(EXAM 1) Lecture 2- establishment of bacterial and fungal infections

pathogen

organism that causes disease in an immunocompetent host

pathogenicity

the ability of an organism to cause disease in an immunocompetent host

virulence

is a relative measure of pathogenicity. For example, a highly virulent vs. a weakly virulent strain of a pathogen

What is ID50?

ID50 (Infectious Dose 50) is the number of bacteria needed to cause an infection in 50% of exposed animals.

If Strain 1 has an ID50 of 100 bacteria and Strain 2 has an ID50 of 10,000 bacteria, which strain is more virulent?

Strain 1 is more virulent because a lower ID50 means fewer bacteria are needed to cause infection.

What is LD50?

LD50 (Lethal Dose 50) is the number of bacteria or toxin required to cause death in 50% of the exposed population.

How is LD50 used in veterinary and medical research?

It helps assess the potency of pathogens or toxins by determining the dose required to cause mortality in half of the test population.

What is MLD?

MLD (Minimum Lethal Dose) is the smallest amount of a pathogen or toxin required to cause fatality in an animal.

What is the MLD of botulinum toxin in mice?

Less than 0.01 ng; 1 mg of botulinum toxin can kill 100 billion mice.

Why is MLD important in toxicology?

It defines the absolute minimum amount of a toxin that can be fatal, helping establish safety and exposure limits.

What is an apparent infection?

An infection with overt disease symptoms.

What is an inapparent infection?

An infection that occurs without overt disease symptoms.

What defines an acute infection?

An infection with a short time course, which may result in death, recovery, or progression to chronic infection.

What is an example of an acute infection?

Canine ehrlichiosis (E. canis).

What defines a chronic infection?

An infection with a prolonged time course.

What is an example of a chronic infection?

Tuberculosis (M. bovis).

What is an exogenous infection?

An infection acquired from an external source.

What is an example of an exogenous infection?

Food-borne Salmonella infection.

How can exogenous infections be controlled?

By modifying the external source, such as cleaning and reducing contact.

What is an endogenous infection?

An infection caused by normal flora present in or on the host's body.

What is an example of an endogenous infection?

Pseudomonas aeruginosa infection in immunosuppressed patients.

How are endogenous infections controlled?

By eliminating the pathogen from tissues, but not necessarily from the host.

What is normal flora?

Microbes that live on the body without causing disease.

What are pathogens?

Microbes that always or sometimes cause disease.

What is an obligate pathogen?

A microorganism that can only cause disease and cannot survive long outside a host.

What is an opportunistic pathogen?

A microbe that usually does not cause disease but can become pathogenic when the host's immune system is weakened.

What is ingress?

Pathogen entry without crossing the epithelial barrier

What are the three main ways pathogens enter via ingress?

Inhalation, ingestion, and infection of mucous membranes.

How does inhalation lead to infection?

Damage to respiratory defenses predisposes to bacterial infections (e.g., BRSV & M. hemolytica in cattle).

How does ingestion lead to infection?

Normal flora plays a protective role, but some pathogens (e.g., Enterotoxigenic E. coli in calves) can still cause disease.

What is penetration?

Pathogen entry by breaching deeper tissues.

What are the two types of penetration?

Pathogen-dependent and pathogen-independent.

How does pathogen-dependent penetration occur?

Bacteria use specific structures (virulence factors) to bind receptors and facilitate penetration.

How does pathogen-independent penetration occur?

Through mechanisms such as vector-borne transmission, catheters, or blood transfusions.

How do nutritional requirements affect bacterial multiplication?

Bacteria require iron, but the host increases transferrin levels to bind free iron, limiting bacterial growth.

How does temperature affect bacterial growth?

Some pathogens cannot grow at specific temperatures (e.g., some ringworm fungi can't survive at 37°C).

What is bacteremia?

The presence of bacteria in the blood.

How does transient bacteremia differ from sustained bacteremia?

Transient bacteremia is quickly cleared by host defenses, while sustained bacteremia can cause disease.

How do some bacteria avoid immune clearance in the blood?

Some bacteria, like Brucella abortus, hide in monocytes, making them less visible to the immune system.

What are the three main ways bacteria spread within the host?

Blood (bacteremia), lymphatics, and fascial planes.

Give an example of a pathogen that spreads through the lymphatics.

Streptococcus equi spreads from the tonsils to submandibular lymph nodes (strangles) and then to mediastinal lymph nodes (bastard strangles).

Give an example of a pathogen that spreads via fascial planes.

Pseudomonas aeruginosa spreads rapidly through tissue planes (e.g., hot tub folliculitis).

What are the three main mechanisms by which microbes cause host damage?

Endotoxin release, exotoxin production, and damaging host response.

What is an endotoxin?

A constitutive part of the Gram-negative bacterial cell wall, also called Lipopolysaccharide (LPS).

What component of LPS is responsible for endotoxin activity?

Lipid A.

How does endotoxin affect the host?

Its action is non-specific and dose-dependent:

- Low levels: Triggers host defenses (fever, inflammation).

- High levels: Can cause septic shock, hypotension, and multi-organ failure.

What are exotoxins?

Secreted bacterial proteins that can be produced by both Gram-positive and Gram-negative bacteria.

How do exotoxins differ from endotoxins?

Exotoxins have specific actions, while endotoxins have non-specific effects.

Exotoxins are actively secreted, whereas endotoxins are part of the bacterial cell wall.

Exotoxins are often encoded by plasmids or bacteriophages.

How does inflammation contribute to microbial damage?

It is a primary host defense but can become harmful in excess (e.g., chronic inflammation).

How do bacteria evade phagocytosis?

Capsule formation → Streptococcus pneumoniae, Klebsiella.

Escape from the phagosome → Rickettsia.

Inhibiting phagosome-lysosome fusion → Ehrlichia.

Phagocyte damage via toxins → Bacillus anthracis.

What is immunopathology?

Host tissue damage caused by immune response to infection.

Examples of immunopathology:

Immune complexes → Purpura hemorrhagica (Streptococcus equi).

Granuloma formation → Rhodococcal pneumonia (Rhodococcus equi), Tuberculosis (Mycobacterium spp.).

What is the main difference between exogenous and endogenous infections?

Exogenous infections originate from external sources (e.g., contaminated environments), while endogenous infections arise from the patient's own microbiome.

How are exogenous infections typically diagnosed?

Through environmental sampling (e.g., swabs from surfaces, water testing) to identify external sources of pathogens.

How are endogenous infections typically diagnosed?

By evaluating patient history, identifying risk factors, and performing targeted testing of body sites where normal flora has overgrown.

What is the treatment focus for exogenous infections?

Targeted antimicrobial therapy and infection control measures to prevent further exposure.

What is the treatment focus for endogenous infections?

Adjusting antibiotics to avoid disrupting normal flora, managing immune function, and using probiotics if needed.

How are exogenous infections controlled?

By prioritizing environmental hygiene (disinfection, hand hygiene) and preventing cross-contamination.