Medically Important Bacteria

Pathogenicity

• The ability of a pathogen to produce a disease by overcoming the host's defense

• Virulence is the degree of pathogenicity

Gram Positive Bacteria

1. STAPHYLOCOCCI

• 3 species are of clinical importance

• STAPHYLOCOCCUS AUREUS:
• - Cause of most staphylococcal infection in men

• STAPHYLOCCOCUS EPIDERMIDIS
• - Normally part of indigenous flora but occasionally cause infection and intravascular catheter-associated infection

• MORPHOLOGY:
• - Gram positive cocci, grow in irregular clusters (grape like)

• TRANSMISSION:
• - Skin, Mucous Membrane, Nose, Fomites contaminated with Lesion

Pathogenesis

• Cause disease both by producing toxins and multiplying in tissue called inflammation

• Typical lesion of S. aureus infection is an abscess

• Produces toxins like enterotoxins which is a protein that cause vomitting and diarrhea

• Its enzyme include coagulase, proteases, etc.

Boils (Furuncles)

Impetigo

Cellulitis

• Showing typical red streaks and swelling

Disease - Staphylococci

4. Osteomyelitis

- Bone infection

5. Pneumonia

- In post-operative patients or following viral respiratory infection

6. Abscesses

- EG: Skin, lung

7. Food Poisoning

- Due to enterotoxin

Pathogenesis

• Transmission:
• - Part of normal flora of human throat, skin, intestines but produce disease when they gain access to tissues or blood

• Produce 6 toxins and enzymes

• Streptokinase (firinolysin) dissolves clots, thrombi and emboli

Diseases

• S.pyogens is the most common bacterial cause of sore throat

• Pharyngitis if untreated can extend as otitis, sinusitis, mastoiditis and meningitis
• - It can also cause scarlet fever and rheumatic fever

• Skin infection can cause cellulitis, erysipelas, impetigo

Erysipelas - Red Skin

Scarlet Fever

Post-Streptococcal Disease

• Acute glomerulonephritis and rheumatic fever

• Immunological cross reaction between streptococcal antigens and antigens of the joint (cartilage), heart (valves) and kidney (filtration membrane)

Clinical Disorders

• Meningitis:

• - Cause fever, shock, purpura, disseminated intravascular coagulation (DIC)

Gonorrhoea

• Cause purulent discharge of urethra in males
• - Dysuria, Urethritis

• In females there is vaginal discharge and may result in salphingitis

Mycobacterium

• M.tuberculosis (TB), M.leprae (leprosy)

• Aerobic acid-fast bacilli (stained by Ziehl-Neelson Stain)

• Transmitted from person to person by respiratory aerosol

• A vacine containing a strain of live attenuated M.bovis (BCB-bacillus Calmette-Guerin) can be used to induce partical resistance to tuberculosis

Pathogenesis

• The infected tissue undergoes necrosis (granuloma) and forms a caseous lesion

• If these heal, they become calcified and show clearly on X-rays

• If the disease is not arrested at this point, caseous lesion progress to liquefaction

• The primary lesion of tuberculosis usually occurs in lung (Ghon complex)

• Secondary tuberculosis can affect other organs
• - Bone, brain, etc.

Caseous Granulomatous Lesion

Ghon Complex

• The parenchymal exudative lesion and the draining lymph nodes together are called a Ghon Complex