📖 General Microbiology

INNATE (NON-SPECIFIC) IMMUNITY

The elements of the innate (non-specific) immune system include anatomical barriers, secretory molecules and cellular components. 

Among the mechanical anatomical barriers are the skin and internal epithelial layers, the movement of the intestines and the oscillation of broncho-pulmonary cilia. 

Associated with these protective surfaces are chemical and biological agents.

A. Anatomical barriers to infections

1. Mechanical factors

The epithelial surfaces form a physical barrier that is very impermeable to most infectious agents. Thus, the skin acts as our first line of defense against invading organisms. The desquamation of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces. 

2. Chemical factors

Fatty acids in sweat inhibit the growth of bacteria. Lysozyme and phospholipase found in tears, saliva and nasal secretions can breakdown the cell wall of bacteria and destabilize bacterial membranes. The low pH of sweat and gastric secretions prevents growth of bacteria. Defensins (low molecular weight proteins) found in the lung and gastrointestinal tract have antimicrobial activity. Surfactants in the lung act as opsonins (substances that promote phagocytosis of particles by phagocytic cells). 

3. Biological factors

The normal flora of the skin and in the gastrointestinal tract can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces.

B. Humoral barriers to infection

Humoral factors play an important role in inflammation, which is characterized by edema and the recruitment of phagocytic cells. These humoral factors are found in serum or they are formed at the site of infection.

1. Complement system – The complement system is the major humoral non-specific defense mechanism (see complement chapter). Once activated complement can lead to increased vascular permeability, recruitment of phagocytic cells, and lysis and opsonization of bacteria. 

2. Coagulation system – Depending on the severity of the tissue injury, the coagulation system may or may not be activated. Some products of the coagulation system can contribute to the non-specific defenses because of their ability to increase vascular permeability and act as chemotactic agents for phagocytic cells. In addition, some of the products of the coagulation system are directly antimicrobial. For example, beta-lysin, a protein produced by platelets during coagulation can lyse many Gram positive bacteria by acting as a cationic detergent.

3. Lactoferrin and transferrin – By binding iron, an essential nutrient for bacteria, these proteins limit bacterial growth.

4. Interferons – Interferons are proteins that can limit virus replication in cells.

5. Lysozyme – Lysozyme breaks down the cell wall of bacteria. 

6. Interleukin -1 – Il-1 induces fever and the production of acute phase proteins, some of which are antimicrobial because they can opsonize bacteria.

C. Cellular barriers to infection

Part of the inflammatory response is the recruitment of polymorphonuclear eosinophiles and macrophages to sites of infection. These cells are the main line of defense in the non-specific immune system.

1. Neutrophils – Polymorphonuclear cells  are recruited to the site of infection where they phagocytose invading organisms and kill them intracellularly. In addition, PMNs contribute to collateral tissue damage that occurs during inflammation.

2. Macrophages – Tissue macrophages  and newly recruited monocytes , which differentiate into macrophages, also function in phagocytosis and intracellular killing of microorganisms. In addition, macrophages are capable of extracellular killing of infected or altered self target cells. Furthermore, macrophages contribute to tissue repair and act as antigen-presenting cells, which are required for the induction of specific immune responses.

3. Natural killer (NK) and lymphokine activated killer (LAK) cells – NK and LAK cells can nonspecifically kill virus infected and tumor cells. These cells are not part of the inflammatory response but they are important in nonspecific immunity to viral infections and tumor surveillance. 

4. Eosinophils – Eosinophils  have proteins in granules that are effective in killing certain parasites.

CROSS INFECTION AND STERLIZATION IN DENTISTRY

Cross infection is defined as the transmission of infectious agents amongst patients and staff with in hospital environment.

Routes of Infection 
Two routes are important : transdermal  and respiratory. 

 In transdermal route microorganisms enter the tissues of the recipient by means of injection through intact skin or mucosa (usually due to an accident involving a sharp instrument) or via defects in the skin e.g. recent cuts and abrasions.
 
Microorganisms causing cross infection in dentistry

Transmitted through skin 

Bacteria : Treponema pallidum, Staphylococcus aureus

Viruses :Hepatitis virus, HIV ,Herpes simplex virus, Mumps, Measles , Epstein-Barr virus

Fungi: Dermatomycoses, Candidiasis, 

Transmitted through aerosols

Bordetella pertussis, Myco.tuberculosis, Streptococcus pyogenes, Influenza virus
Rhinovirus,  Rubella 
 

Types of microscopy used in bacteriology

Light microscopy
Phase contrast microscopy
Fluorescence microscopy
Darkfield microscopy
Transmission electron microscopy
Scanning electron microscopy

Fluorescent microscopy in which ultraviolet rays are used to examine cells after treatment with fluorescent days.

Phase contrast microscope enhances the refractive index differences of the cell components. This microscopy can be used to reveal details of the internal structures as well as capsules, endospores and motility

Electron microscope The resolving power is more than 200 times that of light microscope.