MC- Morphology, pathogenicity - S. aureus

This presentation provides a comprehensive overview of Staphylococcus aureus, an important pathogenic bacterium in medical microbiology. It covers the morphology, cultural characteristics, pathogenic mechanisms, virulence factors, laboratory diagnosis, epidemiology, and treatment options. The slides also highlight clinically significant infections caused by S. aureus, including skin infections, pneumonia, food poisoning, and toxic shock syndrome. Emphasis is placed on antibiotic resistance patterns such as MRSA (Methicillin-Resistant Staphylococcus aureus) and infection control measures. Designed for medical, paramedical, and microbiology students, this presentation serves as a concise and informative learning resource.

1. The Microbiology Corner Spreading knowledge in microbiology and life sciences Subject: Medical Bacteriology and Mycology Topic: Morphology, pathogenicity, laboratory diagnosis of Staphylococcus aureus

2. Learning outcome 1. Identify the characteristics of Staphylococcus aureus. 2. Explain the significance of Gram staining in identifying Staphylococcus aureus. 3. Define catalase and coagulase and their roles in differentiating Staphylococcus aureus from other bacteria. 4. Analyze the conditions under which Staphylococcus aureus can lead to infections.

3. Habitat of  Staphylococcus aureus N atural habitat is mammalian body surfaces. N ormal flora of the skin and mucous membrane. P resent in the nose / the anterior nares. Found in Pharynx. Found in stratified epithelial cells or mucous or serum constituents associated with these cells.

4. Found in skin/nasal passage and axillae of humans. B enign or symbiotic relationship with the hosts. Enterogeneric strains of  S. aureus  are found in various food products. 4. S urvive on dry skin. Thirty percent (30%) of the normal human healthy population is affected by  S. aureus  as it asymptomatically colonizes on the human host’s skin.

5. History Von Recklinghausen (1871)- first to see Staphylococci in pus specimen Louis Pasteur (1880)- first to cultivate in liquid medium Sir Alexander Ongston (1880)- named the bacteria as “ Staphylococcus ” Rosenbach (1884)- named the Golden yellow colony bacteria as “ Staphylococcus aureus ”

6. Morphology of  Staphylococcus aureus Gram-positive, singly, in pairs, or a short chain of 3-4 bacteria. Irregular clusters of cells. 1 um in diameter Spherical colonies in clusters in two planes. Cell wall- very thick peptidoglycan layer Non-Flagellated, Non-Motile and Non-Sporing C apsulated. Grapes-like clusters arrangement.

7. Cultural characteristics of  Staphylococcus aureus Staphylococci grow - under aerobic or microaerophilic conditions. Colonies on solid media are round, smooth, raised, and glistening. S. aureus  usually forms gray to deep golden yellow colonies. Mannitol Salt Agar:   circular, 2–3 mm in diameter, with a smooth, shiny surface; colonies appear opaque and are often pigmented golden yellow.  Tryptic Soy Agar : circular, convex, and entire margin.  Blood Agar : beta-hemolysis. Brain heart infusion agar:   Yellow pigmented colonies.

8. Culture Aerobes and facultative anaerobes Opt. Temp. For growth= 37 °C (10-42 °C) Opt. pH for growth 7.4- 7.6 On Nutrient Agar Golden yellow and opaque colonies: Large, circular, convex, shiny with smooth glistening surface, 2-4 mm in diameter (max. pigment production@22 °C) NA Slope : Oil Paint appearance

9. Culture On Blood agar, golden yellow colonies, surrounded by a clear zone of hemolysis (beta-hemolysis), esp. When incubated in sheep or rabbit blood agar in an atmosphere of 20% CO2 On MacConkey agar, Smaller colonies (0.1-0.5 mm) and are pink-colored due to lactose fermentation

10. Culture (.... contd ) In Liquid Media: Uniform Turbidity is observed Special Selective Media, Salt – Milk Agar (8-10% NaCl), Ludlam’s Media (Lithium Chloride, Tellurite and Polymixin ) is a useful selective medium for recovering S. aureus from faecal specimens. Baird-Parker agar medium is formulated on the principle that staphylococci can reduce tellurite to tellurium  and show  lecithinase reaction in the presence of egg yolk . It is primarily used in processing food, cosmetics, and environmental samples rather than clinical samples.

11. Biochemical characteristics of  Staphylococcus aureus S.No Test Result 1 Catalase Positive 2 Oxidase Negative 3 OF Test Fermentative 4 Coagulase Positive: The Presence Of Free And /Or Bound Coagulase 5 Indole Negative 6 Gas Negative

12. 7 Hydrogen Sulfide Negative 8 Methyl Red Positive 9 Vp Positive 10 Nitrate Reduction Positive 11 Beta Hemolysis Blood Agar 12 Motility Negative 13 Citrate Positive

13. Virulence factors of  Staphylococcus aureus

14. A. Cell wall components Polysaccharide Capsule : inhibits phagocytosis Peptidoglycan : activates complement, IL-1, chemotactic to PMNs Teichoic acid : species-specific, mediate binding to fibronectin Protein A : It binds to the Fc region of IgG and complement, exerting an anti-opsonin effect. Fibronectin binding protein ( FnBP ): promotes binding to, mucosal cells and tissue matrices. Clumping factor: FnBP enhances the clumping of the organism in the presence of plasma.

