Microbe-Human Interactions (copy)

Microbes living in or on the human body consist of a diverse range of organisms including bacteria, fungi, protozoa, and viruses, which play critical roles in maintaining health and influencing disease. Understanding these interactions is essential for grasping how our microbial communities affect our wellbeing and contribute to various diseases.

Types of Microbiota:

• Resident microbiota: These are established microorganisms that are regularly found in specific regions of the body and do not normally cause disease. They help protect the host by outcompeting pathogenic organisms and contributing to immune responses.

• Transient microbiota: These microorganisms are temporary residents that come and go, often influenced by environmental changes, dietary habits, and lifestyle.

• Pathogenic microbes: Defined as those capable of penetrating host defenses, these microbes enter host tissues and multiply, leading to disease, which is characterized by deviations from normal health.

Factors Causing Disease:

Diseases can arise from several interrelated factors:

• Infections: A primary cause of disease where pathogens invade and disrupt host tissues.

• Genetics and aging: Genetic predispositions can influence susceptibility to infections and diseases, while aging often weakens immune responses.

• Environmental agents/chemicals: Exposure to toxins and pollutants can lead to diseases, affecting various organ systems.

• Malfunctions of systems or organs: Dysfunctions in body systems can lead to illness, demonstrating the complexity of disease mechanisms.

• Infectious disease: This is specifically the disruption of tissues or organs driven by microbes or their products.

Key Definitions:

• Pathogen: A microbe capable of causing disease. Examples include bacteria, viruses, fungi, and protozoa.

• Pathology: The study of the nature and causes of diseases.

• Etiology: The specific cause or origin of a disease.

• Pathogenesis: The development of disease and the biological mechanisms involved in progression.

Normal Microbiota:

Microbes colonizing the human body start at birth as the baby acquires microbiota during passage through the birth canal and continues through interaction with the environment, diet, and caregivers.

Colonization Sites:

Major colonization sites include the skin, eyes, respiratory system, mouth, intestines, and urogenital system, as well as previously thought sterile sites such as the lungs and bladder, emphasizing the complexity of human microbiota.

Human Microbiome Project:

This research initiative aims to sequence the complete biota present in humans. Comparatively, humans possess about 21,000 protein-encoding genes, whereas microbial genes constitute approximately 8 million. Importantly, all healthy individuals harbor potentially dangerous pathogens in low numbers, allowing for the study of microbiome dynamics in healthy versus diseased states.

Benefits of Normal Microbiota:

• Development Influence: Normal microbiota play a crucial role in the growth and development of the host, particularly during early life.

• Nutritional Support: Certain gut bacteria are responsible for synthesizing essential nutrients like Vitamin K, which is vital for blood clotting and overall health.

• Microbial Antagonism: Normal microbiota prevent the overgrowth of harmful microorganisms through competition for resources and production of antimicrobial substances.

Types of Pathogens:

• Primary pathogens: These can cause disease in healthy individuals with normal immune function, as they possess specific virulence factors that enable pathogenicity.

• Opportunistic pathogens: These only cause disease when host defenses are compromised or in unsuitable anatomical sites, often exploiting pre-existing vulnerabilities of the host.

Virulence:

• Virulence: This term denotes the degree of pathogenicity of a microbe, reflecting its ability to establish infection and cause damage to the host.

• Virulence factors: These are specific characteristics that enhance the ability of a microbe to cause disease, such as toxins, adhesion molecules, and mechanisms to evade the host immune response.

• Infectious dose (ID50): This refers to the number of microorganisms required to infect 50% of a test population under exposure conditions.

• Lethal dose (LD50): The number of organisms necessary to cause death in 50% of an inoculated group within a specified time, illustrating the lethality of specific pathogens.

Causing Disease: Steps Involved:

1. Entrance: Microbes typically enter the host through designated portals including the skin, gastrointestinal tract, respiratory tract, urogenital tract, and existing biota.

2. Attachment: Pathogens utilize specialized structures such as fimbriae and capsules to adhere to host tissues effectively.

