CHAPTER 1 - INTRODUCTION

CONTENT1) A brief history of medical microbiology2) Host – parasite relationships3) Mechanism of pathogenesis

Pathogenic properties of pathogens

4) Transmission of diseases

A brief history of medical microbiology

Antony van Leeuwenhoek Antony van Leeuwenhoek (1632-1723) designed a single lens microscope and demonstrated the

little agents of disease, which he designated as

animalcules. These animalcules are now well established

entities belonging to bacteria, viruses and several other pathogens. The organisms being invisible to naked

eye are known as microorganisms.

Louis Pasteur

Louis Pasteur (1822-1895), a French chemist generated strong evidence to show that the microorganisms did not rise de novo or spontaneously in the media but were introduced from without. Pasteur showed that these organisms were maximum in the dusty air of towns and minimum in air of hilly areas where human habitation did not exist.

Robert Koch Robert Koch's first contribution to

science was demonstration of the character and mode of growth of causative bacillus of anthrax.

In 1882, Koch discovered tubercle bacillus

1883 the cholera vibrio. For his manifold discoveries in bacteriology, Koch is considered as father of bacteriology.

Host – parasite relationships

Terminologies Pathogen – a parasites capable to cause

disease in a host Symbiosis – associate between two species

(normal flora)* Parasitism – 1 org is parasite (gain benefit) 1

org is harmed by it Contamination – the microorganisms are

present Infection – multiplication of any parasitic org

within or

cont Pathogenicity – the capacity to produce

disease Example – Mycobacterium tuberculosis

frequently cause disease upon susceptible host

- S. epidermidis only in rare instance and poor immunodeficiency host

Mechanism of pathogenesis- Pathogenic properties of pathogens

How microbes cause disease? They act certain ways to harm / destruct

the host Actions – gain access to the host, adhering

to and colonizing cell surface, invading tissues, producing toxins and other harmful metabolic products.

Virulence factors – structural or physiological characteristics that help organisms cause infection and disease (pili, enzymes and toxin)

Direct action of bacteria1. Adherence using (adhesin) protein found

on attachment pili and capsule.2. Colonization is growth of m/org on epit

surfaces or other host tissues3. Invasiveness is to invade and grow in host

hyaluronidase (spreading enzyme)- hold the cell together

4. Damage by coagulase enzyme – clotting of blood or streptokinase – dissolve blood clot

Bacterial toxin Substance that poisonous to other

organism Examples – hemolysin, leukocidins,

leukostatin, toxemia, neurotoxin, enterotoxin and toxoid

1. Invasiveness is the ability to invade tissues. It

encompasses mechanisms for colonization (adherence and initial multiplication), production of extracellular substances which facilitate invasion (invasins) and ability to bypass or overcome host defense mechanisms.

2. Toxigenesis Toxigenesis is the ability to produce toxins.

Bacteria may produce two types of toxins called exotoxins and endotoxins. Exotoxins are released from bacterial cells and may act at tissue sites removed from the site of bacterial growth. Endotoxins are cell-associated substance. (In a classic sense, the term endotoxin refers to the lipopolysaccharide component of the outer membrane of Gram-negative bacteria).

Cont.. However, endotoxins may be released from

growing bacterial cells and cells that are lysed as a result of effective host defense (e.g. lysozyme) or the activities of certain antibiotics (e.g. penicillins and cephalosporins).

Hence, bacterial toxins, both soluble and cell-associated, may be transported by blood and lymph and cause cytotoxic effects at tissue sites remote from the original point of invasion or growth. Some bacterial toxins may also act at the site of colonization and play a role in invasion.

Transmission of diseases

Types of bourne Air borne transmission - a transmission mechanism

in the which the infectious agent is spread as an aerosol and usually enters a person through the respiratory tract

direct transmission - a transmission mechanism in which the infectious agent is transferred directly into the body via touching or biting or kissing or sexual intercourse or by droplets entering the eye or nose or mouth

indirect transmission - a transmission mechanism in which the infectious agent is transferred to the person by a fomite of vector

Airbourne transmission

Examples

Direct transmission

Contaminated water

Indirect transmission