Virulence is the degree to which an organism is pathogenic. Highly virulent pathogens will almost always lead to a disease. Less virulent pathogens may cause an initial infection, but they do not always cause severe illness. Some individuals might even be asymptomatic. Two important indicators of virulence are:
- the median infectious dose (ID50) 
- the median lethal dose (LD50)

Virulence

There are four stages of pathogenesis. The pathogen must be able to gain entry to the host, travel to the location where it can establish an infection, evade or overcome the host’s immune response, and cause disease to the host.
- exposure (contact)
- adhesion (colonization)
- invasion
- infection.

Stages of Pathogenesis

A disease is any condition in which the normal structure or function of the body is damaged or impaired.

Disease

- Opportunistic pathogen
- Primary pathogen

Types of Pathogens

Which of the following statements correctly describes pathogenic bacteria?

Which of the following is a characteristic of pathogenic bacteria?

Which of the following is an example of a disease caused by pathogenic bacteria?

Which of the following methods can be used to prevent infections caused by pathogenic bacteria?

Pathogenic bacteria are bacteria that can cause infectious disease.

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Bacteria can be classified based on their different morphologies.
- Coccus
- Bacillus
- Vibrio
- Coccobacillus
- Spirillum
- Spirochete

Cell Morphologies

In 1884, Koch published four postulates that EXPLAINED his method for determining whether a specific microbe was the cause of a specific disease. Each of these postulates outlines the qualifications that must be met before a disease can be linked with a pathogen.

Koch's postulates

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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These bacterial cells lack a cell wall, thus they do not have a stable shape. They may appear as round bodies or interlacing filaments. 

Mycoplasmas

Coccus bacteria are round and spherical in shape.

Coccus

Bacterial cells that are rod shaped are bacilli. Bacilli are capable of forming endospores. 

Bacillus

These bacterial cells are curved rod shaped. 

Vibrio

Coccobacilli bacteria are short rod shaped bacteria. 

Coccobacillus

Spirilla are rigidly spiraled shaped bacterial cells.

Spirillum

Spirochete cells are longer and helical spiral bacterial cells. They are members of the Spirochaetota phylum of bacteria.

Spirochete

Pathogenic Bacteria

Which of the following bacterial morphologies is characterized by a spiral shape?

Which bacterial morphology is described as rod-shaped?

Which bacterial morphology is described as spherical in shape?

Which bacterial morphology is described as comma-shaped?

The suspected causative agent must be absent from all healthy organisms but present in all diseased organism

Koch's First Postulate

The causative agent must be isolated from the diseased organism and grown in a pure culture.

Koch's Second Postulate

The cultured agent must cause the same disease when inoculated into a healthy susceptible organism.

Koch's Third Postulate

The same causative agent must then be re-isolated from the inoculated, diseased organism

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