Host-Microbe Interactions and Pathogens

Host-Microbe Interactions

• Host-microbe interactions refer to microbes sustaining themselves within other organisms on a molecular, cellular, and population level

Pathogens

• Pathogens are microorganisms that can cause disease
• This can occur even if they don't always cause illness in all hosts

Host

• A host is any organism that harbors another organism or particle like a virus/prion

Types of Pathogens

• True pathogens can cause disease in healthy individuals with normal immune systems
• Examples of true pathogens include influenza virus, plague bacillus, and malarial protozoan
• Opportunistic pathogens cause disease when a host’s defences are compromised
• Opportunistic pathogens may grow in a part of the body that is not natural to them
• Examples of opportunistic pathogens include Pseudomonas spp. and Candida albicans

Bacteria

• Bacteria are one-celled organisms which cause illnesses like strep, throat infections, UTIs, and tuberculosis

Viruses

• Viruses are smaller than bacteria, viruses cause diseases ranging from the common cold to AIDS

Fungi

• Fungi can cause skin diseases like ringworm and athlete’s foot
• Fungi may infect lungs or the nervous system, such as Aspergillus and Histoplasma

Parasites

• Parasites are organisms that depend on each other for survival and nutrients
• Malaria is caused by a parasite transmitted by mosquitoes
• Other parasites can be transmitted by animal feces, like hookworm or tapeworm

Factors Influencing Infectious Diseases

• Pathogenicity is the ability to produce disease in a host
• Virulence is the degree of pathogenicity in a microorganism
• Virulence includes a pathogen’s genetic, biochemical, and any structural features
• Infectivity shows how easily a microorganism can infect or invade a host
• Transmissibility measures a microorganism's ability to spread from host to host

The Body's Normal Flora

• Resident flora survive in the body for extended periods, e.g., Candida spp, S. epidermidis, E. coli
• Transient flora are temporary, e.g., E. coli, Proteus mirabilis, M. tuberculosis
• Flora help defend against pathogens by covering adherence sites
• Other defenses include toxic compounds that kill other organisms, and preventing pathogens from consuming nutrients For pathogens to overcome the normal flora, the body's environment must change
• Changes in the pH of the body or the elimination of normal flora by antibiotics can enable pathogens to overcome the flora

Host Defenses

• Microorganisms must overcome defenses to invade
• Host defenses include skin and mucosal secretions, non-specific local responses (e.g., pH), non-specific inflammatory responses and specific immune responses (e.g., lymphocytes)

Pathogen Defenses

• Pathogens contain virulence factors that promote disease formation and provide the opportunity for infection
• Greater virulence increases the likelihood of disease occurring.
• Factors include ability to adhere to a host, ability to colonize a host and ability to evade host defenses

Mechanisms of Pathogenesis

• Pathogenesis is the process by which disease develops
• Food-borne intoxication can occur if causative agent produces toxins in the body (e.g., botulism)
• Colonization of an invading pathogen on the host surface results in pathogen numbers that produce toxins, which damage cells (e.g., Corynebacterium)
• The relationship between host and pathogen is dynamic
• Disease production occurs through a process of steps

Transmission

• Contact Transmission includes:
  • STD's and cold sores

Contact Transmission: Indirect

• Indirect transfer of pathogens occur via items, organisms etc, to susceptible hosts

Vehicle/Fomite Transmission: Airborne

• Pathogens suspended in droplets or dust, can remain in air for hours/days
• Cold viruses travel in droplets when a person sneezes and talks

Vehicle Transmission

• Vehicle borne transmissions are pathogens which are transmitted from source to another via intermediate item (fomite) like contaminated medical equipment
• Water/food borne transmission is transmitted to individuals via food/water
• Ingestion describes ingestion of pathogenic organism that grows in the GI tract.
  • Example V. cholerae transmitted in water sources, E. coli in hamburgers, Salmonella

Vector-borne transmission

• Pathogens are transmitted to susceptible individuals via an animal or insect
• Mechanical transmission (external) uses a host for transport
• An example of transmission is a fly landing on food at a picnic after it picked up some E. coli
• Biological transmission (internal) of disease is carried inside the vector and uses a vector
• Example malaria parasite, Plasmodium in blood ingested by Anopheles mosquito after bite

Microbial Interaction

• Microbial interaction occurs between two species ("living together")
• Microbial interactions may be beneficial or harmful
• Organisms can be located on the surface of others like 'ectobionts' like S. epidermidis
• Some organisms are located within another organism in a relationship like 'endobionts' such as Plasmodium spp
• Microbial interactions can be positive or negative
• Positive interactions include mutualism and commensalism
• Negative interactions include parasitism, amensalism, and competition

