by other forms of contact. Microbes can spread through direct contact with an infected person, such as shaking hands, hugging, or kissing. For example, the common cold and flu viruses often spread this way Some infections are transmitted from animals to humans, like rabies from an animal bite or ringworm from petting an infected animal. 2\. Airborne Transmission When an individual coughs or sneezes, small droplets of mucus that may contain pathogens are ejected. This leads to direct droplet transmission, which refers to the droplet transmission of a pathogen to a new host over distances of one meter or less. A wide variety of diseases are transmitted by droplets, including influenza and many forms of pneumonia. Transmission over distances greater than one meter is called airborne transmission. Indirect contact transmission involves inanimate objects called fomites that become contaminated by pathogens from an infected individual or reservoir. For example, an individual with the common cold may sneeze, causing droplets to land on a fomite such as a tablecloth or carpet, or the individual may wipe her nose and then transfer mucus to a fomite such as a doorknob or towel. Transmission occurs indirectly when a new susceptible host later touches the fomite and transfers the contaminated material to a susceptible portal of entry. Fomites can also include objects used in clinical settings that are not properly sterilized, such as syringes, needles, catheters, and surgical equipment. Pathogens transmitted indirectly via such fomites are a major cause of healthcare-associated infections  3\. Contaminated Food and Water The term vehicle transmission refers to the transmission of pathogens through vehicles such as water, food, and air. Water contamination through poor sanitation methods leads to waterborne transmission of disease. Waterborne disease remains a serious problem in many regions throughout the world. The World Health Organization (WHO) estimates that contaminated drinking water is responsible for more than 500,000 deaths each year. Similarly, food contaminated through poor handling or storage can lead to foodborne transmission of disease. Dust and fine particles known as aerosols, which can float in the air, can carry pathogens and facilitate the airborne transmission of disease. For example, dust particles are the dominant mode of transmission of hantavirus to humans. Hantavirus is found in mouse feces, urine, and saliva, but when these substances dry, they can disintegrate into fine particles that can become airborne when disturbed; inhalation of these particles can lead to a serious and sometimes fatal respiratory infection. Although droplet transmission over short distances is considered contact transmission as discussed above, longer distance transmission of droplets through the air is considered vehicle transmission. Unlike larger particles that drop quickly out of the air column, fine mucus droplets produced by coughs or sneezes can remain suspended for long periods of time, traveling considerable distances. In certain conditions, droplets desiccate quickly to produce a droplet nucleus that is capable of transmitting pathogens; air temperature and humidity can have an impact on effectiveness of airborne transmission. Tuberculosis is often transmitted via airborne transmission when the causative agent, *Mycobacterium tuberculosis*, is released in small particles with coughs. Because tuberculosis requires as few as 10 microbes to initiate a new infection, patients with tuberculosis must be treated in rooms equipped with special ventilation, and anyone entering the room should wear a mask. 1. Foodborne Illnesses: Eating food contaminated with harmful bacteria, viruses, or parasites, like *Salmonella* or *E. coli,* can cause food poisoning. 2. Waterborne Diseases: Drinking or using water contaminated with microbes, such as *Giardia* or *Vibrio cholerae,* can lead to illnesses like diarrhea or cholera. 4.Vector transmission Diseases can also be transmitted by a mechanical or biological vector, an animal (typically an arthropod) that carries the disease from one host to another. Mechanical transmission is facilitated by a mechanical vector, an animal that carries a pathogen from one host to another without being infected itself. For example, a fly may land on fecal matter and later transmit bacteria from the feces to food that it lands on; a human eating the food may then become infected by the bacteria, resulting in a case of diarrhea or dysentery. Biological transmission occurs when the pathogen reproduces within a biological vector that transmits the pathogen from one host to another. Arthropods are the main vectors responsible for biological transmission. Most arthropod vectors transmit the pathogen by biting the host, creating a wound that serves as a portal of entry. The pathogen may go through part of its reproductive cycle in the gut or salivary glands of the arthropod to facilitate its transmission through the bite. For example, hemipterans (called "kissing bugs" or "assassin bugs") transmit Chagas disease to humans by defecating when they bite, after which the human scratches or rubs the infected feces into a mucous membrane or breaks in the skin.Biological insect vectors include mosquitoes, which transmit malaria and other diseases, and lice, which transmit typhus.  