Process by which a micro-organism establishes a colony in/on the host organism is called infection.

• If infection causes symptoms of disease in the host then – Pathogenic
• Not all infections are pathogenic

Types of Micro-organisms

1. Pathogenic : those that cause infection
2. Non Pathogenic : those that are not harmful
3. Commensals: they live on/in the host (like symbiotic relationship) & non-pathogenic. Eg – Ecoli
4. Opportunistic: they are basically commensals but become pathogenic if the hosts defense mechanism become weak.

Micro-organisms fall into 4 main groups:

1. Bacteria
2. Virus
3. Fungi
4. Protozoa
5. Others – Helminths

BACTERIAL OCULAR INFECTION

What is Bacteria?

A bacterium is a single-celled (unicellular) microorganism that does not have a nucleus or any other membrane-bound organelles. Bacteria are sometimes called ‘prokaryotes.’ In Greek, ‘prokaryote’ literally means ‘before the nut’ (where ‘the nut’ is the nucleus.)

Bacteria adapt to become well-suited to their environments, and therefore come in many shapes and forms. However, they all have a few parts in common.

1. Capsule: A protective, often slimy, coating, often of sugars, that helps to protect the bacterium. It also makes bacteria virulent. This means the bacteria is more likely to cause disease, since it aids the cell in survival against attack. For example, the bacteria may survive an attack from the human body’s immune system.
2. Cell wall: In bacteria, the cell wall is usually made of peptidoglycan, a protein and sugar compound. This structure gives the cell some rigidity and protection.

3. Cell membrane: As in most cells, the bacterium’s plasma membrane acts by coordinating the passage of

molecules into and out of the cell.

4. Cytoplasm: Again, as in many cells, the cytoplasm serves as a medium through which molecules are transported, as well as a system to maintain conditions (like temperature and pH) that are best for the cell.
5. Ribosomes: The main site for the bacterium’s protein synthesis.
6. Nucleosome: A basic unit of chromatin, which won’t be covered in this lesson.
7. Nucleoid: This is the region where the bacterium’s DNA is located. Again, it’s not the same as a nucleus because it’s not surrounded by a membrane.
8. Flagellum: In many bacteria, a flagellum is present, and is the means by which the cell moves around

Classification of Bacteria

• Morphology (Cocci – round shape, Bacilli – rod shape, Vibrio – comma shape)
• Gram stain (Gram positive and Gram negative)
• Respiration (Aerobic & Anerobic)
• Colony (Clusters – Staphylo & Chain – Strepto)