Microbes and Human Diseases | CHAPTER 1 | Microbiology & Parasitology for Nurses

Microbes and Human Diseases – Basic microbiology, parasitology, and immunology; nature, reproduction, growth, and transmission of common microorganisms and parasites in Bangladesh; prevention including universal precaution and immunization, control, sterilization, and disinfection; and specimen collections and examination. Students will have an understanding of common organisms and parasites caused human diseases and acquire knowledge about the prevention and control of those organisms.

Microbes and Human Diseases

Normal Flora

Normal flora is the term used to describe the various micro-organisms (bacteria and fungi) that are permanent residents of certain body sites, especially the skin, oropharynx, colon, and vagina.

Or,

The human body is continuously inhabited by many different microorganisms mostly bacteria, but also fungi and other microorganisms which under normal circumstances in a healthy individual are harmless, and may even be beneficial. These microorganisms are termed “normal flora”.

Sources of Normal Flora:

A healthy newborn enters the world in essentially sterile condition, but after birth, it rapidly acquires normal- flora from –

• Food
• Environment &
• Other humans

Types of Normal Flora;

There are 2 types of normal flora-

Resident flora: These are micro-organisms regularly present in the region, e.g. Viridans streptococci in mouth and throat, E. coli in intestine.

Transient flora: The micro-organisms are present only of hours to weeks, e.g. Streptococcus pyogenes in throat.

Sites of Residence of Normal Flora:

The most common sites of the body inhabited by normal -flora are, as one might expect, those in contact or communication with the outside world, namely –

• Skin
• Eye
• Mouth
• Upper respiratory tract including nose
• Gastrointestinal tract and
• Urogenital tract

Examples of Normal Flora with Their Sites:

The species contained in normal -flora cannot be rigidly defined for all humans, because those species vary from individual to individual, as a result of physiologic differences, diet, age and geographic habitat.

Beneficial Effects of Normal Flora:

1. Prevention of colonization of pathogenic organisms: In mouth and lower bowel an invading pathogen may fail to compete for nutrients & receptor sites with normal flora. Thus normal flora prevents infection.

2. Production of antimicrobial substances; Some bacteria produce bacteriocins or antibiotics, which also prevent infection by pathogenic bacteria.

3. Antigenic stimulation by normal flora : In new born, bacteria act as powerful stimulus for the development of the immune system. It leads to production of IgA that is secreted through mucus membranes & give protection.

4. Nutritional function; Bacteria of the gut provide important nutrients, such as vitamin-K, and aid in digestion and absorption of nutrients

Harmful Effect / Role of Normal Flora in the Causation of Disease;

Clinical problems caused by normal flora arise under the following conditions –

1. When the organisms are displaced from their normal site in the body to an abnormal site. An example is the introduction of the normal skin flora S. epidermidis into the bloodstream where it can colonize catheters and heart valves, resulting in bacterial endocarditis.

2. When potential pathogens gain a competitive advantage due to diminished populations of harmless competitors. For example, when normal bowel flora is depleted by antibiotic therapy leading to overgrowth by the resistant Cl. difficile, which can cause severe colitis.

3. When harmless commonly ingested food substances are converted into carcinogenic derivatives by bacteria in the colon.

4. When individuals are immunocompromised, normal -flora can overgrow and become pathogenic.

Opportunistic Pathogens/ Opportunistic Organisms:

The pathogens which do not produce disease in their normal habitat but may cause infection when the body immunity is decreased are called ‘opportunistic pathogens’.

Example: Normal flora of different body sites, such as

Skin

• Staphylococcus epidermidis

Nose

• Staphylococcus aureus

Mouthia

• Viridans streptococci

Throat

• Viridans streptococci

Colon

• Bacteroides fragilis
• Escherichia coli,

Vagina

• Lactobacillus.
• E. colin
• Group B Streptococci

Urethra

• Staphylococcus epidermidis

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