3. Establishment: Successful evasion of host defenses may lead to colonization and subsequent infection.

4. Pathogenesis: Pathogens employ mechanisms to cause disease, which can involve direct tissue damage as well as evasion and manipulation of host immune responses.

5. Exiting: Pathogens can exit the host via various secretions and excretions such as respiratory secretions, feces, and blood, often continuing the cycle of infection.

Portal of Exit: Types

• Respiratory & salivary: Secreted from the respiratory tract, often containing pathogens expelled through coughing or sneezing.

• Skin: Constant shedding of skin cells carries pathogens away.

• Fecal: Pathogens can exit through feces, posing public health risks, especially if sanitation is inadequate.

• Urogenital tract: Pathogens can be found in bodily fluids such as vaginal discharges and semen, contributing to sexually transmitted infections.

• Blood: Pathogens may escape circulation through bleeding, or puncture wounds, facilitating transmission to new hosts.

Infection Patterns:

• Types of infections: Include localized, systemic, focal, mixed, primary, secondary, acute, and chronic infections.

• Localized infections: Defined to specific tissues, such as boils or skin infections.

• Systemic infections: Spread throughout the body, leading to symptoms that affect multiple organ systems (e.g., measles).

• Focal infections: Occur when an infection, initially localized, breaks loose and affects other tissues (e.g., tuberculosis).

• Primary infections: The first infection in a course of disease.

• Secondary infections: Occur in the aftermath of a primary infection and can complicate recovery.

Signs vs. Symptoms:

• Signs: Observable and measurable changes in the body, such as a fever or rash, that can be quantified during an examination.

• Symptoms: Subjective feelings experienced by the patient, such as pain or fatigue, that cannot be directly measured but are reported by the patient.

Infections That Go Unnoticed:

• Asymptomatic infections: Occur when the host is infected, yet presents no noticeable symptoms (e.g., HIV).

• Latent infections: Involve a dormant state, where the microbe remains inactive but can reactivate later under certain conditions.

• Sequelae- Long term or permanent damage to organs and tissues

  (ex: meningitits)

The Course of an Infection:

• Incubation period: This is the interval from the initial infection to the appearance of the first symptoms, which can vary widely among diseases.

• Prodromal period: Recognizable by the appearance of initial, often vague symptoms, signaling the onset of the illness.

• Acute phase: Characterized by high virulence and active replication of the infectious agent, resulting in pronounced symptoms.

• Convalescent stage: The recovery phase where symptoms lessen, and the individual begins to regain health.

Reservoirs of Infectious Agents:

• Reservoir: The natural habitat from which a pathogen originates, which can be living (humans, animals) or non-living (soil, water), crucial for sustaining pathogen life cycles.

• Carrier: An individual or object that can transmit an infection without displaying symptoms, often contributing significantly to the spread of diseases.

Transmission Types:

• Horizontal transmission: The spread of infection between individuals, which can happen through direct contact or indirect routes (airborne, surfaces).

• Vertical transmission: The transfer of pathogens from parent to offspring, which can occur during pregnancy, childbirth, or breastfeeding.

Zoonosis:

Zoonotic diseases are infections transmitted from animals to humans, typically resulting in significant health risks and are often hard to control. Examples include rabies and the plague, highlighting the importance of monitoring animal health in public health strategies.

Emerging and Re-emerging Infectious Diseases (EIDs):

Newly identified microbes and pathogens can lead to global health challenges, as seen during the COVID-19 pandemic. Factors driving EIDs include antibiotic overuse, climate change altering pathogen habitats, and increased interactions between humans and wildlife.

Epidemiology: Monitoring Diseases:

Epidemiology is the study of the distribution and determinants of health-related states and events in specified populations. Key terms include:

• Prevalence: The total number of existing cases of a disease in a population at a given time.

• Incidence: The number of new cases that occur during a specified time period.

• Mortality rate: The frequency of deaths within a specified population due to a disease, used to assess health impacts and effectiveness of interventions