Esthetician

Microbiology is the study of microorganisms, living organisms too small to be seen without a microscope. Estheticians work directly on clients' skin, so they must understand how microorganisms cause disease and how to prevent their transmission. Microorganisms include bacteria, viruses, fungi, and parasites. Some are harmless or beneficial; those that cause disease are called pathogens.

Bacteria are single-celled microorganisms found almost everywhere. They are identified first by shape.

Bacteria have a simpler structure than human cells. The cell wall gives shape and protection; the cell membrane beneath it controls what enters and leaves. The gel-like cytoplasm is where metabolism occurs, and the nucleoid holds the DNA, which is not enclosed in a membrane. Ribosomes build proteins. Some bacteria move with whip-like flagella; hair-like pili help them attach to surfaces and exchange genetic material.

Bacteria reproduce by binary fission, dividing into two identical daughter cells. Under ideal conditions they can divide about every 20 minutes. Growth requires moisture, nutrients, warmth, and usually oxygen; anaerobes grow without oxygen.

Bacteria have two stages: an active stage, in which they grow and reproduce, and an inactive (spore-forming) stage. Certain bacilli and clostridia form spores when conditions are unfavorable. Spores are dormant, highly resistant structures that survive heat, drying, and many disinfectants, then germinate when conditions improve. Only sterilization reliably destroys spores.

Nonpathogenic bacteria do not cause disease, and many are beneficial. The skin's normal flora, or skin microbiome (for example Staphylococcus epidermidis), competes with pathogens for space and nutrients and supports immune defense.

Pathogenic bacteria cause disease by invading tissue and multiplying or by producing toxins. Exotoxins are secreted by living bacteria; endotoxins are released when bacteria die. The body's response is inflammation: redness, swelling, heat, and pain.

Viruses are far smaller than bacteria and cannot reproduce on their own; they must invade a host cell and use its machinery to make copies of themselves. A virus is genetic material (DNA or RNA) inside a protein coat called a capsid; some also have an outer envelope. Antibiotics do not kill viruses.

Fungi include yeasts and molds. Pathogenic fungi cause infections called mycoses and thrive in warm, moist environments.

• Tinea (ringworm) is caused by dermatophyte fungi, not worms: tinea corporis (body, ring-shaped scaly patches), tinea pedis (athlete's foot), tinea capitis (scalp), and tinea unguium or onychomycosis (nails).
• Tinea versicolor (pityriasis versicolor) is a yeast overgrowth that causes light or dark patches, often on the chest and back.
• Candidiasis is caused by Candida albicans yeast and appears as red, itchy patches with satellite lesions in moist areas and skin folds.

Parasites live on or in a host and draw nutrients from it. Pediculosis is infestation with lice; head lice spread by direct contact or shared items and leave nits (eggs) attached to hair shafts. Scabies is caused by the mite Sarcoptes scabiei, which burrows into skin and causes intense itching, often between the fingers, at the wrists, and in skin folds. Both are contagious and contraindicate services.

Infection occurs when pathogens enter the body, multiply, and damage tissue. A local infection is confined to one area (a pimple or abscess); a systemic infection spreads through the bloodstream to the whole body. An infection that spreads easily from person to person is contagious (communicable).

The stages of disease are the incubation period (exposure to first symptoms), prodromal period (early, general symptoms), acute period (peak illness), and convalescent period (recovery).