Microbiology: Streptococci Identification

Avium

• Resists 6.05% NaCl and tests positive for PYR.
• Common virulence factors include:
  • Extracellular surface protein, serine protease, gelatinase, cytolysins.
  • Can resist antibiotics and antimicrobial agents.
• Associated diseases:
  • Nosocomial infections, UTIs particularly from catheterization, bacteremia in hemodialysis and surgery, and endocarditis with prosthetic heart valves.

Laboratory Tests for Identification of Streptococci

• Bacitracin Susceptibility Test (Taxo A)

  • Positive indicates Group A (susceptible) with zone of inhibition; negative indicates Group B (resistant).

• Sulfamethoxazole-Trimethoprim Susceptibility Test

  • Positive shows susceptibility to 1.25 µg SXT disk with zone of inhibition; negative indicates resistance.

• CAMP Reaction

  • Tests for synergistic hemolysis between Group B Streptococcus and β-hemolytic S. aureus.
  • Positive results show enhanced hemolysis with an arrowhead pattern; negative shows no zone of β-hemolysis.

• Hippurate Hydrolysis

  • Detects hydrolysis of hippurate acid to benzoic acid and glycine; positive shows deep blue-purple color.

• PYR Test

  • Tests ability to hydrolyze L-pyrolidonyl-β-naphthylamide; positive shows bright pink to red color in 5 minutes.

• LAP Test

  • Hydrolyzes leucine-β-naphthylamide; positive shows red color within 1 minute.

• Bile Esculin Hydrolysis

  • Ability to grow in 40% bile and hydrolyze esculin; positive yields blackening of the agar.

Staphylococcus aureus: Associated Infections

• Skin and Wound Infections

  • Folliculitis, furuncles (boils), and carbuncles (multiple furuncles).
  • Bullous impetigo presents as larger pustules with surrounding erythema.

• Toxin-Mediated Infections

  • Scalded skin syndrome caused by exfoliative toxin.
  • Toxic shock syndrome presents with chills, fever, vomiting, diarrhea, hypotension, and rash.

Other Staphylococcus Species

• Staphylococcus epidermidis

  • Virulence factor: Exopolysaccharide slime enhances adhesion to catheters and devices.
  • Associated with nosocomial UTIs and prosthetic valve endocarditis.

• Staphylococcus saprophyticus

  • Adheres to urogenital epithelial cells; commonly causes UTIs in sexually active young women.

• Staphylococcus lugdunensis

  • Gene mec A for oxacillin resistance; causes endocarditis and UTIs.

• Staphylococcus haemolyticus

  • Known for vancomycin resistance; linked to endocarditis and UTIs.

Laboratory Diagnosis for Staphylococci

• Isolation involves cultural characteristics:

  • S. aureus: Medium to large, creamy, beta-hemolytic, with an old sock odor.
  • S. epidermidis: Small to medium, non-hemolytic, translucent gray-white colonies.

• Biochemical Tests

  • Coagulase Test differentiates S. aureus (positive) from coagulase-negative staphylococci.

• Growth in CNA/PEA

  • Selective for gram-positive bacteria; growth indicates Staphylococcus/streptococcus.

• DNAse Test

  • Detects DNA hydrolysis with clearing around colonies indicating positive results.

• Mannitol Salt Agar

  • Tests ability to ferment mannitol; positive shows yellow halo around colonies.

• Novobiocin Susceptibility Test

  • Differentiates coagulase-negative staphylococci; susceptibility indicated by zone of inhibition.

Viridans Streptococci

• Major throat flora with polysaccharide capsule aiding adhesion.
• Associated diseases include subacute bacterial endocarditis, bacteremia, septicemia, and dental cavities.

Nutritionally Variant Streptococci

• Require vitamin B6 for growth; includes Abiotrophia and Granulicatella species.

Group D Streptococcus: Non-Enterococci and Enterococci

• Non-Enterococci: Includes S. bovis; positive in bile esculin test; linked to bacteremia and endocarditis.
• Enterococci: Includes E. faecalis and E. faecium; significant in nosocomial infections.