MCEM Part A Study Guide/Microbiology/Principles of Microbiology/Natural and Innate Immunity

Barriers to Infection

Skin

The skin acts as a physical barrier to invasion of bacteria. It also secretes free fatty acids and sebum which will inhibit bacterial growth.

There is an enhanced risk in psoriasis, burns, eczema due to the breech in physical barrier and dysfunction of the chemical barrier.

Mucociliary

Mucus has polysaccharides with similar antigenic structure to the underlying mucosa, and bacteria bind to these antigens and are removed with the mucus.

Gastric Acid

Gastric Acid will kill ingested pathogens

Urinary

The flushing action of urinary flow helps to keep the urinary tract free of microorganisms. Obstruction to flow (BPH, stones, tumor, scarring) will predispose to urinary tract infections.

Normal Bacterial Flora

The average human body has approximately 100 trillion bacteria colonizing it. These non-pathogenic bacteria compete for sites with pathogenic bacteria. They also produce bacteriocins that will inhibit other bacteria. Anarobes will also produce free fatty acids. Lactobacilli in the female genital tract will produce lactic acid, reducing the pH and making the area unsuitable for other bacteria.

The use of antibiotics can suppress normal non-pathogenic bacteria, allowing inherently resistant organisms to predominate (eg. Candida, C. difficile)