Staphylococci

Ms. Neha Sharma

Introduction

  • Staphylococci are Gram-positive cocci that typically appear in grape-like clusters due to division in multiple planes.

  • They are non-motile, non-spore-forming, and facultative anaerobes.

  • Staphylococci are widely present in nature and form part of the normal flora of skin, anterior nares, and mucous membranes of humans.

  • They are catalase-positive, which differentiates them from Streptococci (catalase-negative).

  • Based on coagulase production, they are divided into:

    • Coagulase-positive (e.g., Staphylococcus aureus)

    • Coagulase-negative (e.g., S. epidermidis, S. saprophyticus)

  • Staphylococci are highly resistant to environmental conditions, surviving drying, heat, and high salt concentrations.

  • They produce multiple virulence factors such as enzymes, toxins, and adhesion molecules, enabling colonization and invasion.

  • Clinically, they are major causes of pyogenic infections, wound infections, abscesses, and device-related infections.

  • Some strains (especially S. aureus) cause toxin-mediated diseases like food poisoning, scalded skin syndrome, and toxic shock syndrome.

  • Staphylococci are significant hospital-acquired (nosocomial) pathogens, including MRSA strains with antimicrobial resistance.

 

General Character

  • Genus: Staphylococcus
  • Family: Staphylococcaceae
  • Gram Staining: Staphylococci are Gram-positive, appearing purple under a microscope due to their thick peptidoglycan layer.
  • Shape and Arrangement:
    • Shape: They are spherical (cocci) in shape.
    • Arrangement: Staphylococci are typically found in clusters resembling grapes due to division in multiple planes without separation.
  • Oxygen Requirements: Staphylococci are facultative anaerobes. This means they can grow in both the presence and absence of oxygen, allowing them to inhabit diverse environments, including human skin and mucosal surfaces.

Morphology

  • Cell Wall Structure:
    • Composed of a thick peptidoglycan layer (approximately 30-80% of the cell wall) provides structural integrity and helps resist osmotic pressure.
    • It contains teichoic acids that play roles in cell wall maintenance and regulation of cell growth.
  • Capsule:
    • Some species, particularly Staphylococcus aureus, possess a polysaccharide capsule that helps evade phagocytosis by immune cells.
  • Surface Structures:
    • Slime Layer: Contributes to biofilm formation, particularly in coagulase-negative staphylococci (CoNS), enhancing adherence to surfaces like catheters and prosthetic devices.
  • Flagella: Staphylococci are non-motile, lacking flagella and using other mechanisms for adherence and colonization.

Cultural Characteristics

  • Growth Media:
    • Blood Agar: Supports the growth of various staphylococcal species and allows for hemolysis observation. S. aureus shows β-hemolysis, while S. epidermidis is usually non-hemolytic.
    • Mannitol Salt Agar (MSA): A selective medium for staphylococci. It contains high salt concentration (7.5% NaCl) and mannitol. S. aureus ferments mannitol, resulting in a yellow colour change, while S. epidermidis does not ferment it and remains red.
  • Colony Appearance:
    • Colonies of S. aureus are typically golden-yellow due to carotenoid pigments, while S. epidermidis colonies are usually white or off-white.
  • Temperature and pH Range:
    • The optimal growth temperature is around 37°C, but they can grow between 15°C and 45°C.
    • They can tolerate a broad pH range, but neutral pH is preferred for optimal growth.

Biochemical Reactions

  • Catalase Test: Staphylococci produce the enzyme catalase, which converts hydrogen peroxide into water and oxygen, resulting in bubbling. This distinguishes them from streptococci, which are catalase-negative.
  • Coagulase Test:
    • S. aureus: Positive for coagulase, causing plasma to clot.
    • Coagulase-negative Staphylococci (CoNS), like S. epidermidis, are usually coagulase-negative.
  • Mannitol Fermentation:
    • S. aureus: Ferments mannitol, producing acid and causing a colour change in MSA.
    • S. epidermidis: Does not ferment mannitol.
  • Other Biochemical Tests:
    • Oxidase Test: Negative.
    • Urease Test: Variable; most strains of S. saprophyticus are urease-positive, while others are typically negative.
    • DNase Test: Positive for S. aureus, indicating the ability to degrade DNA. This is a differentiating feature for identifying S. aureus.

