UNIUYO MCB 314 Bacteriology & Pathogenesis Lecture Notes 300 Level First Semester

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UNIUYO MCB 314 Bacteriology & Pathogenesis Lecture Notes 300 Level First Semester lecture notes study materials covering bacterial mechanisms, disease processes, microbial pathogenicity, Koch’s postulates and virulence factors.

Table of Contents

UNIUYO MCB 314 Bacteriology & Pathogenesis Lecture Notes 300 Level First Semester

PATHOGENESIS OF BACTERIAL INFECTION

8.1 INTRODUCTION

In this chapter we would focus on how bacterias causes disease to human beings. This process of causing disease is termed as Pathogenesis. Pathogenesis is a multi-factorial process which depends on the immune status of the host, the nature of the species or strain (virulence factors) and the number of organisms in the initial exposure.

A limited number of bacterial species are responsible for the majority of infectious diseases in healthy individuals. Due to the success of vaccination, antibiotics, and effective public health measures, until recently, epidemics were felt to be a thing of the past. Due to the development of antibiotic resistant organisms, this situation is changing rapidly.

All humans are infected with bacteria (the normal flora) living on their external surfaces (including the skin, gut and lungs). We are constantly also exposed to bacteria (including air, water, soil and food). Normally due to our host defenses most of these bacteria are harmless. In compromised patients, whose defenses are weakened, these bacteria often cause opportunistic infectious diseases when entering the bloodstream (after surgery, catheterization or other treatment modalities). When initiated in the hospital, these infectious diseases are referred to as nosocomial. Some common bacteria found in the normal flora include Staphylococcus aureus, S. epidermidis and Propionibacterium acnes (found on the skin)and Bacteroides and Enterobacteriaceae found in the intestine (the latter in much smaller numbers).

OBJECTIVES

After reading this chapter, the student will be able to:

describe the term pathogenesis.
explain Koch’s postulates.
differentiate colonization and pathogens
explain steps involved in the bacterial pathogenesis
describe toxins
differentiate endotoxins and exotoxins
discuss the various diseases caused by bacteria

8.2 PATHOGENICITY

Pathogenicity is the capacity to initiate disease. It requires the attributes of transmissibility or communicability from one host or reservoir to a fresh host, survival in the new host, infectivity or the ability to breach the new host’s defenses, and virulence, a variable that is multifactorial and denotes the capacity of a pathogen to harm the host. Virulence in the clinical sense is a manifestation of a complex bacterial–host relationship in which the capacity of the organism to cause disease is considered in relation to the resistance of the host.

Types of Bacterial Pathogens

Bacterial pathogens can be classified into two broad groups, primary and opportunistic pathogens.

Primary pathogens are capable of establishing infection and causing disease in previously healthy individuals with intact immunological defenses. However, these bacteria may more readily cause disease in individuals with impaired defenses.

Opportunistic pathogens rarely cause disease in individuals’ with intact immunological and anatomical defenses. Only when such defenses are impaired or compromised, as a result of congenital or acquired disease or by the use of immunosuppressive therapy or surgical techniques, are these bacteria able to cause disease. Many opportunistic pathogens, e.g. coagulase negative staphylococci and Escherichia coli, are part of the normal human flora and are carried on the skin or mucosal surfaces where they cause no harm and may actually have beneficial effects, by preventing colonization by other potential pathogens. However, introduction of these organisms into anatomical sites in which they are not normally found, or removal of competing bacteria by the use of broad-spectrum antibiotics, may allow their localized multiplication and subsequent development of disease.

The above classification is applicable to the vast majority of pathogens; however, there are exceptions and variations within both categories of bacterial pathogens. Different strains of any individual bacterial species can vary in their genetic makeup and virulence capacity. For example, the majority of Neisseria meningitidis strains are harmless commensals and considered opportunistic bacteria, however, some hypervirulent clones of the organism can cause disease in a previously healthy individual. Conversely, people vary in their genetic make-up and susceptibility to invading bacteria. For example, Mycobacterium tuberculosis is a primary pathogen but does not cause disease in every host it invades.