Positive relationship

• Mutualism is the relationship in which each organism acting in mutual benefit to obtain
• An example is S. epidermidis on the skin, Bifidobacterium in gut
• Commensalism occurs when one organism in benefitted and the remaining other organism is not harmed like Bacillus spp

Negative relationship

• Parasitism occurs when one benefits an organism while harming another, such as viruses
• Microbial population competition can alter populations with respect to their survival and growth
• This occurs when both populations use the same resources such as the same space or nutrition
• Eg., Clostridioides difficile, Methicillin-resistant Staphylococcus aureus (MRSA)
• The overuse of antibiotics can eliminate the body's normal flora which gives room for resistant strains to thrive

Amensalism

• Amensalism occurs when one microbial population generates and releases substances that are inhibitory to another population
• The first population which produces these substances is unaffected while other populations get inhibited
• This chemical inhibition is known as antibiosis
• Example is Staphylococcus epidermidis vs. Staphylococcus aureus

Infection

• Infection- a condition where pathogenic microbes penetrate into host defenses, enter tissues and multiply
• Hosts of infections have sterile body compartments that can be invaded
• Invasion can come contact through the target tissue

Disease

• Disease is any deviation from health, disruption of a tissue or organ that is caused by microbes

Portals of Entry

• Common portals of entry for pathogens include skin, gastrointestinal tract, respiratory tract and the urogenital tract

Portals of Exit

• Common portals for pathogen exit include respiratory, saliva, skin, scales, fecal, urogenital tract and the removal of blood

Patterns of Infection

• Localized infection results in the spread of microbes throughout the body and remain is one tissue
• Systemic infection results in the spread to several sites and tissue fluids throughout the bloodstream
• Focal infection occurs when infectious agent breaks loose from a local infection is carried to other tissues
• Mixed infection occurs when different microbes or species grow simultaneously at infected site
• Primary infection is the first instance of an infection by a pathogen
• Secondary infection occurs after or during a primary infection caused by another microbe Example of a contracting pneumonia after influenza

Hospital Acquired (Nosocomial) Infections

• The onset of a disease when acquired during a hospital stay
• Most bacterial infections usually have incubation that lasts 48 hours; can vary depending on conditions
• Nosocomial infections may not occur fully until after discharged of patient
• Usually surgical incisions, respiratory tract, and urinary tract are most affected
• Most common include E. coli, Pseudomonas spp., and Staphylococcus spp

Factors Influencing the Risk of Transmission of HAIs

• The intensity of care, increased susceptibility to infections and exposure to Environmental Sources
• Long-term care is another factor as well as patients with trauma or burns, patients on the oncology or transplant units

Factors Causing Infections in Hospitalized Patients

• Hospital staff behavior, economic constraints due to high staff to patient ratio, improper use of hygiene
• A lack of focus on the prioritization of the institution can affect rates

Burns

• Damaged skin and exposed tissues provide easy entry for airborne or contact infections
• Isolation rooms must use single positive pressure systems with filtered air Extract all outside air from the rooms
• Burns require PPE, stringent hygiene practices and isolation for over a 30% surface area involvement

Treatment for Burns

• Appropriate wound care needs to consist of aseptic approach with disinfecting as well as cleaning less infected areas first with microbiology samples performed
• Antibiotic staff treatment needs to follow microbiology with sterilized instruments and hepatitis vaccines

Dermatology Wards

• Predisposing causes and types of infections from broken skin include generalized desquamating lesions colonized with:
  • Staphylococcus aureus
  • Multi-drug resistant (MDR) pathogens
• To cover up infections and transmission the patients need separation as well papercloths
• Sterilization of instruments and disposal must occur correctly

Emergency Departments

• Management of biomedical waste is crucial and important that all patients and staff follow proper procedure
• Having proper technique with sanitation is important with signs that show what precautions must occur such as PPE and cough care etiquette

Dialysis Units

• Vascular and blood with nosocomial pathogens infections is more likely. To treat and help prevent a outbreak to occur proper check and screening with proper hygiene
• Use preferred fistulae instead of outside tools to lower exposure to pathogen

Transplant Recipients

• Immunosuppressive meds must occur with proper check-in since low immunity can affect these patients
• Check for outside signs and symptoms and have proper response through testing and hygiene

Immunocompromised Patients

• Patients need strong screening and protection procedures with regular hygiene and management of areas