15. B. Enzymes Catalase enzyme: conversion of hydrogen peroxide into water and oxygen Coagulase enzyme and clumping factor: an enzyme-like protein that clots oxalated or citrated plasma. Other enzymes: hyaluronidase (spreading factor), staphylokinase (fibrinolysis), proteinases, lipases, β- lactamases

16. C. Toxins Exotoxin:  comprised of four toxins α,β,γ,δ: also called hemolysin, – α exotoxins- heterogenous protein acts on a broad spectrum of eukaryotic cell membranes – β exotoxins- degrade sphingomyelin – δ exotoxins- disrupts biological membrane – γ exotoxins- interact with two proteins to form six potential two-component toxins. Panton-Valentine leukocidin :  composed of two components S and F which act synergistically to kill white blood cells

17. Exfoliative toxins:  composed of type A- located on phage and heat stable and type B- plasmid-mediated and heat-labile: yield generalized desquamation scalded skin syndrome Toxic shock syndrome toxins (TSST-1):  prototypical superantigen which binds with MHC-II yielding T-cell stimulation. Toxic is associated with fever, shock, and multisystem involvement. Enterotoxins:  altogether 15 enterotoxins(A-E, G-P), heat stable, resistant to gut enzymes

18. Pathogenesis Adhere to damaged skin, mucosa or tissue surfaces At these sites, they evade defence mechanisms of the host, colonize and cause tissue damage S.aureus produces disease by Multiplying in tissues Liberating toxins, Stimulating inflammation

19. Clinical manifestation of  Staphylococcus aureus A. Localized skin infections Infections are small superficial abscesses involving hair follicles, sweat, or sebaceous glands. Subcutaneous abscesses called furuncles (boils) often formed around foreign bodies. Carbuncles are larger, deeper, multiloculated skin infections that can lead to bacteremia. Impetigo is usually localized, superficial, spreading crusty skin lesions.

20. Deep-seated Infections Osteomyelitis : inflammation of bone Bacteria can get to the bone Via bloodstream Following an injury Clinical features: pain, swelling, deformity, defective healing, in some case pus flow, Diagnosis: X-ray, MRI, bone aspirates

21. Toxic Shock Syndrome Caused when Toxin shock syndrome toxin (TSST) liberated by S.aureus enters bloodstream It is a multisystem illness, characterized by:

22. B. Deep localized infections Metastatic from superficial infections or skin carriage or may result from trauma. Acute infection of joint space in children. C. Acute endocarditis Generally associated with intravenous drug users. Bacteria can be introduced into soft tissue and the bloodstream. D. Septicemia Generalized infection with sepsis or bacteremia associated with a known focus or not.

23. E. Pneumonia F. Nosocomial infections Hospital-associated infections often of wounds or bacteremia associated with catheters. G. Toxin mediated infections Toxic shock syndrome High Fever Rash Vomiting Diarrhea Hypotension Multiorgan involvement Staphylococcal gastroenteritis Nausea Vomiting Diarrhea

24. Mode Of Transmission

25. Laboratory Diagnosis of  Staphylococcus aureus Microscopy Microscopy is useful for pyogenic infections but not blood infections or toxin-mediated infections. A direct smear for Gram staining may be performed as soon as the specimen is collected. The Gram stain showing  typical Gram-positive  cocci that occur singly and in pairs, tetrads, short chains, and irregular  grape-like clusters  can be suspected to be  S. aureus .

26. Culture- Growth medium The organism is isolated by streaking material from the clinical specimen (or from a blood culture) onto solid media such as  blood agar, tryptic soy agar,  or  heart infusion agar . Specimens likely to be contaminated with other microorganisms can be plated on  mannitol salt agar  containing 7.5% sodium chloride, which allows the halo-tolerant staphylococci to grow. The inoculated plates should be incubated at 35°C to 37°C for 24 to 48 hours.

27. Presumptive identification The presumptive identification of  S. aureus  rests on the isolation of: Large mannitol fermenting colonies on MSA Gram-positive cocci in clusters Catalase-positive organisms Coagulase-positive organisms

28. Confirmatory tests Confirmatory tests include biochemical tests, molecular probes, or mass spectrometry. Biochemical reactions Tests for clumping factor, coagulase, hemolysins, and thermostable deoxyribonuclease are routinely used to identify  S. aureus .

29. Treatment of  Staphylococcus aureus  infection Oral therapy can include trimethoprim-sulfamethoxazole, doxycycline or minocycline. Hospital- and community-acquired infections with methicillin-resistant  Staphylococcus aureus  (MRSA) - worldwide problem. Treatment is symptomatic for patients with food poisoning (although the source of infection should be identified so that appropriate preventive procedures can be enacted)

30. Prevention of  Staphylococcus aureus  infection Proper cleansing of wounds. Thorough hand washing. Patients and staff carrying epidemic strains, particularly MRSA, should be isolated. Patients may be given disinfectant baths or treated with a topical antibiotic to eradicate the carriage of MRSA. Infection control programs should be used in hospitals.