5\. Cuts or Injuries 1. Open Wounds: Cuts, scrapes, or other injuries that break the skin can allow bacteria and other microbes to enter the body, potentially leading to infections like tetanus. 6\. Sexual Contact 1. Sexually Transmitted Infections (STIs): Microbes can be transmitted through sexual contact, leading to infections like HIV, chlamydia, and herpes. These pathways highlight the importance of hygiene, safe practices, and preventive measures to reduce the risk of microbial infections. **UNIT 1 (1.2)** a. **An infection and diseases: primary and secondary infection, contagious infection, opportunistic pathogen, zoonoses and vector-borne infection** Primary infection Primary infection is the initial infection caused by a pathogen in a previously healthy individual. This infection is typically the first encounter the host has with the pathogen. Example: The first time someone catches the flu virus and develops flu symptoms. Secondary infection Secondary infection occurs during or after treatment for the primary infection. It is caused by a different pathogen than the one causing the primary infection and often takes advantage of the weakened immune system or damaged tissues from the primary infection. Example: A person who has the flu might develop bacterial pneumonia as a secondary infection. The flu virus weakens the respiratory system, making it easier for bacteria to cause a subsequent infection. Sometimes a primary infection, the initial infection caused by one pathogen, can lead to a secondary infection by another pathogen. For example, the immune system of a patient with a primary infection by HIV becomes compromised, making the patient more susceptible to secondary diseases like oral thrush and others caused by opportunistic pathogens. Similarly, a primary infection by Influenza virus damages and decreases the lungs\' defense mechanisms, making patients more susceptible to secondary pneumonia by a bacterial pathogen like *Streptococcus pneumoniae*. Some secondary infections can even develop as a result of treatment for a primary infection. Antibiotic therapy targeting the primary pathogen can cause collateral damage to the normal microbiota, creating an opening for opportunistic pathogens  Contagious infection Contagious infection: An infection that can easily spread from one person to another, typically through direct contact or close proximity. An infectious illness spreads by an infectious agent entering the body. A contagious illness is spread by coming into contact with someone who is sick (who is infected with an infectious agent). Something contagious (such as a virus or bacteria) is spread from one person or animal to another by touching or by coming into contact with another person or animal\'s germs. All contagious diseases are also infectious because if you can catch it from someone then their germs (viruses or bacteria) are being passed to you. But not all infectious diseases are also contagious. Infectious illnesses are spread by an infectious (infection causing) agent (such as a virus or bacteria) entering the body, but some infectious illnesses, such as food poisoning, cannot be spread to another person. For example, a cold is contagious and infectious: an infectious agent (e.g. a cold virus) enters the body and is passed to other people by contact (e.g. shaking hands or kissing). Food poisoning is only infectious: an infectious agent (e.g. bacteria) enters the body but you cannot spread food poisoning to other people. Chickenpox, spreads through direct contact with an infected person\'s rash or through the air from respiratory droplets. Opportunistic pathogen Opportunistic pathogen: A microorganism that normally does not cause disease in a healthy individual but can cause infections in people with weakened immune systems. They are normally present in the body without causing [disease](https://www.biologyonline.com/dictionary/disease) but cause an infectious disease in certain conditions, such as when the immunological response of the host is low. The low resistance of the body to opportunistic pathogens may occur due to many factors such as malnutrition, [HIV](https://courses.lumenlearning.com/waymakercollegealgebra/chapter/introduction-exponential-functions/), genetic factors, and aging. The opportunistic organism may exist in the body as a commensal. Let's take commensal bacteria as an example. But first, let's define commensal bacteria. Commensal