Lecture 6 Medical Bacteriology Part I PDF

Microbiology & Immunology Unit Lecture 6 Medical Bacteriology (I) (Cocci bacteria) Dr. Rasha Mokhtar Elnagar M.B.B.C, MSc, PhD, MD Medical Microbiology and Immunology Associate Professor of Medical Microbiology & Immunology Consultant M...

Microbiology & Immunology Unit Lecture 6 Medical Bacteriology (I) (Cocci bacteria) Dr. Rasha Mokhtar Elnagar M.B.B.C, MSc, PhD, MD Medical Microbiology and Immunology Associate Professor of Medical Microbiology & Immunology Consultant Microbiology & Immunology  Types of bacteria A. Pathogenic bacteria: - Cause disease in humans: B. Commensal bacteria (normal flora) (Human microbiome):  Normally inhabit the human body.  Examples of Medically Important Bacteria Gram Positive Cocci 1- Staphylococci  Morphology: - Gram positive cocci - Arranged in grape like clusters - Culture & Biochemical characters: - Facultative anaerobes. - Grow at 37ºC. - Grow on Nutrient agar and Blood agar. - All Staphylococci are Catalase positive. Nutrient agar Blood agar Classification of Staphylococci: 1- Coagulase-positive Staphylococci:  Staphylococcus aureus (Most pathogenic) 2- Coagulase-negative Staphylococci:  Staphylococcus epidermidis (Normal skin flora)  Staphylococcus saprophyticus Virulence factors of Staphylococcus aureus: Toxins Enzymes Epidermolytic (exfoliative) toxin Coagulase Enterotoxin Staphylokinase Toxic shock syndrome toxin Catalase Haemolysin Proteases, Lipase, DNase Diseases caused by Staphylococcus aureus: A) Supppurative (pyogenic = pus forming) infections: o Skin infections: Boils, Folliculitis, Impetigo. o Bacteremia. o Endocarditis, Osteomyelitis, Pneumonia, Urinary tract infection. B) Toxigenic diseases: o Scalded skin syndrome (By Epidermolytic toxin). o Food poisoning (By Enterotoxin). o Toxic Shock Syndrome (By Toxic shock syndrome toxin). Impetigo Boils Folliculitis Scalded skin syndrome  2- Streptococci Morphology: Gram positive cocci. Arranged in pairs (S. Pneumoniae) or in chains (S. Pyogenes). Some strains are capsulated. S. Pneumoniae S. Pyogenes - Culture & Biochemical characters: - Facultative anaerobes. - Grow at 37ºC. - Catalase-negative. - Grow on Blood agar. Classification of Streptococci: according to their hemolytic activity on blood agar. Streptococci Alpha-hemolytic Beta-hemolytic Gamma-hemolytic (partial hemolysis) (complete (no hemolysis) hemolysis) S. Pneumoniae Group A Streptococci Enterococcus S. viridans (S. Pyogenes) faecalis Group B Streptococci (S. agalactiae) Gram positive diplococci Gram positive cocci arranged in long chains  1- Streptococcus pyogenes Virulence factors of Streptococcus pyogenes: 1- M protein:  Adherence to cells.  Resistance to phagocytosis. 2- Enzymes:  Streptokinase, Streptodornase, Hyaluronidase  Streptolysins (Streptolysin S & Streptolysin O) 3- Exotoxins:  Erythrogenic toxin Diseases Caused by Streptococcus pyogenes: A) Supppurative infections: o Pharyngitis, Tonsillitis, Sinusitis, Otitis media o Impetigo, Cellulitis. o Bacteremia. o Puerperal sepsis, Endocarditis, Osteomyelitis. B) Toxigenic diseases: - Scarlet fever (fever, skin rash and Strawberry tongue) due to effect of the Erythrogenic toxin. - Streptococcal toxic shock like syndrome C) Post streptococcal sequelae (By cross reaction to M protein): - Acute rheumatic fever - Acute glomerulonephritis Follicular Tonsillitis Scarlet fever 2- Streptococcus agalactiae: (Normal flora of the vagina). Neonatal meningitis Neonatal pneumonia 3- Viridans Streptococci (e.g., Streptococcus mutans): (Normal flora of the mouth) Infective endocarditis. Dental caries 4- S. pneumoniae (pneumococci): Pneumonia Meningitis 5- Enterococcus faecalis: Normal flora in GIT → Urinary tract infection UTI.  Gram Negative cocci Neisseria species Morphology: Gram-negative cocci Arranged in pairs (diplococci). Culture conditions: - Facultative anaerobes. - Grow at 37ºC. -Capnophilic: Need 5-10% CO2. - They grow best on Chocolate agar and Thayer- Martin agar (selective media; supplemented by vancomycin and nystatin). Important species: 1- Neisseria meningitidis: Mode of transmission: Inhalation of infected droplets. Diseases: Meningitis 2- Neisseria gonorrhoeae: Mode of transmission: Sexually transmitted disease. Diseases: Gonococcal Urethritis or Cervicitis. Ophthalmia neonatorum.  Case study A 1-year-old girl admitted to emergency room with fever and generalized convulsion. Physical examination revealed fever (40° C), irritable child on examination, mild throat inflammation with no exudates, marked rigidity of the neck and clear lungs. Microscopic examination of cerebrospinal fluid (CSF) showed Gram-negative diplococci. Which of the following is the most likely causative pathogen of this case? A. Staphylococcus aureus B. Neisseria meningitides C. Haemophilus influenzae D. Streptococcus pyogenes Reference Lippincott Illustrated Microbiology 4th edition, Chapters: 8, 9 &11.