INTEXT QUESTIONS 8.1

The process of bacteria causing disease is termed as ……………………..
Ability to affect the host’s disease is ……………………..
Capacity of a pathogen to harm the host is ……………………..
Pathogens which causes disease in healthy individual is ……………………..
Pathogens that causes disease in immune compromised individual is ……………………..

8.3 KOCH’S POSTULATES (MODIFIED)

Koch forwarded four criteria designed to establish a causal relationship between a causative microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890. Koch applied the postulates to establish the etiology of anthrax and tuberculosis, and now have been generalized to other diseases.

The organism must always be found in humans with the infectious disease but not found in healthy ones.
The organism must be isolated from humans with the infectious disease and grown in pure culture.
The organism isolated in pure culture must initiate disease when re-inoculated into susceptible animals.
The organism should be re-isolated from the experimentally infected animals.

Postulates 3. and 4. are extremely important in definite proof of the role of agent in human disease. However, this depends on the ability to develop animal models that resemble the human disease. In many cases such models do not exist.

Pathogenesis

The process of pathogenesis involves various steps beginning with the transmission of the infectious agent (bacterial) to the host, followed by colonization of the site. After the colonization of host, the bacteria remain adherent at the site of colonization then invades the host system. After surviving the host immune system it is ready to cause the disease.

Steps Involved in the Pathogenesis of the Bacteria:

Transmission
Colonization
Adhesion
Invasion
Survival in the host
Tissue Injury

Transmission

Potential pathogens may enter the body by various routes, including the respiratory, gastrointestinal, urinary or genital tracts. Alternatively, they may directly enter tissues through insect bites or by accidental or surgical trauma to the skin. Many opportunistic pathogens are carried as part of the normal human flora, and this acts as a ready source of infection in the compromised host (e.g. in cases of AIDS or when the skin barrier is breached). For many primary pathogens, however, transmission to a new host and establishment of infection are more complex processes.

Colonization

The establishment of a stable population of bacteria on the host’s skin or mucous membranes is called colonization. For many pathogenic bacteria, the initial interaction with host tissues occurs at a mucosal surface and colonization normally requires adhesion to the mucosal cell surface. This allows the establishment of a focus of infection that may remain localized or may subsequently spread to other tissues.

Frequently Asked Questions (FAQs)

1. What is bacterial pathogenesis?

Bacterial pathogenesis is the process by which bacteria cause disease in humans. It is a multi-factorial process that depends on the immune status of the host, the virulence factors of the bacterial species or strain, and the number of organisms in the initial exposure. The process involves several steps including transmission, colonization, adhesion, invasion, and survival in the host.

2. What is the difference between primary and opportunistic pathogens?

Primary pathogens are capable of establishing infection and causing disease in previously healthy individuals with intact immunological defenses. Examples include Mycobacterium tuberculosis. Opportunistic pathogens, on the other hand, rarely cause disease in individuals with intact immunological and anatomical defenses. They only cause disease when the host’s defenses are compromised, such as in cases of immunosuppressive therapy or AIDS. Examples include coagulase-negative staphylococci and Escherichia coli.

3. What are Koch’s postulates and why are they important?

Koch’s postulates are four criteria designed to establish a causal relationship between a microbe and a disease. They state that: (1) the organism must always be found in diseased individuals but not in healthy ones, (2) the organism must be isolated and grown in pure culture, (3) the isolated organism must initiate disease when inoculated into susceptible animals, and (4) the organism should be re-isolated from the experimentally infected animals. These postulates are fundamental in proving the role of a specific agent in causing human disease.

4. What is the difference between colonization and infection?

Colonization is the establishment of a stable population of bacteria on the host’s skin or mucous membranes without causing disease. Many bacteria that colonize our bodies are part of the normal flora and are harmless or even beneficial. Infection, however, occurs when bacteria breach the host’s defenses, multiply, and cause tissue damage or disease. Colonization is often the first step in the pathogenesis process, but it does not always lead to infection.

5. What are nosocomial infections?

Nosocomial infections are infectious diseases that are initiated in the hospital setting. These infections typically occur in compromised patients whose defenses are weakened, often as a result of surgery, catheterization, or other treatment modalities. Opportunistic pathogens from the patient’s normal flora or from the hospital environment often cause these infections when they enter the bloodstream or other normally sterile sites in the body.