bacteria are bacteria that do not necessarily cause harm to their host. This type of symbiosis occurring between the bacteria and the host where the host is neither generally harmed nor is benefitting in the relationship is referred to as [commensalism](https://courses.lumenlearning.com/waymakercollegealgebra/chapter/introduction-exponential-functions/). Commensal bacteria are essential to the well-being of humans. Their presence may even aid in the absorption of nutrients. Moreover, commensal bacteria can prevent the pathogenic invasion of the body and improve the function as well as the development of the [immune system](https://creativecommons.org/licenses/by/3.0/). Commensal bacteria produce antimicrobials that resist pathogenic invasion and colonization in the body. Under certain conditions, commensal bacteria may become opportunistic pathogenic bacteria. Example: *Candida albicans*, a fungus that can cause thrush or yeast infections in individuals with compromised immune systems, such as those undergoing chemotherapy. Zoonoses: Diseases that can be transmitted from animals to humans or Infections acquired from animal reservoirs are referred to as zoonoses or zoonotic diseases. Humans acquire infection from animals either by direct contact, as in the case of pets or farm animals, by ingestion of the animal or inhalation of bacteria in or around its hide, or through an insect vector that transmits the pathogen from the animal to the human via a bite. Diarrhea caused by *Salmonella* can occur after handling turtles and contaminating one\'s hands with their feces, or ingesting undercooked chicken contaminated with the bacteria, or through other routes such as eating undercooked or raw chicken eggs.  Vector-borne diseases: Infections transmitted to humans and other animals by blood-feeding arthropods, such as mosquitoes, ticks, and fleas. Vector-borne diseases are caused by the bite of infected insects like mosquitoes, ticks, and sandflies, which act as carriers. Most of these vectors are insects that suck human blood, which is when pathogen transmission occurs. These vectors first ingest a disease-causing pathogen from an already-infected host (an animal or human) and transmit it to other humans during subsequent blood meals. Insects from the arthropod species, like mosquitoes, ticks, triatomine bugs, sandflies, and blackflies, typically transmit vector-borne diseases. The spread of vector-borne diseases depends on several factors, like the environment in which it breeds, population density in the region, and frantic urbanization. You're at greater risk of getting a vector-borne disease in areas where the vectors thrive, like still water bodies, tall grass growth, and regions that report large outbreaks. Vector-borne diseases happen due to the action of specific pathogens and vectors. Some vector-borne diseases, their disease-causing pathogens, and their vectors are listed below. 1. Chikungunya, dengue, yellow fever, and Zika. Caused by a virus carried by the Aedes mosquito 2. Malaria.** **Caused by a parasite carried by the female Anopheles mosquito **b. Factors Affecting Infections** Host Factors:- The outcome of exposure to an infectious agent depends, in part, upon multiple host factors that determine individual *susceptibility* to infection and disease. Susceptibility refers to the ability of an exposed individual (or group of individuals) to resist infection or limit disease as a result of their biological makeup. Factors influencing susceptibility include innate, genetic, and acquired factors such as the specific immunity that develops following exposure or vaccination. The malaria resistance afforded carriers of the sickle cell trait exemplifies how genetics can influence susceptibility to infectious disease. Susceptibility is also affected by extremes of age, stress, pregnancy, nutritional status, and underlying diseases. These latter factors can impact immunity to infection, as illustrated by immunologically naïve infant populations, and aging populations experiencing immune senescence. Immune System: The strength and responsiveness of the immune system significantly influence the ability to resist infections. People with compromised immune systems, such as those with HIV/AIDS or undergoing chemotherapy, are more susceptible to infections. Age: Very young children and elderly individuals are more vulnerable to infections due to less mature or weakened immune systems. Immune Status: The state of the host\'s immune system is a critical determinant of susceptibility to infectious diseases. Immunocompromised individuals, such as those with HIV/AIDS, undergoing [chemotherapy](https://www.icliniq.com/articles/cancer/chemotherapy-and-hair-loss), or receiving immunosuppressive therapy, are at increased risk of severe infections due to impaired immune function. Nutritional Status:** **Adequate nutrition is essential for maintaining