Medical Microbiology - 5

Overview of Medical Microbiology

A study of bacteria that cause diseases and how they affect human health.

1. Legionella and Coxiella (Ch. 34)

Legionella pneumophila

Characteristics:

• Thin, flexible rod-shaped bacteria that are Gram-negative.

• Needs air to survive and specific nutrients like L-cysteine and iron to grow.

• Cannot ferment sugars; instead, it uses amino acids for energy.

• Found in water and soil, especially in man-made water systems like cooling towers and hot tubs.

• Prefers to infect lungs, where it can survive and reproduce inside immune cells called alveolar macrophages.

• Can also live inside certain types of amoebas.

Virulence Factors:

• Contains a type of cell wall component called lipopolysaccharide (LPS) that is less harmful than other bacteria, helping it avoid the immune system.

• Uses a special system to inject proteins into host cells, which helps it to survive inside immune cells.

• Prevents immune cells from effectively digesting it by stopping fusion of immune cell compartments.

• Forms special vacuoles in cells that allow it to live and reproduce, creating pores to escape when threatened.

Clinical Manifestations:

• Causes pneumonia, known as Legionnaires' disease, primarily affecting the lungs.

• Pontiac fever is a milder illness that mimics flu symptoms.

• Legionnaires' disease may take 2-10 days to appear, leading to severe lung damage.

2. Pseudomonas (Ch. 35)

Pseudomonas aeruginosa

Characteristics:

• Rod-shaped, Gram-negative bacteria that can move, often found in pairs.

• Needs oxygen to survive and can use minimal nutrients to grow; tests positive for cytochrome oxidase, indicating a functioning energy production system.

• Produces colorful compounds and may develop a thick protective layer known as a mucoid capsule.

• Commonly found in damp places like soil, water, and hospitals, where it can cause infections in patients with weakened immune systems.

Virulence Factors:

• The mucoid capsule helps protect against being eaten by immune cells and dehydration.

• Uses a secretion system to directly introduce harmful proteins into host cells.

• Possesses attachments, tails, and hair-like structures to help stick to surfaces and form biofilms.

• Its toxins, such as Exotoxin A, disrupt protein production in host cells, while others harm immune system functions.

• Produces enzymes that can break down important body proteins and inhibit immune responses.

Antibiotic Resistance:

• Naturally resistant to many antibiotics; can develop increased resistance through genetic changes.

• Produces enzymes that inactivate commonly used antibiotics, complicating treatments.

Clinical Diseases:

• Causes severe infections, especially in patients with weak immune systems, leading to lung infections, skin infections, and urinary tract infections (UTIs).

3. Spirochetes (Ch. 37)

Key Genera:

• Treponema: T. pallidum (Causes syphilis)

• Borrelia: B. burgdorferi (Causes Lyme disease)

• Leptospira: L. interrogans (Causes leptospirosis)

Characteristics:

• Spirochetes are spiral-shaped bacteria that can move easily through fluids in the body.

• Different ways to spread and where they live:

  • T. pallidum: Spread mainly through sexual contact, with humans being the main carriers.

  • L. interrogans: Spread through contaminated water usually from animal urine.

  • B. burgdorferi: Spread by tick bites, causing Lyme disease.

Diseases:

• Treponema pallidum: Causes syphilis; has several stages:

  • Primary: Formation of a sore (chancre).

  • Secondary: Flu-like symptoms and rashes appear throughout the body.

  • Tertiary: Causes long-term problems such as neurological or heart issues.

• Leptospira: Causes mild flu-like symptoms, but severe cases can result in kidney failure, known as Weil’s disease.

• Borrelia: Causes Lyme disease, which has different stages:

  • Early stage: Characterized by a rash (erythema migrans).

  • Can also lead to other problems involving the nervous system and joints.

4. Chlamydia (Ch. 39)

Species:

• Chlamydia trachomatis: Causes genital infections and eye infections.

• Chlamydophila psittaci: Causes pneumonia from bird exposure.

• Chlamydophila pneumoniae: Causes pneumonia spread through respiratory droplets.

General Properties:

• These bacteria are obligate intracellular, meaning they can only live and grow inside host cells.

• They have a structure similar to Gram-negative bacteria but lack a certain cell wall component, which causes resistance to some antibiotics.

• These bacteria have a unique life cycle involving elementary bodies (EBs) that infect host cells and reticulate bodies (RBs) that reproduce inside those cells.

Clinical Relevance:

• C. trachomatis is a leading cause of both preventable blindness and common STDs in the United States, often leading to chronic conditions if not treated.

• It can result in conditions like trachoma (a leading cause of blindness) and lymphogranuloma venereum (LGV), with pneumonia being possible in various groups based on different ways of transmission, such as sexual contact or respiratory droplets.