Pathogenicity

  • Virulence Factors:
    • Toxins:
      • Enterotoxins: Heat-stable toxins responsible for food poisoning, leading to gastrointestinal symptoms.
      • Toxic Shock Syndrome Toxin (TSST-1): A superantigen that can cause toxic shock syndrome characterized by fever, rash, and multi-organ failure.
      • Alpha-toxin: A pore-forming toxin that disrupts cell membranes, causing tissue damage and necrosis.
    • Enzymes:
      • Coagulase: Promotes clot formation, helping to shield the bacteria from immune responses.
      • Hyaluronidase: Breaks down hyaluronic acid in connective tissue, facilitating tissue invasion.
      • Lipase: Allows for colonization of sebaceous glands by breaking down lipids.
    • Adhesins: Surface proteins that promote binding to host tissues, critical for colonization and infection.
  • Clinical Infections:
    • Skin and Soft Tissue Infections: Commonly cause boils, abscesses, impetigo, and cellulitis.
    • Invasive Infections: This can lead to more severe conditions like endocarditis (infection of the heart valves), osteomyelitis (bone infection), and septic arthritis.
    • Respiratory Infections: S. aureus can cause pneumonia, especially in immunocompromised individuals or those with chronic lung disease.
    • Food Poisoning: Resulting from ingestion of food contaminated with enterotoxins, leading to nausea, vomiting, and diarrhoea.
    • Opportunistic Pathogen:
      • S. epidermidis: Often associated with infections related to medical devices (e.g., catheters, prosthetic joints) due to biofilm formation.
      • S. saprophyticus: A common cause of urinary tract infections, particularly in young women.

Laboratory Diagnosis

  • Specimen Collection: Obtaining samples from infected sites (e.g., skin lesions, abscesses, blood, urine). The choice of specimen depends on the clinical presentation.
  • Microscopic Examination:
    • A Gram stain of the specimen reveals Gram-positive cocci in clusters.
    • This initial finding guides further testing and identification.
  • Culture Techniques:
    • Inoculation: Samples are inoculated on blood agar and MSA to promote growth and identify species.
    • Incubation: Cultures are typically incubated at 35-37°C for 24-48 hours.
    • Colony Morphology Examination: Observing colony colour and morphology aids in preliminary identification.
  • Biochemical Testing:
    • Catalase Test: Distinguishes staphylococci from streptococci.
    • Coagulase Test: Essential for differentiating S. aureus from CoNS.
    • Mannitol Fermentation Test: Further identifies S. aureus based on mannitol fermentation.
    • Additional tests may include the DNase and urease tests, depending on the suspected species.
  • Molecular Methods:
    • Polymerase chain reaction (PCR) and other nucleic acid amplification techniques can provide rapid and specific identification of staphylococci and detection of antibiotic resistance genes.
    • These methods are especially useful in outbreaks or cases of severe infections.

Antibiotic Resistance

  • Methicillin-resistant Staphylococcus aureus (MRSA): A significant public health concern due to its resistance to beta-lactam antibiotics. MRSA strains are often associated with severe infections and are prevalent in both hospital and community settings.
  • Vancomycin-Resistant Staphylococcus aureus (VRSA): The emergence of strains resistant to vancomycin complicates treatment options.
  • Coagulase-negative Staphylococci: Increasingly recognized for their resistance to multiple antibiotics, posing challenges in treating infections associated with implanted medical devices.

 

MCQs

1. Staphylococci appear microscopically as:

A. Gram-negative rods
B. Gram-positive cocci in chains
C. Gram-positive cocci in clusters
D. Gram-negative cocci

2. The word “staphyle” means:

A. Chain
B. Grape
C. Spiral
D. Rod

3. Staphylococci are:

A. Motile
B. Non-motile
C. Spore forming
D. Acid-fast

4. Staphylococci are:

A. Obligate anaerobes
B. Facultative anaerobes
C. Strict aerobes
D. Microaerophilic

5. Staphylococci are differentiated from Streptococci by:

A. Oxidase test
B. Catalase test
C. Coagulase test
D. Urease test

6. The most pathogenic species of Staphylococcus is:

A. S. epidermidis
B. S. aureus
C. S. saprophyticus
D. S. hominis

7. Staphylococcus aureus colonies typically appear:

A. White
B. Golden yellow
C. Red
D. Green

8. The enzyme that differentiates S. aureus from other staphylococci is:

A. Catalase
B. Oxidase
C. Coagulase
D. Urease

9. Majority of Staphylococci on skin are:

A. Coagulase-positive
B. Coagulase-negative
C. Acid-fast
D. Motile

10. Catalase-positive organisms include:

A. Streptococcus
B. Staphylococcus
C. Enterococcus
D. Pneumococcus

11. Hemolysis pattern of S. aureus on blood agar is:

A. Alpha
B. Beta
C. Gamma
D. None

12. S. aureus ferments mannitol to produce:

A. Red colonies
B. Yellow colonies
C. Black colonies
D. White colonies

13. Mannitol Salt Agar selects for Staphylococci due to:

A. High sugar
B. High NaCl concentration
C. Low pH
D. Antibiotics

14. S. epidermidis is best known for:

A. Food poisoning
B. Toxic shock syndrome
C. Biofilm formation
D. Skin peeling toxin

15. S. saprophyticus commonly causes:

A. Pneumonia
B. UTI in young women
C. Meningitis
D. Osteomyelitis

16. Coagulase-negative Staphylococci include:

A. S. aureus
B. S. epidermidis
C. S. pyogenes
D. E. coli

17. The major virulence factor of S. aureus that binds IgG is:

A. Exotoxin A
B. Protein A
C. Hemolysin
D. Coagulase

18. Staphylococcal enterotoxin causes:

A. Diarrhea after 6–12 hours
B. Rapid food poisoning within hours
C. Jaundice
D. Meningitis

19. Toxic Shock Syndrome is caused by:

A. TSST-1
B. Coagulase
C. Beta toxin
D. Alpha hemolysin

20. MRSA carries which gene?

A. blaZ
B. mecA
C. vanA
D. tetM

21. mecA gene encodes:

A. Beta-lactamase
B. Altered PBP2a
C. Protein A
D. Coagulase

22. First-line drug for serious MRSA infection is:

A. Penicillin
B. Vancomycin
C. Gentamicin
D. Tetracycline

23. Common colonization site for S. aureus is:

A. Rectum
B. Nasal cavity
C. Mouth
D. Stomach

24. Staphylococci divide along:

A. One plane
B. Multiple planes
C. Spiral axis
D. Horizontal axis

25. S. aureus possesses:

A. Capsule
B. Flagella
C. Spores
D. Fimbriae

26. Staphylococcal alpha toxin causes:

A. Protein synthesis inhibition
B. Membrane pore formation
C. DNA damage
D. Neurotoxicity

27. Enterotoxins of S. aureus are:

A. Heat-labile
B. Heat-stable
C. Inactivated by cooking
D. Antibiotic sensitive

28. A common hospital-acquired infection caused by S. aureus is:

A. Malaria
B. Surgical wound infection
C. Liver abscess
D. Typhoid

29. S. epidermidis infections commonly involve:

A. Catheters and implants
B. Lungs
C. GI tract
D. Kidneys

30. Coagulase test positive means:

A. No clot formation
B. Clot formation
C. Color change only
D. Rapid hemolysis

31. Staphylococci tolerate:

A. 1% NaCl
B. 7.5% NaCl
C. No salt
D. Only low-salt medium

32. Staphylococcal scalded skin syndrome is due to:

A. TSST-1
B. Exfoliative toxin
C. Enterotoxin
D. Hemolysin

33. Food poisoning by S. aureus is due to:

A. Infection
B. Preformed toxin
C. Endotoxin
D. Coagulase

34. Staphylococci are part of normal flora of:

A. Skin
B. Blood
C. CSF
D. Bone marrow

35. Novobiocin resistance helps differentiate:

A. S. aureus
B. S. saprophyticus
C. S. epidermidis
D. S. hominis

36. Biofilm formation is a key virulence factor of:

A. S. aureus
B. S. epidermidis
C. S. saprophyticus
D. S. lugdunensis

37. On Gram stain, Staphylococci appear:

A. Pink rods
B. Purple cocci
C. Pink cocci
D. Purple rods

38. S. aureus is commonly associated with:

A. Impetigo
B. Pneumonia
C. Osteomyelitis
D. All of the above

39. Beta-lactamase in S. aureus causes resistance to:

A. Methicillin
B. Penicillin
C. Vancomycin
D. Linezolid

40. The most common coagulase-negative Staphylococcus causing device infections is:

A. S. aureus
B. S. epidermidis
C. S. saprophyticus
D. S. hominis

41. Staphylococci grow best at:

A. 20°C
B. 25°C
C. 35–37°C
D. 50°C

42. Staphylococcal toxic shock involves:

A. Superantigen activity
B. Endotoxin action
C. Viral mimicry
D. Hemolysis only

43. Staphylococci are:

A. Acid-fast
B. Non–acid-fast
C. Weakly acid-fast
D. Obligate acid-fast

44. Cell wall of Staphylococci contains:

A. Mycolic acid
B. Teichoic acid
C. Lipopolysaccharide
D. Capsule only

45. S. aureus beta hemolysis results from:

A. Protein A
B. Hemolysins
C. Coagulase
D. Capsule

46. The enzyme catalase breaks down:

A. H₂O₂ → H₂O + O₂
B. H₂O → H⁺ + OH⁻
C. CO₂ → CO + O₂
D. Lipids → fatty acids

47. Staphylococci are best cultured on:

A. MacConkey agar
B. Mannitol salt agar
C. SS agar
D. Thiosulfate agar

48. The structure allowing S. epidermidis to stick to surfaces is:

A. Flagella
B. Biofilm
C. Pili
D. Fimbriae

49. Exfoliative toxin causes:

A. Peeling of skin
B. Food poisoning
C. Fever only
D. Kidney injury

50. S. aureus pneumonia is often seen following:

A. Viral influenza
B. COVID-19 only
C. Asthma
D. Common cold

✅ Answer Key

1-C
2-B
3-B
4-B
5-B
6-B
7-B
8-C
9-B
10-B

11-B
12-B
13-B
14-C
15-B
16-B
17-B
18-B
19-A
20-B

21-B
22-B
23-B
24-B
25-A
26-B
27-B
28-B
29-A
30-B

31-B
32-B
33-B
34-A
35-B
36-B
37-B
38-D
39-B
40-B

41-C
42-A
43-B
44-B
45-B
46-A
47-B
48-B
49-A
50-A