a healthy immune system. Malnutrition, whether due to inadequate intake or underlying medical conditions, can compromise immune function and increase susceptibility to infections. Natural Resistance: Physiological Factors: Body temperature, pH levels, and other physiological factors can inhibit the growth and survival of pathogens. Species and Racial Resistance: Species Resistance: Certain species are naturally resistant to diseases that affect other species. For example, humans are not susceptible to many diseases that affect other animals. Racial Resistance: Genetic factors can confer varying levels of resistance to specific infections among different racial or ethnic groups. For instance, certain populations have genetic resistance to malaria due to the presence of sickle cell trait. Individual Resistance: Genetics: Individual genetic variations can influence susceptibility to infections. For example, some people have genetic mutations that provide resistance to HIV. Previous Exposure: Previous infections or vaccinations can provide immunity or partial resistance to certain pathogens. Lifestyle Factors: Hygiene practices, stress levels, sleep patterns, and behaviors such as smoking and alcohol consumption can impact an individual\'s resistance to infections. **c. Microbial virulence factors in adherence, invasion, colonization, and diseases** Virulence is the degree to which a pathogen (such as a bacterium, virus, fungus, or parasite) can cause disease. It encompasses the pathogen\'s ability to infect a host, evade or overcome the host\'s immune defenses, multiply within the host, and cause damage or illness. Microbial virulence factors are specialized molecules produced by pathogens (bacteria, viruses, fungi, and parasites) that enable them to colonize a host, evade or suppress the immune response, enter and exit cells, and obtain nutrients from the host. Virulence factors are molecules or structures produced by pathogens (such as bacteria, viruses, fungi, and parasites) that enable them to establish infections, cause disease, and evade the host\'s immune response. These factors enhance the ability of the pathogen to invade the host, damage host tissues, and evade or suppress the host\'s immune defenses. Adherence Invasion by most microorganisms begins when they adhere to cells in a person's body. Adherence is a very specific process, involving \"lock-and-key\" connections between the microorganism and cells in the body. Being able to adhere to the surface of a cell enables microorganisms to establish a base from which to invade tissues. Whether the microorganism remains near the invasion site or spreads to other sites and how severe the infection is depends on such factors as the following: 1. Whether the microorganism [produces toxins, enzymes, or other substances](https://courses.lumenlearning.com/waymakercollegealgebra/chapter/introduction-exponential-functions/#v27742509) 2. Whether it develops [resistance to antimicrobial medications](https://creativecommons.org/licenses/by/3.0/#v785293) 3. Whether it can block the [body\'s defense mechanisms](https://www.merckmanuals.com/home/infections/biology-of-infectious-disease/defenses-against-infection) 4. How well the person\'s [immune system](https://journalistsresource.org/studies/society/public-health/global-rise-human-infectious-disease-outbreaks) is functioning Many disease-causing microorganisms have properties that increase the severity of the diseases they cause (virulence) and that help them resist the body's defense mechanisms. These properties include the following: 1. Toxins 2. Enzymes 3. Ways to block the body\'s defenses Key virulence factors involved in adherence include: 1. Adhesin: Adhesins are found on bacterial, viral, fungal, and protozoan pathogens. One example of a bacterial adhesin is type 1 fimbrial adhesin, a molecule found on the tips of fimbriae of enterotoxigenic *E. coli* (ETEC). fimbriae are hairlike protein bristles on the cell surface. Surface proteins that bind to specific receptors on host cells. Examples include pili or (fimbriae) in bacteria like *Escherichia coli*. Pili are short, hair-like proteinaceous filaments, anchored to the bacterial cell wall, that extend only a short distance. Invasins: Invasion by microbes refers to their ability to enter and spread within the host\'s body, causing infection. Once adhesion is successful, invasion can proceed. Invasion involves the dissemination of a pathogen throughout local tissues or the body. Pathogens may produce exoenzymes or toxins, which serve as virulence factors that allow them to colonize and damage host tissues as they spread deeper into the body. Pathogens may also produce virulence factors that protect them against immune system defenses. A pathogen's specific virulence factors determine the degree of tissue damage that occurs. Virulence factors Enzymes: Pathogens secrete enzymes that