Introduction to medical microbiology

Introduction to Microbiology

Dr. Waleed Khalid
Lec. : 1

Medical microbiology is the study of the causative agents of infectious diseases of humans and the reaction to such infections. In other words it deals with etiology, pathogenesis, laboratory diagnosis, specific treatment and control of infection (immunization).

Medical microbiology includes:

Bacteriology – The science that study bacteria, the causative agents of a number of infectious diseases.
Virology – The science that study viruses, non-cellular living systems, capable of causing infectious diseases in human being.
Immunology – The science which concerned with mechanisms of body protection against pathogenic microorganisms and foreign cells and substances.
Mycology – The science that deals with the study of fungi .
Protozoology – It deals with pathogenic unicellular animal organisms.


Classification of Microorganisms:-
Classification by structure

Subcellular – DNA or RNA surrounded by a protein coat – viruses

Prokaryotic – simple cell structure with no nucleus or organelles – bacteria

Eukaryotic – complex cell structure with nucleus and specialized organelles – protozoans, fungi, parasites

Naming of Microorganisms:-

Standardized naming
Genus
Category of biologic classification
Example – Staphylococcus
Species of organism
Represents a distinct type of microorganisms
Examples – Staphylococcus aureus and Staphylococcus epidermidis

The genus name is written with a capital letter, and the species name – with a small letter.

How Microorganisms Cause Disease

Microorganisms cause disease in a variety of ways
1- By using nutrients needed by cells and tissues
2- By damaging cells directly
3- By producing toxins


These microorganisms may remain localized or become systemic

Transmission

Direct contact
Indirect contact

Localized symptoms

Swelling
Pain
Warmth
Redness
Generalized symptoms
Fever
Tiredness
Aches
Weakness

Normal flora

Provides a barrier
Can cause an infection when the immunity decrease .


1- Viruses
They are the smallest known infectious agents
They are subcellular microorganisms that :
Have only nucleic acid surrounded by a protein coat
Must live and grow in living cells of other organisms

Hepatitis virus

Illnesses caused by viruses

Colds

Influenza
Hepatitis
Warts
AIDS

Vaccines are available for many viruses

Mumps
Rubella
Measles
Herpes

2-Bacteria

Single-celled prokaryotic organisms that reproduce rapidly .

Classification

Bacteria can be classified according to:-
1- Shape
2- Ability to retain dyes
3- Ability to grow with / without air
4- Biochemical reactions

Bacillus bacterial classification

Classification and Identification
1- Shape
Coccus – spherical, rounded, or ovoid


Bacillus – rod-shaped

Spirillum – spiral-shaped

Virbrio – comma-shaped

Spherical (cocci) bacteria

• Micrococci

• Diplococci
• Streptococci
• Staphylococci
• Tetracocci
• Sarcine

Representatives of pathogenic cocci

1.Scanning Electron Micrograph of Streptococcus pneumoniae

2.Scanning electron micrograph of a Staphylococcus aureus
1
2

Electron Micrograph of Neisseria gonorrhoeae

Classification and Identification (cont.)

2-Ability to retain certain dyes
Gram’s stain
Acid-fast stain

3- Ability to grow in presence or absence of air

Aerobes – grow best in the presence of oxygen
Anaerobes – grow best in the absence of oxygen

4- Biochemical reactions

Special groups
Mycobacteria – bacilli with a cell wall that differs from most bacteria

Rickettsiae

Very small
Live and grow within other living organisms such as mites and ticks
Chlamydiae
Cell wall structure differs from other bacteria
Live and grow within other living cells

Mycoplasmas – completely lack the rigid cell wall

The size of bacteria
The size of bacteria is measured in micrometer (m) or micron () (1 micron or micrometer is one thousandth of a millimeter) and varies from 0.1  to 16-18 . Most pathogenic bacteria measure from 0.1 to 10 .

The other units of measurement of microorganisms is nanometer (nm) (one millionth of a millimeter) .

BACTERIAL CELL

FLAGELLA

Electron Micrograph of Bacteria with Flagella

SPORE

Bacillus megaterium
Bacillus anthracis

Cell wall

In addition to conferring rigidity upon bacteria, the cell wall protects against osmotic damage
Chemically, the rigid part of the cell wall is peptidoglycan
Cell wall first described by Gram in 1884. It is used to study morphologic appearance of bacteria. Gram's stain differentiates all bacteria into two distinct groups:
a. Gram-positive organisms
b. Gram-negative organisms

Gram

Staining
Technique

3- Protozoans

Single-celled eukaryotic organisms, larger than bacteria, they are found in soil and water and they are a leading cause of death in developing countries

Illnesses caused by protozoans are

Malaria
Amebic dysentery
Trichomoniasis vaginitis

Protozoan Trichomonas vaginalis

4- Fungi

Eukaryotic organisms with rigid cell wall
Yeasts
Single-celled
Reproduce by budding
Molds
Large, fuzzy, multicelled organisms
Produce spores

Superficial infections

Athlete’s foot
Ringworm
Thrush

Can cause systemic infections

Yeast: a single-celled fungi

Multicellular Parasites
Organisms that live on or in another organism and use it for nourishment


Parasitic worms
Usually due to poor sanitation
Roundworms
Flatworms
Tapeworms

Parasitic insects

Bite or burrow under the skin
Mosquitoes
Ticks
Lice
mites

Apply Your Knowledge

Matching:
___ Yeast or mold A. Virus
___ Tapeworm / lice B. Bacteria
___ Classified by shape C. Protozoan
___ Subcellular organism D. Fungus
___ May be aerobic or anaerobic E. Multicellular parasite
___ Smallest known organism
___ Found in soil and water
E
ANSWER:
D
A
B
B
C
A
Very Good!