CHAPTER 1 - INTRODUCTION

CONTENT 1) A brief history of medical microbiology 2) Host – parasite relationships 3) Mechanism of pathogenesis  Pathogenic properties of pathogens 4) Transmission of diseases

A brief history of medical microbiology

Antony van Leeuwenhoek  Antony van Leeuwenhoek ( ) 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 ( ), 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 bacteria 1. Adherence using (adhesin) protein found on attachment pili and capsule. 2. Colonization is growth of m/org on epit surfaces or other host tissues 3. 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