break down host tissues. For example, hyaluronidase S, an Enzyme produced by pathogens like *Staphylococcus aureus*, and *Streptococcus pyogenes* degrades the (hyaluronic acid), which acts as an intercellular cement between adjacent cells in connective tissue. Bacteria produce enzymes that break down tissue, allowing the infection to spread through tissues faster. Other bacteria produce enzymes that allow them to enter and/or pass through cells. Capsules: it is a slimy outer layer made of complex sugar. They protect bacteria from being recognized and attacked by the host\'s immune system. capsule makes it difficult for immune cells to engulf and destroy bacteria, allowing them to survive and multiply within the host. Some Staphylococcus aureus strains have a capsule, a protective outer layer that helps them avoid being eaten by immune cells. This capsule is like a shield that protects the bacteria from the host\'s defenses. Toxins: This bacterium can also release toxins that kill host cells. Some microorganisms that invade the body produce toxins. Toxins are chemicals that can harm living things. Toxins can be man-made, such as certain insecticides, or natural, such as those produced by certain organisms. For example, the bacteria *Clostridium tetani* in an infected wound produces a toxin that causes [tetanus](https://creativecommons.org/licenses/by-nd/3.0/). Some diseases are caused by toxins produced by microorganisms outside the body. For example, staphylococci bacteria living in food may produce a toxin that causes [food poisoning](https://www.merckmanuals.com/home/digestive-disorders/gastroenteritis/staphylococcal-food-poisoning) when that food is eaten, even if the staphylococci have been killed. Most toxins contain components that bind specifically with molecules on certain cells (target cells). Toxins play a central role in diseases such as [tetanus](https://journalistsresource.org/studies/society/public-health/global-rise-human-infectious-disease-outbreaks), [toxic shock syndrome](https://creativecommons.org/licenses/by-nd/3.0/), [botulism](https://www.merckmanuals.com/home/infections/bacterial-infections-anaerobic-bacteria/botulism), [anthrax](https://pxhere.com/en/photo/171032), and [cholera](https://www.merckmanuals.com/home/infections/bacterial-infections-gram-negative-bacteria/cholera). In addition to exoenzymes, certain pathogens are able to produce toxins, biological poisons that assist in their ability to invade and cause damage to tissues. The ability of a pathogen to produce toxins to cause damage to host cells is called toxigenicity. Toxins can be categorized as endotoxins or exotoxins. Exotoxins -- These are secreted by bacteria and can target various cells or tissues. They can disrupt cell membranes, inhibit protein synthesis, and cause inflammation. For example- *Clostridium botulinum* produces botulinum toxin which causes botulism by blocking nerve function and leading to paralysis Endotoxins -- These are components of the outer membrane of certain bacteria. They are released when the bacteria die or lysis occurs and can trigger a strong immune response, leading to symptoms like fever, and organ damage. Example -- *E. coli*  Colonization Colonization refers to the ability of the pathogen to multiply and establish a stable presence within the host. Factors aiding in colonization include: Quorum sensing Bacteria communicate with each other using chemical signals in a process called quorum sensing. This enables them to coordinate their actions. Such as the production of virulence factors based on their population density. Ex- E. coli, Vibrio cholera, etc Biofilm formation Some bacteria can form biofilms, complex communities of bacteria embedded in a protective matrix. Communities of bacteria that stick together and to surfaces, protected by a slimy layer. Biofilm enables bacteria to adhere to the surface, evade the immune system, & resist antibiotics, making infections more challenging to treat. Bacteria in biofilms are often able to communicate with one another by a process called quorum sensing and are able to interact with and adapt to their environment as a population of bacteria rather than as individual bacteria. By living as a community of bacteria as a biofilm, these bacteria are better able to: resist attack by antibiotics; trap nutrients for bacterial growth and remain in a favorable niche; adhere to environmental surfaces and resist flushing; live in close association and communicate with other bacteria in the biofilm; and resist and attack by the body\'s immune system. Biofilms are, therefore, functional, interacting, and growing bacterial communities. Biofilms even contain their own water channels for delivering water and nutrients throughout the biofilm community. 7. Example: Dental plaque, formed by various bacteria, can lead to tooth decay and gum disease. 8. Biofilm formation bacteria -- E. coli